Indian Journal of Engineering & Materials Sciences Vol. 8, February 200 I, pp. 1-7

Effects of in-cylinder catalytic coating on the performance of a two-stroke spark ignition engine

N Nedunchezhian*" & S Dhandapanib

aDepartment of Automobile Engineering, Institute of Road & Transport Technology, Erode 638 316, India

bDepartment of Mechanical Engineering, Coimbatore Institute of Technology, Coimbatore-644 014, India

Received 28 October 1999; accepted 21 November 2000

Two-stroke spark ignited engines offer superior power to weight ratio, which is best suited for two-wheeler applications. But, low thermal efficiency and higher emission levels are the main drawbacks of these engines. In-cylinder catalytic coating was applied in a two-stroke single cylinder spark ignited engine for improving thermal efficiency and reducing emission levels. The selected catalyst (copper), which is an oxidation catalyst, was coated over the inside surface of cylinder head and piston crown by plasma coating method. Detailed studies were carried out to evaluate the performance of catalyst under varied operating conditions. The copper coated engine showed improved fuel economy at all operating conditions. The brake thermal efficiency was improved by 10% and the unburned hydrocarbon emission was reduced by 25% at full load condition of the engine. The plasma coating of catalyst showed no deterioration after 50 h of operation and the catalytic action was not affected. Detailed combustion analysis was carried out to find out the effect of catalyst action on combustion parameters. It was found that the rate of burning, P max, and heat release rate were increased. Combustion duration was reduced by 8° of crank angle at full load condition.

Among the internal combustion engines, the two­stroke spark ignited engine has the highest power to weight ratio. Thus, for a given power output, these engines have lowest weight. Due to this reason, the two-stroke SI engines are best suited for two wheeler applications where weight reduction is important. But, these engines have the lowest thermal efficiency and highest emission levels. This is due to the rich mixture operation, poor scavenging efficiency and short-circuiting of fresh charge. The two-stroke engines are operated with rich air-fuel mixture due to their poor combustion performance.

Various methods have been employed to improve the engine performance and reduce the pollutants( ·7. Lean mixture operation is one such promising method where the engine is operated under lean air-fuel ratio. Methods like increasing the air movement inside the combustion chamber and employing direct fuel injection increases the lean limit to some extent. Quader3,4 carried out detailed investigations to investigate the possibility of improving engine performance by extending the lean misfire limit in spark ignited engines. Michael May5.6 employed a fire ball combustion chamber having increased swirl rate for reducing emissions and improving combustion. The chamber had the exhaust valve recessed in a small bathtub shaped bum space and a shallow guide

*For correspondence

from the inlet valve. This configuration allows high compression ratio and burning of lean mixture. Morsee et al.7 modified the engine for lean operation by using a turbulent flow inlet manifold, a carburetor adjusted for lean air fuel mixture, special calibration of ignition timing and exhaust gas recirculation rate.

However, all these methods are quite expensive because they require modifications in the base engine and complicated auxiliary mechanisms and mixture preparation devices involving high cost. Thus, their application to smaller two-stroke engines has been problematic and the advantage of low-cost of these engines is lost. Hence, a low-cost method requiring mInImum modification of the base engine for improved fuel economy and reduced emission is attempted here. The concept of in-cylinder catalytic coating for improved bum rate and flame propagation of the charge has been investigated. Also, detailed experimental studies have been carried out for evaluating the catalyst performance and its effect on the combustion parameters.

The use of catalytic surface to enhance chemical reaction is a well-established and common practice. Catalysts are extensively used for exhaust treatment of automotive engines. However, their use in combustion devices for improving combustion rate is somewhat less common and limited to aircraft combustor applications8

•9

•

2 INDIAN J. ENG. MATER. SCI., FEBRUARY 2001

Thring lO employed platinum in the form of wire mesh inside a pre-combustion chamber for improving fuel economy and reducing emission levels. Both NOx

and HC emissions were reduced and also the catalyst improved multi fuel capability of the engine. Rychter et aL. II used platinum and copper wire mesh in a sub divided combustion chamber of a single compression machine and studied the combustion process of lean fuel mixture. They concluded that the chemical activation of a charge prior to ignition due to the presence of catalyst in the pre-chamber facilitates ignition, reduces ignition delay and strengthens the flame kernel in its initial combustion phase. The presence of catalyst resulted in the extension of misfire limit and significant reduction in cycle-to­cycle variation of pressure development.

