The Experimental Investigation Of Pilot Diesel Autoginition Behavior In A Dual Fuel Engine

The hydrocarbon(HC)emissions are high in conventional diesel/natural gas dual fuel engines at medium and especially low loads,resulting in lower thermal efficiency.The lack and insufficient distribution of ignition centers due to the pilot diesel stratified compression combustion is one of the important reasons.Therefore,the effects of pilot diesel injection timing(Tinj)and pressure(Pinj)on autoignition behavior and the subsequent combustion of natural gas are investigated experimentally in a diesel/natural gas dual fuel engine,and the reasonable ignition mode of dual fuel engines at medium and low loads is put forward.First,the ignition,combustion and emission characteristics of a diesel/natural gas dual fuel engine at different Tinj are investigated,the results show that the Tinj can effectively control the diesel ignition mode,and then the combustion mode.With the conventional Tinj,the combustion of pilot diesel is the stratified compression combustion,and the combustion is the conventional dual fuel combustion mode,which has two distinct heat release rate peaks.According to the percentage energy substitution(PES),the first peak of heat release rate(PHRR1)occurs near crank angle of 15% heat release(CA15),crank angle of 10% heat release(CA10),and crank angle of 5% heat release(CA5),respectively.A significant advancing Tinj can realize the quasi-homogeneous charge compression ignition combustion(QHCCI)of pilot diesel,the autoignition does not occur at a specific place in the spray,but at the whole combustion chamber.The heat release rate curve has only one peak which is located near crank angle of 50% heat release(CA50).At low load,with increasing PES,the fuel injection advance angle where the ignition mode changes is smaller,this shows that increasing PES can accelerate the transformation process of ignition mode.This is due to that the decreased diesel and extended ignition delay speed the mixing process of diesel-air mixtures.Because the limits of the maximum in-cylinder pressure and the maximum pressure rise rate,the QHCCI ignition mode cannot be achieved by advancing Tinj at medium load,when the 85% and 90% PES are utilized.However,this ignition mode can be realized at medium load with 95% PES.With advancing Tinj,the in-cylinder pressure,in-cylinder temperature,heat release rate,start of combustion(SOC),CA50,HC,carbon monoxide(CO)and oxynitride(NOx)emissions show completely different variation trends in different ignition modes.The ignition process is very sensitive to Tinj in QHCCI ignition mode.When the Tinj is too early,the diesel impingement causes the deterioration of diesel combustion,which leads to the poor combustion process of natural gas,higher combustion cyclic variation,THC and CO emissions and lower brake thermal efficiency.Therefore,Tinj should be strictly controlled in QHCCI ignition mode.Generally,the higher thermal efficiency and lower exhaust emissions can be obtained in QHCCI ignition mode before the deterioration of ignition.The brake thermal efficiency can obtain 36.0%,and THC,CO and NOx emissions are 1790,374,45 ppm respectively at A25 condition with 85% PES,when Tinj is 45°BTDC.Under normal diesel mode,they are 38.3%,28,73 and 713 ppm respectively.In addition,the brake thermal efficiency can obtain 38.1%,and THC,CO and NOx emissions are 1016,113,663 ppm respectively at A50 condition with 95% PES,when Tinj is 32.5°BTDC.The corresponding values are 39.9%,64,102 and 776 ppm under pure diesel mode.In this paper,the effects of Pinj on combustion and emission characteristics of a diesel/natural gas dual fuel engine at different Tinj are investigated.The results indicate that Pinj is an important parameter,which influences the pilot diesel autoignition behavior of a diesel/natural gas dual fuel engine.With increasing Pinj,the fuel injection advance angle where the ignition mode transforms from conventional compression combustion to QHCCI mode becomes smaller.This suggests that increasing Pinj can promote the transformation process of pilot diesel ignition mode,which eventually decreases the maximum fuel injection advance angle.In conventional diesel compression combustion ignition mode,increasing Pinj can effectively improve the combustion process at low and medium loads of dual fuel engines.With increasing Pinj,PHRR1,the second peak of heat release rate(PHRR2)and brake thermal efficiency increase,combustion cyclic variation,HC and CO emissions decrease,but the in-cylinder pressure,temperature,pressure rise rate and NOx emissions increase.In addition,with increasing Pinj,the effect of Pinj on engine performance is werk.In QHCCI ignition mode,the diesel impingement will occur in advance with increasing Pinj,which leads to the decreased thermal efficiency and increased HC and CO emissions.A significant advancing Tinj provides adequate mixing time for diesel,thus the mixing of diesel is not dependent on Pinj.with increasing Pinj,the combustion cyclic variation,thermal efficiency,HC and CO emissions have smaller change in QHCCI ignition mode before the deterioration of ignition.Therefore,the low injection pressure should be selected to reduce the fuel injection cost in QHCCI ignition mode.