fisita2010sco04
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F2010-SC-O-04
MODELING THE COMBUSTION OF LIGHT ALCOHOLS
IN SI ENGINES: A PRELIMINARY STUDY
Vancoillie, Jeroen*, Verhelst, Sebastian,Ghent University, Belgium
KEYWORDSAlcohols, Spark Ignition Engine, Thermodynamic, Modeling
ABSTRACT - The use of methanol and ethanol in internal combustion engines forms an
interesting approach to decarbonizing transport and securing domestic energy supply. The
physico-chemical properties of these fuels enable engines with increased performance and
efficiency compared to their fossil fuel counterparts. The development of alcohol-fuelled
engines has been mainly experimental up till now. The application of an engine cycle code
valid for these fuels could help to unlock their full potential. For this reason, our research
group decided to extend its in-house engine code to alcohols. This paper discusses the
requirements for the construction of a two-zone thermodynamic model that can predict the
power cycle, pollutant emissions and knock onset in alcohol engines.
We reviewed the properties of alcohol fuels and their use in dedicated engine technology.
From this information we identified the characteristics relevant to combustion engines and
defined the areas the model should cover in terms of cylinder pressure, temperature, residual
gas fraction, etc. Next, we investigated which building blocks of the current model will need
adaptations. For the laminar burning velocity of alcohol-air mixtures, our literature review
revealed a lack of data at engine-like conditions. Upon inspection of the pollutant formation
models, we found that special attention should be paid to the formation of aldehydes andselected a suitable formation model. Finally we decided that a knock prediction model based
on a one-step Arrhenius-type autoignition reaction is best suited for our purpose.Future workwill further focus on each of these building blocks separately in order to come to a
comprehensive model for the combustion of alcohols in spark-ignition engines.1. INTRODUCTIONLight Alcohols as Alternative Vehicle Fuels
Our present energy supply is based on fossil fuels, which are depletable. Given the growing
world population, increasing energy demand per capita and global warming, the need for along-term alternative energy supply is clear. This is particularly true for the transport sector,
which is extremely dependent on oil. Although transport is currently only the third largest
contributor to energy use and greenhouse gas emissions, it is the fastest growing sector.
Hydrogen and electrification are two approaches to de-carbonizing transport, which receive a
lot of attention these days. However, their inherently low energy densities and high associated
infrastructure costs make it unlikely that these solutions will become competitive with liquid
fuels in the near future. Conversely, sustainable liquid alcohols, such as ethanol and methanol,
are largely compatible with the existing fuelling and distribution infrastructure and are easily
stored in a vehicle. In addition methanol can be synthesized from a variety of sources (fossil
fuels, 1st and 2nd generation biomass, renewably produced hydrogen, etc.).
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Light alcohols can be used in low-cost internal combustion engines with only minor
adjustments. Unlike many other alternative fuels, they have the potential to increase the
engine performance and efficiency over that achievable with gasoline. This is demonstrated in
section 2.
The application of methanol and ethanol in engines is not new. During the oil crises of the1970s and 1980s, many studies and large-scale fleet trials with methanol-fuelled vehicles
were conducted in California and Canada. More recently the focus has shifted towards bio-
ethanol. In Brazil, this fuel has been popular for several decades. Today, millions of flexible
fuel vehicles, capable of running on any mixture of ethanol and gasoline, are in service
around the world. Recent developments are feeding a renewed interest in both fuels: the US
Energy Independence and Security Act of 2007 will incentivize the development of second-
generation biofuels. China on the other hand has declared coal-based methanol as a strategic
transportation fuel to ensure its energy-independence.
Experimental testing of alcohol-fuelled engines has shown some promising results. However,
the real potential of alcohol blended fuels and their impact on engine control strategies remainto be explored. Today, these issues can be addressed at low cost using system simulations of
the whole engine, provided that the employed models account for the effect of the fuel on the
combustion process. The current work investigates the requirements for the construction of an
engine model valid for alcohol fuels, as is discussed in the next section.
