Study of Lubrication Oil Ignition in a Rapid Compression Machine
Under Sporadic Pre-Ignition Conditions
ARCHNES
by
TITLIT
M ASS AC
QF V2<'OLOLG'Y
Morgen Paul Sullivan
JUL 3 0 2015
Bachelor of Science in Mechanical Engineering
The University of Texas at Austin, 2013
LIBRARIES
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE IN MECHANICAL ENGINEERING
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2015
Massachusetts Institute of Technology 2015. All Rights Reserved
Signature redacted
Author
Department of Mechanical Engineering
April 17, 2015
Signature redacted
.
Certified by
Wai K. Cheng, Professor of Mechanical Engineering
Director, Sloan Automotive Laboratory
Thesis Supervisor
Signature redacted
Accepted byDavid E. Hardt, Professor of Mechanical Engineering
Chairman, Department Committee on Graduate Theses
2
To my family
3
4
Study of Lubrication Oil Ignition in a Rapid Compression Machine
Under Sporadic Pre-Ignition Conditions
by
Morgen Paul Sullivan
Submitted to the Department of Mechanical Engineering
on April 17, 2015 in partial fulfillment of the requirements
for the degree of Master of Science in Mechanical Engineering
Abstract
In recent years, the industry has shifted toward down-sizing and turbo-charging spark
ignition (SI) engines in an effort to increase fuel conversion efficiency. However, this has
given rise to a destructive phenomenon known as sporadic pre-ignition (SPI). At low
cranking speeds and high loads, engines have been observed to knock violently for brief
and infrequent intervals. If allowed to continue, these periods of knock will result in a
destroyed engine.
This study looks at the propensity of lube oil vapor appearing in the cylinder as a cause for
this phenomenon. The theory is that a local oil vapor/air mixture pocket may auto-ignite
and start a flame in the charge. The pre-ignition would produce extreme knock. A rapid
compression machine (RCM) was used to simulate this scenario and determine if oil vapor
can cause SPI, and if so, to relate the auto-ignition tendency to the oil properties. The RCM
was used to measure the ignition delay of a cloud of oil vapor in a stoichiometric
gasoline/air mixture. The ignition delays were then correlated to chemical and physical
properties of the oils. Finally, the effect of diluting the mixture was assessed.
The results suggest that lube oil is a plausible source of SPI. The oil ignition delay times
are sufficiently short to produce extreme pre-ignition consistent with SPI. Further
supporting evidence lies in the fact that oil ignition delay times concur with SPI behavior
in engines. It was found that the base stock, degradation, and chemical additives all play a
role in oil ignition delay times. The results also demonstrate. that dilution significantly
slows auto-ignition of the oil.
Thesis Supervisor: Wai K. Cheng
Title: Professor of Mechanical Engineering
5
6
Acknowledgements
A number of people deserve my deepest gratitude for their immense support. The first of
which is my advisor, Professor Wai Cheng. If it were not for his guidance, devotion, and
constant involvement at every step, this work would not have been possible. I am truly
grateful for his invaluable support.
Secondly, I would like to extend my thanks to all of the members of the Engine and Fuels
Consortium for their financial and technical support. A special thanks to Richard Davis
and Justin Ketterer from General Motors, Thomas Leone from Ford, David Roth from
BorgWarner, and Professor John Heywood from MIT, for their vital contributions to
research planning and analysis.
I would also like to thank the staff of the Sloan Automotive Lab. I want to especially thank
Thane DeWitt, the Lab Supervisor, and Raymond Phan, the Lab Technician. Thane
constantly provided helpful advice and assistance throughout the entire project while
Raymond offered his expertise with technical issues.
Also, I could not have completed this work without the valued collaboration with my lab
mates. I would like to especially show my gratitude for Felipe Rodriguez who not only
provided valuable advice throughout my time at MIT but also voluntarily helped prepare
my experimental setup. I also want to thank Jacob McKenzie for offering essential help
during the setup phase.
Finally, I would like to thank my entire family for their unwavering support of my goals.
Without my parents and brothers, I would not be where I am today. I cannot express how
grateful I am to them for everything they have done for me. I owe special thanks to my
aunt and uncle, Maureen and Frank Keton, for their incredible support. Without their help,
moving to Boston would have been impossible.
Morgen Sullivan,
Cambridge, MA, April 2015
7
8
Table of Contents
Abstract...............................................................................................................................
5
Acknowledgem ents.......................................................................................................
7
Table of Contents................................................................................................................
9
List of Figures ...................................................................................................................
11
List o f Tables ....................................................................................................................
13
Nom enclature ....................................................................................................................
15
Chapter 1: Introduction ...................................................................................................
17
1.1 M otivation ...............................................................................................................
17
1.2 Background .............................................................................................................
18
1.3 Objective .................................................................................................................
22
Chapter 2: Experim ental Setup .....................................................................................
23
2.1 Rapid Com pression M achine ..............................................................................
23
2.2 Oil Vaporization Electrode and Power Source ....................................................
26
2.3 M ixture Preparation Setup (M ixing Tank)...........................................................
31
Chapter 3: Procedure.........................................................................................................33
3.1 Oil Sam ple Preparation .......................................................................................
33
3.2 Charge Preparation..............................................................................................
34
3.3 Firing and D ata Acquisition .................................................................................
35
Chapter 4: Results .............................................................................................................
37
4.1 Oil Vapor and Ignition Delay...............................................................................
37
4.2 Am ount of Oil.....................................................................................................
40
4.3 Oil Vapor Ignition Stages and Isolation of Valuable Data...................................
41
4.4 Synthetic Versus Non-Synthetic O il ...................................................................
44
9
4.5 New Versus U sed Oil..........................................................................................
47
4.6 Effect of Oil Additives........................................................................................
48
4.7 Effect of Oil Physical Properties........................................................................
53
4.8 Effect of Charge Dilution...................................................................................
57
Chapter 5: Sum m ary and Conclusions...........................................................................
61
5.1 Oil Vapor and Ignition Delay...............................................................................
61
5.2 Oil Characteristics ................................................................................................
62
5.3 Charge Dilution...................................................................................................
62
5.4 Applicability to Engines......................................................................................
62
5.5 Closure ....................................................................................................................
63
Appendix A : Rapid Com pression M achine ..................................................................
65
A .1 RCM Schem atics....................................................................................................
65
A .2 RCM M echanical Drawings [14] ...........................................................................
69
Appendix B : RCM Operating M anual.............................................................................
81
B. 1 The Control Panel..............................................................................................
82
B.2 Initial Preparations...............................................................................................
83
B.3 Firing Sequence...................................................................................................
87
B.4 Shut-D own Sequence ..........................................................................................
89
B.5 Troubleshooting...................................................................................................
90
Appendix C : Lubrication Oil Classification ...............................................................
93
References .........................................................................................................................
95
10
List of Figures
Figure 1.1: Pressure traces for a single SPI event with multiple SPI cycles [1]............ 17
Figure 1.2: Potential SPI mechanism proposed by Toyota [7]....................................
20
Figure 1.3: Ignition delay times as a function of oil concentration ..............................
21
Figure 2.1: Schematic of the RCM built by Kitsopanidis [14]...................
25
Figure 2.2: Schematic of electrode used to rapidly vaporize oil within the cylinder ....... 26
Figure 2.3: Results of testing the power source performance.......................................
29
Figure 2.4: Filament properties as a function of the transformer dial setting................
30
Figure 4.1: Determination of the influence of wire energy on ignition delay (no oil) .
37
Figure 4.2: Effects of liquid oil versus oil vapor and timing of oil vaporization trigger.. 39
Figure 4.3: Varying the amount of oil applied to the wire...........................................
40
Figure 4.4: Illustration of oil-induced-knock stages (NOT ACTUAL DATA)............ 42
Figure 4.5: Wire-initiated flame propagation to isolate oil-specific ignition delay.......... 43
Figure 4.6: Dependence of flame propagation measurement on wire energy (no oil) ..... 44
Figure 4.7: Preliminary test of synthetic versus non-synthetic oil.................................
45
Figure 4.8: Ignition delay times for all 8 oils in milliseconds .......................................
46
Figure 4.9: Ignition delay times for new versus used oils in milliseconds...................
48
Figure 4.10: Ignition delay versus calcium concentration for all 8 new motor oils ......... 50
Figure 4.11: Ignition delay versus molybdenum concentration for all 8 new motor oils. 50
Figure 4.12: Ignition delay versus phosphorus concentration for all 8 new motor oils.... 51
Figure 4.13: Isolation of the effects of additives in lube oils.......................................
52
Figure 4.14: Published physical data versus oil ignition delay times ...........................
56
Figure 4.15: Dilution with nitrogen (fixed fuel amount and k=l)................................
58
Figure 4.16: Oil ignition delay times versus nitrogen dilution .....................................
59
Figure A .1: R CM setup [14].............................................................................................
65
Figure A.2: Pneumatic (driving) system [14] ...............................................................
66
Figure A.3: Hydraulic (locking-releasing) system [14]...............................................
67
Figure A.4: M ixture preparation setup [14].................................................................
68
Figure B. 1: The rapid compression machine .................................................................
81
11
Figure B.2: RCM control panel ........................................................................................
82
Figure B.3: RCM heater panel..........................................................................................
83
Figure B.4: RCM oil reservoir ......................................................................................
83
Figure B.5: RCM sight tube..............................................................................................
84
Figure B.6: RCM pneum atic tank..................................................................................
84
Figure B.7: RCM m ixture preparation setup ....................................................................
85
Figure B.8: RCM pneum atic chamber valve ................................................................
88
..........................................
89
Figure B.10: Disassembled RCM head.........................................................................
o
Figure B.9: RCM firing valve
12
List of Tables
Table 4.1: Eight preliminary lube oils tested in the RCM ...........................................
45
Table 4.2: Additive elements in ten new and used oils tested in the RCM ..................
49
Table 4.3: Concentrations of additive elements in the four "doped" oils .....................
52
Table 4.4: Published and measured physical properties of the test oils........................
54
Table B. 1: Approximate RCM operating pressures..........................................................
85
Table C. 1: American Petroleum Institute base stock classification [18]......................
93
13
14
Nomenclature
Recurring Acronyms
BMEP
Brake Mean Effective Pressure
CAD
Crank Angle Degree
DAQ
Data Acquisition System
EGR
Exhaust Gas Recirculation
GM
General Motors Company
KV40
Kinematic Viscosity at 40*C
KV100
Kinematic Viscosity at 1 000 C
LSPI
Low-Speed Pre-Ignition
PAO
Polyalphaolefin
MoDTC
Molybdenum Dithiocarbamate
RCM
Rapid Compression Machine
RON
Research Octane Number
RTD
Resistance Temperature Detector
SI
Spark Ignition
SPI
Sporadic Pre-Ignition (or Stochastic Pre-Ignition)
SwRI
Southwest Research Institute
ZDDP
Zinc Dialkyldithiophosphate
15
Electric Symbols
C
Capacitance
R
Resistance
Pr
Resistivity
iT
Time constant
U
Voltage
Geometric Symbols
A
Area
d
Diameter
L
Length
V
Volume
Material Symbols
P
Density
h
Heat transfer coefficient
V
Kinematic viscosity
T
Temperature
k
Thermal conductivity
a
Thermal diffusivity
Thermal expansion coefficient
Cw
Wire heat capacity
cw
Wire specific heat capacity
Dimensionless Symbols
Nu
Nusselt number
Pr
Prandtl number
Ra
Rayleigh number
Miscellaneous Symbols
g
Acceleration due to gravity
Air-fuel equivalence ratio
16
Chapter 1: Introduction
1.1 Motivation
For many decades, spark ignition (SI) engines have been plagued by the existence of
abnormal combustion. In recent years, manufactures have shifted toward down-sizing and
turbo-charging their SI engines in an effort to increase fuel conversion efficiency and
reduce the relative losses associated with pumping work and friction. Unfortunately, this
trend has given rise to a distinct new form of abnormal combustion known as sporadic preignition (SPI), also known as low-speed pre-ignition (LSPI), superknock, or megaknock.
