OREGON
\\o. V)-
OCT
'-- _i
COLLECThy
Optimum Power and
Economy Air-fuel
Ratios for Liquefied
Petroleum Gases
By
W. H. PAUL
and
M. N. POPOVICH
Bulletin Series,
June 1941
Engineering Experiment Station
Oregon State System of Higher Education
Oregon State College
THE Oregon State Engineering Experiment Station was
established by act of the Board of Regents of the College
on May 4, 1927. It is the purpose of the Station to serve the
state in a manner broadly outlined by the following policy:
(1)To stimulate and elevate engineering education by
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investigations of interest to them.
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For copies of publications or for other information address
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Corvallis, Oregon
Optimum Power and Economy
Air-fuel Ratios for Liquefied
Petroleum Gases
W. H. PAUL
Associate Professor of Mechanical Engineering
and
M. N. Por'ovlcH
Graduate Assistant in Mechanical Engineering
Bulletin Series,
No. 14
June 1941
Engineering Experiment Station
Oregon State System of Higher Education
Oregon State College
TABLE OF CONTENTS
Page
I. Acknowledgments and Summary ---------------------------------------------------------------------3
1.
2.
II.
Acknowledgments
------------------------------------------------------------------------------------
3
Summary -----------------------------------------------------------------------------------------------------3
Introduction ---------------------------------------------------------------------------------------------------------- 4
1. Reasons for the Investigation ---------------------------------------------------------------- 4
2. Specific Requirements of a Fuel for Internal-Combustion
Engines----------------------------------------------------------------------------------------------------
4
3. General Requirements of a Fuel for Internal-Combustion
Engines-----------------------------------------------------------------------------------------------------
III.
Properties of Paraffin Hydrocarbons -------------------------------------------------------------
1. Molecular-Structure and Properties -----------------------------------------------------
2. Octane Ratings of the Fuels Used
------------------------------------------------------
8
IV. Method of Conducting Tests ---------------------------------------------------------------------------- 9
General Equipment ----------------------------------------------------------------------------------
9
Procedure -------------------------------------------------------------------------------------------------Results ------------------------------------------------------------------------------------------------------------------
11
1.
2.
V.
11
1. Power and Economy Curves ---------------------------------------------------------------- 11
2. Effect of Molecular Size on Optimum Air-Fuel Ratios
3.
---------------- 11
Air-Fuel Ratio Meter Calibration Curve ------------------------------------------ 15
4. Operating Principle of Thermal-Conductivity Instrument
---------- 15
5. Discussion of Instrument Calibration -------------------------------------------------- 16
6. Explanation of Meter Inaccuracy in the Lean Range
VI.
VII.
Conclusions
References
------------------ 16
----------------------------------------------------------------------------------------------------------
18
------------------------------------------------------------------------------------------------------------
19
ILLUSTRATIONS
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
General View of Apparatus ----------------------------------------------------------------------
9
Thermal-Conductivity Type Air-Fuel Ratio Instrument ------------------ 10
Optimum Power Air-Fuel Ratios for All Fuels Investigated -------- 12
Optimum Economy Air-Fuel Ratios for All Fuels investigated
12
Optimum Air-Fuel Ratios vs. Per Cent Carbon Contained
.
byEach Fuel
------------------------------------------------------------------------------------------ 13
Figure 6. Calibration of Thermal Conductivity Type Air-Fuel Ratio
Instrument for the Various Fuels
14
TABLES
Table 1. Boiling Points and Octane Ratings of Hydrocarbons ------------------------ 6
Table 2. Constants of Liquefied Petroleum Gases ---------------------------------------------- 8
Table 3. Optimum Power and Economy Air-Fuel Ratios for Various
Hydrocarbon Fuels
15
Table 4. Thermal Conductivities of Various Exhaust Gases ---------------------------- 16
Table 5. Typical Exhaust-Gas Analyses for Selected Air-Fuel Ratios ---------- 17
Optimum Power and Economy
Air-fuel Ratios for Liquefied
Petroleum Gases
By
W. H. PAUL
Associate Professor of Mechanical Engineering
and
M. N. PoPovicH
Graduate Assistant in Mechanical Engineering
I. ACKNOWLEDGMENTS AND SUMMARY
1. Acknowledgments. The authors are grateful to the American Liquid
Gas Corporation of Los Angeles, California, for supplying fuel conversion
equipment; to the Standard Oil Company of California for furnishing the
special hydrocarbon fuels used in the investigation; to the Electro-Products
Company of New York for air-fuel ratio instruments; to Harold A. Marble,
senior student in Mechanical Engineering, who gave valuable aid in assisting
with tests; and to Professor S. H. Graf, Director of Engineering Research
and Head of Mechanical Engineering, for suggestions and editorial counsel.
2. Summary. This investigation includes the determination of optimum
power and economy air-fuel ratios for propane, n-butane, iso-pentane, isooctane, gasoline, and commercial mixtures of butane and propane. It also
includes calibration of a commercial thermal conductivity type air-fuel ratio
instrument for each of these fuels. Explanation is offered for the unreliability
of such instruments in the lean-mixture range.
During the past few years utilization of liquefied petroleum gases in internal-combustion engines has undergone considerable expansion. The lighter
hydrocarbons, particularly propane and butane, have certain desirable characteristics that make them attractive as fuels for the carburetting engine. Among
these are high resistance to detonation, uniform manifold distribution, absence
of impurities, and low cost under certain favorable conditions.
This study was carried out by using a single-cylinder, water-cooled engine
of the four-stroke cycle type. A d-c generator formed an integral part of the
unit, and power measurements were made with suitable electrical instruments.
Air was metered accurately through a large "wet" type gas meter, while the fuel
quantities were determined by weighing. The liquid fuels, confined under
pressure in tanks, were converted to gases and reduced in pressure by means of
special apparatus obtainable commercially. Air-fuel ratio readings were
indicated by a thermal-conductivity type instrument connected to the engine
exhaust.
