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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

REFERENCES

1. Verhelst, S. and R. Sierens,"A quasi-dimensional model for the power cycle of hydrogen

fuelled ICE", International Journal of Hydrogen Energy, 32(15), p. 3545-3554, 2007.

2. Verhelst, S., "A study of the combustion in hydrogen-fuelled internal combustion engines.

PhD thesis", UGent,2005. http://hdl.handle.net/1854/337

3. Yates, A., A. Bell, and A. Swarts, "Insights relating to the autoignition characteristics of

alcohol fuels", Fuel, 89(1), p. 83-93, 2010.

4. Brusstar, M.J., et al., "High Efficiency and Low Emissions From a Port-Injected Engine

With Neat Alcohol Fuels", SAE paper no. 2002-01-2743, 2002.

5. McCoy, G.A., J. Kerstetter, and J.K. Lyons, "Alcohol-fueled Vehicles", 1993.

6. Nichols, R.J., "The methanol story: a sustainable fuel for the future", Journal of Scientific& Industrial Research, 62(January-February), p. 97-105, 2003.

7. Clemente, R.C., et al., "Development of an internal combustion alcohol-fueled engine",

SAE paper no. 2001-01-3917, 2001.8. Nakata, K., et al., "The Effect of Ethanol Fuel on a Spark Ignition Engine", SAE paper no.

2006-01-3380, 2006.

9. Pearson, R.J., et al., "Alcohol-Based Fuels in High Performance Engines", SAE paper no.

2007-01-0056, 2007.

10.Bergstrm, K., et al., "ABC - Alcohol based combustion engines - challenges and

opportunities", in 16th Aachener Kolloquium Fahrzeug- und Motorentechnik, 2007.

11.Gillespie, L., et al., "Aspects of laminar and turbulent burning velocity relevant to SI

engines", SAE paper no. 2000-01-0192, 2000.

12.Metghalchi, M. and J.C. Keck, "Burning velocities of mixtures of air with methanol,

isooctane, and indolene at high pressure and temperature", Combustion and Flame, 48, p.

191-210, 1982.

7/30/2019 fisita2010sco04

12/12

13. Glder, O.L., "Burning Velocities Of Ethanol Isooctane Blends", Combustion and Flame,

56(3), p. 261-268, 1984.

14.Westbrook, C.K. and F.L. Dryer, "Prediction of laminar flame properties of methanol-air

mixtures", Combustion and Flame, 37, p. 171-192, 1980.

15.Saeed, K. and C.R. Stone, "Measurements of the laminar burning velocity for mixtures of

methanol and air from a constant-volume vessel using a multizone model", Combustionand Flame, 139(1-2), p. 152-166, 2004.

16.Liao, S.Y., et al., "Laminar burning velocities for mixtures of methanol and air at elevated

temperatures", Energy Conversion and Management, 48(3), p. 857-863, 2007.

17.Gibbs, G.J. and H.F. Calcote, "Effect of Molecular Structure on Burning Velocity",

Journal of Chemical & Engineering Data, 4(3), p. 226-237, 1959.

18.Egolfopoulos, F.N., D.X. Du, and C.K. Law, "A Comprehensive Study of Methanol

Kinetics in Freely-Propagating and Burner-Stabilized Flames, Flow and Static Reactors,

and Shock Tubes", Combustion Science and Technology, 83(1), p. 33 - 75, 1992.

19.Gulder, O.L., "Laminar Burning Velocities Of Methanol, Isooctane And

Isooctane/Methanol Blends", Combustion Science and Technology, 33(1-4), p. 179-192,

1983.20.Bromberg, L., "Benchmarking of Alcohol Chemical Kinetic Mechanism for Laminar

Flame Speed Calculations", PSFC-JA-08-10, 2008.

21.Yano, T. and K. Ito, "Behavior of Methanol and Formaldehyde in Burned Gas from

Methanol Combustion : A Chemical Kinetic Study ", Bulletin of JSME, 26(211), p. 94-

101, 1983.

22.Yano, T., K. Ito, and T. Takahata, "Formaldehyde and Acetaldehyde in Exhaust Gases

Emitted from an Ethanol Fueled S. I. Engine : Clarification of the Formation Mechanism

by Reactor Experiment ", Bulletin of JSME, 25(255), p. pp.3028-3035, 1986.

23.Kusaka, K., et al., "Predicting exhaust emissions in a glow-assisted DI methanol engine

using a combustion model combined with full kinetics", SAE paper no. 961935, 1996.

24.Burluka, A.A., et al., "The Influence of Simulated Residual and NO Concentrations on

Knock Onset for PRFs and Gasolines", SAE paper no. 2004-01-2998, 2004.

25.Lauer, T. and M. Klein, "Mechanisms of the Mixture Preparation and Combustion for an

Engine Operation with the Ethanol Blend E85", in European GT-SUITE user Conference

2009, Frankfurt, 2009.