SPI typically occurs in highly boosted engines operating at low speeds (1,000 to 3,000
rpm) and high loads (above 18 bar BMEP). At these conditions, an SPI event will typically
occur once every 10,000 to 15,000 cycles; however, each event will usually consist of more
than one pre-ignition cycle. Figure 1.1, obtained from the Southwest Research Institute
(SwRI) [1], shows what a typical SPI event looks like.
150-
140-
120-
i
100-
25
0
0
0
*3
s0-
0
0
0
53
25
0~
0
V
C
53M 0
80-
5
Cyd e No,
0
40-
20
TDC"
n
330
345
30
Spa*k
Timing
-5,0deg bTDC
375
390
CAD
405
420
435
450
Figure 1.1: Pressure tracesfor a single SPI event with multiple SPI cycles [1]
17
Figure 1.1 shows several pressure traces for normal cycles and pre-ignition cycles. Five of
the six pre-ignition cycles begin early enough to develop into heavy knock, which is
characterized by extreme peak pressures and pressure oscillations. These five cycles are
considered SPI cycles. One of the six pre-ignition cycles begins slightly later, resulting in
light knock. A key difference between SPI and typical knock is that a much larger portion
of the end gas is auto-ignited, and in extreme cases more than half of the entire charge is
auto-ignited [2]. The cycle peak pressures plotted in the upper right corner of Figure 1.1
demonstrate two points: SPI cycles reach very dangerous peak pressures, and an SPI cycle
is almost always followed by a regular cycle.
SPI is characterized by a very loud and violent "knocking" noise that is usually obvious to
the driver. If allowed to continue, the extreme pressures will result in serious damage, most
commonly to the piston crown and top land. Due to the extremely destructive and
increasingly ubiquitous nature of SPI, it has become the focus of many studies throughout
the world.
1.2 Background
Several theories have been raised regarding the cause of SPI. Most hypotheses include one
of the following fundamental mechanisms, which have been compiled by the French
Institute of Petroleum (IFP) [3]:
1. Hot surface sites
2. Hot soot particles
3.
Fuel pooling sites
4. Interaction with lube oil
Hot surface sites or "hot spots" (e.g. exhaust valves, spark plug, etc...) have been proposed
as possible sources for SPI; however, there is an overwhelming amount of evidence to
suggest that hot spots are not to blame [3]. As shown in Figure 1.1, an SPI event is finite,
and each SPI cycle is typically followed by a regular cycle. If hot spots in the engine were
the source of ignition, then one would expect to see pre-ignition over extended consecutive
18
cycles. In extreme cases, this could even develop into "run-away" pre-ignition, which is a
positive feedback loop whereby heat from the hot spots causes pre-ignition, thus increasing
the temperature of the hot spot.
Hot soot particles or deposits in the cylinder are thought to be another potential source of
ignition. This has been ruled out because SPI has been observed immediately following a
thorough cleaning of the cylinder [4] [5].
Fuel pooling sites are yet another proposed ignition source. The theory is based on the idea
that poor fuel injection techniques are causing liquid fuel to pool in the cylinder, which
creates a very rich vapor diffusion zone that ultimately leads to more rapid auto-ignition
[3]. This theory can be ruled out since SPI has been detected even with port injection using
a gaseous fuel [3].
Interaction with lube oil is the most promising theory and is the focus of this study. The
hypothesis is that lube oil is entering the cylinder where it vaporizes and mixes with the
charge. Having a characteristically short ignition delay, the oil vapor ignites long before
the spark. The resulting heat release starts a flame in the surrounding mixture. As the flame
propagates, the ensuing increase in temperature and pressure forces a significant amount
of fresh charge to auto-ignite. This theory has been supported by an overwhelming number
of studies [4] [5] [6] [7] [8] [9] [10].
Two important questions need to be asked when considering the validity of this theory:
1. How is oil entering the cylinder in the concentrations needed to produce SPI?
2. Can oil vapor in the charge actually produce SPI?
One of the many objectives of this project is to help answer the latter question. The first
question has been extensively studied by several companies including General Motors [4]
[6] and Toyota [5] [7] [8] [10]. Toyota has developed a hypothetical mechanism in hopes
of answering question 1. This mechanism is shown in Figure 1.2.
19
[Step 11 : Dilution
ISten 21 : Accumulation
Oil Film
Accumulation of Fuel / Oil Mixture
in Piston Top Land Crevice
1 2 irect Injection
il Dilution by Fuel Injection
Piston Top Land
M
IStep 31 : Release-Out
IStep 41 : Auto-Ignition
Oil Droplet released out
from Piston Top Land Crevice
Oil Droplet
Vaporization
lame
2
n Lyer
Propagatio
Oxidation
Figure 1.2: PotentialSPI mechanism proposed by Toyota [7]
Several studies have indicated that SPI is strongly correlated to liner wetting and fuel
dilution [4] [8] [11], which supports Step 1 shown in Figure 1.2. Fuel dilution facilitates
oil infiltration into the cylinder by lowering viscosity [12] and compromising film strength
[13]. Once the oil is exposed to the charge, fuel dilution advances the process further by
increasing volatility and promoting vaporization of the oil droplet [13].
While Steps 1 through 3 in Figure 1.2 have received strong support, some authors have
expressed skepticism about Step 4 [10] [11]. One study by Toyota itself even questioned
the mechanism by claiming that the time required to auto-ignite a liquid droplet of oil was
20
too long to be causing SPI at normal temperatures. Therefore, they claim that the droplet
must be exposed to combustion in cycle 'n' before it causes SPI in cycle 'n+l' [10]. This
theory could potentially explain why each SPI cycle is almost always preceded by a regular
cycle. A second study found similar results regarding the time required to auto-ignite a
liquid droplet of oil; however, while believing that oil was indeed contributing to preignition, they were unable to identify a specific ignition source [11].
Regardless of the mechanism by which oil vapor is appearing in the cylinder, it is yet to be
determined whether oil vapor can even ignite fast enough to produce SPI. Measuring the
time required to auto-ignite a liquid droplet of oil only tests the specific theory shown in
Figure 1.2; however, measuring the ignition delay of oil vapor temporarily neglects the
physical mechanism by which the oil vapor appears in the cylinder.
As a result, the precursor to this study sought to measure oil vapor ignition delay times.
The experiment used a rapid compression machine (RCM) to measure the ignition delay
of homogeneous mixtures containing lube oil, fuel, and air. The objective of the experiment
was to determine if oil vapor in the mixture could in fact produce ignition delay times that
were short enough to cause SPI. The results of this experiment are shown in Figure 1.3.
18
16
14
E
12
10
Vm
C
0
8
-4
2
0
I
0%
40%
20%
60%
Oil volume in fuel mixture
Figure 1.3: Ignition delay times as a function of oil concentration
21
The ignition environment for the experiment was about 660 K and 30 bar, which is
comparable to SPI conditions in an engine. As expected, the ignition delay time decreased
as oil concentration increased; however, at an oil concentration above 50% by volume in
the oil-fuel mixture, the oil began to condense onto the walls of the cylinder. This is an
issue because at a concentration of 50% the ignition delay time was still 6 ms, which is too
long to produce SPI. Therefore, a new method had to be devised so that higher
concentrations could be achieved.
1.3 Objective
The main objective of the study is to assess, via ignition delay measurements in an RCM,
the plausibility of the oil mechanism for initiating sporadic pre-ignition in modem engines.
If oil appears to be a viable cause of SPI, then the next step is to determine how oil
characteristics might impact ignition delay times in the RCM and then compare these
results to findings in engine studies. The oil properties that are to be analyzed are base
stocks, deterioration, chemical additives, and physical properties.
Finally, charge dilution will be assessed as a tool for mitigating SPI. The goal is to gain a
more fundamental understanding of how charge dilution is reducing pre-ignition frequency
in engines, especially as it pertains to SPI.
22
Chapter 2: Experimental Setup
2.1 Rapid Compression Machine
As the name implies, an RCM is a device used to compress a mixture of fuel and oxidizer
in a combustion chamber by means of a rapidly moving piston, so that there is negligible
chemical reaction during the compression process. However, the piston compresses the
gases enough to produce an ignition environment resulting in auto-ignition of the charge.
The piston in an RCM is held fixed after compression, yielding a constant-volume
environment.
The RCM used in this study was built at MIT in 2000 by loannis Kitsopanidis [14] based
on a design by Affleck and Thomas at Shell's Thornton Research Centre [15]. A schematic
of the RCM acquired from Kitsopanidis' thesis [14] is shown in Figure 2.1. Compression
occurs in roughly 15 ins. See Appendix A for a detailed schematic of the RCM.
The RCM consists of three fundamental chambers: the combustion chamber, the hydraulic
chamber, and the pneumatic chamber. Each chamber contains a piston attached to the
piston assembly. The combustion chamber piston is analogous to an engine piston and is
used to compress the experimental charge. The hydraulic chamber piston is used to lock
the piston assembly in the retracted (ready-to-fire) state by means of a large pressure
differential being placed across it. The pneumatic chamber piston is responsible for driving
the piston across the RCM during compression.
The combustion chamber is covered with several resistance heaters and resistance
temperature detectors (RTDs) used to control the initial temperature of the charge and to
more closely represent actual engine conditions. The cylinder pressure is measured by a
Kistler 6125A pressure transducer inserted into the wall of the combustion chamber. The
bore is 5.08 cm (2 in) and the stroke is 20.32 cm (8 in).
Silicone oil (Dow Coming 200 series) is used in the hydraulic chamber due to its low vapor
pressure and low toxicity. The hydraulic piston and hydraulic chamber are designed with
a pin and groove (see Figure 2.1), which are used to prevent damage during rapid
23
deceleration of the piston. As the piston nears the end of the stroke, the pin must enter the
groove in the chamber where trapped oil is forced through a 0.7 mm clearance between the
pin and groove. The viscous friction creates a significant deceleration force on the piston,
preventing the assembly from violently slamming into the end of the chamber at full speed.
The opposite side of the hydraulic piston contains an 0-ring to seal off the rear portion of
the hydraulic chamber. When the piston assembly is fully retracted, the hydraulic piston
0-ring contacts the rear surface, sealing the two portions of the chamber from each other.
This allows a pressure differential to be placed across the piston.
The pneumatic chamber is driven by compressed nitrogen stored in a large reservoir, which
is about 50 times the volume of the pneumatic chamber. The reservoir and pneumatic
chamber are connected by a pipe with a very large diameter. The large diameter and large
reservoir volume both promote a relatively constant driving pressure, despite the volume
in the pneumatic chamber rapidly increasing with piston motion.
Overall, the piston assembly is about 6 kg (13.2 lbs). The pneumatic piston has a diameter
of 12.7 cm (5 in), making it 6.25 times the area of the combustion chamber piston and
about 5 times the area of the hydraulic piston. This means that high pressures can be
achieved in the combustion chamber with relatively low driving pressure; however, the
locking pressure applied to the hydraulic chamber must be at least 5 times the driving
pressure in order to create a force balance and lock the piston in the retracted state before
firing. See Appendix B for detailed steps on how to operate the RCM.
24
DETAIL
combustion
combustion
hydraulic
hydraulic
chamber
piston
chamber
piston
pneumatic
chamber
pneumatic
piston
0.22 m
(AM
j
DRAWN IN SCALE
1.17 m
Figure2.1: Schematic of the RCM built by Kitsopanidis [14]
-
pin and groove
mechanism
2.2 Oil Vaporization Electrode and Power Source
As mentioned before, the study needs to produce highly concentrated pockets of oil vapor
within the cylinder to simulate and observe one possible mechanism by which sporadic
pre-ignition is thought to occur. In order to create high concentrations at the ideal moment
(end of compression), a small nichrome filament inside the combustion chamber is used to
rapidly vaporize a coating of liquid oil just when the piston is reaching the end of the stroke.