4
ENGINEERING EXPERIMENT STATION BULLETIN No. 14
In addition to test results, the effect of size and shape of fuel molecules on
the knock resistance of paraffinic hydrocarbons is discussed. Suggestions for
the proper carburetion of the liquefied petroleum fuels are given.
Calibration of the air-fuel ratio instrument for the lighter fuels is pre-
sented and an explanation is offered for the difference between indicated instrument readings and the true values of air-fuel ratio. Explanation is also given
for the fact that thermal-conductivity type meters swing back toward richer
readings when mixtures are very lean.
The investigation showed that the air-fuel ratios for maximum power and
maximum economy for liquefied petroleum fuels were considerably higher than
for gasoline. In general, fuels having smaller molecules yielded higher air-fuel
ratios for maximum power. These air-fuel ratios more nearly approached calculated values for perfect combustion.
II. INTRODUCTION
1. Reasons for the investigation. Little has been written on the subject
of experimentally determined air-fuel ratios for maximum power and maximum
economy for fuels other than gasoline. For gasoline, writers report values
ranging from 12.5 to 13.5 pounds of air per pound of fuel; this lack of agreement depending upon the operating conditions and characteristics of the engine
used. For other fuels, however, only calculated, theoretical air requirements are
generally cited: The theoretical air-fuel ratio for gasoline, the formula for
which is usually taken as pure octane, CH,8, is 15.2, which differs materially
from the actual values cited above for maximum power. With the growing use
of liquefied petroleum gases as motor fuels there is need for accurate knowledge
of these specific air-fuel ratio quantities for each fuel and some of their blends.
Iso-pentane, iso-octane, and gasoline were included along with butane and propane in the present investigation in order to obtain a more complete range of
data for general comparison of actual with theoretical values for the paraffinic
hydrocarbon series. Commercial mixtures of 60-40 butane-propane and 70-30
butane-propane were included because these fuels are sold as commercial
mixtures.
Along with increased use of these liquefied petroleum gases has come some
demand for air-fuel ratio instruments calibrated for these fuels. If optimum
conditions are determined by a test and the air-fuel meters are calibrated for
these fuels, their range of use will be extended and the fuels can be utilized
with greater efficiency. With this in view it was the purpose of this work to
determine the air-fuel ratios for maximum power and maximum economy for
a range of paraffinic hydrocarbon fuels between propane and iso-octane, together
with combinations, and to calibrate a commonly used type of air-fuel ratio
meter for these fuels.
2. Specific requirements of a fuel for internal combustion engines.
The four most desirable characteristics of a fuel for internal combustion engines
are recognized as high resistance to detonation, suitable volatility for good manifold distribution, low sulphur content, and minimum tendency toward gum formation. The liquefied petroleum fuels have a higher detonation resistance than
all but the most expensive aviation fuels. In the matter of cylinder and mani-
fold distribution, gaseous fuels are superior to liquid fuels because they are
completely gasified rather than atomized during carburetion. Due to their
gaseous nature, liquefied petroleum fuels contain little or no gum or sulphur, as
they are recovered in relatively pure form in the refining process.
AIR-FUEl, RATIOS FOR LIQUEFIED PETROI.EUM GASES
Although these gaseous fuels contain fewer heat units per gallon, performance and economy can be augmented by increasing the compression ratio of the
engine. Possible compression ratios for butane and propane have been reported
as 8.5 :1 and 10:1, respectively. The authors have successfully used 9 :1 with
butane-propane mixtures. Unless quite expensive or at least quite heavily
leaded gasoline is used, the octane numbers of the liquid fuels will not approach
those of the gaseous fuels; therefore, compression ratios used with gasoline will
be limited in comparison.
3. General requirements of a fuel for internal-combustion engines.
A fuel is usually considered as a material that is generally available, is sufficientiv cheap, has heating value, and can be used commercially for its thermal
energy. The cost of liquefied petroleum fuels at or near refineries is quite low,
being only a fraction of the cost of gasoline. Since the problem of distributing
these fuels to points far removed from the source of supply has not yet been
worked out to any great extent, the cost per gallon in remote territories is quite
high. At present liquefied petroleum gases fail to comply completely with the
availability requirement except in localities where they are being manufactured.
These fuels are being used at present, however, by commercial vehicles making
long trips on routes where supply stations have been established.
If the utilization of liquefied petroleum gases expands, as expected, the
largest immediate outlet for these fuels will likely be in the heavy-duty motor
transport field.
Mention may here be made of the fact that iso-butane is now being used
with butylene in the alkylation process to manufacture 100 octane aviation
gasoline. The amount of butane needed, at present, however, for this use is
small compared to the total quantity produced.
III. PROPERTIES OF PARAFFIN HYDROCARBONS
1. Molecular-structure and properties. With the exception of gasoline,
fuels used in this work were of the paraffinic type, having the general
This group, or family of hydrocarbons, is the sochemical formula,
all
called "saturated" series, and is the predominant hydrocarbon type in gasoline.
By "saturated" is meant that each carbon atom in the molecule is completely
satisfied with hydrogen. Methane, the first in the series, consists of one atom
of carbon and four of hydrogen, and therefore has the formula CH. Following
methane in order in the series are: ethane, CH; propane, CHs; butane, CH,0;
pentane, CH,; liexane, CH4; heptane, CH,; octane, CH,s; and so on up.
