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timing of 35.51 BTDC. The engine performance was erratic on this timing. The injection was then The 1997 Kyoto-Japan summit focused on the impact of greenhouse gases on the environment, a consequence of global warming. These results in flooding and landslides. ARTICLE IN PRESS The 2005 hurricane Katrina, Rita and Wilma effects in USA been typical examples. The 0960-1481/$-see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2006.12.006 advanced by 3.51. The engine performance was smooth on this timing especially at low loading conditions. The ignition delay was reduced through advanced injection timing but tended to incur a slight increase in fuel consumption. The CO and CO 2 emissions were reduced through advanced injection timing. r 2007 Elsevier Ltd. All rights reserved. Keywords: Carbon monoxide; Carbon dioxide and hydrocarbon emissions; Ignition delay 1. Introduction characteristics of diesel engine running on natural gas O.M.I. Nwafor Department of Mechanical Engineering, Federal University of Technology, Owerri, Imo State, Nigeria Received 30 November 2005; accepted 10 December 2006 Available online 23 May 2007 Abstract There has been a growing concern on the emission of greenhouse gases into the atmosphere, whose consequence is global warming. The sources of greenhouse gases have been identified, of which the major contributor is the combustion of fossil fuel. Researchers have intensified efforts towards identifying greener alternative fuel substitutes for the present fossil fuel. Natural gas is now being investigated as potential alternative fuel for diesel engines. Natural gas appears more attractive due to its high octane number and perhaps, due to its environmental friendly nature. The test results showed that alternative fuels exhibit longer ignition delay, with slow burning rates. Longer delays will lead to unacceptable rates of pressure rise with the result of diesel knock. This work examines the effect of advanced injection timing on the emission characteristics of dual-fuel engine. The engine has standard injection timing of 301 BTDC. The injection was first advanced by 5.51 and given injection Renewable Energy 32 (2007) 23612368 Effect of advanced injection timing on emission ARTICLE IN PRESS issue has been attributed to the combustion of fossil fuel which emits greater proportion of carbon dioxide. Literature review showed quite a number of research work carried out with the aim of identifying greener substitute for the present high pollutant conventional hydrocarbon (HC) fuels Nwafor 1, Lowe and Branham 2 and Horie and Mishizawa 3. There is a great interest in natural gas as alternative fuel for diesel engines. However, its use as viable substitute for diesel fuel has not yet become a reality due to related problems. First, natural gas has high self-ignition temperature (SIT) and requires separate means of initiating combustion. Secondly, it has longer delay period with slow burning rate resulting in pressure fluctuation. Works reported by Nwafor 4 and Stone and Ladommatos 5, constitute some recent research efforts to determine the performance and emission characteristics of gaseous-fuelled engines. Natural gas has high resistance to knock when used in internal combustion engines due to its high octane number (RON 131), Karim and Ali 6. It is therefore, suitable for engines of high compression ratios with possible improvement in performance. This work examines the effect of advanced injection timing on emission characteristics of diesel engine using natural gas as primary fuel. A mixture of gas and air was inducted during the induction stroke and towards the end of compression stroke a metred quantity of pilot diesel fuel was injected into a hot compressed charge to initiate combustion. The maximum quantity of pilot fuel needed is limited by the knocking tendency of the engine, Bari and Rice 7 and Nwafor 8. The knocking tendency is reduced by introducing more pilot fuel and/or reducing primary (alternative) fuel. The advanced injection timing is intended to compensate for the longer ignition delay and slow burning rate of natural gas fuelled engine. The test results showed decrease in CO and CO 2 emissions, and the delay period was also reduced with advanced injection timing compare to standard dual timing. The highest fuel consumption was recorded with the advanced timing. Diesel fuel operation produced the lowest HC and the highest CO 2 emission. The overall results indicate that advanced timing is beneficial at low-speed and low-loading conditions. The system temperature became the dominant factor at high-loading conditions. 