Combustion characteristics of high pressure direct-injected methanol ignited by diesel in a constant volume combustion chamber

To figure out the interaction between direct-injected methanol and diesel during the whole combustion process, this paper studied the direct-injected methanol ignited by diesel through shadowgraph and natural luminescence imaging in a constant volume combustion chamber which allows both fuels directly injected with high pressure. Strategies of diesel injection with pilot methanol (DIPM) and conventional diesel injection (CDI) were included. Results show that in DIPM cases, there were areas where the methanol failed to achieve a complete burning at the end of combustion. These areas may explain the excessive emission of HC in the relevant engine tests. Besides, DIPM ignition delay firstly drops from 1.183 to 1.125 ms as the diesel injection pressure rising from 60 to 120 MPa and then increases to 1.237 ms when diesel pressure is 140 MPa. While ignition delay in CDI cases keeps decreasing with the increase of diesel pressure. It indicates that inhibition effect of methanol injection was not obvious in DIPM at low pressure injected diesel condition due to a poor methanol-diesel mixing but took dominance when diesel injection pressure was high. The inhibition of methanol also led to the result that ignition spot of DIPM was 8.9-17.6% closer to diesel injector nozzle than that of CDI with the increase of diesel pressure. Finally, the SINL of DIPM drops more quickly than CDI. It implies a faster and more complete soot oxidation in DIPM cases caused by the later combustion of methanol.