Two-zone Thermodynamic Engine Modelling
An engine simulation code based on two-zone thermodynamic modelling is a useful tool for
cheap and fast optimization of engines. Such a code is a compromise between non-predictive
zero-dimensional models (type Wiebe-law) and complex multidimensional models (type
CFD). It is best suited for evaluating existing engines, performing parameter studies and
predicting optimum engine settings.
The governing equations of a thermodynamic model are based on conservation of mass and
energy. The two-zone formulation separates the burned from the unburned gases by an
infinitely thin, spherically propagating flame front. In order to close the equations, a number
of additional submodels and assumptions are needed. These are discussed in section 3.
Within our research group, such a code was developed for spark-ignition engines running on
hydrogen, the GUEST code (Ghent University Engine Simulation Tool) (1, 2). The present
research seeks to make this code valid for alcohol engines. Because the properties of alcoholsand their use in engines considerably differ from those of hydrogen, the employed models
will need serious adjustments. In addition, GUEST will be extended to include predictions of
pollutant formation, knock occurrence and the gas dynamics during the breathing cycle.
In this respect, we reviewed the properties of alcohol fuels and their application in dedicated
engine technology. From this information, we identified the characteristics relevant to
combustion engines and defined the areas the model should cover in terms of cylinder
pressure, temperature, residual gas fraction, etc. Next we investigated which building blocks
of the current model will need reworking and selected some interesting approaches to predict
autoignition behaviour, pollutant formation and gas dynamics in alcohol-fuelled engines. The
results of our preliminary research are presented in this paper.
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2. ALCOHOLS AS FUELS FOR THE INTERNAL COMBUSTION ENGINECharacteristics Relevant to Combustion Engines
The most distinct feature of alcohol molecules is the polarity caused by the hydroxyl group.
This polarity is responsible for several interesting physico-chemical properties, most
pronounced in light alcohols. The strong inter-molecular forces caused by polarity, known as
hydrogen bonding, give rise to high boiling points, high heats of vaporization and good
miscibility with other substances having strong molecular polarity, such as water. Polarity,
however, also causes the high corrosiveness of alcohols compared to other fuels. Some
properties of methanol and ethanol, relevant to SI engines, are listed in Table 1 and compared
against other alternative fuels and typical gasoline.
Property Gasoline Methanol Ethanol Methane Hydrogen
Chemical Formula Various CH3OH C2H5OH CH4 H2
Oxygen Content by Mass [%] 0 50 34.8 0 0
Density at NTP [kg/l] 0.74 0.79 0.79 0.00065 0.00008
Lower Heating Value [MJ/kg] 42.9 20.09 26.95 50 120
Volumetric Energy Content [MJ/l] 31.7 15.9 21.3 0.033 0.010
Stoichiometric AFR [kg/kg] 14.7 6.5 9 17.6 34.2
Energy per Unit mass of air [MJ/kg] 2.95 3.12 3.01 2.83 3.51
Research Octane Number 95 109 109 120 130(=2.5)
Motor Octane Number 85 88.6 89.7 120 NA
Sensitivity (RON-MON) 10 20.4 19.3 0 NA
Boiling point at 1 bar [C] 25-215 65 79 -164 -253Heat of vaporisation [kJ/kg] 180-350 1100 838 510 461
Reid Vapour Pressure [psi] 7 4.6 2.3 NA NA
Mole ratio of products to reactants
0.937 1.061 1.065 1 0.852
Ratio of Triatomic to Diatomic Products*
0.351 0.532 0.443 0.399 0.532
Flammability Limits in Air [] 0.26-1.60 0.23-1.81 0.28-1.91 0.59-1.99 0.15-10.57
Laminar flame speed at NTP, =1 [cm/s] 28 42 40 38 210
Adiabatic Flame Temperature [C] 2002 1870 1920 1952 2117
Specific CO2 Emissions [g/MJ] 73.95 68.44 70.99 54.87 0.00
Table 1: Properties of typical gasoline, methanol, ethanol, methane and hydrogen.*
Includes atmospheric
nitrogen. NA=not available.
