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天然气电控发动机设计,天然气,发动机,设计
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Renewable Energy 32 (2007) 23612368Effect of advanced injection timing on emissioncharacteristics of diesel engine running on natural gasO.M.I. NwaforDepartment of Mechanical Engineering, Federal University of Technology, Owerri, Imo State, NigeriaReceived 30 November 2005; accepted 10 December 2006Available online 23 May 2007AbstractThere has been a growing concern on the emission of greenhouse gases into the atmosphere, whoseconsequence is global warming. The sources of greenhouse gases have been identified, of which themajor contributor is the combustion of fossil fuel. Researchers have intensified efforts towardsidentifying greener alternative fuel substitutes for the present fossil fuel. Natural gas is now beinginvestigated as potential alternative fuel for diesel engines. Natural gas appears more attractive dueto its high octane number and perhaps, due to its environmental friendly nature. The test resultsshowed that alternative fuels exhibit longer ignition delay, with slow burning rates. Longer delayswill lead to unacceptable rates of pressure rise with the result of diesel knock. This work examines theeffect of advanced injection timing on the emission characteristics of dual-fuel engine. The engine hasstandard injection timing of 301 BTDC. The injection was first advanced by 5.51 and given injectiontiming of 35.51 BTDC. The engine performance was erratic on this timing. The injection was thenadvanced by 3.51. The engine performance was smooth on this timing especially at low loadingconditions. The ignition delay was reduced through advanced injection timing but tended to incur aslight increase in fuel consumption. The CO and CO2emissions were reduced through advancedinjection timing.r 2007 Elsevier Ltd. All rights reserved.Keywords: Carbon monoxide; Carbon dioxide and hydrocarbon emissions; Ignition delay1. IntroductionThe 1997 Kyoto-Japan summit focused on the impact of greenhouse gases on theenvironment, a consequence of global warming. These results in flooding and landslides.The 2005 hurricane Katrina, Rita and Wilma effects in USA been typical examples. TheARTICLE IN PRESS/locate/renene0960-1481/$-see front matter r 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.renene.2006.12.006issue has been attributed to the combustion of fossil fuel which emits greater proportion ofcarbon dioxide. Literature review showed quite a number of research work carried outwith the aim of identifying greener substitute for the present high pollutant conventionalhydrocarbon (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, itsuse 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 ofinitiating combustion. Secondly, it has longer delay period with slow burning rate resultingin pressure fluctuation. Works reported by Nwafor 4 and Stone and Ladommatos 5,constitute some recent research efforts to determine the performance and emissioncharacteristics of gaseous-fuelled engines. Natural gas has high resistance to knock whenused in internal combustion engines due to its high octane number (RON 131), Karim andAli 6. It is therefore, suitable for engines of high compression ratios with possibleimprovement in performance. This work examines the effect of advanced injection timingon emission characteristics of diesel engine using natural gas as primary fuel. A mixture ofgas and air was inducted during the induction stroke and towards the end of compressionstroke a metred quantity of pilot diesel fuel was injected into a hot compressed charge toinitiate combustion. The maximum quantity of pilot fuel needed is limited by the knockingtendency of the engine, Bari and Rice 7 and Nwafor 8. The knocking tendency isreduced by introducing more pilot fuel and/or reducing primary (alternative) fuel. Theadvanced injection timing is intended to compensate for the longer ignition delay and slowburning rate of natural gas fuelled engine. The test results showed decrease in CO and CO2emissions, and the delay period was also reduced with advanced injection timing compareto standard dual timing. The highest fuel consumption was recorded with the advancedtiming. Diesel fuel operation produced the lowest HC and the highest CO2emission. Theoverall results indicate that advanced timing is beneficial at low-speed and low-loadingconditions. The system