analysis of combustion in a small homogeneous charge ... · 237 analysis of combustion in a small...

237

Analysis of combustion in a small homogeneouscharge compression assisted ignition engineH Ma1*, K Kar1, R Stone1, R Raine2, and H Thorwarth2

1Department of Engineering Science, Oxford University, Oxford, UK2Department of Mechanical Engineering, The University of Auckland, New Zealand

The manuscript was accepted after revision for publication on 8 July 2005.

DOI: 10.1243/146808705X60834

Abstract: Combustion analysis has been conducted on a small two-stroke glow ignitionengine, which has similar combustion characteristics to homogeneous charge compressionignition (HCCI) engines. Difficulties such as unknown ignition timing and the polytropic indexhave been addressed by combining both heat release and mass fraction burn analyses. Resultsfor all operating conditions have shown good correlations between the two methods.

The engine has been fuelled with a mixture of methanol, nitromethane, and lubrication oil.The effect of nitromethane on combustion is difficult to determine, since altering nitromethanecontent also changes the air–fuel ratio under the current experiment set-up. However, it is stillpossible to show that nitromethane shortens the combustion periods beyond the uncertaintycreated by the mixture strength and cycle-by-cycle variations. The results further show that afaster combustion does not necessarily give a higher indicative mean effective pressure(i.m.e.p.) in this engine. This is because the start of combustion can shift away from its optimumvalue when nitromethane is added. The initial combustion period is found to be between 10and 30° CA (crank angle); the main combustion period is between 25 and 50° CA. These com-bustion periods are comparable to a traditional spark ignition engine. In a very rich mixture,the ‘hot’ glow plug has been found to change significantly the combustion characteristics.Further study would be recommended to elucidate the effect of glow plugs. Lastly, in the caseof poor combustion, cycle-by-cycle analysis shows that a misfire or partial burn cycles arealways followed by high i.m.e.p. and fast burn cycles.

Keywords: homogeneous charge compression ignition engine, glow ignition, nitromethane,methanol, combustion analysis, partial burn cycles

1 INTRODUCTION Traditionally, burn rate analysis is widely used forspark ignition engines, while the heat release analysis

This paper is based on continuing work with a is more suitable for compression ignition engines.small homogeneous charge, two-stroke glow ignition This type of miniature engine does not fall into eitherengine of the type used in model aircraft [1, 2]. A category. In spark ignition engines, the mixture isspecial torque balance was devised to be sensitive premixed and the combustion is initiated by a spark,enough to measure torque in the order of 0.1 N m. so the time of the start of combustion is known. InIn the work reported here, the facility has been compression ignition engines, combustion startsextended to instrument for combustion pressure, from autoignition of the fuel. The start of combustionand combustion analysis has been carried out. The is unknown, but it has to be later than the fueldetailed experimental set-up is shown in Fig. 1. injection timing. This miniature engine, with a pre-

mixed mixture and glow plug assisted autoignition isa special case. It is likely that the compression and* Corresponding author: Faculty of Engineering, The University

ignition processes would be most similar to homo-of Auckland, Private Bag 92019, Auckland 1020, New Zealand.

email: [email protected] geneous charge compression ignition (HCCI) engines.

JER03805 © IMechE 2006 Int. J. Engine Res. Vol. 7

238 H Ma, K Kar, R Stone, R Raine, and H Thorwarth

Fig. 1 Sketch of the instrumentation system

In this paper, both analyses of burn rate are con- methane concentration in methanol increased thesidered together to establish correlations between knocking tendency. Ferguson and Kirkpatrick [9]them. The aim is to gain an understanding of the note its wide use as a drag racing fuel.applicability of both analyses to this type of engine. Notably, the authors have not found reported dataSince the combustion process is similar to HCCI on the combustion rate of nitromethane/methanolengines, it is anticipated that the study here would blends. Starkman [6] speculated that nitromethanealso be relevant to those engines. may combust more rapidly than methanol, but did

The two-stroke glow ignition engine has been not have facilities to measure this. In contrast, Gierkefuelled with a mixture of methanol, nitromethane, [10] states that ‘Compared with the snail-like burningand lubrication oil. Methanol (CH

3OH) is widely of nitromethane, methanol is fast burning…’, but

used as an automotive fuel in racing applications gives no quantitative data, nor reference for thisand is being extensively researched as an alternative statement.fuel for internal combustion engines. Glow assisted Glow plug manufacturers typically rate plugs inignition of methanol fuelled, compression ignition three categories, namely ‘cold’, ‘medium’, and ‘hot’engines is an established topic of research and [11], which is assumed to indicate the level ofdevelopment [3–5]. temperature that the plug will maintain between

However, there is little published work on nitro- successive combustion cycles. This implies that amethane (CH

3NO

2) as an engine fuel. Starkman [6, 7] ‘hot’ glow plug is likely to cause earlier ignition than

noted the strong knocking tendency of nitromethane with a ‘cold’ rated glow plug. One aspect of this pro-blends and the high power output that is possible. ject was to determine whether these assumptionsNitromethane will explode if it is subjected to shock. are correct.‘Any attempt to burn nitromethane in undiluted form To summarize, the objectives of this project were:in a reciprocating engine can only lead to disaster’,

(a) to develop a method to combine the burn rateStarkman noted [6], because of its pre-ignition orand heat release analyses in order to investigateknocking tendency. For this reason nitromethane isthe combustion process of engines with pre-only used as an additive to automotive engine fuels.

