Effect of Residual Gas Fraction on the CombustionCharacteristics of Butane-Air Mixtures in the

Constant-Volume Chamber

Myung Yoon Kim, Dae Sik Kim, and Chang Sik Lee*

Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong,Sungdong-ku, Seoul 133-791, Korea

Received March 28, 2002

An experimental study was made to investigate the effect of residual gas on the combustioncharacteristics and flame propagation of butane-air mixtures in a constant-volume combustionchamber. The combustion process and flame propagation are studied under different ratios ofresidual gas and various equivalence ratios in the combustion chamber. The effects of the residualgas ratio on the combustion pressure, heat release rate, burned fraction, and flame propagationphenomena were studied in detail. The experimental apparatus consists of a constant-volumecombustion chamber, a premixture chamber, a data acquisition system, and a laser Schlierensystem with a high-speed camera. With an increase of the residual gas ratio in the combustionchamber, the combustion pressure and the rate of heat release decrease and the burning periodof the fuel-air mixture is reduced by the increase of residual gas. The effects of residual gas onthe combustion characteristics and flame propagation speed are dependent on the amount ofresidual gas. In the case of a higher residual gas ratio (more than 10%) in the chamber, thecombustion pressure and heat release are steeply decreased. It is confirmed that residual gas inthe combustion chamber lowered the rate of heat release as a result of the decrease of combustiontemperature. The flame propagation speed decreases with the increase of residual gas in thecombustion chamber.

1. Introduction

The nitrogen oxide (NOx) concentration of the exhaustgas from the automotive engine is primarily a functionof combustion temperature. So, the most effective wayof reducing NOx emission is to keep the combustiontemperature down. Reduction of NOx formation bydiluting the incoming air-fuel mixture with a smallamount of inert gas is the simplest practical method.Air is available for a diluent gas, but it is not a non-reacting mixture like the exhaust gas.

Tabata et al. investigated the effect of EGR (exhaustgas recirculation) under stoichiometric and lean mixtureconditions and compared it with the effect of lean opera-tion through the exhaust gas recirculation.1 Arcoumaniset al. analyzed the effect of various levels of EGR onthe combustion characteristics in the four-cylinderdirect-injection optical diesel engine.2 Their study re-vealed that the increase of EGR rate showed highercyclic pressure variations during the warm-up periodand reduced flame core temperatures. In the sparkignition engine, the most practical approach for the re-duction of exhaust emission and improvement of enginestability is to control the combustion period by enhancedmixture flow in the cylinder. It is well-known that EGR

is effective in reducing NOx emissions,3 but the problemwith exhaust gas recycling is an increase of the par-ticulate matter. Shiozaki et al. measured the flametemperature under EGR conditions with a two-colorimaging CCD camera.4 Also, Mitchell et al. measuredthe relationship between exhaust gas recirculation andintake air dilution on combustion through the opticaccess in a diesel engine.5 They indicated that flametemperature had a major influence on nitrogen oxideand that carbon monoxide emissions were influencedmainly by the O2 fraction in the intake air. Fundamen-tal studies on exhaust gas recirculation have beencarried out by many researchers, both theoretically andexperimentally.6-8

Most of the previous researchers conducted enginetests to investigate the overall effect of exhaust gasrecycling on emission control. But, these studies ofengine combustion in the case of EGR have manyuncertainties as a result of the difficulties of conductingexperiments in the actual engine. From this point of

* Corresponding Author. Phone: +82-2-2290-0427. Fax: +82-2-2281-5286. E-mail: cslee@ hanyang.ac.kr.

(1) Tabata, M.; Yamamoto, T.; Fukube, T. Soc. Automot. Eng. 1995;No. 950684.

(2) Arcoumanis, C.; Bae, C.; Nagwaney, A.; Whitelaw, J. H. Soc.Automot. Eng. 1995; No. 950850.

(3) Baert, R. S. G.; Beckman, D. E.; Verbeek, R. P. Soc. Automot.Eng. 1996; No. 960848.

