Effect Of Sensitive Combustion Boundary Parameters On Combustion Phase Of Dual-Fuel Engine

Natural gas(NG)engine has been widely concerned because of its extensive resources and relatively clean combustion.Among them,the diesel pilot ignited natural gas dual fuel engine has the advantages of low cost and equal thermal efficiency to diesel at medium and high load,low NO_X and smoke emission.However,its low thermal efficiency and high hydrocarbon emissions at low load have become one of the important reasons restricting the development and promotion of dual-fuel engine.In order to improve the above shortcomings,a 6-cylinder turbocharged intercooler diesel/natural gas dual fuel heavy-duty engine was used in this study.The influence of sensitive combustion boundary parameters on the combustion phase of pilot diesel and natural gas under low load is studied in detail by means of experiments and numerical simulation.It provides theoretical support for organizing reasonable in-cylinder combustion and optimizing combustion process,and points out the direction of optimization.First of all,the pilot diesel injection timing was controlled over a very wide range under the conditions of 25%load,low speed(1335 r/min),medium speed(1656r/min)and high speed(1978 r/min).The results show that the injection timing has an obvious effect on the engine combustion process including combustion phases of the pilot diesel and the natural gas,and which decide the engine thermal efficiency and exhaust emissions level.The chemical reaction of diesel and natural gas before combustion start at the same time,however,the combustion duration of natural gas lags behind that of diesel.In addition,during the combustion overlap period,the in-cylinder combustion is good and the working ability is strong.To advance the pilot diesel injection timing from 5°CA BTDC to 35°CA BTDC,the chemical ignition delay of diesel shows a lengthening trend,and the combustion duration shows a shortened trend.With advancing the pilot diesel injection,the natural gas combustion duration shows a shortened trend,and the shortest is near to 15°CA.There is a clear correspondence between the lengths of combustion period of two fuels.Either the combustion duration or the combustion overlap period shows a shortened trend,but the ratio of the combustion overlap period to the combustion duration is increasing with advancing the pilot diesel injection.The shortest combustion duration corresponds to the lower brake specific energy consumption and emissions.Then,the effects of percentage of energy substitution(85%,90%,95%)on the combustion phase of dual-fuel engine before and after the change of in cylinder combustion mode were studied at 25%load,low(1335 r/min),medium(1656 r/min)and high(1978 r/min)speed.The results show that the percentage of energy substitution determines the distribution range and size of ignition energy,and then affects the combustion phase of diesel and natural gas.With the increase of percentage of energy substitution,the combustion efficiency decreases,but the process of combustion mode transformation is accelerated.When the percentage of energy substitution increases from 85%to 95%,the overall combustion hysteresis of pilot diesel does not change significantly,but the physical hysteresis shortens and the chemical hysteresis prolongs.The combustion center of diesel and natural gas moves backward,and the combustion duration tends to extend.