Karim and Kibrya l2 compared the catalytic activation of eight different metals and found out that platinum and copper show better performance. They employed the catalysts in the form of wire mesh and recommended that further improvement could be achieved if the catalyst was coated inside the combustion chamber.

Zhengyun Hu and Ladommatosl 3 studied the platinum catalyst that was fitted on the piston crown. The results show that the catalyst is able to reduce unburned hydrocarbon emissions to great extent by inducing oxidation of the charge trapped in crevices.

In.the present study, copper was selected as catalyst and It was coated inside of cylinder head and piston crown. Detailed experimental study was carried out to evaluate the catalyst performance and its influence on combustion parameters.

Catalytic Coating The piston crown and inside of the cylinder head

were coated with powdered copper by plasma spraying. Fig. 1 shows the details of the coated piston crown and the cylinder head. A bond coating of NiCoCr alloy was applied for a thickness of about 100 microns using a 80 kW METCO plasma spray gun. Over the bond coating, copper 89.5%, aluminum 9.5% and iron l.0% was coated for 300 microns thickness. The coating had very high bond strength and did not show any sign of wear off after 50 h of operation.

Experimental System The experiments were carried out on a single­

cyli~der air-cooled production two-stroke SI engine havIng the following specifications:

Engine Make & Model

Cylinder Bore x Stroke Displacement Power Connecting Rod Length Compression Ratio Fuel Delivery

Bajaj 150, Single Cylinder, Aircooled, 2-Stroke

57.5 mm x58 mm 150 cc 4.5 kW @ 5500 rpm 105mm 7:1 Down Draft Carburetor

The engine was loaded by an eddy current dynamometer. Fuel flow measurements were carried out using an automatic flow control device and digital stopwatch. Airflow was measured with orifice flow meter. Engine and exhaust temperatures were measured by chrome-alumal thermocouple. Exhaust emissions were measured by HORIBA infra-red gas analyzer.

The cylinder pressure was measured using a Kistler piezoelectric transducer flush mounted in the cylinder head of the engine. The output of the transducer was fed to a charge amplifier possessing a high degree of noise rejection with ground level current attenuation. Cylinder pressure data were recorded using a high speed A VL data acquisition system timed by an optical encoder mounted on the engine crankshaft.

PRESSUUE HtANSDUCER

. CYL.INDER BEAD COATING

COATING

SPARK PLUG

PISTON RINGS

PISTON

GUDGEON PIN

Fig. I - Section of cylinder head and piston showing the pressure transducer. spark plug and catalytic coating

NEDUNCHEZHIAN & DHANDAPANI: TWO STROKE SI ENGINE 3

Sampling intervals of both 0.1 0, 0.5° and 1.0° were examined and the interval of 1.0° was found to be sufficient for the study, which is in agreement with Evans2

• Cylinder pressure traces of 2000 cycles were recorded for each operating condition and it was stored in a hard disc of a personal computer for further analysis. Combustion parameters like peak pressure (P rna .. ), crank angle occurrence of Pmox,

maximum rate of pressure rise and crank angle duration for 5-10%, 5-50% and 5-90% heat release were calculated for the average of 2000 cycles.

Results and Discussion

Engine performance Fig. 2 shows the brake thermal efficiency plotted

against the power output for both engines. It clearly shows that the catalytic coated engine had more thermal efficiency than the base engine at higher range of power output. At lower load range, the improvement was not higher. This may be due to reduced catalytic action at lower temperatures. There was 10% improvement in the thermal efficiency at 2 kW power output. The improvement in thermal efficiency may be due to better and faster combustion of air-fuel charge. ThringlO experimented with platinum and copper wire catalysts in a pre­combustion chamber and concluded that the catalysts accelerate preflame reaction leading to faster and complete combustion. This is reflected further in the Figs 3-4 which depict the carbon monoxide (CO) and unburned hydrocarbon (UBHC) emissions from the engines. The UBHC emissions were reduced by 25%.

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Brake Power, kW

Fig. 2 - Variation of brake thermal efficiency with engine power

Un burnt hydrocarbons from two-stroke engines are due to the rich mixture operation, flame quenching at chamber walls and crevices volumes and absorption and desorption of fuel vapour in to the oil layers on the cylinder walls l 4

.

