Bowl Geometry Optimization Of A Heavy-Duty Diesel Engine In High Density-Low Temperature Combustion

It is a difficult problem to improve the thermal efficiency of heavy diesel engine and control the emission at the same time under full load condition.High-density and low-temperature combustion(HD-LTC)technology has the potential to achieve highefficiency and low-emission combustion under full load conditions.The piston bowl geometry has an important influence on the formation and combustion of combustible mixture,which in turn affects the thermal efficiency and emission performance of the engine.In this paper,the bowl geometry of a heavy-duty diesel engine in high densitylow temperature combustion is optimized,under the condition of low speed and full load.Through the experimental study of single-cylinder bench,it was found that based on HD-LTC strategy,increasing the intake pressure and delaying the closing of the intake valve can greatly increase the charge density within the allowable range of the engine’s maximum explosion pressure,thereby accelerating mixing and burning of oil and gas in-cylinder,reducing the combustion temperature.This helps to achieve the purpose of improving the thermal efficiency of the engine and reducing NOx and soot emissions.Through three-dimensional numerical simulation research,it was found that the original piston bowl(stepped-lip)is not well matched with the spray at the highest thermal efficiency operating point of the original engine under the condition of low speed and full load,which is in HD-LTC strategy.This leads to two problems about the distribution of oil and gas in-cylinder.one is that the distribution of oil and gas above and below the nozzle hole axis is unreasonable,another one is that the oxygen in the relatively upper space of the piston pit is not fully utilized.those problems restrict the mixing and burning in the later stage of combustion.In order to further improve the thermal efficiency of the engine,according to the existing problems of oil and gas distribution in-cylinder with the original piston bowl used,the effect of the variety of three dimensional parameters of the piston bowl(the lip diameter,the lip depth and the concave arc radius)on the mixing,combustion,heat transfer loss and emission in the cylinder was studied,and the three parameters were optimized step by step.The study found that,based on the original piston bowl,proper adjustment of the lip diameter and lip depth can adjust the relative proportion of fuel that enters the upper or lower areas of the nozzle hole axis,making the oil and gas distribution more reasonable.The adjustment can also adjust the vertical distribution of the rich mixture in the piston bowl,making better use of the oxygen in the piston bowl.Those help to accelerate the mixing of oil and gas in-cylinder,and accelerate the heat release during the later stages of combustion,thus increasing the indicated thermal efficiency.On this basis,properly increasing the concave arc radius of the piston bowl can help to reduce the heat transfer loss,thus further increasing the indicated thermal efficiency.Compared with the original bowl,when the optimized piston bowl(called R1)is used,the indicated thermal efficiency is increased from 49.00% to 49.47%,and the soot emissions are reduced by50% while the NOx emissions are basically unchanged.