Bush et al. [8] showed that an increase in nitro- mixed charge, but ignited by compression;

JER03805 © IMechE 2006Int. J. Engine Res. Vol. 7

239Analysis of combustion

(b) to investigate the combustion and perform- 3 THEORYance characteristics of different nitromethane/methanol fuel blends; 3.1 Effects of fuel blend

(c) to investigate the characteristic of different glowThe fuel blends used in this experimental project

plugs.have been made up using a range of methanol andnitromethane proportions, suitable for operation ofthe engine. Castor oil is mixed with the fuel as a

2 EXPERIMENTAL ASPECTSlubricant, and is fixed as 20 per cent by volume ofthe liquid fuel. Proportions of nitromethane and

The engine is fuelled with a mixture of methanol,methanol were varied so that between 0 and 20 per

nitromethane and lubrication oil, using a glow plugcent of the fuel is nitromethane. Thus the proportion

and piston compression to assist the ignition ofof methanol in the fuel was varied between 80 and

the charge. The supplied fuel mass flowrate is varied60 per cent by volume of the liquid fuel.

(not precisely quantitatively) by setting the needleIn these experiments, the absolute air–fuel ratio

valve position. The detailed experimental equipment,was not able to be measured, so it is essential to

arrangement, and results from measured engineunderstand how the fuel flowrate, the stoichiometry

power have been previously reported [2].of the fuel/air mixture, and the energy content of the

With an external sampling rate of 200 points/cycle,mixture supplied to the engine changes with fuel

200 cycles of data at 27 different engine operatingcomposition. The fuel is supplied through a needle

conditions (all at wide-open throttle, or WOT) werevalve by gravity feed. Assuming a constant head is

measured by varying three parameters:maintained upstream of the needle valve and the fuelis incompressible, the fuel mass flowrate is given by(a) three different fuel compositions (nitromethane

0, 10, and 20 per cent);mf=CD

O

AO(2rfDpO)1/2 (1)(b) three different glow plugs (hot, medium, andcold); For a fixed needle setting, the flow area is fixed.

(c) three different fuel-supply needle settings (2, 3, Assuming further that the head (DpO

) and the orificeand 4) – the greater the number, the higher the discharge coefficient remain constant, the effect offuel flowrate. changes in fuel properties is given by

An extended version of the MATLAB combustion mf3√rf (2)

analysis package CoBRA (combustion burn rateanalysis) was used to process the measured com- By using equation (2) and the properties of the fuel

components (Table 1), the relative fuel mass flowrate,bustion pressure data [12]. Extensions that have beenmade allow for the analysis of the two-stroke cycle relative mixture stoichiometry, and relative lower

heating values (LHVs) of fuel mixtures are calculatedengine, heat release, and rate of heat release analysisas well as incorporating completeness of combustion and presented in Table 2 and Fig. 2. The datum

for comparison is a fuel with no nitromethane. Notecalculations.

Table 1 Fuel properties [13, 14]

Fuel Methanol Nitromethane Castor oilFormula CH

3OH CH

3NO

2C

18H

34O

2.9Molar mass (kg/kmol) 32.04 61.04 ~297Density at 20 °C (kg/m3) 787 1138 ~960Viscosity at 20 °C (cP) 0.59 0.65 ~700Enthalpy of vaporization (kJ/kg) 1186.3 628.5Specific heat

Liquid (kJ/kg K) 2.6 1.74 —Vapour (kJ/kg K) 1.72 — —

Higher heating value (MJ/kg) 22.3 11.6 —Lower heating value (MJ/kg) 19.6 10.5 ~37.2LHV of stoichiometric mixture (MJ/kg) 2.63 3.92 —Stoichiometric air–fuel ratio (A/F)

s6.43 1.69 —

Fuel octane ratingRON 106 — —Motor octane number (MON) 92 — —



Table 2 Calculated fuel blend properties based on a fixed needle setting, except thefinal row (A/F, air–fuel ratio)

Fraction of nitromethane by the volume of liquid 0.00 0.05 0.10 0.15 0.20Density of blend (liquid) (kg/m3) 787.0 804.5 821.9 839.4 856.8Relative flowrate of blend (kg/s) 1.00 1.01 1.02 1.03 1.04Blend (A/F)

sby mass 6.43 6.09 5.77 5.46 5.17

(A/F)/(A/F)s

1.00 1.04 1.09 1.14 1.19Relative LHV rate 1.00 0.98 0.96 0.94 0.92Relative LHV per m3 of stoichiometric mixture* 1.00 1.01 1.03 1.04 1.06

*See the text.

Fig. 2 Relative fuel mass flowrate, air–fuel ratio, and LHVs as the fuel blend is varied

that in these calculations, the castor oil has been and modified for research on spark ignition enginecombustion at Oxford University [12]. The com-neglected, since it is assumed that it is unlikely

to contribute to the combustion processes, and it bustion process is approximated by considering aseries of small crank angle intervals (Dh), during eachremains as a constant proportion of the fuel as the

composition of the other components is varied. of which the change in pressure (Dp) is the sum oftwo parts: the pressure rise due to piston motionThese calculations show that, as the nitromethane

content is increased, the density of the fuel blend (Dpv) and the pressure rise due to combustion (Dp

c).