The normal surface temperature of combustion chamber wall (250-300°C) is high enough to activate the catalysts. This leads to the oxidation of fuel mixture adjacent to walls. But, at low load conditions, the surface temperature is less and hence the reduction of HC was lower. This is shown in Fig. 4 where the HC emission reduction was more at higher

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0 .0 0.5 1.0 1.5 2.0 2.5

Brake Power, kW

Fig. 3 - Variation of carbon monoxide emissions with engine power

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1500

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Brake Power, kW

Fig. 4 - Variation of UBHC emissions with engine power

4 INDIAN 1. ENG. MATER. SCI.. FEBRUARY 2001

loads and lower at low load conditions. Also, s lightly higher levels of HC emissions were encountered at low load conditions. The reason may be that at low loads, the combustion chamber wall temperature will be lower and the coated surface lacks in catalytic activ ity. This may result in increased wall quenching effect leading to higher HC emissions.

Combustion analysis

The engine cylinder pressures were measured for 2000 continuous cycles. The combustion parameters were calculated for the average of 2000 pressure traces and the results were plotted .. The engine speed was kept at 3000 rpm and the load was varied . The spark time was set at 30° bTOC.

Figs 5 and 6 show the scatter plot of peak pressures measured for 2000 cycles for base and catalytic coated engi ne. The scattering of PI/UU around the mean value indicates the ex istence of cyclic variation. The mean PIII{Lr value of the base engi ne was 14.6 bar and for the catalytic engine was 15.4 bar. Similar results

were obtained for various load conditions and the mean values were calcul ated and plotted with engine power. Fig. 7 shows the variation of mean values of

Bocine: Base Bn,ine Speed: 3000. Power: 2 .3 kW Mean Pmax: 14.6 'bar

~20-r------------------------------------.D

v ~ I~ f/l f/l V H

0.. 10

.!:c: <0 V 0..

Cycle Number

Fig. 5 - Scatter plot of peak pressure for 2000 continuous cycles

Engine: Cop]><lr Coated Speed: 300D.Power: 2.3 klf )lean Pmax: 15 .4 bar

~ 26.0 .,.------------------------------------,

,Q

V 20.0 H ., f/l

~ 16.0 H

0..

~ 10.0 ell V

0..

Cycle Number

Fig. 6 - Scatter plot of peak pressure for 2000 continuous cycles

peak pressures with engine power out put for base engine and catalytic coated engines. It can be observed that the Pmar values of catalytic engine are higher than base engine. The higher values of peak

pressure may be due to the increased burning rate leading to higher rate of pressure ri se and the occurrence of Pmar closer to top dead center. This is reflected in the Fig. 8, which shows the crank angle of Pillar plotted against power output. The crank angle of

Pillar occurs much earlier for the catalyst engine compared to the base engine. The relation between the Pmax and crank angle of Pillar are explained in Fig.9. It

can be observed that for the same value of crank angle occurrence of Plllax higher values of Plllax were experienced in the case of catalytic coated engine. The maxi mum rate of pressure rise is also plotted in

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Brake Power, kW

Fig. 7 - Variation of peak pressure with engine power

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Brake Power, kW

Fig. 8 - Variation of crank angle occurrence of peak pressure with engine power

NEDUNCHEZHIAN & DHANDAPANI: TWO STROKE SI ENGINE 5

Fig. 10, which shows higher values of rate of pressure rise during higher loads. From these figures it can be

observed that the catalyst accelerates pressure rise rate, which leads to higher Pmax and helps to make the Pmax closer to TDe.

Figs 11 and 12 show the scatter plot of crank angle occurrence of 5%, 10%, 50% and 90% heat release values for 2.3 kW power output of both engines. The mean values for different loads were calculated and their results are plotted. Figs 13-16 show the heat release duration in crank angles for 5%, 5-10%, 5-50% and 5-90% heat release values for different power outputs of the engine, The occurrence of 5% heat release angle can be taken as the delay period of combustion. The duration for 5-10% heat release can be taken as the initial burning period 14 during which

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6.00.0 5.0 10.0 1 ~.O 20.0 2~.0 30.0 35.0 Crank Angle of Pmax, deg

Fig. 9 - Variat ion of peak pressure with crank angle of peak pressure

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0.0 0.5 1.0 1. 2.0 2.5 Brake Power, kW

Fig. 10 - Variation of maximum rate of pressure rise with engine power

Eneme: Base Enjline Speed: 3000 Power: 2.3 kW Pme.x: 14.6 bar

• •••• ~ Heat Release • ••• 0 10. Heat Release • •••• 50. Heal Release • •••• 9011 HEat ReleaQ6

;20.-----------------------------------~ .0

6.~~~~~~~~~~~~~~~~~~~~ 0.0 10.0 20.0 30.0 40.0 60.0 GO.O 70.0

Cronk Angle, deq

Fig. II - Scatter plot of heat release angles for 2000 cycles

Eneme: Copper Coated Speed: 3000 Power: 2.3 kW Yean Pmax: 15.4

• •• '.!lC Heat Release • ••• 0 1011 Heal release ••••• 5011 Heat Release • •••• 90. heat release

