Progressive strategies to avoid and exploit knock limit for optimal performance and stoichiometric operation of a DI hydrogen engine with high CR at WOT conditions.

• All tests are conducted under wide-open throttle conditions. • Two progressive strategies to avoid and exploit borderline knocking are proposed. • The interaction mechanism between strategies and combustion knock is revealed. • Pre-ignition may lead to more severe knock. • The DI hydrogen engine achieves knock-free combustion under stoichiometric condition. Knock is one of the crucial challenges facing direct injection (DI) hydrogen engines. In this study, the rational optimization of operating parameters and the proposal of two progressive knock suppression strategies enable the DI hydrogen engine to achieve knock-free operation at wide-open throttle (WOT) and stoichiometric conditions, which may better control the test system's complexity. The present work uses 0.5 bar knock intensity (KI) as the threshold for occurring knock. The test consists of three parts. In Part 1, KI exceeds 0.5 bar at 1.7 λ. Then, considering the suppression effect of the stratified mixture on knock, a strategy for cooperative control of λ and start of injection (SOI) is proposed in Part 2, which can lower λ to 1.4 within the allowable borderline knocking. However, this strategy fails at 70°CA BTDC SOI. Based on the above results, a strategy to synergistically adjust λ and intake variable valve timing is adopted in Part 3, which suppresses knock mainly by varying the airflow exchange process and effective compression ratio. Finally, the strategies presented in Part 2 and Part 3 allow the engine to control the KI to less than 0.5 bar at WOT and stoichiometric conditions, which ensures power performance. [ABSTRACT FROM AUTHOR]