Study on the potential of injection strategies to suppress knock combustion in direct-injection hydrogen engines under high compression ratio

Knock limits the compression ratio increase of hydrogen engines, and overcoming the limitations of knock combustion under high compression ratios is crucial for the application of hydrogen engines. This study investigates the impact of high compression ratios interfaced with injection strategies on knock in direct-injection hydrogen engines through numerical simulations. The results indicate that when the compression ratio increases from 12.5 to 15, ignition timing demands to be delayed by 9 °CA to achieve acceptable knock levels, resulting in a delay in the combustion phase. At a compression ratio of 15, as injection pressure increases, the knock intensity (KI) first decreases and then increases. At an injection pressure of 6 MPa, a mixture of rich ignition zone and lean end zone is formed in the cylinder, resulting in KI reaching its minimum value. Knock intensity does not change monotonically with injection timing. When injection timing is −100 °CA, under the action of turbulent kinetic energy, hydrogen gas is evenly divided into two beams and moves towards the center of the combustion chamber, forming a relatively enriched mixture near the piston, the air–fuel mixture distribution and combustion zone temperature become more uniform, and KI reaches its minimum. Therefore, optimizing the injection strategy can increase the compression ratio of direct-injection hydrogen engines without significantly delaying the ignition timing and combustion phase.