2. Experimental apparatus A Petter model AC1 single cylinder energy cell diesel engine was used for this work. It is an air-cooled high speed indirect injection four-stroke engine. The dynamometer used to load the engine comprised of a shunt wound Mawdsley d.c generator and load bank. The reaction force and torque were measured by means of a 100C20.5 Newton-spring scale. Measurement of combustion chamber pressure was obtained by installing a kistler type 7063A, sensitivity 79pc/bar, water-cooled piezo-electric pressure transducer into the air cell of the combustion chamber. The cylinder pressure was displayed on a digital oscilloscope (Nicolet 4094) and stored in a diskette for later analysis of maximum rate of cylinder pressure rise. Pressure in the inlet manifold was measured by a normal U-tube manometer. Airflow was measured by means of a viscous flow metre. Thermocouples were installed to monitor gas temperature at inlet and outlet ducts as well as cylinder wall temperatures. Fuel was fed to the injector pump under gravity and the volumetric flow rate was measured by the use of a 50cm 3 graduated burette and stopwatch. Gas flow was measured by a variable area flow rotameter. The relative humidity and ambient temperature were monitored by hygrometer type Vaisala. Natural gasair mixture was O.M.I. Nwafor / Renewable Energy 32 (2007) 236123682362 controlled by the gas control valve with fumigation taking place in the engine inlet natural y. The advanced injection timing showe to standard dual-fuel operation. The sions at low loading conditions and increa in CO concentrations at the exhaust betwe timing for dual-fuel operation. The characteristics. The standard and advan advanced injection timing gave a net ARTICLE IN PRESS reduction in CO production at high-loading conditions. The highest CO production was obtained when running on diesel fuel at high load levels. 3.2. Carbon dioxide (CO 2 ) emissions Figs. 3 and 4 are the plots of CO 2 emissions. The effect of advanced injection timing is evidence for the production of carbon dioxide. The advanced injection timing produced the lowest CO 2 emissions at both speeds. The highest CO 2 concentrations in the exhaust were recorded when running on pure diesel fuel. Standard injection timing at both speeds offere gas at the speeds of 3000 and 2400rpm, respectivel d a significant reduction in CO emissions compared diesel fuel operation produced the lowest CO emis sed with load. There was marked difference en the advanced injection timing and the standard speed of 2400rpm produced different emission ced dual operations showed similar trends. The manifold. The HC emissions were measured by a Rotork flame ionisation detector (FID) analyser model 523. The CO and CO 2 emissions were measured by an Oliver k550 infrared analyser. 2.1. Typical composition of natural gas 2.18% nitrogen, 92.69% methane, 3.43% ethane, 0.52% carbon dioxide, 0.71% propane, 0.12% iso-butane, 0.15% n-butane, 0.09% pentane and 0.11% hexane Gross calorific value 38.59MJ/m 3 Net calorific value 34.83MJ/m 3 Gross Wobbe number 49.80MJ/m 3 Stoichiometric air/fuel ratio 16.65:1 Net calorific value of diesel fuel 42.70 MJ/kg Relative density of diesel fuel 0.844. 2.2. Engine data Bore 76.20mm, stroke 66.67mm, engine capacity 304 cc, compression ratio 17, fuel injection release pressure 183bar, standard fuel injection timing 301 BTDC, advanced fuel injection timing 33.51 BTDC. 3. Test results 3.1. Carbon monoxide (CO) emissions Carbon monoxide production relates to the fuelair ratio and it is a measure of the combustion efficiency of the system. Figs. 1 and 2 compare CO emission characteristics of diesel fuel operation with the standard and advanced injection timing when running on O.M.I. Nwafor / Renewable Energy 32 (2007) 23612368 2363 d a net reduction in CO 2 emissions compared to the results obtained when running ARTICLE IN PRESS O.M.I. Nwafor / Renewable Energy 32 (2007) 236123682364 on pure diesel fuel. The observed trends were increased CO 2 emissions as the A/F ratio decreased. CO 2 and H 2 O are the products of combustion that will appear in the exhaust under an ideal combustion process. The emission of CO 2 is therefore, a measure of combustion efficiency of the system. It is desirable to have high CO 2 and less HC emissions under any operating condition. 3.3. HC emissions Fig. 5 shows the plots of HC emissions in dual-fuel and diesel fuel operations obtained at the speed of 3000rpm. The diesel fuel operation gave the lowest HC emissions. The Fig. 2. Injection advanced effect on carbon monoxide emissions. Engine speed 2400rpm. Fig. 1. Injection advanced effect on carbon monoxide emissions. Engine speed 3000rpm. ARTICLE IN PRESS O.M.I. Nwafor / Renewable Energy 32 (2007) 23612368 2365 advanced injection timing showed low and high HC emissions at low and high loading conditions compared to the standard injection timing operation, respectively. The plots of HC emissions with the dual standard and advanced timing operations at 2400rpm were similar as presented in Fig. 6. Diesel fuel operation offered a remarkable reduction in HC emissions. It was also noted that diesel fuel operation gave the highest CO 2 emissions which reflected on the low HC production. This result is attributed to an efficient combustion realised when running on pure diesel fuel. The overall results indicate that greater proportion of natural gas escaped primary combustion when running on dual system due perhaps, to the slow burning rates of natural gas. HC emissions increase due to several factors including quenched, lean combustion, wall wetting and poor mixture Fig. 3. Injection advanced effect on carbon dioxide emissions. Engine speed 3000rpm. Fig. 4. Injection advanced effect on carbon dioxide emissions. Engine speed 2400rpm. ARTICLE IN PRESS O.M.I. Nwafor / Renewable Energy 32 (2007) 236123682366 preparation. The HC level was high in both advanced and standard operations throughout the load range. The wider valve overlap of diesel engine is likely to result in greater proportion of fresh charge leaving with the products of combustion since a mixture of gas and air is inducted during the induction stroke. 