Fuel 99 (2012) 72–82
Contents lists available at SciVerse ScienceDirect
Fuel
journal homepage: www.elsevier.com/locate/fuel
Ignition timing sensitivities of oxygenated biofuels compared
to gasoline in a direct-injection SI engine
Ritchie Daniel a, Guohong Tian a,b, Hongming Xu a,c,⇑, Shijin Shuai c
a
School of Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, UK
Sir Joseph Swan Centre, Newcastle University, Newcastle Upon Tyne NE1 7RU, UK
c
State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, China
b
a r t i c l e
i n f o
Article history:
Received 4 July 2011
Received in revised form 24 January 2012
Accepted 26 January 2012
Available online 4 April 2012
Keywords:
2,5-Dimethylfuran
DMF
Ethanol
Butanol
Methanol
a b s t r a c t
Global concerns over atmospheric carbon dioxide (CO2) levels and the security of fossil fuel supply have
led to the development of biofuels; a potentially carbon-neutral and renewable fuel strategy. One new
gasoline-alternative biofuel candidate is 2,5-dimethylfuran (DMF). In this paper, the potential of DMF
is examined in a direct-injection spark-ignition (DISI) engine. Focus is given to the combustion performance and emissions sensitivity around the optimum spark timing, especially at 10 crank angle degrees
retard (SR10). Such spark retard strategies are commonly used to reduce catalyst light-off times, albeit at
the cost of reduced engine performance and increased CO2. The results for DMF are compared to gasoline,
ethanol, butanol and methanol so that its sensitivity can be positioned relatively. The overall order of
spark sensitivity at the highest load (8.5 bar IMEP) was: gasoline > butanol > DMF > ethanol > methanol.
The four biofuels widen the spark window due to improved anti-knock qualities and sometimes
increased charge-cooling. This allows the increase of CO2 to be better minimized than with gasoline. Furthermore, DMF is the only biofuel to produce high exhaust gas temperatures, similar to gasoline and
helpful for fast catalyst light-off, whilst maintaining high combustion stabilities. This demonstrates the
potentially favorable characteristics of DMF to become an effective cold-start fuel.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
With ever-increasing concerns of fuel security and the problem
of global warming, there is a greater need to pursue alternative energy sources. Carbon-free fuels, which do not emit CO2 once consumed, are the long-term ideal in order to eradicate carbon
emissions. However, biofuels offer a short- to mid-term solution
in reducing the dependence on mineral oil and life-cycle CO2
emissions.
One particular biofuel candidate, which has benefitted from significant technological breakthroughs in its manufacture is 2,5-
Abbreviations: aTDC, after top dead centre; bTDC, before top dead centre; BUT,
butanol; CAD, crank angle degrees; CFR, cooperative fuel research (engine); CO,
carbon monoxide; CO2, carbon dioxide; COV, coefficient of variation; DISI, directinjection spark-ignition; DMF, 2,5-dimethylfuran; ETH, ethanol; HC, hydrocarbon;
IMEP, indicated mean effective pressure; KL-MBT, knock-limited maximum brake
torque; LCV, lower calorific value; MBT, maximum brake torque; MFB, mass
fraction burned; MON, motor octane number; MTH, methanol; NOx, nitrogen
oxides; PM, particulate matter; RON, research octane number; RPM, revolutions
per minute; SI, spark-ignition; SR10, spark retard (10 CAD); TDC, top dead centre;
ULG, unleaded gasoline.
⇑ Corresponding author.
E-mail address: h.m.xu@bham.ac.uk (H. Xu).
0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.fuel.2012.01.053
dimethylfuran, otherwise known as DMF. In 2007, bioscientists at
the University of Wisconsin–Madison publicized the production
of high yields of DMF [1,2], whose techniques have since benefitted
from further iterations by other institutions [3–7]. These developments have attracted the attention from automotive researchers in
the potential to use DMF as an alternative energy carrier to gasoline [8]. In comparison to ethanol, DMF has a higher energy density
(approximately 40% higher) and is insoluble in water [1]. Currently,
relatively few publications exist on DMF as a gasoline-alternative
fuel. The first reported engine studies were conducted by the
authors’ group [9–11]. This added to the laboratory studies of the
laminar burning velocity [12–15], spray properties [16] and combustion intermediates of DMF [17]. Evidently, this publication contributes to a series of experiments led by the authors’ group to
explore the use of DMF as a fuel for automotive applications.
In spark-ignition (SI) engines, one of the main control parameters is the spark timing. It significantly affects the combustion process, which determines the fuel economy, torque output and
emissions performance [18]. The spark timing is usually optimized
using sophisticated mathematical approaches, including polynomial regression techniques [19,20], radial basis functions and neural networks [21,22], and advanced design of experiment (DoE)
methodologies [23–25]. Each technique requires extensive model
R. Daniel et al. / Fuel 99 (2012) 72–82
tuning in order to find the optimum, or minimum advance for best
torque (MBT) timing. Minimal advance or retard about this point
gives modest variation in power and fuel consumption but can lead
to large changes in NOx and HC emissions. Therefore, in order to
minimize emissions or counteract knock, it is common to employ
a spark retard or knock margin [18,26]. Clearly, when employing
these approaches, an alternative fuel which produces the largest
reduction in emissions whilst achieving competitive performance
has great value in a practical application.
As mentioned, the onset of knock ultimately limits the maximum allowable spark advance and prevents the use of the theoretical optimum (MBT) timing. The knocking tendency of a fuel
depends on its physicochemical properties and is best represented
by the research and motor octane numbers (RON and MON respectively). The Octane (Antiknock) Index, or OI is sometimes preferred
as it combines the effects of RON and MON (OI = [RON + MON]/2)
[18,27]. However, to date, there is no published octane number
(RON or MON) for DMF using the CFR (cooperative fuel research)
engine method. What is known, however, is that DMF has previously been used as an octane enhancer with gasoline [28,29]. Nevertheless, in comparison to pure ethanol, this increased knock
suppression ability of DMF over gasoline has been shown to be less
significant [10].