H
HHHH
HCCCCH
HHHH
I
I
Molecular structure of
nhutane
HCH
H
H
HCCCH
I
III
HHH
Molecular structure of
iso.butane
Due to the characteristic structural pattern, shown at the left above, these
hydrocarbons are commonly referred to as the open-chain type. Starting with
ENGINEERING EXPERIMENT STATION BULLETIN No. 14
6
Table 1. BOILING POINTS AND OCTANE RATINGS OF HYDROCARBONS
Hydrocarbon
Pare ffins
Boiling point
F (760 mm)
Octane number
(CFR-ASTM)
-259
110
n-Butane
2-Methyipropane
33
11
92
99
n.TPentane
97
82
49
61
89
83
n.Hexane
156
141
146
121
137
25
73
75
96
95
209
n-Heptane
194
2-Methyihexane
2 2-Dimethylpentane ----------------------------------------- 175
194
2 3-Dimethylpentane
177
2 4-Dimethylpentane
187
3-Dimethylpentane
3
178
3-Trimethylbutane
2 2
45
93
89
82
84
Ethane----------------------------------------------------------------- -127
2-Methylbutane
Dimethyipropane
2-Methylpentane
3-Methylpentane
2 2-Dimethylbutane
2 3-Dimethylbutane
:
:
:
:
104
100
0
101
n-Octane
3-Methyiheptane
2 3-Dimethylhexane
2 5-Dimethylhexane
4-Dimethyihexane
3
2 2 3-Trimethylpentane
2 2 4-Trimethylpentane
2 3 4-Trimethylpentane
3-Methyl-3-ethylpentane
2 2 3 3-Tetramethylbutane
258
246
240
229
244
230
210
236
245
224
17
n-Nonane
303
45
-152
- 54
97
n-Pentene (-2)
154
n-Hexene (-2)
2.Methylpentene (-2) ------------------------------------------151
154
3-Methylpentene (-2) ---------------------------------------106
2 2-Dimethylbutene (-3)
172
2 2 3-Trimethylbutene
257
n-Octene (-2)
252
3-Methylheptene (-2)
217
2 4 4.Trimethylpentene (-1 and 2)
81
85
80
78
78
79
94
89
55
74
86
:
:
:
:
:
:
:
:
:
Ole/ins
:
:
:
Di-O Ic/ins
Hexadiene (-2:4)
35
76
52
85
102
100
97
91
103
176
77
-----------------------------------------------139
121
Cyclopentane
Methylcyclopentane -------------------------------------------181
Cyclohexane --------------------------------------------------------177
Methylcyclohexane
213
68
83
82
77
71
Naphthenes
Ethylcyclobutane
A romatics
Benzene
--------------------------------------------------------------Toluene--------------------------------------------------------------
176
231
108
104
butane the larger molecules may have different atomic arrangements, yielding
compounds of the same chemical formula, but with slightly different properties.
There are two butanes, shown on page 5, and three pentanes, and as the number
AIR-FUEL RATIOS FOR LIQUEFIED PETROLEUM GASES
of carbon atoms increases, the number of possible combinations increases
rapidly. These iso-paraffins or "isomers" do not vary a great deal from their
open-chain contemporaries in the boiling point, vapor pressure, and other general
physical properties, but the variation in octane number is marked. For instance
in the extreme case of octane, normal-octane has an octane number of 17, but
2-2-4 trimethyl pentane, an isomer, has an octane number of 100. Below are
shown the structural configurations of these two octanes.
HHH HHHHH
HCCCC--CCCCH
HHH HHHHH
I
I
I
I
I
I
I
I
I
I
Octane number, minus 17.
Normal-octane.
H
H
H
H
H HH
HCC
I
-H
1
HC-H
I
2.2-4 trimethyl pentane, or iso-octane, an isomer of
n-octane. Octane number 100.
The Journal of the Institute of Petroleum, June 1940, (1)* shows octane
ratings (CFR-ASTM) for a number of hydrocarbons and their isomers. These
are tabulated in Table 1.
Hydrocarbons in the saturated or paraffinic series become more dense as
the number of carbon atoms increases. Methane, CH4, is a gas and at ordinary
temperatures cannot be liquefied even at extreme pressures. Ethane, C2H6, can
be liquefied at ordinary temperatures, but the pressure requirement is so great
that the container would be too heavy to be practical. Propane, C5H5, is a gas
at atmospheric pressure, but it can be liquefied at ordinary temperatures by a
pressure of about 125 pounds per square inch gage, and is safe in a container of
reasonable strength. Normal-butane, C4H,0, boils at 33 F, so in order to keep
it in the liquid state, it, too, must be kept under pressure. Normal-pentane,
C5H,2, boils at about 97 F and is a very volatile liquid at room temperature.
For that reason it is kept in pressure vessels. Normal-hexane, C6H,, is a liquid
that boils at 156 F and may be stored in ordinary tanks. Isomers of hexane
are a major constituent of gasoline. The lightest hydrocarbon that gasoline
contains in any appreciable quantity is butane, the content being as high as 8
per cent in some winter gasolines.
Table 2 lists some of the more commonly used properties of propane, normal-butane, and iso-butane.
Numbers in parentheses refer to references listed in Section VII.
!si7si'$
8
5i
s-vs
ii.-
-
a
-
e-
-.
kNGINEERING LXPERIMENT STATION Buu.F-rlN No. 14
Table 2. CONSTANTS or
LIQUEFIFD 1'FTROIEUM GASFS
(All values not otherwise designated are at 60 F and 30 inches of mercury)
Propane
Characteristics
Chemical formula ..............
Molecular weight
Percentage composition
-.
.
Normal state at 60 F
and 30 inches mercury ....
Specific gravity gas
(Air=1) ...............................
Specific gravity liquid
at 60 F/60 F
Boiling point liquid:
Degrees F .........................
Critical Data:
(a) Temperature F
pounds
(b) Pressure
per square inch ...
C3H5
CHi.CHCHs
Isohutane
N-butane
CH5
C4H10
CHS.CHS.CHS.CHS
CH3.CH: (CH),
44.0624
58.078
58.078
H-18.2977
C-8L7023
H-17.3525
C-82.6475
(H 17.3525
Gas
)
C-82.6475
Gas
Gas
1.5206
2.0042
2.0042
0.5089
0.5824
0.5665
44
32.9
13.64
206.6
303.4
272.66
648.1
552.4
538.23
192.6
816.3
173.52
841.64
169.92
801.68
Latent heat of vaporization at normal boiling
point:
Btu per pound
Btu per gallon
Pounds per gallon of
liquid-at 60 F
4.2383
4.718
4.8544
Btu tier cubic foot of
vapor at 60 F and 30
inches mercury-dry
Btu per pound ...................
