Effects of Two Pilot Injection on Combustion and Emissions in a PCCI Diesel Engine

The effects of two pilot injections on combustion and emissions were evaluated in a single-cylinder turbocharged diesel engine, which operated in premixed charge compression ignition (PCCI) modes with multiple injections and heavy exhaust gas recirculation under the low load by experiments and simulation. It was revealed that with the delay of the start of the first pilot injection (SOI-P1) or the advance of the start of second pilot injection (SOI-P2), respectively, the pressure, heat release rate (HRR), and temperature peak were all increased. Analysis of the combustion process indicates that, during the two pilot injection periods, the ignition timing was mainly determined by the SOI-P2 while the first released heat peak was influenced by SOI-P1. With the delay of SOI-P1 or the advance of SOI-P2, nitrogen oxide (NOx) generation increased significantly while soot generation varied a little. In addition, increasing Q1 and decreasing the second pilot injection quantity (Q2) can manipulate the NOx and soot at a low level. The advance in SOI-P2 of 5 degrees CA couple with increasing Q1 and reducing Q2 was proposed, which can mitigate the compromise between emissions and thermal efficiency under the low load in the present PCCI mode.