Methanol and ethanol have the potential to increase engine performance and efficiency over
that achievable with gasoline thanks to a variety of interesting properties. Their high heats of
vaporisation, combined with low stoichiometric air-to-fuel ratios, lead to high degrees of
intake charge cooling as the fuel evaporates. This is especially true for engines with direct
injection. The charge cooling not only leads to increased charge density, and thus higher
volumetric efficiency, but also considerably reduces the propensity of the engine to knock.
In fact, the effect of charge cooling causes difficulties when attempting to determine the
octane number of alcohols according to common methods. Yates et al. reviewed several
published values for RON and MON of alcohols and concluded that most of these are affected
by the evaporative cooling effect (3). Based on this observation, they selected octane valuesfrom work which controlled the intake charge temperature, irrespective of the evaporation
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effect. These values are given in Table 1 and reflect the knock resistance of alcohols due to
factors such as chemical autoignition behaviour and flame speed.
The low propensity of alcohol to knock allows for most of the increase in power and
efficiency compared to gasoline engines. It permits the application of optimal values for spark
advance, high compression ratios and opens opportunities for aggressive downsizing withoutthe need for fuel enrichment at high loads. On the other hand, it makes methanol and ethanol
unsuitable for use in conventional diesel engines. Alcohols can be used in conjunction with
another fuel which is more autoignitable, but this falls outside the scope of this paper
Apart from the high knock resistance and volumetric efficiencies, there are some other
properties which bring about minor advantages.
The flame speed of alcohols is about 40% higher than that of typical gasoline. Thiscreates more isochoric combustion and also allows increased levels of mixture
dilution, thus lowering throttling losses (4).
The high molar ratio of products to reactants causes a small increase in work. The heat capacity of the combustion products due to a high ratio of triatomic to
diatomic molecules, combined with the lower combustion temperatures of alcohols,
produce lower heat losses and exhaust temperatures compared to gasoline.
The properties of alcohols also affect the pollutant emissions. Engine-out emissions of NOx
generally decrease because of the lower combustion temperatures. The levels of CO and HC
are comparable to those of gasoline vehicles, although the oxygenated nature of alcohols can
cause minor decreases. For tailpipe emissions, most of these advantages are lost due to the
longer catalyst light-off times caused by lower exhaust temperatures. The low C-number of
methanol and ethanol ensures that PM emissions are very low. Particular for alcohol
combustion is the formation of aldehydes. These intermediate species of alcohol combustion
can attain levels in the exhaust of 2-7 times higher than in gasoline engines (5). Tailpipe CO2emissions are lower because of the lower CO2 formation per unit of energy and the higher
levels of efficiency. Renewably produced alcohols permit near-zero net CO2 emissions.
The lower vapour pressures of alcohols and their high heat of vaporisation raise cold start
problems. When temperatures drop below the freezing point, insufficient alcohol evaporates
to form a combustible mixture. This is the main reason why methanol and ethanol are often
used as mixtures with gasoline. For example, in M85 or E85 15% (by volume) of highlyvolatile gasoline is added to improve the cold start performance of the engine. Also, several
cold start strategies and devices have been proposed, but these will not be discussed here.
The Use of Alcohols in SI Engines
Methanol and ethanol were introduced in the 1970s because of energy security considerations.
However, their favourable properties allowed constructors to build dedicated engines with
increased power and efficiency, which was a nice quality to market these fuels to the public.
In 1981 the M85 Ford Escort had 20% more power and 15% higher efficiency than its
gasoline equivalent (6). Clemente et al. report similar figures for a more recent dedicated
ethanol engine designed for the Brazilian market (7). These figures can be principally
attributed to the increased knock resistance of alcohols. This enables to reach MBT timing
over a wide range of operation points and allows the CR to be raised to 12:1 and above.
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Because the users of dedicated methanol soon complained about a lack of refuelling stations,
flexible fuel vehicles (FFV) were developed during the 1980s. These vehicles were able to
run both on M85 and gasoline, which meant the CR could no longer be increased a lot. Still
FFVs attained about 5% more power and efficiency due to increased volumetric efficiency
and high energy content of stoichiometric mixtures (5). Today, active knock control and
aggressive spark retarding make it possible to combine CR and flexible fuel operation.