2,519
3,274
3,274
21,633
21,331
21,331
Btu tier gallon
91,686
103,465
100,639
Highest useful compres-
sion ratio
Octane number
10 :1
100
8 :1
9 :1
92
Authority: Handbook of Butane-Propane Gases.
Smittenberg, J., JI. Inst. of Pet., 26:294-303, 1940.
2. Octane ratings of the fuels used. The fuels used in this work were
commercially pure propane, normal-butane, iso-pentane, iso-octane, butanepropane mixtures, and gasoline. The octane numbers as reported in Table 1 are:
Propane............................................................ 100
Normal-butane ---------------------------------------------- 92
Iso-pentane ------------------------------------------------------ 89
Iso-octane -------------------------------------------------------- 100
The gasoline used had an octane rating of 74 (CFR-ASTM).
-
AIR-FUEL RATIOS FOR LIQUEFIED PETROLEUM GASES
IV. METHOD OF CONDUCTING TESTS
1. General equipment.
The principal equipment used consisted of a
Delco engine with integral direct-current generator, an "Algas" fuel converter
(universal unit), a drum with rubber heads to dampen intake pulsations, a
fuel-weighing system, a meter for measuring air volume, an air-fuel ratio
instrument, an accurate chronometric tachometer, and an operating panel
equipped with resistors for loading the engine and containing connections for
electrical apparatus to measure power output. A general view of the apparatus,
exclusive of the air meter and pulsation dampener, is shown in Figure 1.
The engine used was a single-cylinder Delco having a bore of 2 inches, a
stroke of 5 inches, and a compression ratio of 6.5. It was directly connected to
a 32-volt, self-excited, direct-current generator. The cooling system was of
the evaporative, water-jacket type, with a reflux condenser to precipitate the
steam formed. The ignition system was the battery-distributor type. A neon
ignition ring, installed permanently on the engine, indicated the spark timing
with the engine operating.
GENERAL VIEW OF AFPARATUS. SHOWING, FROM LEFT TO RIGHT, FUEL MEASURING SYSTEM ON TABLE, FUEL CONVERTER ON STAND, ENGINE ON BASE, AND INSTRUMENT
PANEL WITH CONTROLS, AT RIGHT. AIR METER AND PULSATION SHOCK ABSORBER NOT
Figure 1.
SHOWN.
Air volume consumed by the engine was measured by means of an
American Meter Company "wet" type laboratory test meter having a capacity
of one cubic foot per revolution. Between the meter and the engine was
placed a 20-gallon drum with the heads removed and replaced with rubber
10
ENGINEERING EXPERIMENT STATION BULLETIN No. 14
sheeting to dampen pulsations caused by the air intake of the engine. This
device proved to be very satisfactory.
The fuel, in the case of the more volatile hydrocarbons, was held in a small
metal cylinder adapted from an oil filter and was directly weighed on a 15pound beam-balance. In operating with butane and propane, the volume of the
gasified fuel was measured by a 0.1 cubic foot Sargent wet-test meter, and
these volumetric results were checked with values obtained by direct weighing.
The liquefied petroleum fuels were gasified by means of an "Algas" conversion unit made by the American Liquid Gas Corporation. This unit consists
of a heat exchanger and two diaphram reducing valves, which are connected in
series for the purpose of reducing the gas pressure to atmospheric. Control of
fuel quantity to the engine is accomplished by means of a valve at the outlet of
the conversion unit. The gaseous fuels were introduced into the engine through
a hole at the venturi throat of the standard gasoline carburetor.
The operating panel contained resistors for loading the engine, switches for
starting,, and a field rheostat for controlling generator field current. The panel
also contained outlets for voltmeter and ammeter connections used to measure
power.
A commercial air-fuel ratio instrument of the thermal-conductivity type
made by the Electro-Products Company, Figure 2, was used to indicate air-fuel
relations.
J
Ftgure 2.
THERMAL-CONDUCTIVITY TY?E AIR-FUEL RATIO INSTRUMENT.
AIR-FUEL RATIOS FOR LIQUEFIED PETROLEUM GASES
11
2. Procedure. Before each test, the engine was warmed up for a period
of about 2 hours in order to establish equilibrium conditions. These equilibrium
conditions involved crankcase temperature, generator winding temperature,
steady conditions of air and fuel intake, and constant resistor coil temperature.
A constant speed was chosen in order to bring oil temperatures to equilibrium and maintain constant engine friction. A speed of 675 revolutions per
minute was selected; since, with a heavy load and throttle more than half open,
good speed regulation could be maintained by means of the field rheostat.
Temperature and quantity of water through the converter heat exchanger
were regulated to insure complete evaporation of the fuel without excessive
superheating. Prior to any run with a particular fuel the spark timing was set
for maximum power with an air-fuel ratio very close to that for maximum output as determined from a short preliminary trial. Inasmuch as flame speed
varies with mixture strength, the spark timing should be adjusted for each different air-fuel ratio. In this work, however, the object was to find the air-fuel
ratio for maximum power; therefore, the single spark adjustment fulfilled the
requirement. At the beginning of each day's operation, the fuel container was
filled and tested for leaks, since, on the weight basis, even a very small leak
would introduce objectionable error in the results.
The beginning of a run was marked by the settling of the scale beam past
the midpoint of its swing, at which time a stopwatch was started, and the initial
reading on the air meter was observed. In the case where the small 'wet" type
meter was used in conjunction with weighing, it was read simultaneously. Readings of power output, air temperature, oil temperature, and speed were made
twice during a run. One reading of indicated air-fuel ratio was obtained after
the instrument pointer had settled to a fairly constant position. After the consumption of one-tenth pound of fuel, the run was stopped, at which time final
readings were taken on the air meter, and the fuel-measuring equipment.
The power output, measured air-fuel ratio, and the specific fuel values were
calculated and roughly plotted as the test proceeded.
A sufficient range of air-fuel ratios on either side of the optimum was
investigated to make certain that the optimum point had been found.