The filament is very quickly heated by driving a large but brief pulse of current across the
wire. A schematic of the electrode is shown in Figure 2.2.
ground
termina
-
positive
terminal
electric
insulator
nichrom e
filament
-70
mm
Figure 2.2: Schematic of electrode used to rapidly vaporize oil within the cylinder
The electrode shown in Figure 2.2 is fixed in the head of the RCM with the electrical
terminals external to the machine. When installed, it yields a compression ratio of just
under 9.1. The nichrome wire is about 2 cm (0.8 in) in length and 255 pm (0.01 in) in
26
diameter (#30 AWG) with an electrical resistance of roughly 0.43 Q, which was calculated
with the following equation:
PrL
AX
where pr is the resistivity of nichrome (about 1.1 x 10-6 Q-m), L is the length of the wire,
and AX is the cross-sectional area of the wire. When fully coated, the oil film has a mass of
approximately 1 mg. In order to accurately measure this small mass with a microgram
scale, it was necessary to design the entire part to be under 200 g.
The power source used to drive current across the wire consists of four parts: a transformer,
a rectifying circuit, a capacitor, and a triggering circuit used to release the energy stored on
the capacitor. The four components work together to produce an electric pulse and vaporize
the oil film in the following steps:
1. AC voltage is sent into the transformer which reduces it to a specified voltage.
2. The resulting voltage is then rectified to DC and used to charge the capacitor.
3.
At the precise moment that the oil is ready to be vaporized, a signal is sent to the
triggering circuit which flips a switch, closing the circuit between the charged
capacitor and the oil-coated filament.
4. The current from the capacitor is then allowed to flow across the wire which heats
it enough to vaporize the oil film and create a locally concentrated cloud of oil
vapor around the wire.
The capacitor has a capacitance of about 2040 pF and requires at least 20 seconds to be
sufficiently charged. After the power source has been triggered, the capacitor and wire can
be modeled as a discharging RC circuit. Assuming for simplicity that the wire is adiabatic,
then the electric power in the circuit is used entirely to heat the wire and is modeled by the
following equation:
27
dT
C Wdt
U)
-
2
(
e-t
12
)]
R ;r=R
RC
where C, is the heat capacity of the wire (about 0.0039 J/K), T is the wire temperature, U0
is the initial voltage on the capacitor, T is the electrical time constant of the circuit (about
0.88 ms), and C is the capacitance. After rearranging and integrating, the following
equation can be used to determine the adiabatic wire temperature change as a function of
capacitor voltage and time:
[
(T - TO) =
-[e2t/]
The time constant for the above two equations is half that of the voltage time constant
(Tthermal=
0.44 ms), therefore 90% of the energy is deposited on the wire after 1 ms.
For the above calculations to be accurate, convection losses from the wire must be
negligible for this time scale. Assuming the wire is a horizontal cylinder under free
convection, the Nusselt number can be used to determine the ratio of convective to
conductive heat transfer. This is given by the following equation:
i
0.387R aL/L
Nu = 10.6 + [1 + (0.559/Pr)9/16]8/27
[16]
where Pr is the Prandtl number (taken as 0.7), and Ra is the Rayleigh number governed
by the following equation:
Ra = fATgd
av
[16]
where fl is the thermal expansion coefficient of the gas (1/T for ideal gases), AT is the
difference in temperature between the wire and gas (about 300 K), g is the acceleration
due to gravity, a is the thermal diffusivity of the gas (about 4x10 5 m 2 /s), and v is the
kinematic viscosity of the gas (about 5x 10-5 m2 /s). With these values, the Rayleigh number
28
is about 0.08, resulting in a Nusselt number of 0.66. This suggests that heat transfer is
almost purely via conduction.
The Nusselt number can be used to determine the thermal time constant due to convective
cooling of the wire with the following equations:
Tconvection =
hAs
[17]
k
h = Nud
where h is the heat transfer coefficient, As is the surface area of the wire, and k is the
thermal conductivity of the surrounding gas (about 0.05 W/m-K). This equation gives a
time constant of 1.86 sec, which is orders of magnitude larger than the 0.44 ms associated
with heating the wire. As a result, convective losses from the wire can be neglected.
The calculations above agree very well with real-world measurements obtained from the
power source which can be seen in Figure 2.3.
Tek
TRiGGER
M Pos: -1.000ns
J ea
Jr
Type
Trigger Signal
.E
1Capacitor Voltage
Mode
itch,.ma
RC.
(MOSFET Drain)
CH1
CH3 20NJ9
H
,0 V
M1.00ms
28-Mar-1414:25
*
Coupling
T is about 0.9 ms
CH /4,
H
Figure2.3: Results of testing the power sourceperformance
29
The results in Figure 2.3 show a discharge time constant of about 0.9 ms, which agrees
very well with the calculated 0.88 ms time constant described earlier. Again, the time
constant in Figure 2.3 corresponds to the voltage on the capacitor; the time constant for the
energy deposited on the wire and the corresponding temperature rise are expected to be
half as long.
Since it was demonstrated that convective heat loss is negligible over this time scale, the
wire is assumed to be adiabatic. Under this assumption, the maximum wire temperature
rise can easily be calculated for the range of transformer voltage settings. The results are
tabulated in Figure 2.4.
4500
- 16
- 14
-
- 12
3000
.
..
...
........
~
(j)
-6
- - - ..
- --....
-4
.....
- -...
- -
.
1000
..
~
-
1500
- 10 Energy of
Electric
Pulse
- 8
-
Adiabad ic
2500
Wire Temp
Rise
2000
(0 C)
-
.
...-..
3500
-2
500
-
.....
...
-
4000
0
10
20
I
I
1
1
30
40
50
60
0
70
80
Transformer Dial Setting (V)
Figure 2.4: Filamentpropertiesas a function of the transformerdial setting
Again, the values in Figure 2.4 correspond to the adiabatic temperature increase, so the
actual temperature increase is expected to be lower.
30
The power source is triggered by a digital output signal from the data acquisition system
(DAQ) controlled by LabVIEW. The software uses the pressure measurements from the
combustion chamber to identify the end of compression and then triggers the power source
to heat the wire after a specified delay.
2.3 Mixture Preparation Setup (Mixing Tank)
Each firing of the RCM in this study required the combustion chamber to be filled with a
mixture of gasoline (Haltermann 437; 97 RON) and air. To promote consistency, a separate
heated tank, referred to as the "mixing tank," is used to prepare a relatively large amount
of stoichiometric mixture. Gasoline is injected into a small port in the tank using a syringe.
Compressed air is then metered into the tank until stoichiometry is achieved. This mixing
tank has a much larger volume than the combustion chamber, and the gases are held at a
high pressure to allow for several firings with the same mixture. Other gases can also be
metered into this tank to produce different types of mixtures. The mixing tank contains a
rotating fan to ensure that the various gases are well mixed and homogeneous. See
Appendix B for a more detailed description on how the mixtures are prepared.
31
32
Chapter 3: Procedure
3.1 Oil Sample Preparation
For most of the runs in this study, a sample of oil was being tested in the RCM. There were
six steps in preparing these samples:
Step 1. Detach the power source and remove the vaporization electrode from the RCM
Step 2. Connect a fresh nichrome filament to the electrode
Step 3. Record the electrode weight before oil has been applied
Step 4. Apply the oil sample to the filament using a cotton swab
Step 5. Record the electrode weight after oil has been applied
Step 6. Replace the electrode into the RCM and reattach the power source
Typically, Steps 3 and 5 were omitted, except in cases where it was imperative to know the
mass of the oil on the wire (more on this in Section 4.2). Step 1 required the head of the
RCM to be removed, which is held in by four bolts. Once removed, it was deemed
necessary to use a new wire for each run due to slight oxidation of the wire after each firing,
which leads to a change in electrical and thermal properties. It also was found that used
wires can contain trace amounts of oil residue, which can contaminate the results. The mass
of the oil was determined by measuring the difference in overall mass before and after oil
had been applied. The oil specimen was applied with a cotton swab, and typically the entire
surface area was coated (-1 mg). Finally, the electrode was placed back into the head of
the RCM with its new wire and coating of oil. The power source was reattached to the
terminals making the sample ready to be tested.
In order to promote consistency, while the head was removed, the cylinder and electrode
were very thoroughly cleaned with acetone and then flushed with air to prevent deposits or
contamination from previous oil samples from impacting the results. It should be noted that
while the oil visibly created a film across the entire coated surface of the wire, surface
tension did cause small portions of accumulation to form.
33
3.2 Charge Preparation
Every experiment presented in this study required some sort of gaseous mixture to be
compressed in the RCM. There were six steps to preparing that mixture:
Step 1. Evacuate the mixing tank
Step 2. Inject gasoline into the evacuated tank
Step 3. Meter in air to achieve stoichiometry using partial pressures
Step 4. Evacuate the combustion chamber
Step 5. Fill the combustion chamber with stoichiometric charge from the mixing tank
Step 6. Add any additional gases specific to each experiment (e.g. dilution)
Steps 1 through 3 needed to be performed roughly once every 5 to 10 runs (until the mixture
was used up) while Steps 4 through 6 needed to be repeated for each run. In Step 1, the
mixing tank was evacuated to a pressure reading between 1.0 and 1.5 torr (-0.13 to 0.20
kPa), which depended on temperature since the pressure gauge drifts when heated. Then
roughly 30 to 40 torr (-4.0 to 5.3 kPa) of gasoline were injected into the evacuated mixing
tank. Finally, air was metered into the mixing chamber, accounting for the initial 1.0 to 1.5
torr pressures, until a stoichiometric mixture was produced. At this stage, the mixing tank
contained around 2.5 bar (250 kPa) of fresh air-fuel charge, which is the pressure limit of
the mixing tank. At this pressure, the mixing tank supplied charge for 5 to 10 runs.
Next, the charge had to be transferred to the combustion chamber before each run. This
was done by evacuating the cylinder down to 2.0 torr (0.27 kPa) and then metering fresh
charge from the mixing tank to the evacuated chamber. For nearly every run in this study,
1.490 bar (149.0 kPa) of fresh stoichiometric charge was supplied to the combustion
chamber. Finally, for a select few runs, diluent gases were metered into the chamber.
For nearly every run, the mixing tank and combustion chamber were held at 315 K (42C)
before compression, and all the pressures during Steps 1 through 6 above were monitored
with extreme care to promote consistency. See Appendix B for a more detailed procedure
on mixture preparation.
34
3.3 Firing and Data Acquisition
Once the oil specimen was prepared for testing and the cylinder had been filled with a fresh
charge, the RCM was almost ready to be fired. Before the gases could be compressed, there
were four steps that needed to occur:
Step 1. Apply pressure to the hydraulic chamber to lock the piston in place
Step 2. Add driving pressure to the pneumatic chamber
Step 3. Wait 10 minutes for the gas pressure and temperature to reach equilibrium
Step 4. Release the pressure from the hydraulic chamber to fire the RCM
The locking pressure in Step 1 was about 1750 psi (-12,000 kPa), and the driving pressure
in Step 2 was roughly 275 psi (-1,900 kPa). It was determined that the 10 minute
equilibration period was essential for repeatability. For a more detailed step-by-step
process of operating the RCM see Appendix B.
After the four steps had been completed, the gases in the combustion chamber began to
compress. Vaporization of the oil needed to occur at a specified point after compression
had begun. This was handled by the DAQ. There were four steps performed by the DAQ:
Step 0. The RCM has been fired and compression has begun
Step 1. The DAQ continuously monitors the cylinder pressure
Step 2. Once a specified pressure threshold has been crossed, the DAQ reacts
Step 3. The DAQ triggers the vaporization process after an optional delay
Step 4. Pressure data is recorded and stored over a specified period of time
For Step 1, the DAQ sampled the cylinder pressure at a rate of 10 kHz, and for most runs
in this study the pressure threshold in Step 2 was set to 23 bar, to indicate the approximate
end of compression. In a few experiments, the use of a vaporization delay was required
(Step 3); however, in most runs this delay was not used (i.e. vaporization occurred
immediately after detecting the pressure threshold). The pressure was then recorded at the
10 kHz sampling rate and stored for analysis.