3.4. Ignition delay Ignition delay in diesel engine is defined as the time interval between the start of fuel injection and the start of combustion. The ignition delay for dual-fuel operations is compared with the baseline diesel fuel operation shown in Figs. 7 and 8. The diesel fuel operation had the shortest delay periods at both speeds tested. The standard injection Fig. 5. Injection advanced effect on hydrocarbon emissions. Engine speed 3000rpm. Fig. 6. Injection advanced effect on hydrocarbon emissions. Engine speed 2400rpm. ARTICLE IN PRESS O.M.I. Nwafor / Renewable Energy 32 (2007) 23612368 2367 timing showed the longest delay periods at high load levels, than the advanced timing operation. There was very significant difference between the ignition delay of diesel fuel and dual-fuel operations at 2400rpm. The standard timing also produced the longest delay periods at this speed. In the fumigated dual-fuel engine, the measured data indicate that ignition delay increases with decreased in engine speed. This is contrary to the pure diesel fuel operation as shown in the plots. At low speed, greater proportion of pilot fuel will take part in premixed combustion hence increasing the tendency of diesel knock. The ignition delay of dual-fuel operation is generally longer than those of diesel fuel operations. The SIT of natural gas (7041C) is higher than that of diesel fuel (2451C). A mixture of gas and air was inducted in the cylinder and the temperature attained at the end of compression stroke was lower than the SIT of the gas. The fuel penetration and spray cone angle Fig. 7. Injection advanced effect on ignition delay. Engine speed 3000rpm. Fig. 8. Injection advanced effect on ignition delay. Engine speed 2400rpm. depend on the density of the air in the cylinder. A very poor atomization results in long delay periods due perhaps, to the slow development of very fine droplets. 4. Conclusions The test results showed that alternative fuels exhibit delay characteristics which was ARTICLE IN PRESS O.M.I. Nwafor / Renewable Energy 32 (2007) 236123682368 noted to be influenced by engine load and speed. The test results with advanced injection timing showed that each alternative fuel requires injection advanced appropriate to its delay period. It was found that advanced timing tended to incur a slight increase in fuel consumption. There was a significant reduction in CO 2 emissions with advanced timing. The CO concentrations in the exhaust were considerably reduced with the advanced timing unit compared with the standard timing. The HC emissions of the dual-fuel systems were high throughout the loading conditions. Advanced injection timing showed a marginal improvement in HC emissions over the dual standard unit. The engine ran smoothly at light-load conditions in dual fuel with advance of 3.51 compared to standard timing. A further 1.51 advance tended to produce very erratic behaviour of the engine. At high load, the combustion temperature became the dominant factor, which increases the evaporation rate of the injected fuel with reduction in delay period. Injection advance is not therefore, recommended at high-loading conditions. The emission characteristics of dual-fuelled engine were noted to be influenced by the delay periods. References 1 Nwafor OMI, Rice G. Combustion characteristics and performance of natural gas in high speed, indirect injection diesel engine. UK: WREC; 1994. p. 841. 2 Lowe W, Brandham RT. Development and application of medium speed gas burning engines. IMechE 1971;186:75. 3 Horie K, Mishizawa K. Development of a high fuel economy and performance four-valve lean burn engine. IMechE 1992;C448/014:137. 4 Nwafor OMI. Effect of advanced injection timing on the performance of natural gas in diesel engine. Int J Indian Acad Sci, Sadhana 2000;25:11. 5 Stone CR, Ladommatos N. Design and evaluation of a fast-burn spark ignition combustion system for gaseous fuels at high compression ratios. J Inst Energy 1991;64:202. 6 Karim GA, Ali AI. Combustion, knock and emission characteristics of a natural gas fuelled s.i. engines with particular reference to low intake temperature conditions. IMechE 1975;189(25/75):135. 7 Bari S, Rice G. Knocking in gas-fumigated dual-fuel engine. In: Proceedings of the fourth international conference on small engines, their fuels and the environment. 2124 September 1993. 8 Nwafor OMI. Effect of oxygen supply on dual-fuel engine performance using natural gas as primary fuel. J AMSE, Modelling, Simulation Control, Fr 2002;71(3):29.