V. RESULTS
1. Power and economy curves. Curves showing power and economy
results have been prepared for each fuel investigated. In the case of the power
curves, results for all fuels are plotted in Figure 3, in which per cent of maximum power output is plotted as ordinate, and air-fuel ratio as abscissa. Figure
4 includes the economy curves for all fuels and was constructed with per cent
of minimum fuel consumption as ordinate, and air-fuel ratio as abscissa.
It can be seen that the range of optimum air-fuel ratio values obtained for
the fuels investigated was fairly wide. The air-fuel ratio for maximum power
for propane was found to be 15.5, whereas the corresponding figure obtained
for gasoline was 13.0. For maximum economy, the range was from 17.9 for
propane to 16.0 for gasoline.
2. Effect of molecular size on optimum air-fuel ratios. From the
maximum power and maximum economy values obtained for each fuel (Figures
3 and 4), a curve was constructed with these optimum values plotted against
the respective carbon contents of the fuel molecules (Figure 5). On this plot
are also shown the theoretical air-fuel ratios for the various fuels. Since the
carbon content of hydrocarbons in a particular series is proportional to the size
!HHH
I::
PROPANE
60-40 BUTANE-
PROPANE
70-33 BUTANEPROPANE
GASOLINE
w
Q. BC
SO-OCTAN
ID
0.
SO-PENTANE
I-
D
0
N-BUTANE--
Ui
9
2
4
13
Figure
3.
6
17
16
15
AIR-FUEL RATIO
lB
LB AIR
LB FUEL
OPTIMUM POWER AIR-FUEL RATIOS FOR ALL FUELS INVESTIGATED.
120
I0.
115
______ ______ ______
PROPANE
z
0
0
-
______ ______
60-40 BUTANE-PROPANE
-J
_N-BUTANE
110
105
IIi
CASOLINE
ISO-OCTANF
U.
0
1 100 _______I5O-PENT6NE
z
______ ______
II
w
U
U.
12
13
14
IS
16
AIRFUEL RATIO
Figure
4.
Il
8
LB AIR
LB FUEL
OPTIMUM ECONOMY AIR-FUEL RATIOS FOR ALL FUELS INVESTIGATED.
12
19
AIR-FUEl. RATIOS FOR LIQUEFIED PETROLEUM GASES
13
of the molecule, the curves demonstrate the effect of molecular size on optimum
air-fuel ratio requirements as well as the effect of size on the relative agreement of actual air-fuel ratios with theoretical values. It can be seen that the
points obtained experimentally define the curves quite well, although some deviations are evident.
From Figure 5 it is apparent that with decreasing molecular size, or reduc-
tion in per cent carbon, the air-fuel ratio for maximum power approaches the
theoretical ratio for perfect combustion. Since the fuels showed a general trend
toward higher combustion efficiency as the molecule became smaller, it would
seem that the actual and theoretical values would practically coincide for ethane
and methane, two lighter hydrocarbons not included in this investigation.
(9 - - - - - 0
--
(8 ---
ECONOMY
2
I-
-j
Ui
-
PERFECT COMBUSTION
4
(5
POWER
'IiIIIi
8182838485
PER CENT CARBON
OPrIMUM ATR-FSXEL RATIOS VS PER CENT CARBON CONTAINED BY EACH FUEL.
POINTS FROM LEFT TO RIGHT IN ORDER REPRESENT PROFANE, 60-40 BUTANE-PROPANE,
Figure 5.
70-30 BUTANE-PROPANE, BUTANE, PENTANE, OCTANE, AND GASOLINE.
.--.,
14
ENGINEERING EXPERIMENT STATION BULLETIN No. 14
It is not expected that results obtained with olefines and other nonparaffinic
fuels would fall on these curves, but it is probable they would form curves having the same general shape. The factor controlling the degree to which actual
air-fuel ratio values approach theoretical values is likely to be the molecular
size rather than per cent carbon.
The general shape of the economy curve in Figure 5 is similar to that of
the power curve except for being slightly flatter. It was evident that as the
fuel molecules became smaller, the per cent excess air, over the theoretical
amount for perfect combustion, increased. This is shown by the fact that the
ordinate distances between the theoretical and economy curves increased as molecules became smaller. The maximum economy for gasoline occurred about one
air-fuel ratio higher than the theoretical value. On the other hand, the maximum point for propane occurred over two air-fuel :ratios above the theoretical
value, definitely indicating a greater quantity of excess air in the case of lighter
fuels. This would indicate that leaner mixtures should be used with increasingly light fuels to obtain maximum economy conditions.
The difference in air-fuel ratio between maximum power and maximum
economy for any one fuel was greater in the case of fuels with higher molecular weight, being 3.0 air-fuel ratios for gasoline and 2.4 air-fuel ratios for propane.
This increased difference is explained by the fact that gasoline
is
atomized during carburetion rather than gasified as is propane, and therefore
mixing of gasoline and air is not as thorough as the mixing of propane and air.
In going from the maximum power point to the maximum economy point,
17
- - - - - - -- - - -
--
0
--
- -
GASOLI NE\
15
2
J7 Ii
Q-t77
_13
/
/
ISO PENTANE
c
-
SPPOPANE
9
10
12
14
16
18
20
MEASURED AIRFUEL RATIO
Figure 6.
CALIBRATION OF THERMAL CONDUCTIVITY TYPE AIR-FUEL RATIO INSTRUMENT
FOR THE VARIOUS FUELS.
4
AIR-FUEL RATIOS FOR LIQUEFIED PETROLEUM GASES
15
additional air is introduced until there is an excess. This excess of air causes
more turbulence and better atomization of the gasoline, resulting in greater combustion efficiency and a maximum economy point higher than might be expected.
The excess air added in going from maximum power to maximum economy
has less influence on the smaller molecules, such as propane and butane, since
they are completely gaseous and lend themselves more readily to thorough and
uniform mixing. Therefore the ordinate difference between the power and
economy curves becomes less as the fuels become lighter.
