Influence of Injection Strategies on Engine Efficiency for a Methanol PPC Engine

Partially premixed combustion (PPC) is one of several advanced combustion concepts for the conventional diesel engine. PPC uses a separation between end of fuel injection and start of combustion, also called ignition dwell, to increase the mixing of fuel and oxidizer. This has been shown to be beneficial for simultaneously reducing harmful emissions and fuel consumption. The ignition dwell can be increased by means of exhaust gas recirculation or lower intake temperature. However, the most effective means is to use a fuel with high research octane number (RON). Methanol has a RON of 109 and a recent study found that methanol can be used effectively in PPC mode, with multiple injections, to yield high brake efficiency. However, the early start of injection (SOI) timings in this study were noted as a potential issue due to increased combustion sensitivity. Therefore, the present study attempts to quantify the changes in engine performance for different injection strategies. Simulations were performed on a heavy-duty multi-cylinder compression ignition engine fueled with methanol. Two operating conditions with different engine load were chosen from the European stationary cycle. Three different injection strategies were applied: 1) SOI > −160°ca aTDC 2) SOI > −40°ca aTDC 3) SOI > −25°ca aTDC. The engine settings were selected to maximize the brake efficiency for each case and the sensitivity of combustion to inlet conditions was analyzed. For the high load operating point, the brake efficiency was 2.2 %pt. higher for case 1 compared to case 3, while this difference was only 0.5 %pt. for the low load operating point. However, the combustion phasing for case 1 and 2 at the high load point proved to be very sensitive to inlet temperature, inlet pressure and oxygen concentration.