预喷策略对双燃料发动机燃烧和排放特性的影响

通过一台6缸直喷、高压共轨柴油机改装成的柴油引燃天然气发动机.试验研究预喷正时和预喷油量对燃烧参数和性能参数的影响,结果表明:相对单次喷射,较晚的预喷正时(30°CA BTDC)能提高发动机有效热效率(BTE),降低HC、CO排放,但NOx排放恶化,而较早的预喷正时(60°CA BTDC)能够在提高发动机BTE的同时,降低HC、CO排放,并且NOx排放基本保持不变;当预喷正时为60°CA BTDC,预喷油量适当增多(3~5 mg/cyc)能进一步提高发动机BTE,降低HC、CO和NOx排放;预喷油量进一步增加(6~7 mg/cyc),NOx排放恶化,并且由于着火相位波动导致燃烧稳定性变差.改善双燃料发动机燃烧和排放特性一方面要增加预喷柴油在可燃混合气中的分布,增大柴油与可燃混合气混合的时间,改善混合气的活性,进而提高燃烧速率;另一方面要强化主喷柴油喷射对着火相位的控制,防止着火相位不一致而恶化燃烧稳定性.

Effect of Pre-Injection Strategy on Combustion and Emission Characteristics of Dual-Fuel Engines

An experiment was carried out to investigate the effect of various pre-injection timing and pre-injection quantity on combustion characteristics and exhaust emissions of a diesel/natural-gas dual-fuel engine,which was modified from a six-cylinder direct injection common rail diesel engine.Combustion parameters and emission parameters were investigated with different pre-injection timing and pre-injection quantity.The results show that,compared with the single injection mode,the brake thermal efficiency(BTE)can be improved and the HC and CO emissions are reduced with the late pre-injection timing(30°CA BTDC),but NOx emissions are improved.Besides,the BTE can be further improved and the HC and CO emissions are reduced with the earlier preinjection timing(60°CA BTDC),while NOx emissions remain unchanged.Furthermore,in the condition of earlier pre-injection timing(60°CA BTDC),the BTE can be improved and the HC,CO and NOx emissions are reduced with increasing the pre-injection quantity(3—5 mg/cyc).NOx emissions are improved and the problem of combustion instability,which is caused by inconsistent ignition phase,becomes more prominent with further increasing pre-injection quantity(6—7 mg/cyc).These results indicate that,in order to optimize pre-injection strategy for better combustion and emission characteristics of dual-fuel engines,it is necessary to increase the distribution of pre-injection fuel in the mixture or increase the time of air-fuel mixing process so as to improve the reactivity of mixture and the burning rate,and on the other hand to strengthen ability of main injection to control the ignition phase so as to avoid the worsen combustion stability caused by inconsistent ignition phase.