Evaluation of the variable valve timing strategy in a direct-injection hydrogen engine with the Miller cycle under lean conditions.

• The Miller cycle and variable valve timing are used in a DI hydrogen engine. • The effects of variable valve timing on the airflow exchange process are analyzed. • The engine can reach 42.22% maximum brake thermal efficiency. • Positive effects of exhaust valve timing on a DI hydrogen engine cannot be ignored. The favorable physicochemical characteristics of hydrogen make it an excellent alternative fuel for IC engines. Compared with hydrogen port injection, hydrogen direct injection provides multiple degrees of freedom for injection strategies and mixture formation, which can effectively suppress abnormal combustion and enhance engine performance. Variable valve timing (VVT) and the Miller cycle, as advanced and effective means of improving engine performance, are less investigated based on a direct-injection (DI) hydrogen engine. Hence, to optimize the performance of a DI hydrogen engine, strategies of the Miller cycle and VVT are introduced. In this work, a 1.5 L direct-injection engine with a VVT system was employed to evaluate the effects of the Miller cycle, intake valve closing (IVC) timing, and exhaust valve opening (EVO) timing on the airflow exchange process, in-cylinder charge characteristics, heat release process, dynamics and emission characteristics. The engine speed was stabilized at 1400 rpm and the strategies of lean combustion and late injection were used. The main results are as follows. As the IVC timing is delayed, the ratio of pumping mean effective pressure to indicated mean effective pressure (PMEP/IMEP) first decreases and then slightly increases, and the engine tends to achieve a high level of brake thermal efficiency (BTE) when PMEP/IMEP is lower. The optimization of intake and exhaust VVT can shorten combustion duration and reduce cyclic variation by increasing volumetric efficiency and improving mixture distribution at ignition timing. Furthermore, the engine reaches 42.22% BTE and a high level of brake mean effective pressure by the cooperative control of IVC and EVO timing. NOx emissions can be controlled below 100 ppm by employing the synergistic effect of the Miller cycle and VVT. It is worth mentioning that exhaust VVT as a way to compensate for the performance of the DI hydrogen engine with the Miller cycle should not be ignored. [ABSTRACT FROM AUTHOR]