35
36
Chapter 4: Results
4.1 Oil Vapor and Ignition Delay
The first step was to confirm that the setup performed as expected. In other words, if
everything went to plan the wire should successfully vaporize a small film of oil, and there
should be a significant decrease in ignition delay as a result of rapid auto-ignition of the oil
vapor. Before this could be observed, it was necessary to confirm that the individual effects
of the liquid oil (un-vaporized film) and the wire heat had insignificant impacts on ignition
delay.
First, an appropriate wire energy had to be selected such that the oil was vaporized, but the
wire heat did not impact ignition delay. This was done by repeatedly exposing a dry wire
(no oil-coating) to a stoichiometric environment while sweeping a range of voltage settings
on the transformer. The objective was to observe at what minimum setting the ignition
delay of the charge was decreased (i.e. at which point the heat was no longer negligible).
The results of this experiment are shown in Figure 4.1.
160
Wire Ignites Mixture
V ire Does NOT Ignite Mixture
32 Volt Setting
3 0 Volt Setting (4 trials)
Wire AT: ~660*C
V ire AT: ~58 0 1C
140
-
120
100
80
J
40
S40
20
0
0
40
20
J
80
60
100
120
140
Time (ms)
Figure4.1: Determination of the influence of wire energy on ignition delay (no oil)
37
The range of swept voltage settings between 10 and 30 volts had negligible effects on the
stoichiometric ignition delay; however, Figure 4.1 shows that the 32 volt setting on the
transformer actually started a flame in the mixture. The flame caused ignition to occur
significantly earlier than when lower settings were used. This implies that a transformer
setting of 30 volts was the maximum that could be used. Although the 2 volt difference
seems negligible, it actually corresponds to a change in the adiabatic temperature rise of
800C, which is not negligible.
The 30 volt setting on the transformer corresponds to about 2.2 J of energy being deposited
onto the wire which translates into an adiabatic temperature increase of 5801C in the wire.
This experiment made the maximum limit clear, but it was still unclear if this setting was
above the minimum energy required to vaporize a film of oil. Testing this was much less
quantitative and was determined by simply observing what happened when the wire was
coated and subsequently heated. The 30 volt setting was observed to successfully vaporize
a full coating of oil outside the cylinder of the RCM. As a result, the 30 volt setting was
used from then on to vaporize the oil film.
Next was to test the difference in ignition delays between liquid oil and vaporized oil when
exposed to the stoichiometric ignition environment. This was done by first testing a control
case with no oil and no current sent across the wire. Then, the liquid oil case was tested in
which the wire was coated in a film of oil, but the wire was never heated. Finally, the wire
was coated in oil and heated at three different times during and after compression. The
results of the experiment are shown in Figure 4.2.
38
200Oil
180
End of Compression:
~26 bar
~65O K
Oil Vaporized
at Late Trigger
(3 Trials)
Vaporized
at Normal Trigger
(3 Trials)
160
Oil Vaporized
at Early Trigger
(3 Trials)
140
160
Trials)
(33
C
L)
Normal
S80
60Noi/otwr
NorTmals
40
60
att TrralTrggrgONRO
Early Trigger
20
0
20
40
80
60
100
120
140
Time (ins)
Figure4.2: Effects of liquid oil versus oil vapor and timing of oil vaporization trigger
The results shown in Figure 4.2 suggest that the heat from the wire and the liquid oil did
not independently influence the ignition delay; however, when combined and the oil was
vaporized, a flame was started in the cylinder due to rapid auto-ignition of the oil vapor
pocket. The flame propagated throughout the stoichiometric charge and resulted in violent
end-gas auto-ignition. This produced knock, which is characterized by the extreme peak
pressures and oscillatory pressure waves. The onset of knock is not clear in Figure 4.2,
however, it will become much more evident in later plots.
Finally, the trigger timing had little impact on ignition delay of the oil. This suggests that
the physical processes associated with vaporization and mixing were not significantly
influenced by the final stages of compression. On the same token, this was evidence that
the delay associated with physical processes was not insignificant.
39
The results followed expectations very well. The wire successfully vaporized the film of
oil without significantly effecting the charge. The oil vapor auto-ignited very early,
resulting in end-gas auto-ignition and knock.
4.2 Amount of Oil
Figure 4.2 shows that when the proper wire setting is used, wire heat and vaporization
timing have negligible effects on the results, but up to this point it was not clear how the
mass of the oil was impacting the timing. To find out, a range of oil masses were applied
to the wire and tested. A full coating of the wire corresponds to approximately 1 mg, so it
was only feasible to decrease the amount from there. Three different masses were tested:
1.0 mg, 0.5 mg, and 0.2 mg. The digital scale that was used could only accurately measure
down to 0.2 mg. Nevertheless, the three quantities were tested, and the results are shown
in Figure 4.3.
180
160
11.0
mg of Oil (-90 pM film)
140
10.5
mg of Oil (~50 pm film)
120
a-
10.2 mg of Oil (~20 p.m film)
100
80
Trigger
-o
40
20
0
20
60
40
80
100
120
Time (ms)
Figure 4.3: Varying the amount of oil appliedto the wire
Figure 4.3 shows that the amount of oil over the quantities tested had little effect on ignition
time. This would suggest that the 0.2 mg film was above the minimum amount of oil
40
required to produce a fully concentrated pocket of oil vapor. A mass of 0.2 mg is
characteristic of typical oil droplets observed in a test engine at MIT (about 0.1 to 0.2 mg).
4.3 Oil Vapor Ignition Stages and Isolation of Valuable Data
Before looking at the effects of the different oils, it was important to understand what was
actually happening after the oil was vaporized and to understand which specific portions
of the data were influenced by oil properties. Ultimately, the goal was to isolate the impacts
that were unique to each oil, so that a better understanding of ideal oil properties could be
achieved.
There are four fundamental stages associated with oil-induced-knock:
Stage 1.
Pre-Vaporization Compression
Stage 2.
Oil-Specific Ignition Delay
Stage 3.
Oil-Independent Flame Propagation
Stage 4.
Knock
Stage 2 began at the end of compression when the wire was heated, vaporizing the oil film
in approximately 0.5 ms. The oil vapor had to then diffuse away from the wire and mix
with the oxygen in the charge. Once exposed to the ignition environment, there was a short
chemical delay that was specific to each oil. Finally, the oil vapor auto-ignited, starting a
small flame in the stoichiometric mixture, which initiated Stage 3.
Stage 3 was independent of which oils were used, but instead depended on the properties
of the stoichiometric mixture, which was kept constant for these experiments. During this
stage, the flame spread throughout the stoichiometric mixture. This increased the
temperature and pressure of the end gases, ultimately resulting in detonation (occurring
around 70 bar).
Stage 4 was characterized by the onset of knock, which was visibly apparent as a sudden
change in slope. Knock is characterized by very large peak pressures and pressure
41
oscillations. Substantial heat loss through the cylinder walls occurred during this stage. The
four stages are illustrated in Figure 4.4.
200
GRAPHIC: NOT
ACTUALDATA
180
Oil-Specific
Ignition
Delay
160
Oil-independent
Flame Propagation
Period
140
120
Vaporization
100
Mixing, &
80
Chemcial
Delay
-
CL)
60
I
Oil Vaporization
Trigger
40
Highly
Exaggerated Oil
Auto-Ignition
Bump Starts a
I
Ii.
Onset of
Knock
Flame
20
0
0
5
10
10
15
Time (ms)
20
25
30
Figure4.4: Illustrationof oil-induced-knock stages (NOT ACTUAL DATA)
In actual data, the oil auto-ignition bump was not discemable outside of the noise of the
transducer, which made it difficult to determine the oil-specific ignition delay time. Since
this study is focused on how oil might influence sporadic pre-ignition, we were really only
concerned with the duration of Stage 2 for each individual oil.
The total time from the oil vaporization trigger to the onset of knock can be measured,
since both times are known. Therefore, the combined duration for Stages 2 and 3 can be
obtained from the data. This means that if the duration of Stage 3 can be measured then the
duration of Stage 2 can be calculated, which is exactly the purpose of the next experiment.
In the next experiment, the heat from the wire was used to simulate auto-ignition of the oil
and initiation of flame propagation (Stage 3). By significantly increasing the heat of the
42
wire, a flame could be started at the time of the trigger allowing us to isolate and measure
Stage 3. Subtracting the measured duration of flame propagation (Stage 3) from the total
time required for the onset of knock (Stages 2 & 3), we were left with the oil-specific
ignition delay time (Stage 2). The results of this experiment are shown in Figure 4.5.
200
Oil-Specific Oil-independent
Flame Propagation
Ignition
(7.7 ms)
Delay Time
180
160
140 A
Black Curve is
WITH Oil
(Vapor-Ignited)
120
S120
(A00Tige
100
C
~0
80
Hot Wire
Oil Vaporization
Trigger
(30V Setting)
80(70V
Flame Initiation
Trigge
Setting)
60
Red Curve is
NO Oil
(Wire-Ignited)
40
20
00
5
15
10
20
25
30
Time (ms)
Figure4.5: Wire-initiatedflame propagationto isolate oil-specific ignition delay
Using the wire to initiate the flame resulted in a flame propagation duration measurement
of 7.7 ms. To ensure that the heat was actually igniting the mixture at the trigger point, the
transformer dial setting was increased to 70 volts. At lower voltages, there was a slight
delay between the trigger and when flame propagation actually began. To make sure that
the 70 volt setting was immediately starting a flame, a range of voltage settings were tested
between 40 and 70 volts. The corresponding flame propagation measurement for each
setting was plotted versus wire energy, and the results are shown in Figure 4.6.
43
11
E
40V
10
Time to knock is no longer
dependent upon
45V
transformer voltage setting
0
0
E
55V
0
2
4
6
8
-65V
10
'70V
12
14
Wire Energy (J)
Figure4.6: Dependence offlame propagationmeasurement on wire energy (no oil)
Since the time between the trigger and onset of knock reached an asymptotic minimum
around 8 J, this minimum was attributed to the time required for the flame to propagate.
This would mean the time to initiate the flame had approached zero. Therefore, it can be
inferred from the results that flame propagation took about 7.7 ms, under the current
conditions.
4.4 Synthetic Versus Non-Synthetic Oil
It has been proposed by some automotive companies that different base oils lead to
different SPI behavior in engine tests. The American Petroleum Institute (API) has divided
all base stocks into five categories, which are detailed in Table C. 1 in Appendix C. Groups
I, II, and III are refined from petroleum crude oil and are considered mineral oils while
Group IV is made up of polyalphaolefins (PAO) which are 100% synthetic. Group V is a
catch-all for any oil that does not fall into the first four groups [18].
General Motors (GM) has observed significantly lower SPI frequency when using a PAO
base stock (synthetic) instead of a standard hydrocrack base stock (non-synthetic) [6]. A
similar claim has been made by Toyota in that the use of Group IV (synthetic) base oils
resulted in lower frequency of SPI than Groups I or II (non-synthetic) [5]. In theory, these
results should translate into synthetic PAO oils having longer ignition delays.
44
Eight different oils were tested in the RCM: 2 non-synthetics, 3 synthetic blends, and 3 full
synthetics (one of which was a gear lube). Two of the oils were recommended by GM after
producing notably dissimilar SPI behavior in engine tests. The oils are listed in Table 4.1.
Table 4.1: Eight preliminary lube oils tested in the RCM
|
Oil B
15W-40
oil C
Oil D
Oil E
Gear Lube
5W-20
5W-30
15W-50
75W-90
Blend
Blend
Full synthetic
Full synthetic
GM A
GM B
5W-30
5W-30
Blend
Full synthetic
Non-synthetic
Each of the eight oils were vaporized in the RCM at the end of compression, and the total
time from the vaporization trigger to the onset of knock was compared between them.