Although the octane number can provide an insight into the
sensitivity to spark variations, it does not take in account the
charge-cooling effect made possible with modern direct-injection
(DI). Furthermore, for fuels which outperform iso-octane (100
RON) the values can only be extrapolated. As such, the CFR engine
octane methods, developed in 1930, have received criticism for
their relevance to the modern situation [27,30–32]. Therefore, in
an effort to further establish the antiknock performance of DMF,
the authors have proposed an alternative method, which is closer
to modern reality. By analyzing the effect of spark timing sweeps
at various loads and fixed engine speed, it is possible to determine
the spark sensitivity of each fuel, or the spark window for a given
decrease in load. Low sensitivity is ideal, as a wide spark window
provides a greater opportunity to reduce the NOx and HC
emissions.
Spark timing retard strategies are employed during cold-starts
for fast catalyst light-off (defined as the temperature to reach
50% efficiency [18]), because the combustion phasing shifts towards the expansion stroke and raises the exhaust temperature.
It is also used in turbocharged engines to allow a more rapid buildup of boost pressure, especially at low loads [33,34]. The extent of
raising the exhaust temperature for fast catalyst light-off is limited
by the reduced combustion stability and efficiency during spark retard. As the cold-start HC emissions can contribute to 80–90% of
the total during the FTP test cycle, the need for fast catalyst
light-off is paramount [35,36]. Although researchers have analyzed
the effect of HC emissions with spark retard [35,37], little is documented about the reduction limitations due to the spark sensitivity
of the fuel.
Through the assessment of the spark sensitivity of DMF, it is
possible to further hypothesize the octane rating. Therefore, the
objective for this investigation is twofold: (1) to position the knock
suppression ability of DMF amongst other alternative SI fuels and
(2) to study the effect of spark retard on modern engine performance and emissions with such fuels. In an effort to achieve these
aims, the authors have examined the behavior around the optimum spark timing between 3.5 bar (low load) and 8.5 bar (high
load) indicated mean effective pressure (IMEP) in 1 bar intervals
at a fixed engine speed of 1500 rpm. The performance of DMF is
benchmarked against gasoline and compared to ethanol, butanol
and methanol. The experimental system is described in the next
section and then the results are presented and discussed. A summary of the conclusions is given at the end of the paper.
73
2. Experimental setup
2.1. Engine and instrumentation
The experiments were performed on a single-cylinder, 4-stroke
SI research engine, as shown in Fig. 1. The 4-valve cylinder head includes the Jaguar spray-guided direct-injection (DISI) technology
used in their V8 production engine (AJ133) [38]. It also includes
variable valve timing technology for both intake and exhaust
valves, which, for this study, was kept constant.
The engine was coupled to a DC dynamometer to maintain a
constant speed of 1500 rpm (±1 rpm), regardless of the torque output. The in-cylinder pressure was measured using a Kistler 6041A
water-cooled pressure transducer which was fitted to the side-wall
of the cylinder head. The signal was then passed to a Kistler 5011
charge amplifier and finally to a National Instruments data acquisition card. Samples were taken at 0.5 CAD intervals for 300 consecutive cycles, so that an average could be taken. The crankshaft
position was measured using a digital shaft encoder mounted on
the crankshaft. Coolant and oil temperatures were controlled at
85 °C and 95 °C (±3 °C) respectively using a Proportional Integral
Differential (PID) controller. All temperatures were measured with
K-type thermocouples.
The engine was controlled using software developed in-house
written in the LabVIEW programming environment. High-speed,
crank-angle-resolved and low-speed, time-resolved data was also
acquired using LabVIEW. This was then analyzed using MATLAB
developed code so that an analysis of the combustion performance
could be made.
2.2. Emissions and fuel measurement
The gaseous emissions were quantified using a Horiba MEXA7100DEGR gas tower. Exhaust samples were taken 0.3 m downstream of the exhaust valve and were pumped via a heated line
(maintained at 191 °C) to the analyzer.
Particulate matter (PM) emissions were measured using a 3936
Scanning Mobility Particle Sizer Spectrometer (SMPS) manufactured by TSI. This comprises of a 3080 Electrostatic Classifier, a
3775 Condensation Particle Counter (CPC) and a 3081 Differential
Mobility Analyzer (DMA). PM samples were taken from the same
position as the Horiba analyzer but measured asynchronously.
A heated (150 °C) rotating disc diluter (Model 379020A, supplied by
TSI) was used at a dilution ratio of 67:1. The SMPS measured
particles from 7.23 to 294.3 nm in diameter and the sample and
sheath flow rates were 1 and 10 l/min, respectively.
The fuel consumption was calculated using the volumetric air
flow rate (measured by a positive displacement rotary flow meter)
and the actual lambda value (Bosch heated LSU wideband lambda
sensor and ETAS LA4 lambda meter). The LA4 lambda meter uses
fuel-specific curves to interpret the actual air-fuel ratio (AFR) using
the oxygen content in the exhaust. Before each test, the user inputs
the fuel’s hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C)
ratios, as well as the stoichiometric AFR, so that the fuel composition can be used to characterize the fuel curves.
2.3. Test fuels
The DMF used in this study was supplied by Shijiazhuang Lida
Chemical Co. Ltd., China at 99.8% purity. This was benchmarked
against commercial 97 RON gasoline and to ethanol, which were
both supplied by Shell Global Solutions, UK. These three fuels are
hereby referred to as the primary fuels used in the study. Both
methanol and butanol constitute the secondary fuels and were
supplied by Fisher Scientific, UK with 99.5% and 99% purity,
74
R. Daniel et al. / Fuel 99 (2012) 72–82
Pressure Gauge
(150bar)
Fuel
Accumulator
to Injector
Compressed Nitrogen
Cylinder
Exhaust
VVT
Throttle
Intake
VVT
Intake
Plenum
Chamber
Exhaust
Plenum
Chamber
Lambda
Meter
VAF
Meter
Intake
Filter
Air In
Kistler Pressure Sensor
Horiba
MEXA -7100DEGR
Emissions Analyser
(HC, CO, CO2, O2, NOx)
Crank Angle Encoder
Oil/Water
Cooler
Scanning Mobility
Particle Sizer
(SMPS)
Exhaust
Control Tower
High/Low Speed
Data Acquisition
Fig. 1. Schematic of engine and instrumentation setup.
respectively. The high octane gasoline was chosen as this represents the most favorable characteristics offered by the market
and provides a strong benchmark to the four biofuels. The fuel
characteristics are shown in Table 1.
