Study of single and split injection strategies on combustion and emissions of hydrogen DISI engine

Abstract

Hydrogen energy shows excellent potential as an alternative fuel for internal combustion engines (ICE) in terms of cleanliness and efficiency. However, the specificity of hydrogen density and diffusion rate leads to further optimization of the working process of the hydrogen direct injection spark ignition (DISI) engines. In this study, the effects of single injection and split injection strategies on the combustion and emissions of a hydrogen DISI engine were investigated at medium/low load. The specific results showed that premature hydrogen injection could trigger pre-ignition events, and it was appropriate to set the earliest hydrogen injection timing to 180° CA BTDC after the intake valve closing. For the single injection strategy, the brake thermal efficiency (BTE) increased first and then decreased with the delay of the start of injection (SOI). When SOI = 100° CA BTDC, the optimal BTE (36.1 %) could be obtained, and nitrogen oxide (NOx) emissions could still be limited to less than 0.1 g/(kW·h). For the split injection strategy, Delaying the secondary end of injection (SEOI) beyond 30° CA BTDC rapidly worsened NOx emissions to over 1.5 g/(kW·h). Delaying SEOI achieved higher efficiency but slightly worse the coefficient of variation COVIMEP than the single injection strategy. In addition, increasing the secondary injection mass fraction (SIMF) to 30 % could achieve an 18 % higher peak heat release rate than a single injection and shorter combustion duration. However, over 20 % SIMF resulted in an increase of NOx emissions to over 4 g/(kW·h). The energy distribution indicated that adjusting the injection strategy to increase BTE was mainly achieved by reducing power of heat loss (PHL).