Figure 4.7 shows representative curves for Oil A and Oil E.
200
180
Oil A
Non-Synthetic
15W-40
160
140
(2 trials)
120
MJ
L
(2 trials)
100
80
C
Oil E
Full Synthetic
15W-50
Trigger
LI-
60
40
20
0
0
5
15
10
20
25
Time (ms)
Figure4.7: Preliminarytest of synthetic versus non-synthetic oil
45
30
After testing for all eight oils had been thoroughly repeated in the RCM, an average time
to the onset of knock was calculated for each. Using the method explained in Section 4.3,
the auto-ignition time for each oil was calculated by removing the time associated with
flame propagation. The resulting oil-specific auto-ignition times for all eight oils are
tabulated in Figure 4.8.
These times include vaporization, mixing, and
chemical ignition delay.
5.9
6.1
5.4
5.1
4.4
6.0
4.6
4.0
Oil C
Oil A
(5W-2 D) (15W-40)
GM B
Oil B
GMA
(5W-30) (15W-40) (5W-30)
Oil D Gear Lube Oil E
(5W-30) (75W-90) 15W-50)
Figure 4.8: Ignition delay times for all 8 oils in milliseconds
Figure 4.8 shows that in general the oils with full synthetic base stocks had longer ignition
delays than those with non-synthetic base stocks. There was no definitive difference in the
ignition delays between non-synthetics and blends.
The difference between the two GM recommended oil ignition delay times was about 0.8
ms, which corresponds to just under 10 CAD in an engine operating at 2000 rpm. This was
small compared to the 2.1 ms difference between Oil C and Oil E, which is approximately
25 CAD at 2000 rpm. This suggests that Oil C is significantly more likely to initiate pre-
46
ignition than Oil E when exposed as vapor to the ignition environment. It was not clear
why the GM recommended oils produced such similar ignition delays in the RCM after
behaving very differently in GM's engine tests. One speculation is that since the oils used
at GM and the two used here at MIT were purchased at different times and locations, there
may have been some influential difference between the batches tested.
Overall, there was a correlation between the base stocks and ignition delay times measured
in the RCM. In an engine, we would expect the synthetic oils (longer delays) to have a
lower probability of producing SPI while the use of non-synthetic oils (shorter delays)
should be more likely to produce SPI. These findings agree with those published by GM
[6] and Toyota [5].
4.5 New Versus Used Oil
The results in Section 4.4suggested that the base oil had some effect on ignition delay in
the RCM. It was likely then that degradation of the base oil should have some sort of impact
as well. Over time, repeated exposure to heat and pressure causes lube oils to become
thermally unstable and oxidize, especially after the depletion of oxidation inhibitors (such
as zinc dithiophosphate) [19]. Toyota has suggested that this oxidation instability is a key
element in SPI [5].
A new and used version of two different oils were tested in the RCM. The two oils tested
were Oil D and GM A. The used Oil D sample came from a GM LNF experimental engine
in the Sloan Automotive Lab at MIT, and the used sample of GM A was provided by GM.
The two used samples had very similar fuel dilution of about 8% by weight, and both had
a KV100 (kinematic viscosity at 100 0C) below the allowable limit for 5W-30 grade oils.
This amount of fuel dilution is considered very high [13]. The results from the RCM are
shown in Figure 4.9.
47
5.9
4.6
4.1
3.8
GM A
Oil D
Figure4.9: Ignition delay times for new versus used oils in milliseconds
Although the used version of both oils produced shorter ignition delays, Figure 4.9 shows
a much more significant change for Oil D (-2.1 ms) than for GM A (-0.5 ms). One possible
explanation for the discrepancy is that both Oil D samples (new and used) originated from
the same 5 gallon bucket, while the new and used versions of GM A came from different
batches. Therefore, variations in the sample batches might have been influencing the results
for GM A.
Nevertheless, degradation of the oil decreased ignition delay, which should result in a
higher probability of SPI occurring in an engine. This would suggest that the likelihood of
SPI happening should increase with extended use, until the oil is replaced. Due to variations
in base stocks and additives amongst different oils, we would expect to have a range of
sensitivity to degradation. This would imply that some oils can maintain a level of SPI
resistance for longer than others.
4.6 Effect of Oil Additives
As mentioned in Section 4.5, lubrication oils contain a variety of chemical additives which
promote maintenance of the engine such as detergents, corrosion inhibitors, anti-wear
additives, and many more. It was proposed by Toyota that several of these additive
48
compounds have conflicting catalytic effects on oxidation, and they have found a strong
correlation between oxidation catalysis and SPI frequency [8].
General Motors provided their resources to perform an elemental analysis on all ten oils
tested in the RCM so that the additives could be compared to ignition delay times. The
results of that analysis are tabulated in Table 4.2.
Table 4.2: Additive elements in ten new and used oils tested in the RCM
Oil
6
98
<10
28
531'"
1953
10
603
8
3.8
219 11885
<2
<5
<10
32
649
2554
1
828
0
4.0
6
52
1552
<2
59
2
64
580
2065
17
62
Oil A
<3
49
2616
<2
<5
301
<5
1013
5209
3
1085
0
4.4
GMA
<3
192
1860
<2
<5
<10
64
630
1912
3
733
0
4.6
Oil B
<3
488
1415
<2
<5
372
88
1069
2686
5
1184
0
5.1
GM B
<3
95
1165
<2
<5
918
74
663
1563
4
821
0
5.4
Oil D
<3
144
1645
<2
<5
11
31
623
2071
3
792
0
5.9
Gear Lube
<3
252
16
<2
<5
<10
<5
1839
12170
<1
10
0
6.0
USED
D
Oil C
USED GMV A
6"
60
<3
1L270
7.
4.
There are four additives that have been linked to SPI promotion: calcium, iron,
molybdenum, and phosphorus. Toyota has found that the first two increased SPI through
oxidation catalysis, while the second two decreased it through peroxide decomposition [8].
In order to minimize competing effects, only the new motor oils were plotted (i.e. excluding
the gear lube and used oils). Since the base stock had an influence on ignition delay, it was
not reasonable to compare oils with dramatically different bases. The effect of calcium is
plotted in Figure 4.10.
49
6.5
Used oils and gear lube are
excluded from this plot
6
,
55
*
0
Sb
45
**
0
3.5 4- ---------0.15%
0.10%
-
5%
0.20%
0.25%
0.30%
Calcium Concentration (mass %)
Figure4.10: Ignition delay versus calcium concentrationfor all 8 new motor oils
Although the correlation was weak, delay times decreased as calcium concentrations
increased. It was not surprising that the correlation was weak since there were so many
competing effects involved
(e.g. other additives and variations
in base stocks).
Nevertheless, the trend agreed with claims made by Toyota [8].
The same plot was made for molybdenum concentrations and is shown in Figure 4.11.
6.5
6
0
5...
55t
0
o
Used oils and gear lube are
excluded from this plot
4
0.000%
0.002%
0.004%
0.006%
0.008%
0.010%
Molybdenum Concentration (mass %)
Figure 4.11: Ignition delay versus molybdenum concentrationfor all 8 new motor oils
50
Once again, there was a weak correlation; however, the ignition delay times increased with
increasing molybdenum. Again, it was likely a weak correlation due to the multitude of
competing effects. The direction of the trend agreed with Toyota's findings [8].
A third plot was made for the phosphorus concentrations and is shown in Figure 4.12.
6.5
6
5.5
0
5
0
0
Used oils and gear lube are
excluded from this plot
4
3.5 ,
0.00%
------0,10%
0.05%
---0.15%
0.20%
Phosphorus Concentration (mass %)
Figure 4.12: Ignition delay versus phosphorus concentrationfor all 8 new motor oils
Phosphorus concentrations produced no correlation with the ignition delay times of the
oils. At this point, it was not clear whether the competing effects were simply negating any
potential correlation; however, from Toyota's results [8] we would expect to see ignition
delay times increase with increasing phosphorus concentration.
Iron concentrations were not plotted since only the used oils contained measurable traces
of the element. Toyota found iron to have similar effects to calcium [8], so we would expect
to see ignition delay times decrease with increasing iron concentrations.
It was clear that the effects of each of these additives needed to be isolated before any
conclusions could be made. This would be done by artificially "doping" each of the oils
with additives to see how the ignition delay times would be effected. The range of additive
element concentrations and base oils are shown in Table 4.3.
51
Table 4.3: Concentrationsof additive elements in thefour "doped" oils
Calcium
GM B
Oil B
GMIA
Oil D
Calcium Sulfonate
I
Phosphorus
Iron
ZDDIP
Iron Naphthenate
1165
1415
630
<5
5000
5000
40GO
500
Each oil was selected for having a low initial concentration of the element to be added.
Compounds were added so that a range of concentrations could be tested up to the
maximum concentrations tabulated in Table 4.3. The results are plotted in Figure 4.13.
6.5
6.5
1~
i 6
2.2 ms
drop
5.5
6
0.9 ms
drop
5.5
5
5
0
0
-E4.5
:E 4.5
Calcium
GMB
4
3.5
-- -r--
0
Calcium
04
-
-
r------
....
-.....
......
3.5
0
1000 2000 3000 4000 5000
Calcium Concentration (ppm)
6.5
6.5
E
0.2 ms
range
S6
4
0.3 ms
range
5.5
5.5
0
5
5
4.5
4.5
Phosphorus
4
3.5
-
0
-r--
1000
Iron
Oil D
34
GM A
-
0
1000 2000 3000 4000 5000
Calcium Concentration (ppm)
-
2000
---
3000
...-.
r --.
r --- r - -
3.5
,
4000
0
100
200
300
400
Iron Concentration (ppm)
Phosphorous Concentration (ppm)
Figure 4.13: Isolation of the effects of additives in lube oils
52
500
The results show that calcium did have an influence on ignition delay times measured in
the RCM. The effect was especially significant in GM B, which saw a decrease in delay
time that was nearly equal to the entire range of oils tested. The 2.2 ms drop for GM A
would correspond to about 26 CAD in an engine operating at 2000 rpm. The same trend
was observed for Oil B. While the 0.9 ms drop was less significant, it corresponds to about
11 CAD at 2000 rpm, which is not trivial. It should be noted that GM B was a fully
synthetic oil while Oil B was completely non-synthetic. Furthermore, the two oils had
different initial concentrations and delays. The results show that the oils had dissimilar
sensitivities to the effects of the additives.
As for phosphorus and iron, the effects claimed by Toyota [8] were undetectable in the
RCM measurements. In both cases, relatively high concentrations were added with
negligible changes in ignition delay times (<0.5 ms spreads). The effects of varied
molybdenum
concentration
were never
tested
in the RCM
since molybdenum
dithiocarbamate (MoDTC) was never sourced.
Overall, the RCM results for calcium and potentially molybdenum helped explain why
these elements have been observed to influence SPI frequency in engine tests. Calcium
decreased ignition delay, which would increase the probability of SPI, while the correlation
for molybdenum indicated that higher concentrations may be increasing delay times, which
should decrease SPI frequency. These findings agree with claims from a study done by
Toyota [8]; however, concentrations of phosphorus and iron had little effect on ignition
delay in the RCM, which does not support corresponding claims made by the same Toyota
study [8].
4.7 Effect of Oil Physical Properties
The previous three sections focused on the chemical characteristics of the oils. It was
logical then to look into their various physical properties. Since all of the oils tested are
commercially available, physical data is published and readily accessible. Five physical
parameters are commonly published for each oil:
53
* NOACK Volatility (ASTM D5800) -measured wt% loss due to evaporation [20]
*
Pour Point (ASTM D97) - min. temperature associatedwith oilflow [21]
*
Flash Point (ASTM D92) - min. temperature associatedvaporflammability [22]
*
KV40 (ASTM D445) - kinematic viscosity at 40'C measured in cSt
*
KV100 (ASTM D445) - kinematic viscosity at 1000 C measured in cSt
The material safety data sheets (MSDS) for each oil were studied to collect information on
the five parameters. Any information that was not readily available was obtained through
correspondence with the manufacturers for each oil. The published data along with KV100
values measured by GM are tabulated in Table 4.4.
