Experimental Research on Performance Development of Direct Injection Hydrogen Internal Combustion Engine with High Injection Pressure

<div>As a carbon-free power with excellent performance, the direct injection (DI) hydrogen-fueled internal combustion engine (H₂-ICE) has the potential to contribute to carbon dioxide (CO₂)-neutral on-road transport solutions. Aiming at high thermal efficiency, the influences of key factors on thermal efficiency over wide operating conditions of a turbocharging DI H₂-ICE were investigated under the lean-burn strategy. And the nitrogen oxides (NO_x) emission characteristics region was clarified in the high efficiency. The results confirm the optimal ignition strategy with the CA50 of 8–9 crank angle degrees after top dead center (°CA ATDC). The late-injection strategy manifests a significant advantage in brake thermal efficiency (BTE) compared with the early-injection strategy, and this advantage can be amplified by the increased load or injection pressure. The effects of injection (EOIs) pressure on BTE exhibit different laws at different EOIs. Under the early-injection strategy, the lower injection pressure improves BTE due to a more sufficient mixing. While under the late-injection strategy with strong mixture stratification, the high injection pressure conditions exhibit a higher BTE due to reduced compression work. In terms of air-fuel ratio, the BTE is improved monotonically with increased λ at low and medium loads. But there is an optimal λ value limited by the oxygen concentration at a high load. The late-injection strategies with high BTE perform a high level of NO_x emissions, which confirms the strong trade-off relationship between the thermal efficiency and NO_x emissions of H₂-ICEs. A moderate late-injection strategy with an EOI of about 40°CA BTDC can significantly reduce the NO_x emissions with a slight loss in BTE. The injection pressure shows different effects on NO_x emissions in different EOI ranges, depending on the mixture distribution. In addition, ultra-lean burn and lower intake temperature are effective means to reduce NO_x emissions without losing thermal efficiency.</div>