Indian Journal of Engineering & M ateri als Sciences Vo l. 9. A ugust 2002. pp. 250-254

Lean-knock limits for dual-fuel combustion of natural gas in diesel engi ne

OM I wafol'

Dcpartment of M cchan ical Engineering. Federal Universi ty ofTcchnology. Owerri. Imo state. igeri a

Receil'ed 6 Jllll e 2001; accepted 10 May 2002

The use of natural gas as energy source for internal combusti on (lC) engincs has an attract ive intuit ive appeal due to i ts environmentall y friendly nature. Whil st success ful commerciali sat ion of thi s encrgy source, it requires that i t offers cost advantages over competing fuel s. It has been considered as potcntial source o f fuel encrgy for compress ion ignition (CI) engincs due to its high oc tane va lue (RON 13 1) necessitating for the use of high compress ion rati os. In th is paper, the operating range (lean-knock li mits) for dual-fuel combusti on of natural gas in an unmodifi ed diesel engine has been in ves ti gated. The factors influencing the knock and lean limits have been identifi ed . The delay peri od has been noted to feature in defin ing knock limit and high conccntrations of hydrocarbon (HC) in the exhaust. The cylinder pressure diagram indica ted longer igni tion delay and reduccd maximulll peak cy linder pressure when operating beyond these limi ts, w ith an increased pressure fluctu ati on.

The fuel system of the natural gas engine is partly different from that of the liquid fue l engine. In the case of spark ignition (SI ) engine, petrol is atomised and mixed with high veloc ity air stream passing through the venturi in a carburettor. In compress ion igni tion engine the air-fuel mi xture is formed in the combusti on chamber before combusti on takes placet. [n both cases, mixing is not generall y as mixing air and natura l gas. Many researchers2-~ have documented enough in formati on on the use of natural gas in SI engines. The use of natural gas in an IC engines offers greater interest for two reasons. First, it is a clean fuel and availab le as waste products in considerabl e quantiti es th roughout many parts of the world. Secondly, it is known to have high res istance to knock when used as a fuel in IC engines resulting from its high octane value. It is therefore, suitable for engines of high compress ion ratios w ith possible improvement in engine performance. Natura l gas ha~ a high auto-ignition temperature and requires means of a separate source of ignition when used in CI engines . In thi s study, ign ition is brought about by diesel fuel pilot injection. The research work was designed to establi sh knock free operating range for dual-fuel combusti on of natural gas using pil ot injection ignition.

atural gas exh ibits longer ignition delay and slower burning rates; Karim5

, and eedham and Doy le6

. The longer burning rate of the gas allows more ti me for heat transfer to the end gas to take place with a tendency to knock. The use of natural gas in CI eng ine involves an evolution of two stages of ignition

and combusti on processes resulting in three types of kn ()l'~ : the diesel knock, spark igni tion knock and knock du ~ to secondary igni tion delay of the gas. The combusti on processes of dual-fuel engine lie between that of SI engine and CI engine. It, therefore, becomes quest ionable to rely on pressure transducer alone to detect the onset of these knocks. The author resorted to es tabli sh the onset of knock through visual observati on of oscil lati on around the peak of cy linder pressure displayed on the Nicolet 4094 storage oscilloscope recorder, together with the audible meta llic noise of the eilg ine. The lean-knock l imits estab li shed in thi s scheme of work were noted to

depend on operating conditions such as load, speed. combustion temperature and mixture strength.

Expedmental Procedure Tests were conducted on Petter model AC I single

cy linder, energy cell diese l engine. It is an air-coo leel . high speed, indirect injection. four-stroke engine. The energy cell consis ts of major and minor chambers which open into the main combust ion chamber. The cel l induces a secondary turbulence wh ich aids more complete combusti on resulting from good mixi ng. The ex peri mental system used for measuri ng engi ne performance characteristi cs has been illustrated elsewhere7

. The dynamometer used to load the eng ine compri sed a shunt wound M aurdsley d.c. generator and load bank. Measurement of combustion chamber pressure was obtained by installing a Kistl er type 7063A , sensiti vity 79 pc/bar, water-cooled piezo­electric pressure transducer into the air ce ll of the

..