NOACK
USED Oil D
PourPoint
FlashPoint
KV40
KV100
-----
XV100
,
Table 4.4: Publishedand measuredphysical properties of the test oils
6.868
3.8
oil C
14.3
-39
185
67.0
11.1
8.94
4.0
Oil A
12.00
-30
199
118.0
15.2
15.59
4.4
GM A
7.90
-39
225
63.3
10.9
11.17
4.6
oil BB
18.34
-30
204
1625.0
153
15372
5.1
Oil D
14.70
-42
135
63.5
10.5
10:24
5.9
Gear Lube
N/A
40
165
125.0
14.6
14.55
6.0
oil E
6.40
-39
232
125.0
18.0
1.64
6.1
It should be made clear that the published values are expected to differ somewhat from the
actual physical properties for the specific oils tested in the RCM. Since there are variations
between batches, it is highly unlikely that the test oils will match published data exactly.
This was evident when comparing the measured and published KV100 values. Oil C was
54
measured to have approximately a 20% lower KV100 value than what is published while
Oil E had a roughly 10% higher KV100 value than what is published.
Nevertheless, the ignition delay times were plotted against the five published physical
properties, and the results are shown in Figure 4.14. The used oils were excluded since
their physical properties were expected to be drastically different from their new
counterparts.
Figure 4.14 shows that there was no correlation between published physical data and
ignition delay times measured in the RCM. It is not clear if the discrepancy is due to
disparities between published and actual values or whether the experimental procedure is
possibly making the physical characteristics non-limiting, but there is no discernable trend
for any of the physical properties.
55
NOACK
6.5
6
5.5
0
Gear lube does not have a
published NOACK value
5
4.5
4
0
3.5
4.0%
19.0%
14.0%
9.0%
NOACK (wt% loss)
Pour Point
Flash Point
6.5
6.5
6
6
55
5.5
5
5
4.5
4.5
0
4
-
-
3 .5
-45
-40
- - - - --35
Pour
-30
- ..
-. - - - --.-
3.5
---25
100
150
200
250
KV0sh
Pint (C)
Pnint (oC)
KV40
KV100
6.5
6.5
6
60
55
5.5
0
50
5
4.5
4.5
40
0
4
3.5
3.5
0.0
50.0
100.0
150.0
9.0
KV40 (cSt)
11.0
13.0
15.0
17.0
KV100 (cSt)
Figure 4.14: Publishedphysical data versus oil ignition delay times
56
19.0
4.8 Effect of Charge Dilution
It has been observed that charge dilution could reduce SPI. Dilution is typically in the form
of exhaust gas recirculation (EGR). The Southwest Research Institute has claimed that
reasonable levels of EGR produce a substantial reduction in SPI frequency [1]. Since the
RCM ignition delay results correlate well with SPI observations in engines, observing
dilution effects on ignition delay should help to fundamentally understand how EGR is
mitigating engine knock.
There are three potential explanations for how dilution could prevent pre-ignition,
assuming oil vapor is the ignition source:
Scenario 1. Auto-ignition of the oil is fully suppressed by the inert gases
Scenario 2. Auto-ignition occurs but a flame does not start in the diluted mixture
Scenario 3. Auto-ignition occurs and starts a flame but dilution slows the flame
propagation, hence mitigating the compression ignition of the end gas
To find out if any of the above are occurring, dilution in the form of nitrogen gas was added
to the mixture in the RCM. Nitrogen was added in increments of 10% by volume up to
50% in the air-nitrogen mixture. In other words, at 50% dilution, the mixture would have
approximately 10.5% oxygen by volume and about 89.5% nitrogen by volume, assuming
air is composed entirely of oxygen and nitrogen. The fuel mass was kept constant, and k
was fixed at I for every case.
Since ignition delay times are highly sensitive to temperature, it was imperative to keep
the end of compression temperatures the same, despite reaching higher pressures. This was
done by lowering the pre-compression temperatures such that the gases reached a
temperature of about 650 K after compression, assuming adiabatic conditions.
Oil A (reference oil) was the oil used in this experiment, which again had a non-synthetic
base oil and produced a relatively short ignition delay time in the RCM (4.4 ms). The results
for dilution with nitrogen up to 50% are shown in Figure 4.15.
57
-
180
Piston retr action and push-back
due t o overpressure
20%
-
160
1 0%
30%
-
140
40%
50%
0%
Dilution
-
120
100
80
Trigger
60
20
-
40
0
0
20
40
80
60
100
120
140
Time (mns)
Figure 4.15: Dilution with nitrogen (fixedfuel amount and2=1)
As expected, the total times to end-gas auto-ignition increased with increasing dilution. It
can also be seen from the slopes of each curve that flame propagation occurred in every
case, which would imply that the oil vapor auto-ignited in each case. This would suggest
that we are observing Scenario 3: oil auto-ignition occurred and started a flame but the
entire process was significantly delayed by dilution.
In an engine, it is expected that at high levels of dilution the time required for the end-gases
to auto-ignite would be so long that the piston would already be travelling down the
expansion stroke, thus preventing auto-ignition from occurring. Although it was very clear
from the results that dilution delayed knock, it was not obvious whether that delay was
mostly due to slower flame propagation or if the oil auto-ignition was also substantially
delayed. Flame propagation is not comparable between the RCM and an engine because
the RCM utilizes constant-volume combustion, simple geometry, and research grade fuel,
58
while in an engine, volume is always changing, turbulence is promoted, and fuels vary. As
a result, the effect of dilution on oil auto-ignition time is much more valuable when
comparing RCM results to engine data.
As described in Section 4.3, the oil-specific ignition delay times for dilution up to 30%
were calculated by subtracting measured flame propagation times from the total times to
knock for each dilution case. The results from this experiment are shown in Figure 4.16.
-
30
-
32.1
35
Oil A
(Reference Oil)
23.1
25
am
Time to Knock Onset
e
agation
Prop
Dura tion
16.4
12.7
15
12.1
010
t
91.
00
W
E
I
at
6.3
-
10
M *
,
0
.
-
do -
-
-
O 20
-""a
4.41
,
Time to Oil Auto-Ignition
00'-
0
0%
5%
15%
10%
20%
25%
30%
Nitrogen Dilution
Figure4.16: Oil ignition delay times versus nitrogen dilution
Looking at the range of oil ignition delays, there was a very significant effect from the
dilution. Even at 10% dilution, which is typically considered a modest amount, the ignition
delay for Oil A was already slowed from 4.4 to 6.3 ins, which was a longer delay than any
of the oils tested. At 30% dilution, the ignition delay time was almost tripled. In this
experiment, flame propagation and oil auto-ignition were proportionately delayed since the
59
oil-specific delay accounts for roughly 40% of the total time to knock for all four cases. It
should be noted that there was more opportunity for error in the dilution cases, mostly due
to mixture preparation uncertainties associated with adding nitrogen; however, the trends
are still very clear.
Overall, dilution had a much more significant impact on ignition delay times than chemical
or physical properties of the oils. This finding agrees with a claim made by SwRI that 10%
EGR totally eliminated SPI from the 0% case, which produced approximately 4 SPI events
per 30,000 cycles, with each event averaging about 4 pre-ignition cycles [1] (i.e. relatively
severe SPI).
60
Chapter 5: Summary and Conclusions
5.1 Oil Vapor and Ignition Delay
It has been observed that oil vapor and air mixture in the cylinder can initiate combustion
and lead to compression ignition of the end gas. Thus, lube oil is confirmed as the ignition
source. The amount of oil required to initiate a flame in the mixture is consistent with
masses seen in typical oil droplets found in the engine cylinder.
The oil ignition delay measurements in the RCM use oil vapor evaporated from a heated
wire. Therefore, the delay includes both physical and chemical processes. The wire must
be heated, the oil vaporized, the vapor mixed, and finally chemical reactions must occur
before the oil vapor can auto-ignite. As shown in Section 2.2, wire heating is short, on the
order of 0.5 ms; however, the mixing time is not trivial. The chemical ignition delay of the
oil is of the order of milliseconds since the overall ignition delay is around 5 ms.
Evidence that mixing time constitutes a substantial part of the delay lies in Figure 4.2. Since
the overall time from the trigger to the onset of knock remained relatively unchanged
between the "Normal Trigger" and "Early Trigger," the physical processes are on the order
of roughly 3 or 4 ms. The chemical processes are heavily dependent upon temperature,
therefore, if the physical processes were negligible the "Early Trigger" would produce a
longer delay since the vapor-air mixture is prepared at a low temperature and pressure
(beginning of compression). For example, if we assume for a moment that vaporization
and mixing occur instantaneously and chemical delay accounts for 100% of the oil ignition
delay, then in the "Early Trigger" case the ignition delay would be much longer since for
about 4 ms the oil vapor is exposed to the lower temperatures and pressures seen during
compression, which would increase the delay. However, this is not the case, which means
that for the majority of that 4 ms the ignition delay is relatively independent of temperature,
suggesting that mixing is taking place, rather than substantial chemical reactions. The
reason this is important is that if indeed the physical processes are taking about 3 to 4 ms
then the chemical ignition delay would be on the order of 1 to 2 ms, which would
undoubtedly cause SPI in an engine.
61
5.2 Oil Characteristics
In general, ignition delay times correlated fairly well with engine results. The findings
suggest that oil base stock and deterioration effect the ignition delay, with synthetic oils
being less likely to produce SPI than non-synthetics, and new oils being less likely to
produce SPI than their used counterparts.
Additives in the oils also play a role. Calcium significantly increases the likelihood of SPI.
Any effect from phosphorus and iron was undetectable in the RCM results. Molybdenum
could potentially have a positive effect in mitigating SPI; however, its effects were not
isolated in the RCM experiments, and no meaningful conclusion can be made.
The physical properties of the oils play little role as no correlation has been seen between
published physical data and ignition delay times. It should be noted that the published data
may differ from the actual values of the particular samples of oil tested. The heated wire
vaporization process may also render the physical properties to be non-limiting.
5.3 Charge Dilution
Charge dilution significantly delays flame propagation and oil auto-ignition. Even at
modest levels of dilution, oil ignition was significantly delayed. At 10% dilution with
nitrogen the oil
uto-igrnition was delayed by nearly 45% from the 0% dilution case.
The effect of dilution far outweighed any effects due to variations in oil properties. A
maximum difference in delays of 2.3 ms was seen across the wide variations in oil base
stock, deterioration, chemical additives, and physical properties, while more than 8 ms was
achieved just through dilution.
5.4 Applicability to Engines
The end of compression temperature and pressure in the RCM were chosen to match SPI
conditions seen in engines. After compression, the gases are roughly 650 K and 26 bar,
which compares well with SPI conditions seen by GM in which the ignition environment
is also about 650 K and between 20 and 30 bar.
62
The range in delay times of 2.3 ms between the best and worst of the ten oils corresponds
to 27.6 CAD at a speed of 2000 rpm, which is fairly substantial. This wide range should
impact SPI behavior. In addition, several of the ignition delay correlations presented in this
study agree reasonably well with actual engine tests, suggesting that the ignition delay
measurements in the RCM are a decent indicator of SPI probability.
5.5 Closure
The results of this study indicate that oil vapor is an ignition source for initiating flame
propagation, which leads to end gas knock. The observed ignition delay of approximately
5 ms results from both physical and chemical processes. The delay, which corresponds to
60 CAD at 2000 rpm, is short enough so that oil vapor that is introduced during the intake
or early compression can ignite the charge. This delay is sensitive to oil composition.
Calcium additives have been found to decrease the delay.