temperature became the dominant factor at high-loadingconditions.2. Experimental apparatusA Petter model AC1 single cylinder energy cell diesel engine was used for this work. It isan air-cooled high speed indirect injection four-stroke engine. The dynamometer used toload the engine comprised of a shunt wound Mawdsley d.c generator and load bank. Thereaction force and torque were measured by means of a 100?0.5 Newton-spring scale.Measurement of combustion chamber pressure was obtained by installing a kistler type7063A, sensitivity 79pc/bar, water-cooled piezo-electric pressure transducer into the aircell of the combustion chamber. The cylinder pressure was displayed on a digitaloscilloscope (Nicolet 4094) and stored in a diskette for later analysis of maximum rate ofcylinder pressure rise. Pressure in the inlet manifold was measured by a normal U-tubemanometer. Airflow was measured by means of a viscous flow metre. Thermocouples wereinstalled to monitor gas temperature at inlet and outlet ducts as well as cylinder walltemperatures. Fuel was fed to the injector pump under gravity and the volumetric flow ratewas measured by the use of a 50cm3graduated burette and stopwatch. Gas flow wasmeasured by a variable area flow rotameter. The relative humidity and ambienttemperature were monitored by hygrometer type Vaisala. Natural gasair mixture wascontrolled by the gas control valve with fumigation taking place in the engine inletARTICLE IN PRESSO.M.I. Nwafor / Renewable Energy 32 (2007) 236123682362manifold. The HC emissions were measured by a Rotork flame ionisation detector (FID)analyser model 523. The CO and CO2emissions were measured by an Oliver k550 infraredanalyser.2.1. Typical composition of natural gas2.18% 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% hexaneGross calorific value 38.59MJ/m3Net calorific value 34.83MJ/m3Gross Wobbe number 49.80MJ/m3Stoichiometric air/fuel ratio 16.65:1Net calorific value of diesel fuel 42.70 MJ/kgRelative density of diesel fuel 0.844.2.2. Engine dataBore 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 results3.1. Carbon monoxide (CO) emissionsCarbon monoxide production relates to the fuelair ratio and it is a measure of thecombustion efficiency of the system. Figs. 1 and 2 compare CO emission characteristics ofdiesel fuel operation with the standard and advanced injection timing when running onnatural gas at the speeds of 3000 and 2400rpm, respectively. The advanced injection timingshowed a significant reduction in CO emissions compared to standard dual-fuel operation.The diesel fuel operation produced the lowest CO emissions at low loading conditions andincreased with load. There was marked difference in CO concentrations at the exhaustbetween the advanced injection timing and the standard timing for dual-fuel operation.The speed of 2400rpm produced different emission characteristics. The standard andadvanced dual operations showed similar trends. The advanced injection timing gave a netreduction in CO production at high-loading conditions. The highest CO production wasobtained when running on diesel fuel at high load levels.3.2. Carbon dioxide (CO2) emissionsFigs. 3 and 4 are the plots of CO2emissions. The effect of advanced injection timing isevidence for the production of carbon dioxide. The advanced injection timing producedthe lowest CO2emissions at both speeds. The highest CO2concentrations in the exhaustwere recorded when running on pure diesel fuel. Standard injection timing at both speedsoffered a net reduction in CO2emissions compared to the results obtained when runningARTICLE IN PRESSO.M.I. Nwafor / Renewable Energy 32 (2007) 236123682363on pure diesel fuel. The observed trends were increased CO2emissions as the A/F ratiodecreased. CO2and H2O are the products of combustion that will appear in the exhaustunder an ideal combustion process. The emission of CO2is therefore, a measure ofcombustion efficiency of the system. It is desirable to have high CO2and less HC emissionsunder any operating condition.3.3. HC emissionsFig. 5 shows the plots of HC emissions in dual-fuel and diesel fuel operations obtainedat the speed of 3000rpm. The diesel fuel operation gave the lowest HC emissions. TheARTICLE IN PRESSFig. 