Effect Of Pre-injection Timing On Combustion And Emission Characteristics Of Methanol-diesel Dual-fuel Four-cylinder Diesel Engine

Methanol has received widespread attention due to its wide range of sources,high oxygen content,clean combustion and other advantages.In recent years,using methanol as an alternative fuel for diesel engines,and combusting on diesel engines in the form of dual fuels has become a research topic for scholars.This article is based on a Weichai WP4G154E330 electronically controlled common rail four-cylinder diesel engine,installing a methanol supply system and methanol injection control system to achieve individual control of methanol injection.And introducing diesel pre-injecting strategy,through bench tests,comprehensively analyze the effects of diesel pre-injection timing on the combustion and emission characteristics of methanol-diesel dual-fuel engines.This paper studied the effects of pre-injection timing on the combustion characteristics of methanol-diesel engines under various operating conditions.The results show that:1)Under light load conditions,the introduction of the pre-injection strategy and the pre-injection time is 10°CA ahead of the main injection,the increase in cylinder pressure is more obvious;in the medium and heavy load conditions,the introduction of the pre-injection strategy will increase the pressure in the cylinder,both are higher than the single injection mode,and the pre-injection time is 5-10°CA earlier than the main injection at medium load,which increases the cylinder pressure more obviously.2)Under all working conditions,with the ahead of the pre-injection timing and the increase of the co-combustion ratio,the pressure rise rate in the pre-injection combustion stage and the main injection combustion stage changes with the crank angle basically the same.The introduction of the pre-injection strategy can reduce the combustion noise to a certain extent and make the combustion more gentle in the medium and light load conditions.3)Under all working conditions,the dual-fuel mode can improve the instantaneous heat release rate,and with the enhance of the methanol co-combustion ratio,the maximal heat release rate also improve.After the introduction of the pre-injection strategy,with the ahead of the pre-injection timing,pre-injection combustion phase advances,when the interval of main injection and pre-injection reaches 10-20°CA,combustion phase tend to stable.4)Under light load conditions,in pure diesel mode,COV_IMEPis the largest when the main injection and pre-injection gap is 10°CA,after mixing with methanol,especially under the conditions of high co-combustion ratio,the introduction of pre-injection strategy can reduce COV_IMEP.When the main pre-injection interval is 20°CA,COV_IMEPincreases to a large extent.Under medium and heavy load conditions,the COV_IMEPof dual-fuel mode is greater than that of pure diesel mode,as the co-combustion ratio increases,COV_IMEPalso increases.When the pre-injection strategy is introduced and the pre-injection timing changes,the COV_IMEPhas a larger variation range under the condition of a large co-combustion ratio.5)Under light load conditions,in pure diesel mode,when the interval of main injection and pre-injection is 10°CA,the average temperature in the cylinder is higher.When the co-combustion ratio is less than 40%,after the introduction of the pre-injection strategy,when the pre-injection time is 5°CA ahead of the main injection,the maximum temperature in the cylinder decreases significantly compared with the single injection mode,after the gap between the main injection time and the pre-injection time increases,the temperature in the cylinder rises,and as the gap further increases,the maximal temperature in the cylinder reduce.When the co-combustion ratio is 40%,the temperature in the cylinder decreases in the single injection mode,after the introduction of the pre-injection strategy,the temperature in the cylinder rises,as the pre-injection timing continues to advance,the maximum temperature in the cylinder decreases.Compared with light load conditions,medium load and heavy load conditions,the introduction of the pre-injection strategy has less impact on the cylinder temperature.This paper studied the effects of the pre-injection timing on the emission characteristics of methanol-diesel engines under various operating conditions.The results show that:1)Under medium and light load conditions,co-combustion methanol can reduce the total concentration of ultrafine particles significantly,under high load conditions,only when the methanol co-combustion ratio is large（30%,40%）,can the ultrafine particles total concentration be reduced.Under light load conditions,the number concentration of ultra-fine particles varies with the pre-injection timing is more complicated.Under other conditions,the trends of the number concentration and average particle size of the ultrafine particles with the pre-injection timing are all rises after introduction of the pre-injection strategy,and then decreases as the pre-injection timing continues to advance,but the extent of the rise and fall changes slightly.2)Under light load conditions,the introduction of the pre-injection strategy will reduce BSPN,and as the pre-injection timing continues to advance,the BSPN will continue to decrease.Under medium load and large load conditions,after the introduction of the pre-injection strategy,BSPN will follow the change trend of the pre-injection timing is similar,rising and then falling.3)Under all working conditions,the methanol co-combustion ratio can effectively reduce the BSNO_x;after the introduction of the pre-injection strategy,with the change of the pre-injection timing,the change of the BSNO_xis relatively complicated,which is compared with the temperature and instantaneous heat release rate are inextricably linked.