NWAFOR: DUAL-FUEL COMBUSTION OF NATURAL GAS IN DI ESEL ENGINE 251

combustion chamber. The cy linder pressure was displayed on a digital storage oscilloscope. Natural gas fuel mi xture was controlled by the gas control valve with fumigation taking pl ace in the engine inlet manifo ld .

Natural gas typical composition 2. 18% nitrogen, 92 .69% methane, 3.43 % ethane,

0 .52% carbon diox ide, 0.71 % propane, 0 .12% iso­butane, 0.15 % N-butane, 0.09% pentane and 0 .11 % hexane .

Gross calori fic value = 38 .59 MJ/m3

et calori fic value = 34 .83 MJ/m3

Gross Wobbe number = 49.80 MJ/m3

Stoichiometric AlF ratio = 16.65 : I Relative density of diesel fuel = 0.844 Net calorific value of diesel fuel = 42.70 MJ/kg

Engine data Bore = 76.20 mm, stroke = 66.67 mm, engine

capaci ty = 304 cc, compression ratio = 17: 1, fuel injection release pressure = 183 bar and fuel injection timing = 30 deg. BTOC.

Rated power (using diesel fuel ) 2.0 kW at 1500 rev/min, 2.45 kW at 1800 rev/min,

3.7 kW at 2500 rev/min, 4 .45 kW at 3000 rev/min and 4.8 kW at 3600 rev/min .

Results and Discussion Pilot injection ignition

The research work was designed to es tabli sh ' lean­knock limited lines ' as a means of describing the operating range of a gas engine using pilot injection igni tion. The limits offer practical advantage to gas engine designers and operators. In the gas-fumigated dual-fuel engine, the primary fue l is mi xed outside the cy linder before it is inducted into the cy linder. A mi xture of gas and air is compressed during the compress ion stroke . The self igniti on temperature of natural gas (704°C) is high compared to the prevailing temperature in the cy linder (about 600°C) at the end of compress ion stroke. The mi xture in the cylinder wi ll not ignite under thi s condition and therefore, requires a separate means of initiating combustion. In this programme o f work , ignition is brought about by the injection of pilot quantity o f diesel fuel (about 20% depending on the load) to initiate combustion. Ignition delay exists and it is longer than when running on pure diesel fuel.

Cylinder pressure, heat release and ignition delay The measurement of cy linder pressure served as a

valuable aid in the detection of irregul ar combustion under certain conditions. The heat release analys is provides a method fo r screening the fuels for the perfo rmance and durability characteristi cs , Figs 1 and 2 of the pressure crankangle and heat release di agrams of normal dual fuel operation compare Figs 3 and 4 of pure diesel fuel operation. Fi gs 2 and 4 are the heat release di agrams of Figs 1 and 3. These diagrams indicate that the dual-fuel delays the start of combustion as detec ted by the point of rapid pressure ri se. Peak pressure was decreased with the addition of natural gas and the initi al rate of pressure ri se was also decreased. These diagrams also indicate that with the dual-fuel combusti on process, the ignition delay period becomes longer. It is due perhaps, to the reduction in oxygen concentration resulting from gas substitution for air. The ignition delay in this study is

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Crankangle, deg ree

Fi g. I-P-8 diagram of dual-fuel combustion at 3000 rev/min ; (engine torque output = 9.65 Nm)

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Fig. 2-Normal dual- fuel combustion at 3000 rev/min ; (engine torque output = 9.65 Nm)

252 INDIAN J. ENG. MATER. SCI. , AUGUST 2002

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Fig. 3-Pressure-crankangle diagram of diesel fuel operation; (engine speed = 3000 rev/min, engine torque output = 9.65 Nm)

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Crankangle) deg ree

Fig. 4--Heat released diagram of diesel fuel operation; (engine speed = 3000 rev/min ; engine torque output = 9.65 Nm)

defined in terms of the time from the start of fuel injection to the point where the rate of pressure rise becomes positive. The initial rate of heat release was also less than it was the case when running on diesel fuel. Fig. 4 of diesel fuel operation shows an apparent negative heat release prior to the main start of combustion. This i ~ due to the cooling effect of the injected liquid fuel. This effect is not apparent with the dual-fuel operation as shown in Fig. 2. It is thought that peroxidation of the gas had started before pilot fuel injection began, The pilot fuel then flattened the heat release rate within this region. These diagrams indicate ti.,at diesel fuel operation released heat before top dead centre (TOC) whilst dual-fuel operation released beat about 10°C after TOe. The power output of the engine is less when running on natural gas.

