| Reactivity controlled compression ignition(RCCI)is a new combustion mode of internal combustion engine,which has the advantages of clean and efficient combustion.At present,the problem of load expansion of RCCI engine,especially the problem of incomplete combustion at low loads,is the key factor restricting its practical application.The dual-direct injection strategy can control the in-cylinder mixture distribution more accurately and has the potential to optimize the combustion characteristics of RCCI.While hydrogenated catalytic biodiesel(HCB)can improve engine ignition performance due to high cetane number.Therefore,in this paper,by using three-dimensional CFD software CONVERGE,simulation study on gasoline/ HCB RCCI combustion characteristics with dual-direct injection strategy at low loads was carried out,in order to improve the RCCI low-load combustion characteristics.Firstly,the combustion characteristics of conventional RCCI at different loads were studied.It was found that with the reduction of load,the problem of incomplete combustion caused by too lean mixture of conventional RCCI gradually become prominent.Then the combustion characteristics of conventional RCCI and dual direct injection RCCI at low loads were compared.The results showed that,compared with conventional RCCI,dual direct injection strategy makes the gasoline centrally distribute in the piston recess,forming a mixture with moderate equivalent ratio which can be ignited easily,thus improving the incomplete combustion phenomenon.In addition,direct injection of gasoline can enhance the air flow in the cylinder,which can accelerate the diffusion and mixing of HCB.The fuel injection timing is an important parameter to control the mixture distribution and optimize the combustion process.The results showed that keeping the HCB injection time at-25° CA ATDC,with the delay of gasoline injection timing,the diffusion and mixing of HCB become worse,the mixture stratification degree reduces,ignition time is delayed,combustion duration is shortened,combustion temperature reduces,combustion efficiency reduces,indicating thermal efficiency reduces first and then increases while finally reduces,HC,CO and soot emissions increase,NOx emissions decreases.When the gasoline injection time is-45°CA ATDC,with the delay of HCB injection timing,stratified combustion intensifies,heat release process is advanced,combustion temperature increases,combustion efficiency increases,indicating thermal efficiency increases first and then decreases,HC,CO and soot emissions decrease,NOx emissions increases.While HCB injection timing is 25 ~ ~ 20 ° CA ATDC and gasoline injection timing-50 ~-45 ° CA ATDC,the engine exerts better performance,with more than 92% of the combustion efficiency,more than 45% of the indicated thermal efficiency,low NOx and soot emissions that can meet National Ⅵ standards.Based on the dual-fuel injection timing optimization,the effects of intake temperature and engine speed on combustion and emission characteristics of dual-direct injection RCCI at low loads were studied,which provides theoretical basis for further optimization of low-load performance of dual-direct injection RCCI engine.The results showed that the increase of intake temperature can accelerate the combustion reaction,advance the heat release process,shorten the combustion duration,increase the combustion efficiency,increase indicated thermal efficiency first and then decreases,decrease HC,CO and soot emissions,increase NOx emissions.While adopting HCB late injection strategy can reduce the sensitivity of combustion to the intake temperature.With the increase of the engine speed,the cycle time decreases,the peak heat release rate and the peak cylinder pressure decrease,the heat release process is delayed,the combustion duration is prolonged,the combustion efficiency and indicating thermal efficiency decrease,HC and CO emission increase,but the power increases with the increase of the engine speed.When the injection timing of HCB is-15° and-20°CA ATDC,NOx emission is slightly higher and decreases with the increase of the speed.With other HCB injection timing,NOx emission is very low and is little affected by engine speed.Soot emission is also little affected by engine speed,increasing slightly with the increase of engine speed only in the early and late HCB injection strategy. |