For convenient reference in the selection of an optimum point for a particular fuel, the values obtained from Figure 5 for maximum power and economy for the fuels investigated are shown in Table 3.
Table 3.
OPTIMUM POwER AND ECONOMY AIR-FUEL RATIOS FOR VARIOUS HYDROCARBON
FUELS.
Fuel
Propane----------------------------------------------------------------------60-40 Butane-propane ------------------------------------------------70-30 Butane-propane -------------------------------------------------
n-Butane -------------------------------------------------------------------Iso-pentane ---------------------------------------------------------------Iso-octane -------------------------------------------------------------------
Gasoline---------------------------------------------------------------------
Air-fuel ratio for
maximum power
15.5
15.3
15.2
15.0
14.5
13.7
13.0
Air.fuel ratio for
maximum economy
17.9
17.8
17.7
17.5
17.2
16.6
16.0
The values tabulated for butane were not in complete agreement with
those reported by Vogt (2) in an earlier investigation. Greatest differences
from his values were in maximum economy. Maximum economy for any one
fuel, however, is largely dependent upon engine characteristics.
For comparable
air rates in the two investigations maximum power values are in good agreement.
3. Air-fuel ratio meter calibration curve.
Although points were
somewhat scattered, curves of indicated air-fuel ratio plotted against measured
air-fuel ratio, Figure 6, for the fuels investigated, show a definite trend in
relation to percentage carbon and hydrogen in the fuel. In order to produce
more accurate calibration curves for the air-fuel ratio meter, additional data
were used that had been obtained from previous tests made in the Oregon State
College automotive laboratory on multi-cylinder engines. Actually the curves
are a result of composite runs on both single and multi-cylinder engines, using
the same type and make of air-fuel ratio instrument.
4. Operating principle of thermal-conductivity instrument. In order
to facilitate understanding of the discussion dealing with air-fuel ratio meter
results, a brief description of the operating principle of the thermal-conductivity
type of instrument will be reviewed. The fundamental principle of operation
deals with the difference in thermal conductivity of the exhaust gas from that
of a standard gas. The fact that the thermal conductivity of hydrogen is considerably higher and the thermal conductivity of carbon dioxide is somewhat
lower than the conductivity of carbon monoxide, nitrogen, and oxygen, results
in definite characteristic changes in thermal conductivity of different exhaustgas samples. The exhaust gas enters a cell containing a coil of resistance wire,
which is an arm of a Wheatstone bridge circuit. A second cell, sealed and
containing a standard gas, together with a coil of resistance wire, makes up
another arm of the Wheatstone bridge circuit. The two remaining resistance
16
ENGINEERING EXPERIMENT STATION BULLETIN No. 14
arms are made equal. A controlled amount of current from a dry-cell battery
is passed through the bridge circuit and warms it to equilibrium temperature. In
passing exhaust gas of different thermal conductivity from the standard gas
through the test arm, difference in cooling of the rcsistance wire in the test cell
from the cooling of the resistance wire in the standard cell causes a temperature change that affects the resistance in the test cell and unbalances the bridge.
Tins deflects the galvanometer over a scale graduated in air-fuel ratio.
It is a known fact that rich mixtures yield exhaust gases of high hydrogen
and relatively low carbon (hioxide volumetric percentages, thereby producing a
gas of high thermal conductivity (7). Inasmuch as a definite correlation exists
between hydrogen content and air-fuel ratio, (3) it is possible to graduate a
galvanometer scale in terms of air-fuel ratio.
5. Discussion of instrument calibration. The calibration curves for
the air-fuel ratio meter showed a definite trend toward richer readings as the
density of the fuels decreased; i.e., the instrument indicated lower and lower
readings for a given measured air-fuel ratio. As thc hydrocarbon fuel molecules
become smaller (lighter fuels), the weight percentage of carbon decreases,
while the proportion of hydrogen increases. Corresponding samples of exhaust
gas from these successively lighter fuels will show decreasing percentages of
carbon dioxide and increasing percentages of free hydrogen. Explanation of
this increase in free hydrogen involves the water-gas reaction and analysis of
chemical equilibrium in the combustion process (4) (5). The greater hydrogen
content increases the thermal conductivity of the exhaust gas for any particular
air-fuel ratio as compared with gasoline. It follows that the smaller the fuel
molecule, the richer the reading would be for the given air-fuel ratio, since the
smaller molecules have more hydrogen. For a measured air-fuel ratio of 14, the
indicated air-fuel ratio for gasoline was 14; for iso-pentane, 12.6; for n-butane,
12.1; and for propane, 11.3, results which are in agreement with the above
explanation.
6. Explanation of meter inaccuracy in the lean range. It was noticed
that in the very lean ranges the air-fuel ratio instrument had a tendency to
return to readings of lower air-fuel ratio. An examination of thermal conductivities of the exhaust gas constituents and typical exhaust-gas analyses
explains why this occurs. The following table shows the thermal conductivities
of exhaust gases as obtained from "Combustion," American Gas Association,
1939 (6).
'fable 4.
THERMAL C0NDUcTIvITIEs OF VARIOUS ExHAusT GASES
Gas
Thermal
conductivity
Btu per
F
per in. per sec
Carbondioxide --------------------------------------------------------------------------------------------Carbon monoxide -----------------------------------------------------------------------------------------
0.000305
0.0000447
0.0000438
0.0000283
0.0000412
The thermal conductivities of oxygen, nitrogen, and carbon monoxide are
about the same, but the thermal conductivity of hydrogen is approximately seven
times that of oxygen, nitrogen, or carbon monoxide, and the conductivity of
carbon dioxide is about 40 ver cent less than that of oxygen, nitrogen, and
carbon monoxide.
Ic
AIR-FUEL RATIOS FOR LIQUEFIED PETROLEUM GASES
17
The following table gives typical exhaust-gas analyses for gasoline at different air-fuel ratios
Table 5.
TYPICAL EXHAUST-GAS ANALYSES FOR SELECTED AIR-FUEL RATIOS
A/F
13
14
15
16
17
18
19
H1
-------------------------------------------------------- 1.6
-------------------------------------------------------
0.8
0.2
....