Charge dilution is found to be a much more effective tool for SPI mitigation than alteration
of oil characteristics. On the other hand, the range of delay times measured is large enough
that the different oils should exhibit different SPI behavior in an engine. Base stock,
degradation, and additives all impact ignition delay. Fully synthetic oils generally produced
longer ignition delays, hence they are less likely to produce SPI. Degraded oils produced
shorter ignition delays making them more likely to produce SPI. Calcium in the oil acts as
a catalyst for oil auto-ignition and would increase the likelihood of SPI. New fully synthetic
oil with low concentrations of calcium combined with the use of moderate levels of EGR
are expected to significantly help reduce the likelihood of SPI.
63
64
Appendix A: Rapid Compression Machine
A.1 RCM Schematics
All of the following images have been adapted from the thesis of Ioannis Kitsopanidis [14].
.............
....... ...............
0-
-
LAJ
_j
>-J"
LY
>
LJ
17
CL
C.
17
C,
CL
II
Ofa
a_
65
vENT
VACUUM
PNEUMATIC
CHAMBER
BYPASS
HIGH
RELIEF
VALVE
PRESSURE
N2
PRESSURE
GAUGE
HIGH PRESSURE TANW
DRAIN
FigureA.2: Pneumatic (driving) system [14]
VENT
r7
PRESSURE
GAUGE
HIGH
PRESSURE
N2
DAMP
OILOIL
SIGHT
TUBE
OIL -N2VA
SEPARATION
TANK
HYDRAULIC
LOCK
OIL
PUMP
PRESSURE
GAUGE
-
C-lF IRING
E
PIN
GROOVE
MECHANISM
OIL
DRAIN
A
Hydrau=:
(
OIL RESERVOIR
FigureA.3: Hydraulic (locking-r-eleasinig) syvstemn [14]
67
MIXING
FAN
METERING
VALVE
AIR
L
VACUUM
PUMP
PRESSURE
Q
GIASE DLiNE
INE CTIEN
P RI
1IXIN
TPANK
TEMPERAT URE
(THERMIST OR)
VENT
TEMPERATUR E
(THERMISTO
VACUUM
PUMP
PRESSURE
(I mbar)
TEMPERATURE
(THERMISTOR)
TEMPERATURE
(THERMISTOR)
N2
PRESSURE
(0,1 Torr)
TEMPERATURE
(THER ISTOR)
COMBUSTION CHAMBER
ALL LINES & VESSELS ABOVE ARE HEATED AND INSULATED
Figure A.4: Mixture preparation setup [14]
68
A.2 RCM Mechanical Drawings [14]
No.
1
2
3
4
5
PART NAME
HEAD WINDOW
OTY
1
1
1
228 U-RING
HEAD PLATE
113 D-RING
4
I"-L14-/2" HOLLOW
SOCKET SET SCREW
4
1
6
7
8
5-938 0-RNG
COMBUSTION CYLINDER
1/8"-18 NPT CONNECTOR
9
10
3/8"-16-1 1/2" SOCKET HEAD CAP SCREW
HYDRAULIC STOP RING
3/8"-16-2 3/4" SOCKET HEAD CAP SCREW
350 0-RING
11
12
13
14
15
16
17
I
1
6
1
8
1
1
HYDRAULIC CYLINDER
1/2"-14 NPT CONNECTOR
ACKNG
2
1
RING
1
342 0-RING
1/2"-13 HEX NUT & 1/2" FLAT WASHER
6
1
18
19
STROKE ADJUSTMENT RING
20
21
1/4"-18 NPT CONNECTOR
1/2"-13-5" STUD
1
6
22
PNEUMATIC CYLINDER
23
358 U-RING
1
24
25
26
27
28
FLANGE
1/2"-13 HEX NUT L 1/2" FLAT WASHER
1/4"-20-1 1/4" SOCKET HEAD CAP SCREW L 1/4" LOCK WASHER
STROKE ADJUSTMENT SPACER
1/2"-t3-5" SOCKET HEAD CAP SCREV & 1/2" FLAT WASHER
29
30
31
32
33
34
SPACER
HEAD) CAP SCREW & 1/2" FLAT WASHER
STAND
3" SCH. 80 PIPE SECTION
PNEUMATIC PISTON
1/2"-13-4" STUD
1
6 L
6
I I I
6
6
4
2
1
38
PNEUMAT1C SHAFT
1
39
40
41
SLC4300A90BT12501000-250 SEAL
8-32-1 1/4" SOCKET HEAD CAP SCREW
SHAFT COUPLER
1
4
1
1
210
STUD
2
4
42
1/4"-20-1"
43
45
HYDRAULIC LOCK RING
141 0-RING
4-40-1/4" FLAT HEAD SCREW
46
210
47
48
49
HYDRAULIC P[STON
1/2~-13-I 1/2" SOCKET HEAD LOCK CAP SCREW S LOCK WASHER
HYDRAULIC SHAFT
SLC4300A90BT12501000-250 SEAL
52
BRACKET
53
54
1/2" FLAT COPPER L RUBBER WASHER
SIDE WINDOW
HEAD WINDOW RETAINING RING
1/2"-13-5" SOCKET HEAD CAP SCREW
SOCKET HEAD CAP SCREW
L
LOCK WASHER
1
57
58
0-RING
1
1
1
L
4
1
4
4 &4
4
iMSSACHUSETTS
INSTITUTE OF TECHNOLDGY
SLOAN AUTOMOTIVE LABORATORY
ASSEMBLY SECTION AND VIEWS
RAPID COMPRESSION MACHINE
69
1
1& 1
4
4
HEX NUT & FLAT WASHER
3/8"-16-1 1/2" HEAD CAP SCREW
DRAWN BY
IOANNIS KITSOPANIDIS
DATE. 08/21/00
1
2
1/2"-13
HNITSi in
&1
1
1
8
50
51 N427418701625 SEAL
56
6
L4
6
N4274A854622E40804500 SEAL
0-RING
1/2"-13-3"
55
L
1
1
35
36
37
44
6
1
1/2"-13-1 1/2"
1/2"-13 NUT
L
6
Duv
r%
I OF 10
11
9
7
5
55
15
13
16
18
22
20
24
23
25
26
3732
53
51
s
49
36 30 4
46
44
42
40
39
36
35
33
34
31
3-10 1/4'
-.1
0
VIEW A
57
56
VIEV B
55
19
27
20
OML
18
52
58
Note: Part 1 has been replaced with the electrode for this study
28
At'.
30
29
SECTION A-A'
DtA 4751
213 040 MOiVE
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PART No. 54
No.
19
PART No. 19
A-A'
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SECTION A-A'A
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43
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Appendix B: RCM Operating Manual
The following operation manual has been adapted from the original operation protocol
outlined by loannis Kitsopanidis [14], who is the creator of the RCM at MIT. These
guidelines are specific to the RCM in the Sloan Automotive Laboratory shown in Figure
B.1, which was built in the summer of 2000.
Figure B.1: The rapid compression machine
81
B.1 The Control Panel
Figure H.2: HCM control panel
82
B.2 Initial Preparations
Heating the RCM
In experiments where the initial temperature is needed to
be
ambient
above
temperature,
there
are
eight
temperature controllers used to regulate the temperature
of the combustion chamber, mixing tank, and supply
network. This is the first step in preparation due to the
large amount of time required for the RCM to heat up and
reach thermal equilibrium. Use the following steps to heat
FigureB.3: RCM heaterpanel the RCM:
1. Turn on the main "POWER" for the controllers
2. Turn on the power to the 8 sets of heating elements (6 switches)
3. Use the controller menus to select and initiate the temperature set-point
4.
Allow at least an hour for equilibrium to be achieved (usually longer)
Filling the RCM with Oil
The RCM must be filled to the "suggested level" of oil
before it can be operated. The oil prevents damage to the
RCM by gradually decelerating the piston at the end of
compression.
First, the oil reservoir below the RCM must have enough
Figure B.4: RCM oil reservoir
oil. If oil is not visible within the plastic sight tube on the
side of the reservoir then perform the following steps:
1. Remove the 4 bolts (7/16" wrench) holding the top plate of the reservoir
2. Fill the reservoir to the line marked on the plastic sight tube with oil (Dow Corning
Series 200 silicone oil is recommended)
3. After filling, reattach the top plate with the 4 bolts
83
When the reservoir has enough oil, the next step is to ensure that there is
enough oil in the RCM. Check the glass sight tube on the control panel
and if the oil is well below the "suggested level" perform the following
steps to supply oil to the RCM:
'
1.
Make sure the "Compressed N2 Vent" valve (2) is open
2.
Make sure the Firing Valve (11) is set to "VENT'
3.
Make sure the "Oil-N2 Separation" valve (1) is open
4.
Make sure the "Sight Tube Supply" valve (5) is open
5.
Turn on the "Oil Pump"
6.
Open the "Oil Supply" valve (4) to begin supplying the oil
7.
Use the "Oil Flow Regulation" valve (6) to slow the oil flow
8.
Supply to "suggested level" and close the "Oil Supply" valve (4)
9.
Allow the oil to settle and repeat steps 6 through 8 as needed
10.
Close the following valves in this order: 6, 2, 11, 5
11.
Turn off the "Oil Pump"
Note that due to variations in pipe cross sections, the oil level in the sight
Figure B.5:
tube will fall slightly after the "Oil Supply" valve has been closed.
RCM sight
tube
Filling the Pneumatic Driving Tank
Before
each
run,
there
must
be
adequate driving pressure in the large
pneumatic tank beneath the RCM. The
driving pressure generally should not
exceed
275
psi.
At
250
psi,
Figure B.6: RCMpneumatic tank
compression time is roughly 15 ms.
The pneumatic tank has a pressure relief valve set to 450 psi. Driving pressure should be
calculated knowing that the area ratios between the pneumatic piston, hydraulic piston, and
84
combustion chamber piston are roughly 6.25:1.25:1 respectively. Use Table B.1 as an
approximate guide for RCM pressures.
Table B.1: Approximate RCM operatingpressures
750
50
150
900
60
175
200
225
1050
1200
1350
70
80
90
250
1500
100
275
1650
110
300
1800
120
125
To fill the pneumatic tank to the desired pressure, perform the following steps:
1.
Set the desired driving pressure using the pressure regulator on the right N2 bottle
2. Turn the "CompressedAir" valve (7) to "Supply"
3. Allow ample time for the desired pressure to be reached
4.
Close the "CompressedAir" valve (7)
Preparing the Mixture
The mixing tank allows for fuel and
oxidizer
to
be
well
mixed
and
homogenous before compressing them in
the RCM. This process will likely vary
greatly from project to project; however,
the fundamentals should be similar. It
FigureB. 7: RCM mixture preparationsetup
should be noted that it is not ideal to
prepare the mixture while the mixing tank is heating up since metering pressures will not
85
be consistent while the temperature is in a transient state. Follow these steps to prepare the
mixture:
1. Vent the mixing tank
2. Replace the gasoline injection port septum if it is worn
3. Turn on the "MIXING FAN"
4. Make sure the combustion chamber valve is closed
5. Open the valves between the 1000 torr Baratron pressure gauge and the tank
6. Turn on the mixing tank "VAC PUMP" and open the vacuum pump valve
7. After sufficient vacuum has been made close the valve to the vacuum pump
8. Turn off the mixing tank "VAC PUMP"
9. Use the syringe to inject liquid fuel into the tank using the 1000 torr pressure gauge
10. Close the valves between the 1000 torr gauge and mixing tank to avoid damage
11. Meter in any desired gases using the 4 bar LEO 2 pressure gauge and meter valves
12. Do not exceed 2.5 bar in the mixing tank or the fan seal may fail
Retracting the Piston
In order to fully retract the piston in the RCM (equivalent to BDC in an engine), a vacuum
must be applied to the pneumatic chamber so that the piston is essentially sucked to the
back of the RCM. This also allows for contact to be made between the "HydraulicLock 0ring" and the back of the hydraulic chamber to enable good sealing. To fully retract the
piston assembly, perform the following steps:
1.