First, the charge is assumed to be compressedincreases, with a corresponding increase in the fuelmass flowrate according to equation (2). At the same polytropically by the piston; then a small amount of

charge burns causing a pressure rise. In the nexttime, the stoichiometric air–fuel ratio of the blend(l

s) decreases due to the much lower value of this crank angle interval, the pressure again changes

polytropically and another portion of charge burns;parameter for nitromethane than for methanol. Therelative change in the stoichiometric air–fuel ratio is this continues throughout the combustion period.

The pressure rise due to combustion at the ithshown by the ratio l/ls. This shows that with an

increasing nitromethane content in the fuel, the interval can be calculated as [12]stoichiometry becomes leaner, so that the 20 per centnitromethane blend is 19 per cent leaner relative to Dpc,i=p

i+1−piA V

iVi+1Bn (3)

stoichiometric than the 0 per cent nitromethane fuel.As the nitromethane content is increased, the LHV

Since combustion is not taking place at constantdecreases, even though the mass flowrate increases,volume, the cylinder volume at TDC (top deaddue to the lower LHV of nitromethane than ofcentre) is used for normalizationmethanol, per unit mass of the component.

3.2 Mass fraction burned Dp*c=Dpc,iA Vi

VTDCB (4)

The technique used here for computing the massfraction burned (MFB) is based on the method first Assuming that the cumulative normalized pressure

rise due to combustion is proportional to the massdeveloped by Rassweiler and Withrow in 1938 [15]



fraction burned (x), then for N total increments R=cp−c

v), then equation (10) can be written as

dQndh=

dQhrdh−

dQhtdh=c

c−1p

dV

dh+

1

c−1V

dp

dh(11)

x=∑i

0Dp*c

∑N

0Dp*c

(5)where dQ

n/dh=the net heat release rate. Then the

cumulative net heat release Qn

is an integration ofthe results of the net heat release rate.The total normalized pressure change due to

Since the mass is assumed to be constant, calcu-combustion is defined as Y=WN0Dp*

c. Further

lation of the net heat release is only valid betweenassumptions and key observations can be found inthe inlet port closure (IPC) and the exhaust portreference [12].opening (EPO).

3.3 Heat release analysis

3.4 Completeness of combustionHeat release analysis is generally used for diesel com-bustion processes, where the timing of first ignition

The completeness of combustion x is defined as theis not known a priori. This method of analysis com-fraction of the inducted fuel that is usefully burnedputes how much heat would have to be added to theduring the cycle [17, 18]. Earlier work [12] hascylinder contents in order to produce the observedintroduced the termpressure variations [16]. It is usually assumed that

the products and reactants are fully mixed.Y=∑

N

0Dp*cApplying the First Law of Thermodynamics to a

control volume in which there is no mass transferfor the ideal conditions, which are the basis of thegives the heat released by combustion (dQ

hr) as

Rassweiler and Withrow analysis. This is expecteddQhr=dU+dW+dQht (6) to yield the adiabatic combustion pressure rise at

the reference volume. If this term is evaluated forwhere dQ

ht=the heat transfer with the chamber any combustion cycle and compared with some

walls. ‘maximum’ of Y, referred to as Ym

, which representsAnother simplification of equation (6) is that there ‘complete combustion’, then an estimate of the

is no allowance for differences in the properties of relative completeness of combustion for a singlethe reactants and products and that there is a uni- cycle can be obtained byform temperature. Each of the terms in equation (6)has to be evaluated

x=Y

Ym(12)

dU=mcvdT (7)The frequency distribution of Y departs from a

From the equation of state ( pV=mRT) normal distribution, so the Kaplan–Meier approach[19] is adopted to estimate the empirical cumu-lative distribution function (CDF). According to thism dT=

p dV+V dp

R(8)

approach, the calculated data array of Y is firstsorted in ascending order, forming a new array Y∞.Equation (7) together with equation (8) givesThen the interval of cumulative distribution, [0, 1],is divided into several subdivisions by the totaldU=cv

p dV+V dp

R(9)

number of cycles N, for instance [50–55 per cent],[95–100 per cent], etc., when N=20. The correspond-

Substituting equation (9) into equation (6) and ing values of the cumulative distribution Sn

(discrete)noting that dW=p dV gives, on an incremental angle to Y∞ are given bybasis

Sn=1− a

n−1

j=0A1− 1

N− jB= n

N(13)dQhr

dh=

cvR Ap dV

dh+V

dp

dhB+pdV

dh+

dQhtdh

(10)

where n is the number of subdivisions involved.For given results of Y, interpolation is used toThe dV/dh term is defined from the geometry of the

engine and the dp/dh term can be obtained from the obtain the consecutive CDF. A significant point atwhich S

n=97.5 per cent is used to define Y

m. Thisengine combustion pressure data. If semi-perfect

gas behaviour is assumed (such that cp

/cv=c and is considered to be a satisfactory measurement of



complete combustion [20]; i.e. 97.5 per cent of the fore, 145° CA ABDC (after bottom dead centre) wasselected as the ‘nominal’ spark timing in the sub-Y data calculated from the engine data is below Y

m.