The reported values for tailpipe emissions of both ethanol and methanol vehicles are
comparable to those of gasoline vehicles. They are mainly dependent on the catalyst light off
time and its influence on cold start emissions. Also, aldehyde emissions have been shown to
be controllable using conventional TWC aftertreatment (5).
Recent work on modern alcohol engines has demonstrated significant potential for increasing
both efficiency and performance.
Nakata et al. used E100 in a high compression ratio (13:1) naturally aspirated port-fuelinjected SI engine (8). They were able to run MBT timing and found that engine
torque increased by 20% compared with operation on 92 RON gasoline. The full-loadthermal efficiency at 2800 rpm was 39.6% and 31.7% on E100 and gasoline
respectively. Even in operating points which were not knock limited, efficiency
improvements of over 3% were possible due to other favourable properties of ethanol.
Pearson et al. used E85 in a supercharged flexible fuel vehicle (9). The use of optimalignition timing increased the peak engine power by 14% compared to RON 95
gasoline. The authors took advantage of the high degrees of charge cooling by
injecting part of the fuel upstream of the supercharger, thus lowering compression
work. This allowed the thermal efficiency to be increased by 16% at maximum torque.
Bergstrm et al. took full advantage of the evaporative cooling effect by using E85 ina production turbocharged SIDI flex-fuel engine (10). Operation on E85 increased the
engines torque by 16% and power by 20%. The peak cylinder pressures were limited
to 120 bar because of structural reasons, rather than to avoid knock. With boosted
SIDI ethanol engines, BMEP of over 30 bar can be realised without knock, but then
the base engine structure must be designed for 140 bar peak pressure.
The greater dilution limit of alcohols was exploited by Brusstar et al. (4). Theyconverted a production 1.9 litre turbocharged diesel engine with a CR of 19:1 to run
on neat methanol and ethanol. The diesel injectors were replaced with spark plugs and
a PFI system was installed for alcohol injection. The high compression ratio enabledpeak brake thermal efficiencies comparable to the baseline diesel engine (40%) for
operation on ethanol and even higher on methanol (42%). High levels of EGR (up to
50%) were used to spread the high efficiency regions to part-load operating points.Throttle-less operation was possible down to a BMEP of 6 bar. Stoichiometric fuelling
throughout the entire operating range made it possible to use conventional TWC
aftertreatment to control emissions of NOx, CO and HC to extremely low levels.
From the discussion above we can conclude that the pressure conditions in state-of the-art
alcohol engines range from below atmospheric (throttling) to 140 bar peak pressure. The
associated temperatures can rise well above 1000 K. Stoichiometric fuelling is used on most
engines. EGR levels of up to 50% have been reported. These conditions will have to be
covered by our model for alcohol-fuelled engines.
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3. TWO-ZONE THERMODYNAMIC ENGINE MODELLINGModelling Framework and Assumptions
This section gives a brief overview of the assumptions and submodels of a two-zone
thermodynamic engine model and the GUEST code in particular. The purpose is to line outthe areas that will need adjustments in order to make the code valid for alcohol-fuelled
engines. The interested reader is referred to publications of Verhelst et al. to learn more about
GUEST and two-zone thermodynamic models (1, 2).
In addition to some standard assumptions to derive the equations for the rate of change of
cylinder pressure, unburned and burned gas temperatures, a two-zone thermodynamic model
needs some submodels to close these equations:
Trapped conditions: i.e. the conditions at the start of compression for pressure,temperature, equivalence ratio, etc. These are currently obtained through
measurements, but could be calculated using a gas dynamics model.
Gas properties, burned gas composition and blow-by Heat transfer: Annands model is used and will need recalibration for alcohol engines. Turbulence model: this is currently based on experimentally derived data, valid for
one particular engine. A better approach would be to use a k- type turbulence model.
Mass burning rate: this crucial submodel will need a lot of adjustments, because it isrelated to the combustion behaviour of the fuel. This is discussed in the next section.