2.4. Experimental procedure
The engine was considered warm once the coolant and lubricating oil temperatures had stabilized at 85 °C and 95 °C, respectively.
All tests were carried out at the stoichiometric AFR (k = 1) with
fixed injection timing (280° bTDC), ambient air intake conditions
(approximately 25 ± 2 °C) and constant valve timing (see Table
2). The pressure data from 300 consecutive cycles was recorded
for each test using the in-house developed LabVIEW code.
When changing fuels, the high pressure fuelling system was
purged using nitrogen until the lines were considered clean. Once
Table 1
Test fuel properties.
Chemical formula
H/C ratio
O/C ratio
Gravimetric oxygen
content (%)
Density @ 20 °C (kg/
m3)
Research octane
number (RON)
Motor octane number
(MON)
Octane Index, (K = 0.5)
Stoichiometric air fuel
ratio
LHV (MJ/kg)
LHV (MJ/L)
LHV stoich. mix (MJ/
m3)
Flash point (°C)
Heat of vaporization
(kJ/kg)
Initial boiling point (°C)
a
b
DMF
Ethanol
Methanol
Butanol
Gasoline
C6H8O
1.333
0.167
16.67
C2H6O
3
0.5
34.78
CH4O
4
1
50
C4H10O
2.5
0.25
21.6
C2–C14
1.795
0
0
889.7a
790.9a
792
811
744.6
106
98
96.8
b
b
n/a
107
n/a
89b
92b
84
85.7
n/a
10.72
98
8.95
99
6.47
91
11.2
91.25
14.46
32.89a
29.26a
3.49
26.9a
21.3a
3.29
19.83a
15.7a
3.16
32.71a
26.5a
3.33
42.9
31.9
3.4
1
332
13
840b
12
1103b
36
430
-40
373
92
78.4
65
118
32.8
Measured at the University of Birmingham.
Heywood [18].
the line was re-pressurized to 150 bar using the new fuel, the engine was run for several minutes. This removed any previous fuel
from the injector tip and in any combustion chamber crevices before the data was acquired. The ETAS LA4 lambda meter settings
were changed for each fuel using the stoichiometric AFR, O/C and
H/C ratios in Table 2.
2.5. Spark advance
In this study, the MBT, or optimum ignition timing is defined as
the ignition timing to produce the maximum IMEP for a fixed
throttle position. If audible knock occurred, the MBT timing was retarded by 2 CAD, an arbitrarily safe margin, as advised by key engine researchers [18,26] and is then referred to as the knocklimited MBT timing (KL-MBT). For this work, the MBT/KL-MBT timings were determined for each fuel from spark sweeps generated
between 3.5 bar and 8.5 bar IMEP, in 1 bar IMEP intervals and at
a fixed engine speed of 1500 rpm. At each load, the spark timing
was advanced to find the knock limit or until a significant drop
in performance or stability was seen (IMEP decrease >5% or COV
of IMEP increase >3%). Retarding the timing further for emissions
preservation was not used, in order to eliminate subjectivity and
better isolate the effect of spark sensitivity. Similarly, the spark
timing was retarded until the aforementioned drop in performance
was found. While performing each spark sweep, the fuel and air
flow rates were kept constant for each fuel once the required load
and stoichiometric AFR was achieved at the anticipated MBT point
(estimated from the spark sweep at the previous load). Firstly, the
throttle position was adjusted and then the fuel injection pulse
Table 2
Engine specification.
Engine type
Combustion system
Swept volume
Bore stroke
Compression ratio
Engine speed
Injector
Fuel pressure and timing
Intake valve opening
Exhaust valve closing
4-Stroke, 4-valve
Spray guided DISI
565.6 cm3
90 89 mm
11.5:1
1500 rpm
Multi-hole nozzle
150 bar, 280° bTDC
16° bTDC
36° aTDC
75
R. Daniel et al. / Fuel 99 (2012) 72–82
1500rpm, λ = 1
8.8
8.7
ETH
DMF
ULG
8.6
8.5
IMEP (bar)
width was adjusted finely (±1 ls) to find stoichiometry. Three repeats were made with each fuel to produce an average.
Once the spark timing sweeps were analyzed, the engine was
run again at each load using the chosen MBT/KL-MBT timings
and at a spark timing retard of 10 CAD. This allowed the engine
performance and emissions’ sensitivity to be analyzed under substantial spark timing retard conditions. Once more, three sets of
tests were carried out for repeatability. Each fuel was tested over
three consecutive days; however, the test order was varied each
day in order to minimize the effect of engine drift, as recommended by leading engine researchers [39]. Error bars have been
used where applicable in order to highlight such variations.
8.4
8.3
8.2
8.1
8.0
3. Results and discussion
7.9
3.1. Spark advance
7.8
37.5
The MBT/KL-MBT timings for each fuel are shown in Fig. 2. At
each load, the MBT/KL-MBT timings are shown by the individual
data points which were observed experimentally. Polynomial
trend lines have then been applied to highlight the differences
and more clearly present the relationship with respect to load.
Throughout the entire load range, ethanol and methanol require
the most advanced spark timing. At the highest load, the optimum
(MBT) timing is 11 CAD more advanced than with gasoline and 5
CAD more than with DMF. Until 6.5 bar IMEP, DMF and ethanol
are separated by less than 1 CAD. Despite this, the maximum IMEP
when using DMF is limited by audible knock and the theoretical
maximum (MBT) cannot be achieved. Although DMF is believed
to have a high octane number, the spark timing is relatively more
retarded than with ethanol, due to this onset of knock. However,
the best spark timings for gasoline are clearly the most retarded,
once again largely limited by knock.
When using 97 RON gasoline, a knock margin (2 CAD retard)
was enforced as early as 4.5 bar IMEP, which restricted the ability
to find the theoretical optimum (MBT) timing. For DMF, however,
this safety margin was not enforced until 5.5 bar IMEP and reaffirms the knocking behavior discovered in earlier experimental
work [9]. What is interesting is that the knock margin was also employed for butanol at this same load, despite having a marginally
lower OI than gasoline (see Table 1). This suggests the higher
charge-cooling effect of butanol, which can be interpreted from
the higher heat of vaporization, has a more pronounced effect on
the overall knock suppression, which is obviously not accounted
for in the CFR tests. Based on this trend, it is possible that the OI
for DMF is noticeably higher than 97 RON (86 MON) gasoline
(OI = 91.25) as the heat of vaporization of DMF is lower. Clearly,
this is hypothetical and no substitute for real CFR engine data.