For those 2.5 per cent Y above Ym

, truncation is sequent analysis.In the heat release analysis, c is a function of themade such that they are all set to Y

m, in order to

treat such cycles as ‘complete’ ones. temperature and air–fuel ratio and has an influenceon the magnitude of Q

nand dQ

n/dh according to

equation (11). In order to investigate the effects of c,results for calculated Q

nand dQ

n/dh from ‘0-4-Cold’4 ANALYSIS ISSUES

are shown in Table 3 and Fig. 4. Table 3 shows that,with c varying between 1.2 and 1.4, the changes inFor convenience, the names of the data files are used

instead of listing their corresponding operating the phasing of Qn

and dQn

/dh were only within1.8° CA (one sample point). It is also noted that theconditions in the following discussion. For example,

‘0-4-Cold’ represents the condition of 0 per cent CA of Qn,min

and the CA of (dQn

/dh)|0

are not exactlythe same. This difference is probably because of thenitromethane concentration and needle setting ‘4’,

and the cold plug. This naming system will be used numerical error from the computation in MATLAB.However, it is the phase of the start of combustionhereafter.

In this glow ignition engine, there are issues with (SOC) that is of most interest, and hence it wasassumed that the effect of changes in c within aunknown timing for the start of combustion, choice

of polytropic index for heat release analysis, and typical range on the phasing of Qn

and dQn

/dh isnegligible. A value of c=1.4 was therefore selectedsampling rate. They will be addressed as follows.

The burn rate analysis in CoBRA requires the spark in the subsequent analysis.The cylinder pressure signal was measured attiming to calculate the compression polytropic index

prior to combustion. This is chosen to be the poly- 200 samples/cycle, which is lower than that typicallyused for heat release analysis in the literature [16].tropic index (polynomially) fitted from a point half-

way between the inlet closure and spark timing to However, since heat release analysis is usuallyapplied in compression ignition engines with athe point of spark timing. Given the fact that this

engine has no spark ignition, the effect of varying much higher compression ratio and so much fasterpressure rise, it is difficult to judge whether thethe ‘nominal’ spark timing specified in the code

on the phase of 1 per cent mass fraction burnedwas investigated. Figure 3 shows the result of this

Table 3 Effect of c on the phasing of Qninvestigation for the data set for mean values for

and dQn

/dH, ‘0-4-Cold’case ‘0-4-Cold’. This data set was chosen as it is oneshowing extreme variations in cycle-by-cycle pressure c 1.2 1.3 1.4data, so that it was felt this would give the most

CA of Qn,min

176.4 176.4 176.4rigorous test of the effects being studied. As shown CA of Qn,max

280.8 280.8 282.6in Fig. 3, a variation of 30° CA (crank angle) in the CA of (dQ

n/dh)

min167.4 165.6 163.8

CA of (dQn

/dh)max

217.8 217.8 219.6‘nominal’ spark timing resulted in a variation ofCA of (dQ

n/dh)|

0176.4 177.3 177.8

only 3° CA in the phase of 1 per cent MFB. There-

Fig. 3 Correlation between ‘nominal’ spark timing and 1 per cent MFB timing, ‘0-4-Cold’



Fig. 4 Sample results of cycle-averaged Qn

and dQn

/dh, ‘0-4-Cold’ (top) and ‘0-4-Hot’ (bottom),calculated with c=1.4

current sampling rate is sufficient without furtheranalysis. Too low a sampling rate will introducealiasing in the signal and truncation errors whenevaluating dp/dh. Figure 5 shows the spectrum of thecylinder pressure signal in one data set. There aredistinct peaks at the fundamental frequency andhigher harmonics, indicating the cyclic nature of thesignal. The spectrum flattens off above 0.08° CA−1,indicating that there is little or no power above thatfrequency. This ensures that the sampling rate is fastenough to capture most information contained inthe signal.

Errors also arise when approximating the derivativeof a continuous signal by finite difference. In CoBRA,dp/dh is calculated by the forward difference scheme,where the truncation error (e) is in the order of a Fig. 5 Power density spectrum of the pressure signalcrank angle step (k), i.e. e=O(k). Three-point and in ‘10-4-Cold’, normalized to the direct current

(DC) valuefive-point central difference schemes have been



considered, which have e in the order of k−2 and k−4 refers to the ‘nominal’ spark timing of 145° CA ABDC.respectively. The three schemes have been compared This correlation implies that features of the initialagainst each other. The percentage differences combustion can be described by either of the twobetween them are typically ±0.1 per cent. When durations. However, in order to obtain an absolutedp/dh is close to zero, because of the quantization value for the rate of combustion, the start ofand electrical noise, the difference can go up to ±1 combustion (SOC) needs to be defined.per cent. This is, however, irrelevant because heat Figure 7 shows that the MFB analysis and heatrelease analysis is only valid between IPC and EPO. release analysis have a close correlation except forThis is when dp/dh values are high. Therefore, it is two anomalies (represented by the hollow symbolsconcluded that the truncation error is insignificant in Fig. 7). Note that both the minimum of Q

nand

compared to random noise in dp/dh and that the 1 per cent MFB are indicators of the SOC. Sincesampling rate is sufficient. the minimum of Q

nis an absolute timing and it

correlates well with 1 per cent MFB, it will be usedto define the SOC to complement the burn rate

5 RESULTS AND DISCUSSION analysis. The anomalies correspond to ‘0-4-Cold’ and‘0-4-Medium’, both of which contain several misfire