Make sure the "Rear Chamber Vent" valve (8) is open
2. Turn on the "Vacuum Pump"
3.
Turn the "Pneumatic Chamber" valve (9) to the "Vacuum" position
4. Make sure the blue retraction mark on the piston shaft is visible
5. Close the "Rear Chamber Vent" valve (8)
6. Close the "Pneumatic Chamber" valve (9)
7. Turn off the "Vacuum Pump"
86
Supplying the Charge
Once the piston is fully retracted, the chamber is ready to be filled with the mixture from
the mixing tank. Follow these steps to fill the combustion chamber with the test gases:
1.
Open the valve between the combustion chamber and the 1000 torr pressure gauge
2. Turn on the "Vacuum Pump" from the control panel
3. Open the valve between the chamber and vacuum to evacuate the cylinder
4. After sufficient vacuum has been made close the valve to the vacuum pump
5. Turn off the "Vacuum Pump"
6. Close the valve to the 1000 torr gauge to avoid damage
7. Meter in the mixture from the mixing tank using the 4 bar LEO 2 pressure gauge
8. Meter in any additional gases
9. Close the combustion chamber valve
10. Allow to sit for 10 minutes to reach thermal equilibrium
B.3 Firing Sequence
Locking the Piston
Once the piston assembly is retracted, it is ready to be locked in place with a pressure
differential being applied across the hydraulic chamber. This process should be
performed with care since high pressures are involved and there is potential for
damage to the RCM.
After the O-ring on the back of the hydraulic piston makes contact with the back of the
hydraulic chamber, the two halves of the chamber are sealed off from one another.
Applying pressure to the forward end of the chamber places a differential across the two
sides of the piston, thus locking it in place. This hydraulic locking allows driving pressure
to be applied to the pneumatic chamber without the piston moving. The locking pressure
is applied to the oil using compressed nitrogen gas. Follow these step to lock the retracted
piston in place:
87
Set the desired locking pressure using the pressure regulator on the left N2 bottle.
1.
(See Table B. 1 for recommended hydraulic pressures)
2. Make sure the "Sight Tube Supply" valve (5) is closed to avoid damaging it
3. Make sure the "Compressed N2 Vent" valve (2) is closed
4. Make sure the Firing Valve (11) is closed
5. Make sure the "Oil-N2 Separation" valve (1) is open
6. Ensure a good seal is being made by retracting the piston as described before
7.
Carefully open the "CompressedN2 Supply" valve (10) to lock the piston
8. Close the "Oil-N2 Separation" valve (1)
9. Close the "Compressed N2 Supply" valve (10)
Loading the RCM
Once the piston assembly is locked in
place, driving pressure can be applied to
the pneumatic chamber without moving
the piston. Once this pressure is applied,
the RCM is analogous to a loaded gun and
is essentially ready to fire. The RCM has
been known to fire unexpectedly when
in this state, so
FigureB.8: RCM pneumatic chamber valve
use caution
(See
Appendix B.5 for why this might happen). Follow these steps to bring the RCM to the
ready-to-fire state:
1. Slowly turn the "Compressed Air" valve (7) to "Bypass" to remove the pressure
differential across the yellow 3 inch pneumatic ball valve
2. Slowly open the yellow 3 inch pneumatic ball valve
3. Close the "Compressed Air" valve (7)
4. OPTIONAL: Turn the "Compressed Air" valve (7) to "Supply" to compensate for
pressure loss due to filling the pneumatic chamber
88
Firing the RCM
After following the procedures above, the RCM is ready
to fire. The piston is rapidly forced across the RCM by
removing the hydraulic locking pressure from the
hydraulic chamber, allowing the force from the driving
pressure to quickly accelerate the piston. Follow these
FigureB.9: RCMfiring valve
steps to fire the RCM and begin compression:
1. Make sure the combustion chamber valve is closed
2. Make sure the yellow 3 inch ball valve is open
3. Make sure each valve on the control panel is pointing toward the red "F"
4.
Set the charge amplifier to "Operate" mode
5.
Initiate the data acquisition system ("RCM Pulse Trigger.VI" in LabVIEW)
6.
Quickly rotate the Firing Valve (11) counter-clockwise to release the hydraulic
pressure and fire the RCM
7.
Turn the Firing Valve (11) 180 degrees to the "VENT' position
B.4 Shut-Down Sequence
Venting the RCM
Once the RCM has been fired, the first step is to relieve the pressures from the machine so
that the piston can be returned to the retracted position. Follow these steps to vent the RCM:
1.
Set the charge amplifier to "Reset" mode
2. Close the yellow 3 inch pneumatic ball valve
3.
Slowly open the "Compressed N2 Vent" valve (2) until the pressure is ambient
4. Open the "Oil-N2 Separation" valve (1) after pressure in step 3 is fully relieved
5.
Open the "Sight Tube Supply" valve (5)
6.
Open the "Rear Chamber Vent" valve (8)
7.
Turn the "CompressedAir" valve (7) to "Supply" only to prepare for the next run
89
8. Turn the "Pneumatic Chamber" valve (9) to "Vent" to begin retraction of the piston
9. Once fully vented, turn on the "Vacuum Pump" on the control panel
10. Turn the "Pneumatic Chamber" valve (9) to "Vacuum" to complete retraction
11. After retraction is complete, close the "Pneumatic Chamber" valve (9)
12. Turn off the "Vacuum Pump"
13. Exhaust the combustion chamber with the three-way chamber valve
Cleaning the Cylinder
are complete,
Once the experiments
it is
essential to clean the combustion chamber to
ensure that corrosion and deposits do not form.
Follow these steps to clean the cylinder:
FigureB.10: DisassembledRCM head
1. Remove the 4 head bolts (3/4" wrench)
2. Using acetone and lint-free paper clean the cylinder walls and piston head
3. Clean the crevices (e.g. window plugs, 0-ring grooves, etc...)
4. Replace the head plate and 4 bolts
B.5 Troubleshooting
Issue
Runs are
inexplicably
inconsistent
(if these fixes
do not help
look at other
issues as many
things effect
consistency)
Combustion
chamber is
leaking
Fixes
Possible Causes
Mixtures are not consistent
Use more care when
metering and injecting fuel
Wait for 10 minutes after
filling the combustion
chamber with charge
Gases are not at thermal
equilibrium
Likelihood
Hi
High
Combustion chamber is dirty
Clean the combustion
chamber and/or replace the
piston seals and 0-rings
High
Head is not fully tightened down
Tighten the four head bolts
Medium
Piston seals have failed
Pressure transducer seal failed
Replace the piston seals
Replace the transducer seal
High
Medium
Plate 0-ring has failed
Head 0-ring has failed
Window plug 0-rings failed
Replace the 0-ring
Replace the 0-ring
Replace the 0-rings
Medium
Low
Very Low
90
Mixing
chamber is
leaking
Needle getting
clogged
Temperature
sensors are not
working
Piston will not
retract
Large pressure
spikes during
compression
The RCM
unexpectedly
fires
The RCM
sounds
significantly
different
Low
Vent is slightly open
Vacuum valve is slightly open
Ensure the vent is closed
Ensure the valve is closed
Mixing fan shaft seal has failed
Check for brown liquid
around the shaft and if so
have the seal repaired
Medium
Top bolts are loose
Tighten the top bolts
Very Low
Gasoline injection port septum is
too worn
Replace the septum
High
Wires are disconnected
Ensure that all temperature
wires are connected
Medium
Controllers have become uncalibrated
Temperature sensors are
damaged
Use the controller menus to
"self tune" the controllers
Medium
Replace the temperature
sensors
High
Wires are damaged
Piston seals are too worn
Replace the wires
Replace the piston seals
Low
High
Pneumatic chamber seal has
failed
Replace the pneumatic
chamber seal
Low
The piston is much hotter than
Insulate or even heat the
High
The hydraulic pressure is too
low
Safely increase the pressure
regulator pressure
High
The N 2 bottle is empty
Replace the N 2 bottle
High
The hydraulic pressure lines are
leaking
The piston was not sufficiently
Use Snoop to check leaks
in the lines
Medium
flsure atthe piston is
Medium
retracted before locking
locking
The hydraulic lock 0-ring failed
Replace the O-ring
Medium
The yellow 3 inch ball valve was
not open before firing
The N 2 bottle is "empty"
The pneumatic chamber is
Open the valve before
firing
Replace the N 2 bottle
Replace the pneumatic
High
leaking
piston seal
Low
the hydraulic chamber
hydraulic chamber
91
Low
High
... continued
Data was not
recorded
The pneumatic tank was not
sufficiently supplied with N 2
Ensure proper regulator
pressure and check tank
pressure
Low
The head plate was not
sufficiently tightened down
There is little oil in the system
Tighten the head bolts more
Low
Check the sight tube and
add oil if none is seen
Very Low
LabVIEW was not initiated
Press start in the top left
corner of the front panel
before firing
High
The LabVIEW pressure
threshold is too high
Enter a lower pressure
trigger on the front panel
High
The charge amplifier is on
"Reset" mode
Set the charge amplifier to
"Operate"when firing
High
The DAQ glitched due to
filtering
No fix necessary, rare
anomaly
Very Low
The combustion chamber valve
was open during firing
Make sure the valve is
closed
Low
The piston seals are extremely
damaged
Replace the piston seals
Medium
The head was not tightened
down
Tighten the head down
better
Medium
An O-ring is missing from the
Ensure that all
cylinder
The presur
ressure
traces look
significantly
different than
normal
The pressure
gauges are not
working
in place
O-rings are
Medium
The driving pressure is too low
Use Table B. 1 to determine
proper pressures
High
The cylinder is contaminated
The pressure transducer wire is
damaged
The pressure transducer is
damaged
chamber
Medium
Replace the transducer wire
Medium
Replace the pressure
transducer
Low
The charge amplifier is damaged
Replace the charge
amplifier
Very Low
The DAQ system is damaged
Replace the DAQ
Replace the batteries
Ensure the proper valves
are open
Very Low
Replace the pressure gauge
High
The batteries are dead
The valves to the gauge are
closed
The pressure gauge is damaged
92
High
Medium
Appendix C: Lubrication Oil Classification
Table C.]: American Petroleum Institute base stock classification[18]
Base
Stock
Group
Typical
Process
Saturates
(wt%)
Group I
Seivent
Refined
<90
Group 11
Hydrocracked
Group Ill
Hydrocracked
Group IV
Synthesized
Group V
Various
Gr__p VIVarious
Sulfur
(wt%)
Viscosity
Index
and/or
>0.03
80-120
90
and
50.03
80- 120
90
and
50.03
120
Mineral/
Synthetic
Mineral*
Polyalphaolefins (PAO)
Synthetic
Any base stock not included in the Groups I
through IV
Various
Various
*In the US Group III is typically marketed as synthetic due to significant processing and high performance
93
94
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for Heat and Fluid Flow, Chichester: John Wiley & Sons, 2004, p. 152.
[18] American Petroleum Institute, "Engine Oil and Licensing Certification," in API
Publication1509, Seventeenth Edition, 2012.
[19] K. Inoue and Y. Yamanaka, "Change in Performance of Engine Oils with
Degradation," SAE Technical Paper 902122, 1990, doi: 10.4271/902122.
[20] ASTM Standard D5800, 2014, "Standard Test Method for Evaporation Loss of
Lubricating
Oils by the Noack Method,"
West
Conshohocken,
PA, 2014,
doi: 10.1520/D5800-14E02, www.astm.org.
[21] ASTM Standard D97, 2012, "Standard Test Method for Pour Point of Petroleum
Products," West Conshohocken, PA, 2012, doi: 10.1520/D0097-12, www.astm.org.
[22] ASTM Standard D92, 2012, "Standard Test Method for Flash and Fire Points by
Cleveland Open Cup Tester," West Conshohocken, PA, 2012, doi: 10.1520/D009212B, www.astm.org.
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