GO.O

;26~----------------------------~ .0

10.0 20.0 30.0 40.0 Cronk Angle, bar

60.0 SO.O

Fig. 12 - Scatter plot of heat release angle for 2000 cycles

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Brake Power, kW

Fig. 13 - Variation of crank angle occurrence of 5% heat release with engine power

6 INDIAN J. ENG. MATER. SCI., FEBRUARY 2001

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0.0 0.5 1.0 1.5 2.0 Brake Power, kW

Fig. 14 - Variation of crank angle duration of 5-10% heat release with engine power

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0.0 0.5 1.0 1.5 2.0 2.5 Brake Power, kW

Fig. 15 - Variation of crank angle duration of 5-50% heat release with engine power

the flame kernel spreads as spherical ball. The period between 5-50% heat release is the rapid burning period where the flame spreads at a much faster rate. The duration of 5-90% heat release can be taken as the combustion duration.

It can be observed from Fig. 13 that the delay period for catalytic engine is lower for all loads. The preflame reaction is accelerated by the catalyst and hence the delay period is reduced leading to higher burning rate. This is reflected in the subsequent Fig. 14, which shows the 5-10% duration of heat release at various loads. It is observed from the figure that the initial burning period, which corresponds to 5-10% heat release rate, is less for catalyst engine compared to the base engine at higher loads.

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0.0 0.5 1.0 1.5 2.0 2.5 Brake Power, kW

Fig. 16 - Variation of crank angle duration of 5-90% heat release with engine power

From Fig. 15, it is observed that the rapid burning period gets reduced for the catalyst engine even at low loads and the reduction is higher at higher loads. Also, Fig. 16 indicates the reduction in combustion duration for the catalytic engine compared to the base engine. The copper coating accelerates the combustion rate and reduces the combustion duration. This is in agreement with the earlier observations of accelerated combustion rate in the presence of catalysts·9. ,' . When a gaseous fluid comes in contact with a catalyst surface, its preflame reaction and flame velocity are increased. The action of catalyst depends mainly on the surface temperature, when other factors remain constant. Hence, at low load conditions, the improvement in the combustion rate is less compared to higher load conditions where the combustion chamber surface temperatures are much higher. The higher rate of heat release leads to increased P max,

pressure rise rate and better fuel economy. By using the simple technique of in-cylinder catalytic coating, both better fuel economy and cleaner exhaust can be achieved.

Conclusions The in-cylinder coating of copper catalyst reduces

the fuel consumption. There is 10% improvement in fuel economy at 2.0 kW load condition achieved. The catalytic activity is higher at higher load conditions. The combustion rate is increased at all load conditions.

The combustion parameters indicate a faster and cleaner combustion when the engine is coated with

NEDUNCHEZHIAN & DHANDAPANI: TWO STROKE SI ENGINE 7

catalyst. The HC emission levels are reduced to a significant level.

Acknowledgment The authors thank the Ministry of Human

Resources Development, Government of India, for the financial support given for the project on Development of Catalytic Combustion Chamber for SI Engines.

References 1 Inoue T, Matsushita S, Nakanishi, K & Okano, H, SAE Tech

Paper, (1993) 930873. 2 Evans R L, SAE Tech Paper, (1992) 921545. 3 Quader A, SAE Tech Paper, (1974) 741055. 4 Quader A, SAE Tech Paper, (1976) 760760.

5 May M G, SAE Tech Paper, (1979) 790386. 6 Federal German Republic German Patent No: pp. 2404

395.6-13, 1974. 7 Morsee F J, Olree R M & Adams W F, SAE Tech Paper,

(1977) 770640. 8 Rosfjord T J, A1AA Paper, 76-46, Washington D.C, Jan

1976. 9 Pfefferle W C, J Energy, 2(1) (1978) 142-146.

10 Thring R H, Platinum Metal Review, 24 (4) (1980) 126-133. 11 Rychter T J, Saragih R, Lezanski T & Wojcicki S,

Eighteenth Symposium (International) on Combustion, The Combution Institute, 1981, 1815-1824.

12 Karim G A & Kibrya M G, Trans ASME, 108 (1986) 446-449.

13 Zhengyun Hu & Ladommatos N, SAE Tech Paper, (1995) 952419.

14 Heywood J B, Internal combustion engines fundamentals (McGraw-Hili, New York), 1988.