Comparative study of effects of pilot injection and fuel properties on low temperature combustion in diesel engine under a medium EGR rate

Comparative study of effects of pilot injection strategy and properties of fuels on the spray, combustion and emissions of a four-cylinder diesel engine was investigated under an intermediate exhaust gas recirculation (EGR) ratio. Four different fuels including pure diesel (D100) and three blended fuels consisting of diesel/gasoline (in a volume ratio of 70:30, D70G30), diesel/n-butanol (in a volume ratio of 70:30, D70B30) and diesel/gasoline/n-butanol (in a volume ratio of 70:15:15, D70G15B15) were investigated. The pilot injection strategies were accomplished by varying the pilot injection ratios and the pilot-main intervals. The results showed that, although the addition of gasoline or n-butanol to diesel had little influence on the spray characteristics, it can greatly reduce soot emissions and lead to an increase in maximum pressure rise rate (MPRR) and brake specific fuel consumption (BSFC). Using the pilot injection strategy with a larger pilot injection ratio and a shorter pilot-main interval can effectively reduce MPRR. A smaller pilot-main interval can remarkably reduce BSFC. Compared to single injection, pilot injection results showed that soot, THC and carbon monoxide (CO) emissions for four fuels increased, while the corresponding NOx emissions decreased. Both the decrease of pilot injection ratio and increase of pilot-main interval can lead to a decline in soot emissions. Nevertheless, the addition of gasoline or n-butanol to diesel can greatly offset the deterioration of soot emissions, induced by the adoption of pilot injection strategy. Additionally, compared to D100, the variations in soot emissions from the combustion of three blended fuels are less sensitive to variations in pilot injection ratios and pilot-main intervals. For all the studied fuels, effects of pilot injection ratios on NOx emissions are less obvious than the effects induced by the pilot-main interval. A decrease in pilot-main interval can further reduce the NOx emissions. For all the studied fuels, decrease in pilot injection ratios or pilot-main intervals can reduce the emissions of THC and CO, whereas the difference among four fuels is quite small. However, the pilot-main interval shows a more remarkable effect on the THC emissions than the pilot interval ratio.