5.1 Correlation between burn rate and heat cycles. These two sets of data also have the greatestrelease analyses cycle-by-cycle variation and extremely slow com-

bustion. This was established from the i.m.e.p.Historical research on engine thermodynamics has(indicated mean effective pressure) and 0–90 pershown that in conventional spark ignition engines,cent MFB results, to be discussed later. For these twothe 0–10 per cent MFB is well correlated to the initialcases, the rate of combustion will be less certainperiod of combustion and the 10–90 per cent isbecause the SOC is not well defined.representative of the main combustion period during

It has been noted from data such as that in Fig. 4which most of the fuel burns. On the other hand, thethat the maximum of Q

nremains almost constantminimum of Q

n(where the net heat release rate

(280.8–282.6° CA) under all operating conditions andbecomes positive, as shown in Fig. 4) and the maxi-hence it can be inferred that this estimates themum of Q

n(where the net heat release rate becomes

exhaust port open timing. In summary, therefore,negative) are commonly defined as the start andfor this engine the initial period of combustion isend of combustion for compression ignition enginesdefined as the difference between Q

n,minand 10 perrespectively. As the combustion process in this

cent MFB and the main combustion period isengine is not the same as either spark ignitiondefined as 10–90 per cent MFB.engines or compression ignition engines, the initial

Correlation between Qn,min

–10 per cent MFB andperiod and main combustion need to be defined10–90 per cent MFB is shown in Fig. 8. As expected,when employing heat release analysis.a quicker initial period of combustion tends to beFigure 6 shows the highly linear correlationfollowed by a quicker main combustion. Again, thebetween the 0–1 per cent MFB and 0–10 per centcorrelation cannot apply well to the data fromMFB durations based on data for all engine operating

conditions. In this figure, the time for 0 per cent MFB ‘0-4-Cold’ and ‘0-4-Medium’ (represented by hollow

Fig. 6 Correlation between 0–1 per cent MFB and 0–10 per cent MFB



Fig. 7 Correlation between 1 per cent MFB and Qn,min

Fig. 8 Correlation between Qn,min

–10 per cent MFB and 10–90 per cent MFB

symbols in Fig. 8) because both of them present by Qn,min

–10 per cent MFB, is a strong function oflaminar burning velocity in spark ignition engines,abnormally slow initial and main periods ofand more generally a function of chemical kineticscombustion.[21]. As the needle setting is reduced, less fuel isThis analysis, making use of both ‘mass fractionsupplied, so the relative air–fuel ratio (l) is increased.burned’ and ‘heat release’ methodologies, has estab-Figure 9 shows that the initial combustion periodlished that both give useful results for the homo-consistently reduces as the needle setting is reducedgeneous, compression assisted engine under study.for the ‘cold’ and ‘medium’ plugs. Experiments [16]Comparison between results from the two methodsshow that the laminar burning velocity of methanolshows good agreement in those parameters that canpeaks at a relative air–fuel ratio (l) of about 0.9, andbe compared.it decreases monotonically either side of that ratio.For l to increase, and at the same time increase the5.2 Effects of fuel blend and flow plugslaminar velocity, the mixture must be richer than

The air–fuel ratio was not monitored or controlled stoichiometry. Furthermore, the reduction in Qn,min

–in the experiment, other than keeping fixed needle 10 per cent MFB is almost linear. This can onlysettings for the fuel supply. This makes the inter- happen for very rich mixtures, as the closer it ispretation of the results difficult, particularly when to l=0.9, the smaller the change in the laminarthe nitromethane content changes, since both fuel velocity. The fact that ‘0-4-Cold’ and ‘0-4-Medium’and mixture strength effects are present. The problem have very poor combustion also indicates that thecan be first isolated by looking at the results of pure mixture is near the rich burning limit; hence l ismethanol as shown in Fig. 9. much smaller than 0.9. ‘0-4-Hot’ does not follow the

general trend. With a needle setting of 4 and 0 perThe initial period of combustion, as indicated



Fig. 9 Qn,min

–10 per cent MFB for various needle settings and plug types using pure methanol

cent nitromethane, l will be the lowest among all cent [16]. Given the cycle-by-cycle variations (limitbars) indicated in Fig. 10, five conditions (‘3-Cold,tested conditions. It is likely that the hot plug hasMedium and Hot’, and ‘4-Cold and Medium’)facilitated the combustion by supplying enough heatdefinitely have shorter Q

n,min–10 per cent MFB. All ofto vaporize the methanol, while the cold and medium

these have about a 40 per cent reduction in Qn,min

–plugs cannot. In other words, a high-temperature10 per cent MFB. Assuming that Q

n,min–10 per centplug effectively extends the rich burning limit of

MFB reduces by the same order as the laminar burn-the fuel.ing velocity, then nitromethane certainly has speededWhen 20 per cent nitromethane is added to theup the combustion. For the other four conditionsmethanol, the initial period of combustion changes,(all needle settings at ‘2’ and ‘4-Hot’), the initial com-as shown by the filled bars in Fig. 10, relative to thebustion period has changed by an insignificantvalues for methanol only. The figure also shows limitamount given that the cycle-by-cycle variation isbars for the cycle-by-cycle variations. This figurelarge compared with the changes in the mean values.indicates that in five out of nine cases, the initialInterestingly, these four conditions also have theperiod of combustion is shorter with nitromethaneshortest initial combustion periods (~15° CA), asadded than without. As discussed previously, addingshown in Fig. 9. For ‘0-4-Hot’ (rich limit), the hot plug20 per cent nitromethane to the fuel will lean thenot only helps to initiate the combustion, it alsomixture by 19 per cent. Hence the shorter initialspeeds up the combustion. In contrast, needle settingperiod of combustion could be caused by the leaner2 may be close to or leaner than the stoichiometricmixture and/or by the nitromethane. However, forcondition, such that the benefit of nitromethane ismethanol and other hydrocarbon fuels, the maxi-offset by the slower combustion of the weak mixture.mum increase in the laminar burning velocity for a