1500rpm, λ = 1
Spark Advance (°bTDC)
40
35
30
25
20
ETH
DMF
ULG
BUT
MTH
15
10
5
0
3
4
5
6
7
8
9
IMEP (bar)
Fig. 2. MBT/KL-MBT spark timings at various loads for ethanol, DMF, gasoline,
butanol and methanol.
32.5
27.5
22.5
17.5
12.5
7.5
2.5
Spark Timing (°bTDC)
Fig. 3. Effect of spark timing at high load when using ethanol, DMF and gasoline.
For gasoline, the octane rating is largely governed by the aromatic content (fractions of benzene, toluene, etc.). However, for
pure, oxygenated fuels another relationship prevails. Gautam and
Martin have shown that the knock suppression capability of oxygenated fuels can be related to the relative oxygen content [40].
For DMF, a non-benzene ring aromatic, the oxygen content is lower
than the other oxygenated compounds used in this work (see Table
1). This could explain why the knocking tendency occurs at lower
loads when using DMF. For ethanol and methanol, fuels with relatively high oxygen content, no knock margin was required at any
load. In addition to their high oxygen content, these fuels also burn
with high velocity and produce a greater charge-cooling effect (see
Table 2), which helps to lower the combustion temperature and
discourage end-gas pre-ignition.
3.2. Spark timing sensitivity
The variation of IMEP using DMF, ethanol and gasoline at the
highest load spark sweep (approximately 8.5 bar IMEP) is shown
in Fig. 3 (the data for methanol and butanol has been omitted in
order to clearly present the methodology). At this load, there is a
clear difference between the three fuels. Ethanol combustion,
which is uninhibited by knock at this compression ratio, permits
a wide spark sweep and allows the IMEP to be analyzed either side
of the MBT timing (21° bTDC). DMF and gasoline on the other hand,
are much more sensitive to the onset of knock and only the retarded timing from KL-MBT can be observed. With comparison,
there appears to be a relationship between the MBT/KL-MBT location and rate of change of IMEP. It is evident that the more retarded
the MBT/KL-MBT spark timing is, the higher the rate of IMEP decay
becomes with spark retard. This rate of decay can be used as an
indicator of the spark timing sensitivity for each fuel.
When normalizing the IMEP and spark timing data (by their
respective MBT/KL-MBT values) from Fig. 3, these fuel effects become more obvious. This is shown in Fig. 4, using the term spark
retard, which represents the number of retarded CAD from MBT/
KL-MBT. As the term suggests, a positive value represents retarded
timing from MBT/KL-MBT, whereas a negative value is advanced.
This term has been previously used by Ayala et al. [41] to help develop their combustion retard parameter.
When using ethanol, the rate of decay is symmetrical about its
MBT and decreases at a lower rate than with DMF and gasoline.
This is largely explained by the knock suppression superiority of
ethanol. The data suggests that the initial rate of decay can also
76
R. Daniel et al. / Fuel 99 (2012) 72–82
1500rpm, λ = 1
1.01
a
ETH
DMF
ULG
1.00
IMEP/IMEPMBT/KL-MBT
0.99
0.98
0.97
0.96
0.95
0.94
0.93
0.92
0.91
-15
-10
-5
0
5
10
15
Spark Retard (θMBT/KL-MBT - θST, CAD)
Fig. 4. Effect of spark retard on normalized IMEP at high load when using ethanol,
DMF and gasoline.
b
indicate how far away the KL-MBT timing is from the theoretical
MBT timing (if unhindered by knock). For instance, the initial rate
of decay using DMF is less than with gasoline, which suggests the
KL-MBT timing for DMF is much closer to its theoretical MBT timing. Within this range of IMEP decay, ethanol is the least sensitive
to spark timing variations. This is clearly shown in Fig. 4 at the
arbitrary 5 CAD spark retard location, or SR5 (most retarded point
for gasoline). Here, when using ethanol, there is a loss in IMEP of
approximately 1% from MBT. However, when using DMF and gasoline this loss increases to 2.5% and 7% respectively from their
KL-MBT timings. Evidently, it is gasoline which is the most sensitive to spark timing at this load, which is largely a function of its
relatively low OI (see Table 1).
This normalization method is applied to the entire load range in
Fig. 5 (each fuel has been separated for clarity). There is a clear
trend in spark sensitivity with load, which is best shown with gasoline (Fig. 5c). As the load increases, the rate of decay of IMEP from
the MBT/KL-MBT point also increases. At the lowest load of 3.5 bar
IMEP, the spark sensitivity is also relatively low. For instance, at
SR5, the loss in IMEP is <1%. However, with each 1 bar increment
in load, the loss in performance rapidly increases to a maximum
of 7.2% at 8.5 bar initial IMEP. The increase in sensitivity with respect to load is also evident for DMF and ethanol but in an increasingly subtle manner. The spark sensitivity of ethanol is also
symmetric either side of MBT due to the benefit of a higher octane
number and greater charge-cooling effect (see Table 2).
For DMF, the spark timing sensitivity lies closer to that of ethanol than to gasoline, which is shown by the lower spread between
the loads. However, it is difficult to examine this link more closely
by solely observing these graphs. Therefore, the arbitrary SR10
location is used to help quantify the spark timing sensitivity when
analyzing other key combustion performance and emissions
parameters. Although the SR5 location demonstrates clearly the
spark sensitivity at the higher loads, the authors have chosen a
more retarded timing of 10 CAD in order to emphasize the trend
at the lower loads.
c
Fig. 5. Effect of spark retard on normalized IMEP when using (a) ethanol, (b) DMF
and (c) gasoline.