(4) Shiozaki, T.; Nakajima, H.; Kudo, Y.; Miyashita, A.; Aoyagi, Y.Soc. Automot. Eng. 1996; No. 960323.

(5) Mitchell, D.; Pinson, J. A.; Lizinger, T. A. Soc. Automot. Eng.1996; No. 932798.

(6) Durnholz, M.; Eifler, G.; Endres, H. Soc. Automot. Eng. 1996;No. 920725.

(7) Mouqallid, M.; Lecodier, B.; Trinite, M. Soc. Automot. Eng. 1994;No. 941990.

(8) Ropke, S.; Schweimer, G. W.; Strauss, T. S. Soc. Automot. Eng.1995; No. 950213.

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10.1021/ef0200774 CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 04/29/2003

view, the effect of residual gas on combustion in a con-stant-volume combustion chamber is important to un-derstand as the influencing factor for the reduction ofcombustion temperature and formation of emissions.

The purpose of this study is to investigate the influ-ence of residual gas ratio on the combustion and flamepropagation characteristics in the constant-volume com-bustion chamber. The information obtained from theexperimental results provides detailed combustion char-acteristics such as combustion pressure, the rate of heatrelease, mass fraction burned, and the visualization ofthe flame propagation under the various equivalenceratios and residual gas ratios.

2. Experimental Section

2.1. Experimental Apparatus. In a spark ignition engine,the combustion happens ideally near the top dead center atthe end of the compression stroke, and this period of the strokecycle is regarded as constant-volume combustion. In this study,the constant-volume combustion chamber was made to inves-tigate the influence of the residual gas ratio on combustionphenomena. Figure 1 shows the schematic diagram of experi-mental apparatus. The test rig consists of the constant-volumecombustion chamber, the fuel supply system, the ignitionsystem, and the data acquisition system. A signal generatorwas used for synchronization of the ignition system, pressuredata acquisition system, and high-speed camera.

Butane and air were premixed in a premixture chamberwhere their equivalence ratio was determined on the basis oftheir partial pressures measured by a diaphragm pressuretransducer. The total volume of the premixture chamber wasmeasured to be 3290 cm3, and the fan at the bottom of thepremixture chamber was operated by electronic control. Aheater of 1 kW was installed to avoid fuel liquefaction underhigh pressure in the chamber.

The constant-volume combustion chamber consists of acylindrical chamber, an intake and exhaust valve, and an igni-tion system, as shown in Figure 2. The combustion chamberis a cylindrical shape with a diameter of 100 mm and a depthof 40 mm. It has extensive optical windowssa pair of 130 mmdiameter and 30 mm thickness quartz windows mounted onboth sides of the combustion chamber, which has a volume of341 cm3. Two plate electrical heaters (300 W) were installedon the outside walls of the combustion chamber and coupledto a K-type thermocouple. This system allows the temperatureof the chamber to be kept constant at 363 K in order to preventwater condensation on the windows after combustion.

Ignition was achieved by a transistorized coil ignitionsystem (TCI) with electronic ignition using a 12 V powersupply. To ignite the mixture in the center of the combustionchamber, the spark plug was elongated to the center of thecombustion chamber.

2.2. Experimental Procedures. The instantaneous pres-sure variation in the combustion chamber was monitored bya piezoelectric pressure transducer (Kistler, 6061B) connectedto a charge amplifier. The output of the amplifier was inputto the data acquisition system (Keithley, DAS-58), whichdigitized and stored the voltage signal with a sampling rateof 1 kHz.

Figure 3 shows a schematic diagram of the optical systemfor combustion visualization. The system of combustion visu-alization was achieved by a high-speed Schlieren system,which consists of a light source, concave mirror (300 mm), anda high-speed camera (maximum 3000 fps, Phantom). The high-speed Schlieren system and optical system were used tovisualize the flame propagation in the combustion chamber.A He-Ne laser was used as a light source, with maximumoutput of 10 mW and light wavelength of 632.8 nm. Flamepropagation speed was calculated from the Schlieren images(512 × 512 pixel, 1000 fps).

