An experimental study of knock in a DI hydrogen engine: The synergistic effects of the deep Miller cycle and oxygen-enriched atmosphere

There are few investigations on the knock characteristics of direct -injection (DI) hydrogen engines, so to reveal the influencing mechanism of synergistic effects of the deep Miller cycle and oxygen-enriched atmosphere on the DI hydrogen engine ' s knock characteristics, an experimental study was carried out at 1600 rpm and wide-open throttle conditions. Overall, the Miller effect provides good suppression of combustion knock even at high oxygen concentration ( phi O 2 ), while the oxygen-enriched atmosphere can mitigate the negative effects of the deep Miller cycle on the combustion process. The results demonstrated that the Miller cycle mitigates the excitation effect of elevated phi O 2 and reduced lambda on combustion knock. For example, even if phi O 2 was raised to 27.1 %, the knock intensity (KI) is only close to 1.0 bar at 1.6 lambda . It was found that the deep Miller cycle achieved by regulating the intake valve timing to the engine could further suppress the combustion knock, which may be related to the reduced effective compression ratio and volumetric efficiency. Specifically, under the deep Miller effect, KI did not exceed 0.8 bar even when phi O 2 was elevated to 32.0 %, indicating that knock was effectively suppressed, although about 190 cycles corresponding to knock durations exceeding 10 degrees CA.