There is a similar picture for the main com-19 per cent increase in l is of the order of 15 perbustion period (Fig. 11). For those conditions thathave shorter initial combustion periods, the maincombustion period is also shorter. Therefore, nitro-methane can also increase the rate of main com-bustion. The cycle-by-cycle variations in the maincombustion period are larger than for the initialperiod. This is expected as other factors such asturbulence and flame development also affect themain combustion period.

Two three-dimensional graphs of cycle-averageddata from all the engine operating conditions areshown in Figs 12(a) and (b), giving information aboutthe effect of the nitromethane content, the fuelsupply needle setting, and the glow plug type oni.m.e.p. and Q

n,min(SOC). In each graph, the x axisFig. 10 The effect of nitromethane on Q

n,min–10 per cent

(projecting into the page) presents the nitromethaneMFB for various needle settings and plug types;change relative to 0 per cent nitromethane fuel concentration of the fuel. The y axis (horizontal)



the hot plug either has little effect or has enhancedthe combustion rate. The answer lies in the com-bustion phasing. The start of combustion (SOC orQ

n,min) in Fig. 12(b) shows that using cold and

medium plugs, the SOC is close to TDC (~170° CA)and occurs earlier when the nitromethane contentincreases; thus the ignition time is closer to the MBTtiming. However, when the hot plug is used, theSOC is generally early (~160° CA), before any nitro-methane is added. The SOC stays approximatelythe same once the nitromethane content increases.Hence there is no improvement in i.m.e.p. due tobetter ignition timing.

Fig. 11 The effect of nitromethane on 10–90 per centMFB for various needle settings and plug types;

5.3 Comparison with other engineschange relative to 0 per cent nitromethane fuel

Although the absolute air–fuel ratio is not known forthe tested engine, it is still possible to compare thepresents the needle setting and the z axis (vertical)

gives the output parameter from the analysis of com- combustion characteristics of this engine to a tradi-tional engine. Bouchard et al. [22] reported the fuelbustion data. Different symbols are used to represent

the three different glow plug types. burning rate of a CFR (cooperative fuel research)L-head engine for a wide range of air–fuel ratios (l).Results presented in Fig. 12(a) suggest that i.m.e.p.

generally increased with increased nitromethane The MFB was measured by observing the flame travelthrough an optical slot in the head. Instead of com-content. This is likely to be the result of faster and

more efficient combustion in the case of higher paring the burning rate at a given l, the rangesof MFB duration are compared in Table 4. It isnitromethane content. Figures 10 and 11 have shown

that faster combustion results. The combustion is surprising to see two engines with very differentoperating speeds, size, and combustion mechanisms,more efficient because all operating conditions are

rich relative to the stoichiometry, so burning at yet the ranges of initial and main combustionperiods are markably similar. Taking account of thecloser to the stoichiometric point would have fewer

incomplete combustion products and less unburnt higher engine speed but smaller combustion lengthsin the small engine relative to the CFR engine,fuel.

The results for the hot plug do not follow the trend the initial and main combustion periods (in realtime) are much shorter in the former engine. Thiswell. The i.m.e.p. does not change much as the nitro-

methane content increases, especially for needle suggests that the combustion is not only enhancedby engine speed (turbulence intensity) but possiblysettings 2 and 3. The burn rate analysis has shown

Fig. 12 Effect of nitromethane content, fuel needle setting, and glow plug type on (a) i.m.e.p.and (b) Q

n,min



Table 4 Comparison between the CFR L-head engine [22] and the engine under study

CFR engine Engine under study

Initial combustion period (°CA) 14–34 (0–10% MFB) 10–31 (Qn,min

–10% MFB)Main combustion period (°CA) 40–62 (10–95% MFB) 24–47 (10–90% MFB)Bore (mm) 111.1 21.6Stroke (mm) 114.3 20.3Swept volume (cm3) 1109 7.44Speed (r/min) 900 ~9000Fuel type Gasoline Methanol/nitromethanel range 0.7–1.4 Not measuredEngine details Single-cylinder, spark ignition, Single-cylinder, glow assisted compression ignition,

four-stroke, WOT two-stroke, WOT

by HCCI combustion with multiple flame kernels and the same fuel and mixture strength, the type of glowplug used could have a profound effect on the com-propagation.bustion process. An optical study of this glow ignitionThe heat release rate results (Fig. 4) are comparedengine would elucidate the actual combustion pro-with the results (Fig. 13) obtained from a sparkcess and how it is affected by different glow plugs.ignition (SI), single-cylinder engine [23]. This engine

In many ways this small glow assisted engine ishad optical access with pure methanol injected torather uncommon in the wider engine market, butthe inlet manifold. Kajitani et al. [23] noted that forits combustion characteristics are comparable tol=0.8 and 1, there were two modes of combustion.traditional research engines. Hence the methodologyFirst, premixed combustion gives a rapid heatdeveloped for analysing the combustion in thisrelease, which manifests itself as the peak in Fig. 13.engine should be applicable to a wide range ofAfter that the heat release is much slower andengines.endures for a long time. During that period, com-

bustion photos showed droplet diffusion flames. The5.4 Cycle-by-cycle resultstest ‘0-4-Cold’ exhibits a similar heat release profile

