Numerical study on the stratified combustion characteristics of methanol-diesel dual direct injection for marine engine

Dual direct injection (DDI) is the optimal way to use methanol in diesel engine, but its control is complicated by numerous adjustable parameters. The paper aims to provide intrinsic guidance for the control simplification and optimization of DDI. Thus, the effect of two key injection parameters on stratified combustion characteristics has been numerically studied on a marine engine model to reveal the formation and regulation mechanism of the stratification of equivalence ratio.

Results show that the injection strategy of methanol-methanol-diesel is suitable for DDI with a three-layer stratification of equivalence ratio from high to low along the center to edge of the combustion chamber. The processes of combustion and pollutant generation can be flexibly controlled by adjusting the stratified state. Properly advancing the start of diesel injection (DSOI) can improve indicated thermal efficiency by more than 13 percentage points and obtain extremely low emissions of hydrocarbon and carbon monoxide, but excessive advance still leads to serious diesel homogenization and has an adverse effect on the initial ignition. Meanwhile, the ratio of the second methanol injection (MR2) determines the stratified state of equivalence ratio. The increase of MR2 is beneficial to improving the combustion speed and combustion quality with an 8 times reduction of combustion duration, but excessive increase also causes combustion roughness and sharp rise of nitrogen oxides. The appropriate values of DSOI and MR2 are around −8 degree of crank angle after top dead center and 55 %. At that condition, the indicated thermal efficiency is as high as 49.3 %, while the ringing intensity and nitrogen oxides are as low as 2.5 MW·m⁻² and 1.84 g·(kW·h)^-1.