The experiments were performed for 5 kinds of residual gasratio that included the range of 0.8 to 1.2 of equivalence ratioand initial mixture pressure from 1 to 5 bar. The fuel and airwere mixed in a premixing chamber, where their equivalenceratio was determined on the basis of the partial pressures ofcomponent gases such as air and fuel as measured by apressure transducer.

A constant-volume combustion chamber is filled with abutane-air mixture at each equivalence ratio, and the mixtureis allowed enough time to decay completely in the chamber tomake a quiescent flow condition. When the burned gastemperature decreased to the same temperature as that ofthe combustion chamber, the burned gas was discharged andthen as much as the amount of burned gas that correspondsto the residual gas ratio. At the same initial pressure con-dition, the fixed amount of fresh mixture is supplied to thecombustion chamber. The fuel used in this experiment isbutane.

Figure 1. Schematic diagram of experimental apparatus.

Figure 2. Constant-volume combustion chamber.

Figure 3. Schematic diagram of the optical system.

756 Energy & Fuels, Vol. 17, No. 3, 2003 Kim et al.

2.3. Residual Gas Ratio and Heat Release. In thisexperiment, the residual gas ratio rRG in the combustionchamber is defined as

where mRG and mmix are the mass of residual gas and mass ofbutane-air mixture (without residual gas), respectively. Thebutane-air mixture with residual gas composition was ob-tained as follows. A constant-volume combustion chamber isfilled with a butane-air mixture at each equivalence ratio,and it is ignited by an electric spark plug. After the burnedgas temperature was decreased to that of the test condition,the burned gas was discharged with the state of test residualgas ratio. And the burned gas and fresh fuel-air were mixedin a combustion chamber where the residual gas ratio wasdetermined on the basis of their partial pressure.

Also, initial pressure in the combustion chamber increaseswith the increase of residual gas fraction as follows:

where Pi is the initial pressure in the combustion chamber(with residual gas), Pmix is the pressure of the fuel-air mixturegas, and PRG is the pressure of the residual gas.

The experiments were carried out for each condition, whichincluded five residual gas ratios (0, 5, 10, 15, and 20%)at various equivalence ratios from 0.8 to 1.2 and variousfuel-air mixture pressures. Also, the residual gas ratiowas controlled from 0 to 20% with 5% intervals at eachexperimental condition. The combustion characteristics wereobtained from the pressure data in the combustion cham-ber, and the flame propagation characteristics were ana-lyzed by the optical system and high-speed digital camerasystem.

The mass fraction burned Mb(τ), was calculated from themeasured pressure trace on the basis of the assumption thatcombustion pressure corresponds to the mass fraction, and itcan be expressed as follows:

where Mb(τ) is the mass fraction burned at time τ, P(τ) is theinstant pressure in the combustion chamber, Pi is the initialpressure in the combustion chamber before combustion, andPmax is the maximum combustion pressure in the combustionchamber.

In this paper, the combustion duration was defined aselapsed time required to reach the maximum pressure fromspark ignition in a constant-volume combustion chamber.

Also, in order to determine the heat release rate, if we ignorethe heat transfer to the combustion chamber wall, the firstlaw equation can be written as follows:

where dQ is the gross heat energy released as a result ofcombustion, m is the mass of the mixture, cv is the specificheat at constant volume, dT is the gas temperature change inthe combustion chamber, P is the pressure in the combustionchamber, and dV is the change in the cylinder volume.

Applying the ideal gas equation to the gas mixture in thecombustion chamber and differentiating about time

Figure 4. Time history of combustion pressure at differentequivalent ratios.

rRG )mRG

mmix + mRG× 100 (%) (1)

Pi ) Pmix + PRG (2)

Mb(τ) )P(τ) - Pi

Pmax - Pi(3)

Figure 5. Effect of residual gas on the combustion pressureat stoichiometric equivalence ratios.