2. Injection advanced effect on carbon monoxide emissions. Engine speed 2400rpm.Fig. 1. Injection advanced effect on carbon monoxide emissions. Engine speed 3000rpm.O.M.I. Nwafor / Renewable Energy 32 (2007) 236123682364advanced injection timing showed low and high HC emissions at low and high loadingconditions compared to the standard injection timing operation, respectively. The plots ofHC emissions with the dual standard and advanced timing operations at 2400rpm weresimilar as presented in Fig. 6. Diesel fuel operation offered a remarkable reduction in HCemissions. It was also noted that diesel fuel operation gave the highest CO2emissionswhich reflected on the low HC production. This result is attributed to an efficientcombustion realised when running on pure diesel fuel. The overall results indicate thatgreater proportion of natural gas escaped primary combustion when running on dualsystem due perhaps, to the slow burning rates of natural gas. HC emissions increase due toseveral factors including quenched, lean combustion, wall wetting and poor mixtureARTICLE IN PRESSFig. 3. Injection advanced effect on carbon dioxide emissions. Engine speed 3000rpm.Fig. 4. Injection advanced effect on carbon dioxide emissions. Engine speed 2400rpm.O.M.I. Nwafor / Renewable Energy 32 (2007) 236123682365preparation. The HC level was high in both advanced and standard operations throughoutthe load range. The wider valve overlap of diesel engine is likely to result in greaterproportion of fresh charge leaving with the products of combustion since a mixture of gasand air is inducted during the induction stroke.3.4. Ignition delayIgnition delay in diesel engine is defined as the time interval between the start of fuelinjection and the start of combustion. The ignition delay for dual-fuel operations iscompared with the baseline diesel fuel operation shown in Figs. 7 and 8. The diesel fueloperation had the shortest delay periods at both speeds tested. The standard injectionARTICLE IN PRESSFig. 5. Injection advanced effect on hydrocarbon emissions. Engine speed 3000rpm.Fig. 6. Injection advanced effect on hydrocarbon emissions. Engine speed 2400rpm.O.M.I. Nwafor / Renewable Energy 32 (2007) 236123682366timing showed the longest delay periods at high load levels, than the advanced timingoperation. There was very significant difference between the ignition delay of diesel fueland dual-fuel operations at 2400rpm. The standard timing also produced the longest delayperiods at this speed. In the fumigated dual-fuel engine, the measured data indicate thatignition delay increases with decreased in engine speed. This is contrary to the pure dieselfuel operation as shown in the plots. At low speed, greater proportion of pilot fuel will takepart in premixed combustion hence increasing the tendency of diesel knock. The ignitiondelay of dual-fuel operation is generally longer than those of diesel fuel operations. TheSIT of natural gas (7041C) is higher than that of diesel fuel (2451C). A mixture of gas andair was inducted in the cylinder and the temperature attained at the end of compressionstroke was lower than the SIT of the gas. The fuel penetration and spray cone angleARTICLE IN PRESSFig. 7. Injection advanced effect on ignition delay. Engine speed 3000rpm.Fig. 8. Injection advanced effect on ignition delay. Engine speed 2400rpm.O.M.I. Nwafor / Renewable Energy 32 (2007) 236123682367depend on the density of the air in the cylinder. A very poor atomization results in longdelay periods due perhaps, to the slow development of very fine droplets.4. ConclusionsThe test results showed that alternative fuels exhibit delay characteristics which wasnoted to be influenced by engine load and speed. The test results with advanced injectiontiming showed that each alternative fuel requires injection advanced appropriate to itsdelay period. It was found that advanced timing tended to incur a slight increase in fuelconsumption. There was a significant reduction in CO2emissions with advanced timing.The CO concentrations in the exhaust were considerably reduced with the advanced timingunit compared with the standard timing. The HC emissions of the dual-fuel systems werehigh throughout the loading conditions. Advanced injection timing showed a marginalimprovement in HC emissions over the dual standard unit. The engine ran smoothly atlight-load conditions in dual fuel with advance of 3.51 compared to standard timing. Afurther 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 evaporationrate of the injected fuel with red
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