...
-------------------------------------------------------
.
-------------------------------------------------------- .
02
0.9
0.7
1.0
1.9
3.0
3.9
4.8
N2
CO1
81.4
83.3
84.4
84.5
84.2
84.1
83.0
10.7
12.4
13.1
12.8
12.1
11.5
10.8
CO
4.8
2.3
1.1
0.6
[
06
0.4
0.5
(Any residual percentage is methane.)
It is evident that for rich mixtures the hydrogen content increases and the
carbon dioxide decreases, thus increasing the thermal conductivity considerably.
For lean mixtures the hydrogen content approaches zero, while the carbon
dioxide remains high. Therefore the thermal conductivity is lowest at air-fuel
ratios of 15-16. For very lean mixtures, excess air is introduced and the carbon
dioxide content decreases. Oxygen, nitrogen, and a little carbon monoxide, in
addition to the carbon dioxide, are all that remain in this very lean range. Since
the conductivities of these remaining gases are the same, and the carbon-dioxide
content is decreasing, the total conductivity will tend to become greater again.
This, of course, causes the meter to swing back toward the lower air-fuel ratio
values.
curve.
In Figure 6 this characteristic is plainly shown by the droop in each
VI. CONCLUSIONS
1. Optimum air-fuel ratios for the hydrocarbcn fuels with which this investigation is concerned are:
Air-fuel ratio
Air-fuel ratio
maximum power maximum economy
Propane............................................................ 15.5
17.9
n-Butane .......................................................... 15.0
17.5
Iso-pentane ...................................................... 14.6
17.2
Iso-octane ........................................................ 13.7
16.6
Gasoline ............................................................ 13.0
16.0
Fuel
2. As the number of carbon atoms in the paraffinic hydrocarbon fuels becomes smaller, the air-fuel ratio for maximum power approaches the theoretical
air-fuel ratio for perfect combustion. In the case of propane, which contains
only three carbon atoms, maximum power occurred at an air-fuel ratio of 15.5,
whereas the theoretical air-fuel ratio for perfect combustion is 15.7.
3. Air-fuel ratio for maximum economy in fuels having larger molecules is
not greatly in excess of the theoretical air-fuel ratio for perfect combustion. The
maximum economy air-fuel ratio for gasoline was found to be 16.0. The theoretical air-fuel ratio for perfect combustion is 15.0.
4. Maximum economy air-fuel ratio for the liquefied petroleum fuels is considerably higher than for gasoline, showing that leaner mixtures should be used
with the very light fuels in order to obtain maximum economy conditions.
5. When the thermal-conductivity type air-fuel ratio meter is used for
determining air-fuel ratios for the lighter fuels, indicated air-fuel ratios are
lower than the true air-fuel ratio. The difference between the indicated and
actual air-fuel ratio becomes greater as the molecule becomes smaller, due to the
fact that the smaller molecules in a given hydroca:rbon series contain a lower
percentage of carbon and a higher percentage of hydrogen.
6. The pointer of the thermal-conductivity type air-fuel ratio meter has a
tendency to swing back toward lower air-fuel ratio readings when air-fuel ratios
are very high. This is explained by the fact that there is an absence of hydrogen and a decreasing amount of carbon dioxide in the very lean range, resulting
in a net increase in thermal conductivity.
18
VII. REFERENCES
1. SMITTENBERG, J.
Octane ratings of a number of pure hydrocarbons and of
some of their binary mixtures; CFR-ASTM Motor Method. Ji. Inst.
of PetI, 26 :294-303, 1940.
2. VOGT, C. J. Some characteristics of internal-combustion engines when operating with butane-air mixtures. Oil and Gas JI., 34 :52-56, 1935.
3. GRAF, S. H., GLEaSON, G. W., and PAUL, W. H. Interpretation of exhaust
gas analyses. Bull. 4, Engr. Expt. Station, Oregon State College, 1934.
4. GLEESON, G. W., and PAUL, W. H. Water gas reaction apparently controls
engine exhaust gas composition. Nat. Pet. News, 28 :25-6, 1936.
5. GLEESON, G. W., and W000FIELD, F. W. Stoichiometric calculations of exhaust gas. Nat. Pet. News, 31 :R401-2, 1939.
6. AMERICAN GAS ASSOCIATION.
Combustion, 3rd ed. Easton, Pa., Mack
Printing Co., 1939.
7. DILWORTIT, J. L. Characteristics of exhaust gas analyzers.
48:234-239, 1941.
19
S.A.E. Jl.,
OREGON STATE COLLEGE
ENGINEERING EXPERIMENT STATION
CORVALLIS, OREGON
LIST OF PUBLICATIONS
BulletinsNo. 1. Preliminary Report on the Control of Stream Pollution in Oregon, by C. V.
Langton and H. S. Rogers. 1929.
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1934.
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CircularsNo. 1. A Discussion of the Properties and Economics of Fuels Used in Oregon, by
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1930.
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20
AIR-FUEL RATIOS FOR LIQUEFIED PETROLEUM GASES
21
Reprints
No. 1. Methods of Live Line Insulator Testing and Results of Tests with Different
Instruments, by F. 0. McMillan. Reprinted from 1927 Proc. N. W. Elec. Lt.
and Power Assoc.
Twenty Cents.
No. 2.
Some Anomalies of Siliceous Matter in Boiler Water Chemistry, by R. E.
Summers. Reprinted from Jan. 1935, Combustion.
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G. W. Gleeson and W. H. Paul. Reprinted from Feb. 1936, National Petro.
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Aug. 1937, Electrical Engineering.
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Concrete.
Reprinted from
Reprinted from Nov. 1937,
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58, A.I.E.E. Transactions, Mar. 1939.
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22
ENGINEERING ExrERIIIENT STATION BULLETIN No. 14
Research Papers
(Published as indicated. Not available from the Station.)
No. 1. Electric Fish Screens, by F. 0. McMillan.