3.3. Combustion performance at SR10
In this section the spark sensitivity between the tested fuels is
examined in more detail in terms of combustion performance
specifically at SR10. As previously mentioned, these tests were performed once the spark sweeps were analyzed and the MBT/KL-MBT
timings were known. Error bars have been used where applicable
for the three primary fuels (ethanol, DMF and gasoline) but have
been omitted for the secondary fuels (butanol and methanol) in
order to maintain clarity. Similarly, solid lines have been used to
position DMF between gasoline and ethanol, whereas dashed
77
R. Daniel et al. / Fuel 99 (2012) 72–82
1500rpm, λ = 1
1.01
0.98
0.99
IndEffSR10/IndEffMBT/KL-MBT
IMEPSR10/IMEPMBT/KL-MBT
1500rpm, λ = 1
1.00
0.96
0.94
0.92
0.90
0.88
ETH
DMF
ULG
BUT
MTH
0.86
0.84
0.82
0.97
0.95
0.93
0.91
0.89
ETH
DMF
ULG
BUT
MTH
0.87
0.85
0.83
0.80
0.81
3
4
5
6
7
8
9
IMEPMBT/KL-MBT (bar)
3
4
5
6
7
8
9
IMEPMBT/KL-MBT (bar)
Fig. 6. Effect of SR10 on normalized IMEP with increasing engine load between
ethanol, DMF, gasoline, butanol and methanol.
Fig. 7. Effect of SR10 on normalized indicated efficiency with increasing engine
load between ethanol, DMF, gasoline, butanol and methanol.
lines are used for the secondary fuels. Firstly, the effect of spark
sensitivity on IMEP is quantified in Fig. 6 at SR10 for each load with
each fuel.
As surmised from Fig. 5, the loss of IMEP at SR10 with increasing
initial load, quantifiably decreases from gasoline, to DMF and finally to ethanol. This is more clearly shown in Fig. 6. When fuelled
with ethanol, the loss of IMEP is always less than 7% (60.5 bar)
across the entire load range, suggesting that the exact MBT timing
for ethanol is less critical than the other two primary fuels. For
DMF, the decay of IMEP is much closer to ethanol than it is for gasoline. Up to 6.5 bar IMEP, the SR10 performance is almost identical
to that seen with ethanol. Above this load, the sensitivity increases
and ethanol outperforms DMF. In comparison to gasoline, this loss
is less significant. At 8.5 bar IMEP, the decay of IMEP when using
DMF is only 0.76 bar, or 9%. However, for gasoline this loss increases to 1.54 bar (18.3%). Evidently, gasoline is much more sensitive to spark retard in terms of IMEP, than both ethanol and
DMF, which is largely a function of its relatively low OI (see Table
1).
The performance of butanol and methanol reside below that of
DMF and above ethanol, respectively. When using methanol, the
fuel which exhibits the greatest knock resistance (OI = 99, see Table 1), the IMEP decay is less than 4% at all loads and is consistently
superior to ethanol. Although the difference in OI between ethanol
and methanol is marginal, the greater charge-cooling effect of
methanol plays a key role in further knock suppression. This is also
true for butanol, despite a similar OI to gasoline; due to the greater
heat of vaporization (see Table 1), the spark sensitivity is far superior. This observation helps us to explain the performance of DMF
and hypothesize its OI. It is possible that DMF produces a relatively
high OI because its lower heat of vaporization would not be taken
into account in the CFR engine tests. In reality, the low chargecooling effect when using DMF would counterbalance the benefit
of the increased OI to suppress knock. Therefore, as observed by
other researchers, it is important to consider the charge-cooling effect and not only the OI, when selecting a fuel to improve knock
suppression [42].
The indicated efficiency is a measure of the fuel conversion efficiency and compares the total work done to the theoretical energy
available from the fuel supplied. The experimental study reveals
the reduced effect on indicated efficiency at SR10 when using ethanol and DMF compared to gasoline due to their lower spark sensitivity. Fig. 7 shows the loss of indicated efficiency for the three
fuels at SR10, which demonstrates a similar trend between the
fuels seen in Fig. 6. Once again, the low decay in indicated efficiency of ethanol (and methanol) reiterates the low sensitivity to
spark timing retard. At 8.5 bar IMEP, when using gasoline, the normalized indicated efficiency drops by 18%, almost double the loss
experienced with DMF (10%) and a factor of 3.6 more than with
ethanol (5%). The low sensitive fuels benefit from an earlier optimum, where the effect of spark retard has less of an impact. Nevertheless, there is a clear difference in spark sensitivity between
ethanol and methanol, despite a similar MBT timing. This could
be explained by the faster burning rate of methanol, which enables
the energy from the air-fuel mixture to be more fully utilized earlier in the expansion stroke. Clearly, the varying degree of efficiency losses due to spark retard between the fuels will also have
a detrimental impact on the fuel consumption rate.
The effect of spark sensitivity on the combustion stability and
exhaust temperature is shown in Fig. 8. Both graphs use absolute
and not normalized units, in order to show the negative and positive effects of spark retard, respectively. Fig. 8a highlights the
advantage of the oxygenated fuels on combustion stability over
gasoline. Although the effect of spark sensitivity for all fuels decreases with load (with the exception of butanol), the instability
of gasoline remains the highest. This is due to reduced combustion
durations resulting from more readily available oxygen molecules
and more advanced spark timing (at SR10). In general, it is methanol that offers the highest stability (lowest COV of IMEP) through
spark retard. For gasoline and DMF, the MBT/KL-MBT timing is
more retarded and closer to top dead centre (TDC). At this point
the in-cylinder turbulence is slightly reduced which subsequently
compromises the burn rate [26]. For ethanol, the MBT timing is
more advanced, so the combustion at SR10 occurs during higher
turbulence intensity, which enhances the burn rate. Despite this,
during the mid-loads (4.5–7.5 bar IMEP), DMF offers slightly improved combustion stability over ethanol. This is possibly due to
the offset of improved fuel droplet vaporization because of the
low heat of vaporization of DMF (see Table 1). In fact, DMF is
known to produce smaller fuel droplets than ethanol at 150 bar
injection pressure and with increasing distance from the injector
nozzle [16]. Furthermore, the lower charge-cooling effect of DMF
results in higher initial combustion temperatures. This also helps
to promote mixture homogenization prior to ignition because the
marginally elevated temperatures (compared to ethanol) improve
the rate of molecular diffusion of the fuel vapor within the air.