(Fig. 4 (top)), which possibly suggests that both pre- Cycle-by-cycle data have been analysed from themixed and diffusion combustion occurred. However, four extreme fuel/needle data sets (the four ‘corners’‘0-4-Hot’ in Fig. 4 (bottom) is more akin to the case of the x–y plane in Fig. 12 representing mixtureof l=1.2 in Fig. 13, where the slow diffusion phase strengths from the richest ‘0-4-Cold, Medium, Hot’of combustion is absent. It can be seen that given to the leanest ‘20-2-Cold, Medium, Hot’). Thus,

needle setting 4 with 0 and 20 per cent nitromethanecontent as well as needle setting 2 with 0 and 20 percent nitromethane content were investigated and theresults are shown in Fig. 14.

Figure 14 presents cycle-by-cycle data for thecalculated i.m.e.p. versus the SOC. The graphs showa characteristic of maximum i.m.e.p. against SOC,analogous to the MBT timing (maximum ignitionadvance for best torque) study commonly applied toSI engines. Each graph consists of three operatingconditions using different plugs. Particularly inthe ‘0-4-Cold, Medium, Hot’ graph, an operatingenvelope has been defined/sketched such that allthe outlier points represent poor engine operatingconditions. In this case, both cold and mediumplugs show a much wider scatter of points than thehot plug. This clearly means large cycle-by-cyclevariations in both indicated power and the start ofcombustion. Apart from several misfire cycles show-ing very low i.m.e.p., a noticeably high percentage ofpoints in these two cases lie in the right bottomFig. 13 Cycle-averaged Q

nof an SI engine running on

corner of the plot, confirming that partial and latepure methanol at a varied dimensionless air–fuel ratio [23] combustion leads to low power.



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Generally speaking, 20 per cent nitromethane the (n+1)th cycle. The upper graph of Fig. 15, whichis for the case ‘0-4-Cold’ with a very large CoV(the lower pair of Fig. 14) gives a higher i.m.e.p., less

variation, and an earlier start of combustion than of i.m.e.p., clearly shows the trend that a partiallyburning (nth) cycle (or even misfire cycles, when0 per cent nitromethane. Furthermore, the hot plug

presents a relatively more stable range of the SOC CoC=0) is always followed by a cycle (1+n)thwith a higher than average i.m.e.p. (3.05 bar). Thisthan the other two plugs. Therefore, to optimize the

operating regime of this engine at WOT, 20 per cent is likely to be because for a partially burningcycle (probably slow burning as well) the trappednitromethane and a hot plug would be selected as

the best compromise. in-cylinder residuals tend to add to the air–fuel ratioand the initial temperature of the following cycle,Inspection of original pressure versus crank angle

traces raises questions about how, in this type of which is then likely to perform a more completecombustion (i.e. better scavenging). The lower graphcombustion system, one cycle might affect the sub-

sequent cycle; i.e. if the nth cycle has late or incom- of Fig. 15 gives a dense cluster of points randomlypositioned on the right top corner of the plot.plete combustion, does this affect the (n+1)th cycle?

In order to address this question, comparisons in This means that the combustion of these cycles isrelatively stable and complete, as expected due toterms of completeness of combustion (CoC) were

made between two cases: ‘0-4-Cold’ and ‘0-4-Hot’, the low cycle-by-cycle variation.Figure 16 presents plots of cycle-by-cycle crankwith coefficients of variation (CoVs) of i.m.e.p. of 31.3

and 4.5 per cent respectively. angle of Qn,min

and crank angle of 90 per cent MFBfrom the above two sets of data (excluding misfireFigure 15 gives the cycle-by-cycle correlation

between the CoC and the i.m.e.p., with the x axis cycles in ‘0-4-Cold’). With Qn,min

and 90 per cent MFBrepresenting the SOC and end of main combustion,presenting the completeness of combustion of the

nth cycle and the y axis presenting the i.m.e.p. of the top graph agrees with the perception that cycles

Fig. 15 Correlation between the completeness of combustion of the nth cycle and the i.m.e.p.of the (n+1)th cycle: ‘0-4-Cold’ (top) and ‘0-4-Hot’ (bottom)



Fig. 16 Scatter plot of cycle-by-cycle data: Qn,min

versus 90 per cent MFB of the same cycle (top)and Q

n,minof the (n+1)th cycle versus 90 per cent MFB of the nth cycle (bottom)

with early ignition tend to finish combustion quickly effect on the calculated results on the phasing ofheat release.as well. The bottom graph shows data for a crank

3. Nitromethane improves both the initial and mainangle of 90 per cent MFB for cycle n against thecombustion rate of pure methanol, beyond thecrank angle of Q

n,minfor cycle (n+1) and implies

uncertainty created by the unknown mixturethat a slow burning cycle (the right bottom wing ofstrength and cycle-by-cycle variations.‘0-4-Cold’, marked with an ellipse) is more likely to

4. The i.m.e.p. generally increases with increasedtrigger a following cycle with early ignition. Datanitromethane content, only if the start of com-from ‘0-4-Hot’ in the bottom graph support thebustion occurs earlier so that the combustionobservation in the previous paragraph that stablephasing is approaching the optimum.combustion features a random and tight distribution

5. Comparison with other engines shows that theof data points.combustion period of the engine under study isremarkably similar to traditional research engines,even though the engine design and operating6 CONCLUSIONSregimes are quite different.