Bulletin of the U. S. Bureau of
Fisheries, vol. 44, 1928. Also in pamphlet form, U. S. Bureau of Fisheries,
Document No. 1042.
No. 2. Water Control of Dry Mixed Concrete, by G. W. Gleeson. Concrete Products,
December 1929.
No. 3. High.voltage Gaseous Conductor Lamps, by F. 0. McMillan and E. C. Starr.
Trans. American Institute of Electrical Engineers, vol. 48, no. 1, pp. 11-18,
1929.
No. 4. The Influence of Polarity in High.voltage Discharges, by F. 0. McMillan and
E. C. Starr. Trans. American Institute of Electrical Engineers, vol. 50, no. 1,
pp. 23-35, 1931.
No. 5. Progress Report on Radio Interference from High.voltage Transmission Lines
Pin and Pedestal Type Insulators, by F. 0. McMillan. Trans. 8th annual
general meeting, Engineering Section, Northwest Electric Light and Power
Assoc., 1931.
No. 6. Aggregate Grading for Tamped Concrete Pipe, by G. W. Gleeson.
Concrete,
June 1932. Rock Products, 1932. Concrete Products, June 1932 and MayJune 1934.
No. 7. Water Control of Dry Mixed Concrete, by G. \V. Gleeson. Concrete Products,
September 1932, and Rock Products, November 1932.
No. 8. Litharge and Glycerine Mortars, by G. W. Gleeson.
October 13, 1932.
No. 9. Radio Interference from Insulator Corona, by F. 0.
Paper Trade Journal,
Trans. American
Institute of Electrical Engineers, vol. 51, no. 2, pp. 385-391, 1932.
No. 10. The Coordination of High-voltage Transmission Lines with Radio, by F. 0.
McMillan. Trans. 9th annual general meeting, Engineering Section, Northwest Electric Light and Power Assoc., 1932.
McMillan.
No. 11. Asphalt Emulsion Reduces Insulator Radio Troubles, by F.
Electrical World, vol. 102, no. 6, August 5, 1933.
No. 12.
0.
McMillan.
Silicon, a Major Constituent of Boiler Scales in Western Oregon, by R. E.
Summers and C. S. Keevil. Paper presented at annual meeting, American
Society of Mechanical Engineers, 1933. Abstracts published in Mechanical
Engineering, vol. 55, p. 720, November 1933; Power, vol. 77, p. 687, midDec. 1933; and Power Plant Engineering, vol. 37, p. 519, December 1933, and
vol. 38, p. 219, May 1934.
No. 13. Study of the Frequency of Fuel Knock Noises, by P1. H. Paul and A. L.
Albert. National Petroleum News, August 9, 1933.
No. 14. The Pollutional Character of Flax Retting Wastes, by G. W. Gleeson, F. Merryfield, and E. F. Howard. Sewage \Vorks Journal, May 1934.
No. 15. Siliceous Scales in Boilers of Western Oregon and Washington, by R. E.
Summers and C. S. Keevil. The Timberman. vol. 35, p. 30, May 1934.
No. 16. How Much Phosphate? by R. E. Summers. Power, vol. 78, p. 452, August
1934.
No. 17. The Carbon Dioxide-Hydrogen Ratio in the Products of Combustion from
Automotive Engines, by G. \V. Gleeson and W. H. Paul. National Petroleum
News, September 15, 1934.
Exhaust Gas Analysis, by G. W. Gleeson and W. H. Paul. Parts I, II,
and III. National Petroleum News, September 26, October 3 and 10, 1934.
No. 19. Simplified Measurements of Sound Absorption, by A. L. Albert and T. B.
No. 18.
Wagner. Electrical Engineering, vol. 53, no. 8, P. 1160, August 1934.
No. 20. Treatment and Recovery of Sulfite Waste, by F. Merryfield. Civil Engineering, June 1936.
No. 21. Industrial Wastes in the Willamette Valley, by F. Merryfield. Civil Engineer.
ing, October 1936.
No. 22. Flow Characteristics in Elbow Draft-Tubes, by C. A.
Mockmore.
Proc.
American Society of Civil Engineers, vol. 63, no. 2, pp. 251-286, Feb. 1937.
No. 23. Some Simple Experiments Dealing with Rates of Solution, by G. W. Gleeson.
Journal of Chemical Education, vol. 15, no. 4, April 1938.
No. 24. Heat Transfer Coefficient in Boiling Refrigerant, by W. H. Martin. Refrigerating Engineering, vol. 36, no. 3, September 1938.
No. 25. Kiln Drying Guaranteed Moisture Content Spruce Lumber, by Glenn Voorlmies.
West Coast Lumberman, vol. 66, no. 1. January 1939.
No. 26. Steam Demand in Drying Douglas Fir Lumber, by Glenn Voorhies. The Timberman, vol. 40, no- 4, February 1939.
No. 27. Aircraft Precipitation-Static Radio Interference, by E. C. Starr. American
Institute of Electrical Engineers, Preprint, May 1940.
AIR-FUEL RATIOS FOR LIQUEFIED PETROLEUM GASES
No. 28. High-Voltage D.0 Point Discharges, by E. C. Starr.
Electrical Engineers, Preprint, May 1940.
23
American Institute of
No. 29. Humidity in the Freezing Chamber, by W. H. Martin. Western Frozen
Foods, voi. 1, no. 7, May 1940.
2Jan'42
THE ENGINEERING EXPERIMENT
STATION STAFF
R. H. DEARBORN, Dean and Director of Engineering.
S. H. GRAF, Director of Engineering Research.
A. L. ALBERT, Commimication Engineering.
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G. W. GLEESON, Chemical Engineering.
BURDETTE GLENN, Highway Engineering.
J. R. GRIFFITH, Structural Engineering.
F. 0. McMILLAN, Electrical Engineering.
W. H. MARTIN, Mechanical Engineering.
E. G. MASON, Forestry.
FRED MERKYFIELD, Sanitary Engineering.
C. A. MOCKMORE, Civil and Hydraulic Engineering.
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