Figure 6. Effect of residual gas on the combustion pressureat Φ ) 0.8 and 1.2.

dQdτ

) mcvdTdτ

+ PdVdτ

(4)

PV ) mRT (5)

PdVdτ

+ VdPdτ

) mRdTdτ

(6)

Combustion Characteristics of Butane-Air Mixtures Energy & Fuels, Vol. 17, No. 3, 2003 757

Substitution of eq 6 into eq 4 and rearrangement of theterms give the usual form of the first law of heat releaseequation:

where κ is the ratio of specific heats.

3. Experimental Results and Discussion

3.1. Effect of Residual Gas on Combustion Char-acteristics. Figure 4 shows the effect of the equivalenceratio on the combustion pressure history in the combus-tion chamber at an initial mixture pressure of 3 bar(without residual gas). The maximum pressure appearsin the equivalence ratio Φ ) 1.2. Figure 5 shows theeffect of residual gas on the combustion pressure in thechamber at the mixture pressure of 3 bar and equiva-lence ratio of 1.0. As shown in Figure 5, the peak valueof combustion pressure decreases with an increase ofthe residual gas fraction in the chamber because of theinert effect of the residual gas. Also, the length of timerequired to reach the maximum value of the combustionpressure is retarded in accordance with the increase ofresidual gas ratio. The effects of residual gas on thecombustion pressure in the chamber at different equiva-lence ratios are illustrated in Figure 6. This figure

shows that the combustion pressure decreases with anincrease of residual gas ratio. This is a result of theincrease of the inert gas fraction that the residual massleft over from the previous combustion in the chamber.As a result of the residual fraction, the combustion dura-tion is increased with the increase of residual gas ratioand equivalence ratio. As indicated in Figure 6, thereare very small variations of maximum pressure with theincrease of the residual gas ratio at low equivalenceratio of Φ ) 0.8. In the range of lean mixture, it cannotbe fired at over 15% of the residual gas ratio. Also, thetiming at which the maximum pressure appears isretarded in proportion to the increase of the residualgas fraction in the mass of mixture.

The effects of initial pressure on the total combustionduration under various residual gas conditions areillustrated in Figure 7. The total combustion period wasproportional to the increase of initial pressure.

Figure 7. Effect of initial pressure on combustion duration.

Figure 8. Effect of residual gas ratio on the maximumcombustion pressure at Pmix ) 3 bar.

dQdτ

) 1κ - 1

VdPdτ

+ κ

κ - 1PdV

dτ(7)

Figure 9. Effect of residual gas ratio and equivalent ratio onthe combustion duration (Pmix ) 3 bar).

Figure 10. Effect of residual gas on the mass fraction burned.

758 Energy & Fuels, Vol. 17, No. 3, 2003 Kim et al.

Figure 8 shows the effect of equivalence ratio on themaximum pressure with different residual gas ratios.The residual gas ratios are slightly affected by the maxi-mum pressure at the range within 10% of the residualfraction. However, in the case of higher residual gasratios, the maximum combustion pressures are steeplydecreased.

Figure 9 shows the influence of the residual gas ratioon the combustion duration for different equivalentratios. As the residual gas fraction in the chamber isincreased, the combustion duration is longer than thatof the lower ratio of residual gas. As illustrated in thefigures, the difference of the combustion durationsbetween 0% and 20% of residual gas ratio is verylarge since a high residual gas ratio brings about thedecrease of combustion temperature and an increase ofheat loss.

3.2. Mass Fraction Burned and Heat-ReleaseRate. Thermodynamic analysis of measured cylinderpressure data is a very powerful tool used for quantify-ing combustion parameters.9,10 There are two mainapproaches, which are often referred to as “burn-rateanalysis” and “heat-release analysis”. Burn-rate analy-sis is used mainly to obtain the mass fraction burned,which is a normalized quantity with a scale of 0 to 1.Heat-release analysis is used to produce absolute en-ergy. The rate of heat release is a very importantparameter because this has a very significant influenceon pressure-rise rate and NOx emissions.

Figure 10 shows the mass fraction burned at theresidual gas ratios rRG ) 0% and rRG ) 10%. With an

(9) Kodah, Z. H.; Soliman, H. S.; Abu Qudais, M.; Jahmany, Z. A.Appl. Energy 2000, 66, 237-250.