Turbulent Burning Velocity Model
In thermodynamic models, the mass burning rate is derived from a turbulent combustion
model. In the GUEST code, a two equation model is used, to account for the finite flame
brush thickness, resulting in a set of equations similar to the entrainment framework:
[1]
where is the entrained mass, is a mean flame front surface and is the turbulent
entrainment velocity. The entrained mass is then burned with a rate proportional to the
amount of entrained unburned gas:
[2]
The time constant is given by , where is a turbulent length scale and is the
laminar burning velocity. With the mass burning rate given by equation 2, the well-known S
shaped curve results when the burned mass fraction is plotted against crank angle .
The equations for the rate of change of , , , and are initialized at the time of
ignition as described in (1) and are integrated throughout the combustion. Obviously a
turbulent entrainment velocity is needed for closure of equation 1. A number of turbulent
burning velocity models are implemented in the GUEST code to procure values of , forexample the model of Glder, used in the form below:
[3]
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where is the rms turbulent velocity, is a turbulent Reynolds number, a calibration
constant and is the stretched laminar burning velocity. The laminar burning velocity is aphysico-chemical property of the air/fuel/residuals mixture and is a fundamental building
block of the turbulent combustion model. This is discussed in more detail below.
Laminar Burning Velocity
Turbulent burning velocity models need laminar burning velocity data of the air/fuel/residuals
mixture at the instantaneous pressure and temperature. Most models use the laminar burning
velocity as the local burning velocity, which means the stretched laminar burning velocity
should be used. This means that a model for the effect of stretch is needed.
The effects of stretch are mostly embodied with a factor . The local flame speed is
calculated from the stretch-free laminar flame speed according to . Several
models for , tailored for use in SI engine modelling, have been proposed. Naturally, all of
this requires stretch-free laminar burning velocity data at engine conditions. As of today,
insufficient data is available for any fuel. Stretch and instabilities hamper the experimentaldetermination of stretch-free data at engine-like pressures (11). Not only the laminar burning
velocity as such, but also its sensitivity to the effects of flame stretch are important for the
turbulent burning velocity. This is characterized by Markstein numbers. The lack of data
regarding Markstein numbers is even greater.
The majority of the currently used correlations for laminar burning velocity were derived
from the pressure development recorded in a constant volume combustion bomb, e.g. the
correlations for methanol/air of Methgalchi and Keck (12) and for ethanol/air of Glder (13).
The correlations are actually not for stretch-free burning velocities and should not be used in
combination with a stretch factor, since they already encompass stretch effects. More
importantly, high pressure flames are prone to instabilities, such as cellularity (see Figure 1).
Cellularity can lead to a considerable overestimation of the measured burning velocity.
Figure 1: Development of a cellular flame structure in a H2-air flame at p=1bar, T=365 K, =0.7 (2)
Alternatively, the properties of alcohol-air flames can be calculated using a chemical kinetic
oxidation mechanism. However this also involves large uncertainties due to errors in reaction
rates, transport coefficients and limits on the numerical resolution of the flame (14).
Our literature review of the published values for laminar burning velocities of methanol and
ethanol revealed that data at elevated (engine-like) pressures and temperatures are scarce.
Also, almost no studies investigated the effect of diluents on the flame properties. A few
recent papers report values for Markstein numbers under a restricted set of conditions (15,
16). The majority of the published data is for mixtures at atmospheric pressure and initial
temperatures between 300 K and 500 K. Some data for the unstretched laminar burning of
methanol at p=1 bar and T = 300 K are shown in Figure 2. The figure includes data obtained
from burner experiments (17, 18), pressure measurements in a constant volume bomb (12, 15,19), Schlieren visualisation in a constant volume bomb (16) and chemical kinetic oxidation
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calculations (20). The various methods and their associated uncertainties bring about a large
scatter in the published data. High speed Schlieren photography can capture the effects of
stretch and instabilities and is deemed to produce the best results (11).
Figure 2: Comparison of reported data for the laminar burning velocity of methanol-air mixtures at p=1 bar,T=300 K, illustrating the large scatter in reported data.