78
R. Daniel et al. / Fuel 99 (2012) 72–82
1500rpm, λ = 1
5.0
COV of IMEP (%) SR10
4.5
4.0
3.5
3.0
2.5
2.0
1.5
ETH
DMF
ULG
BUT
MTH
1500rpm, λ = 1
800
Exhaust Temperature (°C) SR10
a
b
ETH
DMF
ULG
BUT
MTH
750
700
650
600
550
500
1.0
3
4
5
6
7
8
3
9
4
IMEPMBT/KL-MBT (bar)
5
6
7
8
9
IMEPMBT/KL-MBT (bar)
Fig. 8. Effect of SR10 on (a) coefficient of variation of IMEP and (b) exhaust gas temperature with increasing engine load between ethanol, DMF, gasoline, butanol and
methanol.
1500rpm, λ = 1
PmaxSR10/PmaxMBT/KL-MBT
0.75
a
0.70
0.65
0.60
ETH
DMF
ULG
BUT
MTH
0.55
0.50
3
4
5
6
7
8
9
IMEPMBT/KL-MBT (bar)
1500rpm, λ = 1
1.30
SPK-MFB5SR10/SPK-MFB5MBT/KL-MBT
Similarly to the combustion instability, the exhaust temperature should not exceed a component protection limit. However, it
should be high enough to improve cold-start performance (for catalyst light-off) and enable rapid boost pressure build-up through
spark retard. Although these tests have been performed in a warm
condition, the trends can help us to understand the impact on a
cold engine. The high exhaust temperatures with load when using
DMF in Fig. 8b, demonstrates its suitability to potentially meet
cold-start (fast catalyst light-off) and boosting design requirements, whilst maintaining high combustion stability. At the lowest
load, the exhaust temperature at SR10 matches that of gasoline
(the most favorable fuel to meet the aforementioned demands)
and, with increasing load, remains close to gasoline and the highest
between all oxygenated fuels. Methanol, on the other hand, despite
offering high combustion stability with low spark sensitivity in
terms of IMEP, produces the lowest exhaust temperature and demonstrates its unsuitability as a cold-start fuel.
These differences in performance and efficiency can be more
clearly explained when analyzing the in-cylinder pressure data,
in particular, the maximum pressure (Fig. 9a) and resulting heat
release data. In this instance, the initial combustion duration (defined as the CAD from ignition to 5% mass fraction burned (MFB))
has been selected, in order to best highlight the detrimental impact
at SR10 (Fig. 9b). For each combustion (and emissions) parameter,
the absolute values when using ethanol, DMF and gasoline, have
been compared in a previous publication by the authors [10]. This
examines the absolute behavior at fuel-specific MBT timings and
retarded gasoline KL-MBT timings. Therefore, it is the aim of the
present work to present the relative decay in combustion (and
emissions) from these optimum conditions, in order to examine
the robustness of each fuel. In general, the effect of retarded ignition timing with load is a dramatic reduction in the maximum
in-cylinder pressure and increase in the change in normalized initial combustion duration (see Fig. 9). At low load, the decay in maximum in-cylinder pressure is similar between fuels; the range at
3.5 bar IMEP is <2%. However, as the load increases, the differences
become self-evident, whereby gasoline exhibits the greatest
changes at SR10. At 8.5 bar IMEP, the ignition timing at SR10 for
gasoline is TDC. This delays the combustion phasing towards the
expansion stroke and produces a 48% reduction in maximum incylinder pressure. Amongst the oxygenated fuels, butanol and
DMF follow similar reductions in maximum in-cylinder pressure
mainly due to their similar knock suppression abilities. Up to
5.5 bar IMEP, ethanol also behaves similarly, but is less affected
b
ETH
DMF
ULG
BUT
MTH
1.25
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
3
4
5
6
7
8
9
IMEPMBT/KL-MBT (bar)
Fig. 9. Effect of spark retard on In-cylinder pressure and MFB at the highest engine
load between gasoline and DMF.
at higher loads. Methanol, which has the greatest OI, is least affected at SR10.
79
R. Daniel et al. / Fuel 99 (2012) 72–82
Clearly, this change in pressure impacts the initial combustion
duration. At low loads for gasoline (64.5 bar IMEP) and DMF
(66.5 bar IMEP), and almost all loads for ethanol, butanol and
methanol, the initial combustion duration actually reduces at
SR10. This is due to greater in-cylinder pressures as the point of
ignition approaches TDC. However, because gasoline requires the
most retarded MBT/KL-MBT timings, this benefit is rapidly lost
above 5.5 bar IMEP. With methanol, however, the most retarded
ignition timing at SR10 is 11°bTDC (at 8.5 bar IMEP). Therefore,
combustion originates later in the compression stroke when the
piston is closer to TDC and the in-cylinder pressure is higher (compared to MBT). For DMF, the initial combustion duration is more affected than the other oxygenated biofuels at 8.5 bar IMEP, but this
effect is still less than with gasoline. Here, the increase in initial
combustion duration when using DMF is 6.7% (0.96 CAD), whereas
for gasoline the effect is much worse (27% increase, or 3.93 CAD).
3.4. Gaseous emissions at SR10
The sensitivity of the regulated indicated specific emissions and
carbon dioxide (CO2) to variations in spark timing is shown to be as
critical as the performance criteria. This section analyses the impact of spark retard at SR10 on the emissions, as well as on particulate matter (PM), with fuel and load.
Fig. 10a shows the decrease in indicated specific nitrous oxide
(isNOx) emissions at SR10. The formation of NOx is strongly related
to the combustion temperature [26]; as the ignition is retarded, the
1500rpm, λ = 1
0.75
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
5
6
7
8
0.99
0.96
0.93
0.90
0.87
0.84
b
0.78
9
3
4
IMEPMBT/KL-MBT (bar)
6
7
1.8
1.6
8
9
1500rpm, λ = 1
1.23
ETH
DMF
ULG
BUT
MTH
isCO2SR10/isCO2MBT/KL-MBT
isCOSR10/isCOMBT/KL-MBT
5
IMEPMBT/KL-MBT (bar)
1500rpm, λ = 1
2.0
BUT
MTH
0.81
a
4
ETH
DMF
ULG
1.02
0.65
3
1500rpm, λ = 1
1.05
BUT
MTH
isHCSR10/isHCMBT/KL-MBT
ETH
DMF
ULG
0.70
isNOxSR10/isNOxMBT/KL-MBT
peak in-cylinder pressure and temperature lowers and the isNOx is
reduced. This effect is shown for every fuel across the entire load
range at SR10. Between the primary fuels at SR10, the isNOx reduction using ethanol is the most effective, closely followed by gasoline (especially P7.5 bar IMEP) and finally DMF. At 5.5 bar IMEP,
the isNOx emissions for ethanol, gasoline and DMF reduce by
62%, 53% and 44%, respectively at SR10. Therefore, not only does
spark retard have a minimal impact on efficiency when using ethanol, it also produces the greatest benefits in isNOx reductions. For
gasoline, these benefits are outweighed by the severe performance
and efficiency losses, especially at higher loads.