(10) Krieger, R. B.; Borman, G. L. ASME 1966; 66-WA/DGP-4.

Figure 11. Effect of residual gas ratio on the rate of heat release at different equivalence ratios.

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increase of residual gas ratio, the total combustionduration increases, as shown in the comparison of massfraction burned. Moreover, the combustion durationswere considerably affected by residual gas in the richequivalence ratios.

Figure 11 shows the effect of residual gas ratio on theheat-release rate. As indicated in the figures, the peakvalue of the heat-release rate was rapidly decreasedand its timing was retarded at high residual gasratios. From these results of pressure and heat-releaserate, it can be inferred that the increase in residualfraction in the mass of the mixture plays an importantrole in the decrease of maximum temperature in thechamber.

3.3. Effect of Residual Gas on Flame Propaga-tion. The speed of flame propagation is dependent onthe residual gas fraction and equivalence ratio. Theeffects of residual gas fraction on the flame propagationare shown in Figure 12. The flame behaviors of amixture are obtained from a continuous recording of asingle spark event. In the case of rRG ) 20%, flame coreshape is an ellipsoidal trace because of interactionbetween spark energy and heat transfer with thebuoyancy. With an increase in the residual gas ratio in

the chamber, the flame propagation speed decreases, asshown in the pictures.

Figure 13 shows the measured flame speed obtainedby a Schlieren picture at three equivalence ratios andvariable residual gas ratio. The flame speed was calcu-lated from digitized video images consisting of a 512 ×512 pixel array. Also, image processing software wasused to calculate the diameters of the flame. Under theeffect of buoyancy, the center of the flame was raisedand the flame shape was distorted with a high residualgas ratio. Therefore, the diameters of the flame werecalculated from the left and right end of the flamesurface and flame speed was calculated from the diam-eters. As illustrated in the flame pictures, flame speedis fastest when the flame passes through the middleregion of the combustion chamber. In the case of higherresidual gas ratios, the flame speed is very low com-pared to the same condition without residual gas. Thespeed of early flame propagation is lower than theintermediate stage because of higher heat loss at theearly stage of combustion. Also, influenced by the rise

Figure 12. Schlieren pictures of flame propagation in variousresidual gas ratios at Pmix ) 3 bar.

Figure 13. Effect of residual gas ratio on the flame propaga-tion speed.

760 Energy & Fuels, Vol. 17, No. 3, 2003 Kim et al.

of unburned gas pressure, flame propagation speed isslightly lower in the final stage than in the intermediatestage.

4. Conclusions

An experimental study was carried out to investigatethe influence of the residual gas on the combustion char-acteristics and flame propagation in a constant-volumechamber. The effect of residual gas on the combustioncharacteristics and flame propagation speed are ana-lyzed by using the constant-volume chamber with anoptical arrangement and a high-speed Schlieren system.

The main results of this work are summarized asfollows:

(1) The combustion pressure and heat release rate ofthe butane-air mixture were decreased in accordancewith an increase of residual gas ratio. It is confirmedthat the residual gas effect shows a lowered rate of heatrelease as a result of the decrease of combustiontemperature.

(2) From the result of mass fraction burned, theincrease of residual gas fraction in the constant-volume

chamber influences the burning period. In the case of ahigher residual gas ratio in the chamber, the combustioncharacteristics such as combustion pressure and heatrelease are steeply decreased.

(3) The residual gas ratios are slightly affected at themaximum pressure at the range within 10% of the resid-ual gas fraction. In the case of a higher residual gasratio, the maximum pressure of gas in the combustionchamber is steeply decreased, compared to the case ofa lower residual gas ratio.

(4) The flame propagation speed is dependent on theresidual gas ratio and equivalence ratio. With an in-crease of the residual gas portion in the chamber, theflame propagation speed decreases.

Acknowledgment. This work is supported by thefund of National Center for Cleaner Production of KoreaInstitute of Industrial Technology (Project No.: 99-1-K-34).

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