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Brake Mean Effective Pressure (KN/m2)

Fig . 5- Lean-knock limits for dual-fuel combustion at engine speed of 3000 rev/min

Lean-limited point The lean-limit was established through oxygen (0 2)

enrichment. The choice of oxygen was based on two reasons. First, when supplied within flammability limit it acts as additive which promotes combustion and stabilises the combustion of lean mixtures. The oxygen acts as oxidant and increases flame temperature hence the burning rate. Secondly, when supplied beyond flammability limit it acts as a diluent. Since natural gas has longer ignition delay and slower burning rates, CO2 and N 2 were not considered as suitable diluent. The audible noise and peak cylinder pressure trace on the oscilloscope screen were observed. It was noted that for a given load and gas flow, the ripples on the peak pressure wave gradually disappeared as O2 enrichment progressed. As oxygen was further supplied, a point was reached when the engine started vibrating followed by a rapid drop in speed, thus establishing the 'lean-limited point' (i.e. the lower limit of stable combustion of natural gas­pilot injection ignition). Similar resul ts were obtained at different loads and gas flow whilst the speed was maintained at 3000 rev/min. At lower loads and gas flow, the engine ran smoothly with an increased quantity of O2 supply up to this lean-limit line producing a wider range of operation as shown in Fig. 5. At higher engine loads and gas flow together with increased combustion temperature, supply of oxygen beyond this line resulted in a rapid reduction in engine speed and the engine was producing a whistling noise. It was noted that O2 enrichment in excess of 27% at light loads and about 16% at high load levels resulted in this erratic behaviour of the engine. It was also noted that O2 enrichment within the operating range brought the engine back to normal diesel fuel

NWAFOR: DUAL-FUEL COMBUSTION OF NATURAL GAS IN DIESEL ENGINE 253

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Crank angle , de g ree

Fig. 6-Lean-limit pressure-crankangle diagram at 3000 rev/min ; (engine torque output = 9.65 Nm)

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-2~2LOO--------....Ll00--------..L0--------l00L--------.J200

Crankangle , deg ree

Fig. 7-Lean-limit heat release di agram at 3000 rev/min ; (engine torque output = 9.65 Nm)

operation as shown in Figs 6 and 7 . The diagrams showed reduced ignition delay with an increased in maximum peak cylinder pressure. It was observed that as load and gas flow increased, less and less O2

was needed to establish the lean-limited point. The resu lt is contrary to the expectation that additional oxygen supply wou ld have improved combustion efficiency at high load levels and gas supply. It was then thought that the mixture was loo weak to burn at thi s operating conditions. These results indicated that apart from load, speed, combustion temperature and mixture strength, there are other operating variables such as the level of turbulence in the cy linder together with the intake conditions that influence combustion process of dual-fuel engine. The exhaust gas temperature measurements indicated that the combustion temperature dropped rapidly when operating beyond the lean-limited line.

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Fig. 8- Knock-limit P-8 di agram at 3000 rev/min ; (engine torque output = 9.65 Nm)

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Fig. 9-Knock-limit heat release diagram at 3000 rev/min ; (engine torque output = 9.65 Nm)