Some authors have proposed correlations for the laminar burning velocities of methanol and
ethanol mixtures, which can be used in turbulent combustion models (12, 15, 16).
These correlations are only valid within the range of the measurements, but are often used
beyond that range. The correlation is generally expressed as a simple power law relation:
[4]
where , and are dependent on the fuel-air equivalence ratio. The graph below showsthat the values for given by the various correlations differ a lot, especially at high
temperatures. The high values obtained with the correlation of Metghalchi & Keck are
probably related to cellular flame structures at those conditions. The existing correlations
cover pressures from below atmospheric up till 40 bar and temperatures from 300 K till 800
K. The validity of the correlations beyond that range is very doubtful.
Figure 3: Laminar flame speeds of stoichiometric methanol-air mixtures at p=1 bar. Note the large scatter
between the different correlations at high temperatures
0
10
20
30
40
50
60
0,6 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,5
ul [cm/s]
Liao et al. (16) Saeed & Stone (15)
Methgalchi & Keck (12) Glder (19)
Gibbs & Calcote (17) Egolfopoulos (18)
Li et al. (20) Marinov et al. (20)
0
20
40
60
80
100120
140
160
180
200
300 400 500 600 700
ul [cm/s]
Tu [K]
Metghalchi & Keck (12)
Saeed & Stone (15)
Westbrook & Dryer (14)
Li et al. (20)
Marinov et al. (20)
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For brevity, the published values for the laminar burning velocity of ethanol have not been
included. However, the discussion above would be very similar for ethanol. It is clear that
there is a need for a new correlation for the laminar burning velocity of alcohol mixtures,
valid at higher pressures and temperatures than current correlations. In addition, the new
correlation should capture the effect of diluents on burning velocities. This can be done by the
inclusion of a new factor in the correlation: , where f is the concentration of thediluents. Our future work will focus on this correlation. To this end, measurements will be
done in a constant volume bomb using Schlieren photography.
Prediction of Emissions
As mentioned earlier, the regulated emissions from alcohol vehicles are comparable to those
of their gasoline-fuelled counterparts. This is because the formation mechanisms are very
similar. Consequently, the emissions models can be based on those commonly used for
gasoline-fuelled engines. This usually involves the prediction of 10 species (N, O, N2, O2,
CO, CO2, H2, H, OH, H2O) using equilibrium chemistry. The chemical kinetics of NOx
formation are represented by 3 reversible reactions known as the extended Zeldovichmechanism. Unburned hydrocarbons can be predicted by the combination of a crevice volume
model and a kinetic model for the post-flame oxidation of unburned hydrocarbons.
Particular pollutants from alcohol engines are aldehydes. Formaldehyde and acetaldehyde are
believed to originate from the oxidation of methanol and ethanol during the exhaust stroke
(5). Yano et al. used a detailed chemical kinetic reaction scheme of methanol oxidation to
investigate the formaldehyde formation mechanism (21). They concluded that neither
chemical equilibrium nor in-flame chemistry can explain the measured levels of formaldehyde
emissions. Instead, the partial oxidation of unburned methanol during the exhaust stroke is the
main source of formaldehyde. The authors also noted that it was important to include the N-
series reactions involving NO and NO2 in the methanol oxidation scheme. Later work by the
same authors yielded similar conclusions for the formation of acetaldehyde from ethanol
oxidation (22). The validity of this modelling approach was confirmed by Kusaka et al., who
successfully predicted the emission of unburned methanol and formaldehyde from a glow-
assisted methanol compression-ignition engine (23).
Knock Modelling
As mentioned before, the high levels of charge cooling and high flame speeds are two
important reasons for the increased knock resistance of alcohols. A third reason lies in the
particular chemical autoignition behaviour of alcohol fuels. The autoignition of gasoline istypically a two-stage process. At temperatures below 1000 K low-temperature oxidation of
the mixture takes place. These so-called cool-flame reactions release heat, which boosts the
high-temperature oxidation responsible for knock. At temperatures between 700 K and 1000
K the low-temperature oxidation is inhibited by degenerate chain branching. This results in
rising autoignition delay times in this temperature frame. This is called the negative
temperature coefficient (NTC) behaviour of gasoline. Neat alcohols do not exhibit cool-
flames. Consequently, they do not have a NTC region. As alcohol is added to gasoline, the
cool-flame temperature rise decreases and the NTC region gradually diminishes (3).