The effect of spark retard on the indicated specific hydrocarbons
(isHC) is shown in Fig. 10b. Although it is believed that FID analyzers could have a reduced sensitivity to oxygenated fuels [43,44],
the results in Fig. 10b show a trend in the remaining total hydrocarbon emissions. Future work will include a detailed hydrocarbon
emissions investigation for more accurate quantification using
Fourier Transform Infrared Spectroscopy (FTIR). Nevertheless, the
results in Fig. 10b provide a good starting point. Here, as the ignition timing is retarded away from the MBT/KL-MBT location, the
isHC production decreases. This is due to the increased time for
the mixing of induced air and injected fuel, which generates a more
homogenous mixture. This method is very effective in reducing
isHC for the primary fuels (less so for ethanol when above
5.5 bar IMEP due to compromised mixture quality). The results
for DMF consistently show more competitive reductions than with
ethanol across the entire load range. For instance, at 8.5 bar IMEP,
1.4
1.2
1.0
0.8
c
0.6
ETH
DMF
ULG
BUT
MTH
1.20
1.17
1.14
1.11
1.08
1.05
1.02
d
0.99
3
4
5
6
7
IMEPMBT/KL-MBT (bar)
8
9
3
4
5
6
7
8
9
IMEPMBT/KL-MBT (bar)
Fig. 10. Effect of SR10 on normalized indicated specific (a) NOx (b) HCs (c) CO and (d) CO2 with increasing engine load between ethanol, DMF, gasoline, butanol and methanol.
80
R. Daniel et al. / Fuel 99 (2012) 72–82
the isHC reduction is 17.1% compared to 5.6% with ethanol. Previous testing also showed how DMF produces lower isHC emissions
than gasoline because of the oxygen contained in the fuel [9,10].
This positive impact of spark retard on isHC emissions is coupled
by the lower loss in IMEP when using DMF, than with gasoline.
For butanol and methanol, the reduction is less impressive, despite
showing large reductions in isNOx under the same conditions. This
suggests the unburned HCs for the secondary fuels are already
highly oxidized and the use of spark retard is an ineffective way
to further minimize their emissions.
When using oxygenated fuels, the indicated specific carbon
monoxide (isCO) emissions (see Fig. 10c) general decrease with
ignition retard (except for some instances above 7.5 bar IMEP).
However, with gasoline, the isCO emissions dramatically increase
from 5.5 bar IMEP. Although a lower combustion temperature
helps to reduce NOx, the effect is detrimental to CO, especially at
high loads. As the spark is retarded at higher loads, combustion occurs very late in the expansion stroke for gasoline, which reduces
the temperature and pressure. These sub-optimal conditions result
in pockets of localized oxygen-deprived mixtures which generate
higher CO emissions as a result of incomplete combustion. At
6.5 bar IMEP, the gasoline isCO emission increases by 22%, whereas
no increase is observed with the oxygenated fuels. This rapidly
climbs to 76% for gasoline, as the load is increased to 8.5 bar IMEP.
Nevertheless, when using ethanol there are always isCO reductions
at SR10, regardless of the load, and until 7.5 bar IMEP when using
DMF. Once more, the large emissions reductions with spark retard
3.5. PM emissions at SR10
In addition to the CO2 emissions, the monitoring of PM number
emissions from gasoline engines is set to be enforced. Currently,
PM number emissions do not form part of the emissions legisla-
1500rpm, λ = 1
6
10
STMBT = 21°bTDC
IMEPMBT = 8.5bar
4
10
3
10
2
DMF KL-MBT
DMF SR10
b
5
STKL-MBT = 16°bTDC
IMEPKL-MBT = 8.5bar
10
dN/dLogDp (#/cm3)
5
10
1500rpm, λ = 1
6
10
ETH MBT
ETH SR10
a
dN/dLogDp (#/cm3)
when using biofuels is attractive when their performance decay is
so low.
Although CO2 is a non-toxic gas, which is not classified as an engine pollutant, it is one of the substances responsible for global
temperature rises through the greenhouse effect. Therefore, a consideration of the indicated specific CO2 (isCO2) emissions with
spark retard is made between the fuels in Fig. 10d. Unlike with
the previous emissions (except for some instances with isCO),
the isCO2 emissions increase at SR10 and with increasing engine
load. This emissions penalty is due to the increase in fuel consumption and reduction in indicated efficiency, as shown in Fig. 7. In
fact, the inverse of the CO2 emissions almost equals the trend in
indicated efficiency at SR10. As discovered with indicated efficiency, spark retard with gasoline results in the highest change
in isCO2 emissions, while ethanol produces the least and DMF produces only slightly more than with ethanol. At 8.5 bar IMEP, the
isCO2 increase with gasoline is 19.4%, whereas with ethanol and
DMF it is 5.6% and 10.7%, respectively. However, the low spark sensitivity of methanol results in the lowest change in isCO2 emissions
amongst all five fuels (3.2% at 8.5 bar IMEP). In summary, the increase in isCO2 is a function of the spark sensitivity of each fuel.
4
10
3
10
2
10
10
Accumulation
Mode
Nucleation
Mode
1
Nucleation
Mode
1
10
Accumulation
Mode
10
10
100
10
100
Particle Diamater (nm)
Particle Diamater (nm)
1500rpm, λ = 1
6
10
ULG KL-MBT
ULG SR10
c
STKL-MBT = 10°bTDC
IMEPKL-MBT = 8.5bar
5
dN/dLogDp (#/cm3)
10
4
10
3
10
2
10
Nucleation
Mode
1
Accumulation
Mode
10
10
100
Particle Diamater (nm)
Fig. 11. PM size distributions at high load using (a) ethanol, (b) DMF and (c) gasoline to compare the effect at MBT/KL-MBT and SR10 spark timings.
