Experimental investigation into the combustion characteristics of a methanol-Diesel heavy duty engine operated in RCCI mode

This study examines combustion in a dual-fuel methanol-Diesel heavy duty engine using three different methanol injection configurations: port injection into the intake manifold; direct injection during the intake stroke (DI_E) and direct injection during the compression stroke (DI_L). The latter two methanol direct injection configurations were used in the attempt to reduce HC and CO emissions, which were considerably high in the port-injected high-octane fuel RCCI combustion. Engine experiments were performed using a double Diesel injection strategy with two pilot Diesel injections (PI1 and PI2) and a constant engine speed of 1500 rpm. The effects of three parameters - the PI1 and PI2 injection timings, and the PI2/PI1 duration ratio - were investigated at 5 bar IMEP for the three methanol injection configurations. The onset of unstable combustion and excessive combustion phasing advancement imposed lower or upper limits on the sweeps over the studied parameters. The DI_L configuration achieved lower net indicated thermal efficiencies than the other two methanol injection configurations. The influences of the methanol injection pressure and methanol substitution percentage (MSP) were also investigated for the DI_L configuration at 5 bar IMEP, revealing that the combustion process was relatively insensitive to the methanol injection pressure but was adversely affected by increasing the MSP. Finally, the port and DI_L configurations were tested at various loads. Neither configuration offered any advantage over pure Diesel combustion in terms of net indicated thermal efficiency nor emissions of HC and CO, but both offered lower greenhouse gas emissions at all load points. However, only the methanol port injection configuration achieved ultra-low NOx and soot emissions at 12 bar IMEP.