Experimental Study On The Effect Of Combustion Boundary Conditions On Spark-assisted Gasoline Compression Ignition At Low Load Conditions

Gasoline compression ignition is an efficient and clean combustion method,but it faces the bottleneck of unstable combustion at low load.The key reasons are low reactivity of gasoline fuel,and unfavorable thermodynamic condition in the cylinder at low-load.What’s more,this combustion mode is sensitive to boundary conditions.The spark ignition is a feasible and potential way for the stable gasoline compression ignition under low-load conditions,which can greatly reduce the heavy dependence on the thermodynamic state in the cylinder.Therefore,based on the experimental study of the effect of combustion boundary conditions on the spark-assisted gasoline compression ignition,this paper explores the technical approach to stable,efficient and clean gasoline compression ignition combustion of of heavy-duty engines under low-load conditions.First,combined with CFD simulation,the spark-assisted compression ignition combustion chamber structure and spark plug position is designed,and then the experiment is carried out,mainly to study the effect of fuel injection and spark strategies on the low-load combustion and emissions of spark-assisted gasoline compression ignition.Studies have shown that there is a controllable range of injection timing and spark timing for stable combustion under this low-load operating condition,and the spark timing needs to be properly matched with injection timing to maintain stable combustion.As the fuel injection pressure increases,the variant range of injection timing and spark timing for stable combustion can be expanded.The start of injection and spark timing ranges are-34 to-28°CA ATDC and-14 to-26°CA ATDC at fuel injection pressure of 40MPa,-34 to-22°CA ATDC and-12 to-28°CA ATDC at fuel injection pressure of 60MPa,-34 to-20°CA ATDC and-12 to-28°CA ATDC at fuel injection pressure of 80MPa.Affected by the diversion of the piston profile,each injection pressure has a special injection timing at which the combustion has the shortest ignition delay.This special timing is postponed when the injection pressure increases,and it is-32°CA ATDC for 40MPa,-30°CA ATDC for 60MPa,-28°CA ATDC for 80MPa.At this special injection timing,matched with a later spark timing within the controllable range for stable combustion,the highest thermal efficiency point is achieved.Because this injection timing ensures a reasonable mixture distribution near the spark plug,and the combustion phase is delayed to a more suitable position after the top dead center,hence the negative compression work is reduced,and the thermal efficiency is improved.Except for the effect of expanding controllable range,the thermal efficiency is improved by increasing injection pressure together with a suitable injection and spark timing.When the intake gauge pressure is 67.7k Pa,intake temperature is 40℃and injection pressure is 80MPa,a thermal efficiency of 33.89%is achieved.Due to the increased heat release rate,the NOx emissions at the most efficient point increases.Study on the effect of the intake boundary conditions on spark-assisted gasoline compression ignition was further carried out under low load conditions.The cylinder temperature,the loss of heat transfer and pumping loss are reduced by increasing intake pressure,thus the thermal efficiency is significantly improved,and NOx emissions are reduced.But at the same time,the increase in excess air coefficient will narrow the controllable range of injection timing and spark timing for stable combustion.When the intake air temperature is 40℃,through reasonable and optimized fuel injection and spark strategies,the maximum indicated thermal efficiency of 37.83%can be achieved at 102k Pa intake pressure,and the NOx is 8.57g/(k W·h)while maintaining a combustion efficiency of 95.53%.When the intake air volume is fixed,although increasing the intake air temperature cannot improve the indicated thermal efficiency due to the increased loss of heat transfer,it can advance the combustion phase,increase the compression-ignition heat release ratio,reduce the unburned hydrocarbons and improve combustion efficiency,and significantly extend the controllable range of injection timing and spark timing for stable combustion.The introduction of EGR reduces the average temperature and oxygen concentration in the cylinder,which significantly reduces the NOx production in the cylinder.At the same time,the reduction of the loss of heat transfer also improves the thermal efficiency to a certain extent,but its effect on improving thermal efficiency is relatively weaker than that on increasing the intake pressure.On this basis,an exploratory study was conducted to improve the thermal efficiency and NOx emission of spark-assisted gasoline compression ignition by appropriately increasing intake pressure and temperature and introducing EGR under low-load conditions.The final results show that when the intake air temperature is 60℃and the intake pressure is 133k Pa,a stable combustion with the highest indicated thermal efficiency of 39.47%can be achieved,NOx emission is reduced to 4.16g/(k W·h),the combustion efficiency is up to 94.68%,and the highest pressure rise rate is down to 0.24MPa/~oCA.Under the conditions with 98k Pa intake pressure,80℃intake temperature,and EGR rate of 30%,an in dicated thermal efficiency of 37.6%can be achieved,as well as NOx is only 2.33g/(k W·h),the combustion efficiency is up to 96.84%,and the highest pressure rise rate is down to 0.16MPa/~oCA.This research has important value as a reference for improving the combustion stability of spark-assisted gasoline compression ignition and approaching efficient and clean combustion under low-load conditions.