81
1500rpm, λ = 1
8
7
MBT/KL-MBT
SR10
46
IMEPMBT/KL-MBT = 8.5bar
Mean Diameter (nm)
Total Concentration (#/cm3 x104)
R. Daniel et al. / Fuel 99 (2012) 72–82
6
5
4
3
2
43
1500rpm, λ = 1
MBT/KL-MBT
SR10
IMEPMBT/KL-MBT = 8.5bar
40
37
34
31
28
1
25
0
ETH
DMF
ULG
ETH
a
DMF
ULG
b
Fig. 12. (a) PM total concentrations and (b) mean particle diameters at high load using ethanol, DMF and gasoline to compare the effect at MBT/KL-MBT and SR10 spark
timings.
tions for gasoline spark-ignition engines in Europe or the US. However, control of these emissions is expected to commence in European regulation in 2014 [45]. This will require not only the
monitoring of particulate mass, but also the particulate number
for all light-duty vehicles. Therefore, an understanding of these
emissions will become much more important, especially when
using biofuels. In this section, the PM emissions between the three
primary fuels only are studied at MBT and SR10 at the highest target load (8.5 bar IMEP). The PM size distributions are shown in
Fig. 11. Typically, the PM size distribution is bimodal and consists
of a nucleation and an accumulation mode. The former constitutes
liquid particles, whereas the latter constitutes solid carbonaceous
species. Although the separation between these two modes is illdefined [46], in this study, a particle diameter of 50 nm has been
applied to separate the nucleation (<50 nm) and accumulation
modes (>50 nm) as used in previous publications by the authors
[9,10].
The separation between the nucleation and accumulation
modes is shown clearly by the inflection in size distributions
around 50 nm for all fuels in Fig. 11. Clearly, the nucleation mode
is the dominant mode for all three fuels. The total concentration of
this mode is higher when using the two biofuels, compared to gasoline but the accumulation mode is much smaller, a similar result
found by other authors [47]. The PM emissions variation with
spark retard appears to be the most sensitive when using ethanol,
whereas with DMF, it is the least. At SR10, the peak number concentration using ethanol is 359,614 particles/cm3, with a particle
diameter of 38.5 nm, which is 46% and 33% more than at MBT,
respectively. However, with DMF, the increase in particle concentration and diameter is less than half of this (20% and 15%, respectively). This trend might be a function of the in-cylinder
temperature, whereby its change using ethanol at SR10, is greater
than that with DMF (surmised from the NOx emissions in Fig. 10a).
Although this helps to reduce the isNOx emissions, it conversely affects the PM nucleation mode with little effect on the accumulation
mode. Overall, spark retard at SR10 largely affects the nucleation
mode and not the accumulation mode distribution.
Fig. 12 shows the total PM concentration and mean particle
diameter for the three fuels at the highest initial load (8.5 bar
IMEP). At SR10, the total PM concentration and particle diameter
increases in almost every case. As shown with the size distributions in Fig. 11, the change in total concentration when using
DMF is the lowest, albeit at a greater absolute value. For instance,
from KL-MBT timing to SR10, the total PM concentration with DMF
increases by 1429 particles/cm3 (2.1%), whereas with ethanol this
is 12,620 particles/cm3 (26.6%). However, the two biofuels have
larger total concentrations compared to gasoline, which is mainly
due to the dominant nucleation mode. Nevertheless, in terms of
mean particle diameter, the two biofuels produce a lower mean
particle diameter than with gasoline. Ethanol, which shows the
greatest sensitivity to spark retard, has a low mean diameter at
MBT timing of 29.6 nm, whereas for gasoline this is 42 nm. However, for ethanol this rapidly increases to 38.7 nm at SR10 highlighting its sensitivity, whereas there is minimal change with
gasoline.
4. Conclusions
This study compares the spark sensitivity of three primary
fuels: DMF (2,5-dimethylfuran), commercial gasoline and ethanol,
with two secondary fuels: butanol and methanol. The experimental engine tests were performed on a single cylinder DISI engine
from 3.5 bar to 8.5 bar IMEP in 1 bar IMEP increments and at a
fixed engine speed of 1500 rpm. The engine was first tested using
each fuel under various spark sweeps and then an arbitrary 10
CAD spark timing retard was chosen, denoted SR10. Based on these
experiments, the following conclusions can be drawn:
1. All five fuels have different spark sensitivities with respect to
engine load. In terms of IMEP and indicated efficiency, the order
of increasing sensitivity is: methanol > ethanol > DMF >
butanol > gasoline.
2. When selecting a fuel to improve knock suppression, it is
important to consider the charge-cooling effect and not only
the OI.
3. At SR10, DMF produces high combustion stability and
high exhaust temperature. However, gasoline and ethanol
suffer from either low stability or exhaust temperature,
respectively.
4. Both isNOx and isHC decrease for all fuels at SR10. The isCO
emissions are largely reduced for all oxygenated fuels, but not
so for gasoline, which increased to a maximum of 76%.
5. The trend in isCO2 is inversely proportional to that seen with
indicated efficiency. Once more, gasoline is the most sensitive
to spark retard and DMF performs similarly to ethanol.
6. The PM emissions increase at SR10 (largely nucleation mode
particles) for ethanol, DMF and gasoline at the highest, whereby
ethanol is the most sensitive.
7. The widened spark window when using oxygenated fuels can
help to improve calibration flexibility by increasing the range
in which to maximum the reduction of emissions.
82
R. Daniel et al. / Fuel 99 (2012) 72–82
In summary, these experiments highlight the benefit of biofuels
over commercial gasoline, in terms of spark sensitivity. Gasoline is
hindered by the onset of knock, which requires more accurate control of the spark timing. However, it has been shown that this is
less critical with certain biofuels, as large NOx and HC emissions
benefits are achieved with little performance degradation.
Acknowledgments
The present work is part of a 3-year research project sponsored
by the Engineering and Physical Sciences Research Council (EPSRC)
under the grant EP/F061692/1. The authors would like to acknowledge the support from Jaguar Cars Ltd., Shell Global Solutions and
various research assistants and technicians. The authors are also
grateful for the financial support from the European Regional
Development Fund (EUDF) and Advantage West Midlands
(AWM). Finally, the authors would like to acknowledge the support
from their international collaborators at Tsinghua University,
China.
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