Knock prediction models are a valuable help for engine designers to find ways to reduce
knock, e.g. by evaluating high octane like alcohols. To predict knock onset in engines, a goodestimation is required of the main flame propagation in order to correctly calculate the state of
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the end gas. In addition a model is needed of the autoignition behaviour of the fuel under
consideration. There are three main directions in which to model this behaviour (24):
A full chemical kinetics mechanism of the fuels low and high temperature oxidation. A reduced representative scheme of the autoignition reactions An Arrhenius reaction representing rate-limiting step of the main autoignition reaction
[5]
Where is the autoignition delay. , and are the fuel/air equivalence ratio,pressure and temperature of the end gas. The exponents and coefficients have to be
empirically tuned for a particular fuel.
Because the local temperature excess throughout the end gas, caused by 3D and
inhomogeneous flow, may be as high as 100-150 K, 0D models are actually not suitable for
the exact description of autoignition in SI engines. Consequently, there is little use for a full
chemical kinetics mechanism if similar prediction accuracy can be obtained with a one-step
reaction approach (24). Therefore it is advised to reduce the information in a full kineticsmechanism to a one-step Arrhenius type reaction. This was recently done by Yates et al. for
both methanol and ethanol (3).
Gas Dynamics
To incorporate the gas dynamics during the breathing cycle, the GUEST code will be coupled
to the commercial gas dynamics code GT-Power. As can be expected from the high latent
heat of alcohols, the model will need to include the temperature drop caused by alcohol
evaporation. This is confirmed by a recent publication on a GT-Power model for an engine
operating on E85 (25). In fact, the authors report that the effect of fuel evaporation on
volumetric efficiency is so big that a separate puddling and evaporation model was needed to
make accurate estimations of the gas dynamics possible.
4. CONCLUSIONSThis paper discussed our investigation of the requirements to build a two-zone
thermodynamic engine model valid for alcohol fuels. Based on a review of past and present
alcohol-fuelled engine technology we concluded that the model should cover cylinder
pressures from below atmospheric up to 140 bar peak pressure. Most alcohol engines run
stoichiometric over the entire operating range. High EGR rates of up to 50% are possible and
have been used in some engines.
Our review of the published data for the laminar burning velocities of methanol-air and
ethanol-air mixtures revealed a lack of data at engine-like conditions. For the Markstein
lengths the data are even scarcer. Moreover, there is a large scatter among published values.
For the prediction of emissions, we found that similar models can be used as those common
for gasoline engines. The only particularity was the formation of aldehydes, for which a
suitable model was identified. A look into the autoignition behaviour of alcohols revealed that
these fuels exhibit a single-stage autoignition, as opposed to the two-stage behaviour of
gasoline. Within the framework of a thermodynamic engine model, a knock model based on a
one-step Arrhenius reaction is thought to be the best option. In future work, we will focus on
the development of a correlation for the laminar burning velocity of alcohol-air mixtures.
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ACKNOWLEDGEMENTS
This research is funded by a Ph. D. fellowship grant of the Research Foundation -Flanders
(FWO)
DEFINITIONS, ACRONYMS, ABBREVIATIONS
BMEP Brake mean effective pressure
CR Compression ratio
E85 Mixture of 85% ethanol and 15% gasoline (by volume)
FFV Flexible fuel vehicle
GUEST Ghent University Engine Simulation Tool
HC Hydrocarbons
M85 Mixture of 85% methanol and 15% gasoline (by volume)
MBT Minimal spark advance for best torque
MON Motor octane number
NTC Negative temperature coefficientNTP Normal temperature and pressure
PM Particulate matter
RON Research octane number
SI Spark-ignition
SIDI Spark-ignition direct injection
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