6. The combustion analysis has demonstrated thatConclusions can be drawn as follows from the workthe glow plug has a marked effect in a very richcarried out on this two-stroke glow ignition engine.mixture (‘0-4’). An optical study is recommended

1. The burn rate and heat release analyses have pro- to investigate this effect on combustion.duced useful results from the measured pressure 7. Two sets of data from needle setting 4 and 0 perdata. cent nitromethane show the greatest cycle-by-cycle

2. The ‘nominal’ spark timing and ratio of specific variation and extremely slow combustion. In thesecases, a partially burning cycle is always followedheats specified in the program have no significant



ignition engine combustion. Parts 1 and 2. Proc.by a cycle with a higher than average i.m.e.p.,Instn Mech. Engrs, Part D: J. Automotive Engineering,probably because for such a cycle the trapped in-1998, 212(D5), 381–399 and 212(D6), 507–524.cylinder residuals and initial temperature enhance

13 Washburn, E. W. (Ed.) International criticalcombustion in the following cycle tables of numerical data, physics, chemistry and

technology, 1st Electronic edition, 2003 (Knovel);These results may provide an insight into the per-www.knovel.com.formance and combustion process of such engines

14 Kroschwitz, J. I. (Ed.) Kirk–Othmer encyclopediaand facilitate more efficient applications.of chemical technology, 4th edition, 1993 (Wiley–Interscience, New York).

15 Rassweiler, G. M. and Withrow, L. Motion picturesACKNOWLEDGEMENTS of engine flames correlated with pressure cards. SAE

Paper 800131, 1980.16 Stone, R. Introduction to internal combustionThe authors wish to thank the following for useful

engines, 3rd edition, 1999 (Macmillan, New York).discussions and suggestions in furthering this work:17 Sztenderowicz, M. L. and Heywood, J. B. Mixture

Professor H. C. Watson, University of Melbourne nonuniformity effects on S.I. engine combustionProfessor N. Collings, University of Cambridge variability. SAE Paper 902142, 1990.

18 Sztenderowicz, M. L. and Heywood, J. B. Cycle-to-Professor D. B. Kittelson, University of Minnesotacycle IMEP fluctuations in a stoichiometrically-

RRR is grateful to the University of Auckland for fueled SI engine at low speed and load. SAE Paperallowing him sabbatical leave, during which this 902143, 1990.

19 Cox, D. R. and Oakes, D. Analysis of survival data,work was progressed.1984 (Chapman and Hall, London).

20 Davis, C. S. and Stephens, M. A. Approximatepercentage points using Pearson curves. In AppliedREFERENCESstatistics, 1983 (Royal Statistical Society).

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glow plug ignition of a methanol-fueled IDI dieselengine. SAE Paper 930935, 1993.

4 Agama, J. R., Abata, D. L., and Mullins, M. E.APPENDIXCatalytic ignition of methanol in a diesel engine with

a platinum-coated glow plug. SAE Paper 911737,1991. Notation

5 Mueller, C. J. and Musculus, M. P. Glow plug assistedABDC after bottom dead centreignition and combustion of methanol in an opticalc specific heatDI diesel engine. SAE Paper 2001-01-2004, 2001.

6 Starkman, E. S. Nitromethane as a piston engine CA crank anglefuel. SAE Paper 540186, 1954. CDF cumulative distribution function

7 Starkman, E. S. Nitroparaffins as potential engine CoBRA combustion burn rate analysis programfuel. Ind. Engng Chemistry, December 1959, 51(12). CoC completeness of combustion8 Bush, K. C., Germane, G. J., and Hess, G. L.

CoV coefficient of variationImproved utilization of nitromethane as an internalC

Ddischarge coefficientcombustion engine fuel. SAE Paper 852130, 1985.

DC direct current9 Ferguson, C. R. and Kirkpatrick, A. T. Internalcombustion engines – applied thermosciences, 2nd EPO exhaust port openingedition, 2000 (John Wiley, New York). HCCI homogeneous charge compression

10 Gierke, C. D. Two-stroke glow engines for R/C air- ignitioncraft, 1994, Vol. 1 (Air Age Inc., Wilton, Connecticut). i.m.e.p. indicated mean effective pressure

11 O.S. Engines (2003). [Online]. Available:IPC inlet port closurewww.osengines.com/accys/glowplugs.htmlk crank angle step between successive[2004, June 7].

samples12 Ball, J., Raine, R., and Stone, R. Combustionanalysis and cycle-by-cycle variations in spark LHV low heating value



m mass flowrate h crank anglel relative air–fuel ratioMFB mass fraction burnedr densityn polytropic indexx completeness of combustionN number of intervals in finite differenceY total normalized pressure change due tocalculations

combustionp pressurePSD power spectrum density

SubscriptsQ heat transferSI spark ignition c due to combustionSOC start of combustion f fuelTDC top dead centre hr heat releaseR gas constant m, max maximumT temperature min minimumU internal energy n netV volume O orificeW work p due to pressure

s stoichiometricc ratio of specific heats v due to volume

* normalized valueDp pressure difference


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