Knock-limited point

The knock-limited point was established through fuel enrichment. The pi lot fue l and gas flow were adj usted simu ltaneously whi lst maintaining the same speed and load until the onset of knock intensity became intolerable. It was noted that fuel enrichment in excess of 24.6% at light loads and 18.4% at high load levels resu lted in objectionable knock menti oned. Fig. 5 shows the knock-limited line establi shed from the test results. The measured exhaust temperature was taken as an indicator of combustion temperature change relating to heat release. This temperature was noted to increase rapid ly when operating beyond the knock-limited point, while the peak cy linder pressure diagram decreased as shown in Fig. 8. The diagram indicated longer ignition delay when operating at or beyond this point. The heat release diagram Fig. 9 ill ustrated that combustion was well into the expansion stroke and since the energy content of the

254 INDIAN 1. ENG. MATER. SCI. , AUGUST 2002

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Brake Mean Effective Pressure (KN/m2)

Fig. 10-- Effect of lean·knock limits on b.s. f.c. at 3000 rev/min

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Brake Maun Effective Pressure (KN/n1>

Fig. ! 1- Effect of lean-- knock limits on brake thermal efficiency at 3000 rev/min

burning air-gas mixture was not effectively converted to mechanical work, it then appeared in the form of high exhaust temperature. It is implicit that energy released by the fuel increases with gas flow and there was an increased h,~at transfer to the walls of the combustion chamber. There was also an increase in peak pressure and it is postulated that these conditions are likely to lead to an increase in temperature of the end gas resulting to engine knock. This increased peak pressure was, however, moderated by an increas ing cylinder volume and therefore did not cause any damage to the engine other than loss of engine power resulting from knock.

Brake specific fuel consumption and brake thermal efficiency Figs 10 and 11 show the effect of operating at or

beyond the lean-knock limits on engine performance. The plots depict the poor performance of natural gas operation on brake specific fuel consumption (b.sJ.c.). The b.sJ.c. was consistently lower when running on pure diesel fuel than those of gas engine operations. The vruiations between operations were significant ly high at light loading conditions and converged exponentially with increased load. At high

temperature and load level s, there was no significant difference in b.s J .c. between dual-fuel operation and when running on pure diesel fuel. It was thought to be due to improved combustion at high engine load and temperature. The results in general , show reduced brake thermal effic iency when running on gas compared to diesel fuel operation . There was a significant reduction in efficiency when operating at or beyond the lean-knock limited lines. There was no significant di fference between the normal dual-fuel and the lean-limit operations. The knock-limited test produced the lowest brake thermal efficiency due perhaps, to the longer ignition delay observed during the operation resulting in peak cylinder pressure occurring well into the combustion chamber. The hydrocarbon (HC) emissions were observed to increase rapidly when operating beyond these limit lines. It was also noted that carbon monoxide (CO) concentrations in the exhaust increased sharply when operating beyond these lines.

Conclusions The test results showed that natural gas can be used

as a fuel in CI engine using pilot injection ignition . The results reported in this paper illustrate the importance of operating between the lean-knock limit lines. The maximum power output of the engine running on natural gas is less and was fu rther reduced when operating at/or beyond these limits compared to pure diesel fuel operation. The b.sJ.c was significantly high for various dual-fuel operations. The thermal efficiency was remarkably reduced when running on dual-fuel systems, and was further reduced when running beyond the limited lines. In dual-fuel operation there seems to be little difference in engine efficiency between lean-limit and normal dual·fuel operation. The knock-limit test results showed significant reduction in efficiency. The hydrocarbon (HC) emissions of gas-fumigated engine were significantly high throughout the load range, perhaps due to wider positive valve overlap of diesel engine. The si tuation became more pronounced when running beyond these li mit lines. These limit lines were noted to depend on operating conditions such as engine load, speed, combustion temperature and mixture strength.

References I Heywood 1 B, Ed, (McGraw Hill , New York) , 1988. 2 Stone G R & Ladommatos N, Ins1 Energy, xiv (1991) 202. 3 Shiells W, Raine R R & Garia P, SAEpaper, 892 137, 1989. 4 Karim G A & A I Ali , IIlS1 Mech Eng, 189 (1995) 139. 5 Karim G A, Inst Mech Eng, 174 (1969) 660. 6 Needham 1 R&D M Doyle, SAE, 852101,1985.

7 Nwafor 0 M I & Rice G, World Renewable Energy Congress, 2 (1994) 84 1.