Strategies of hydrogen-air mixture formation for suppressing backfire, pre-ignition, and knock in full-load operations of hydrogen heavy-duty engines

This study integrated hydrogen spray visualization and multi-cylinder combustion diagnostics to establish hydrogen-air mixture formation strategies that simultaneously suppress backfire, pre-ignition, and knock in full-load operations of a hydrogen port-fuel injection, heavy-duty engine. Using the Schlieren imaging, the actual injection duration of hydrogen spray according to hydrogen injection pressure was determined. Based on an injection duration limit, only injection pressures ranging from 1.0 to 1.2 MPa could be utilized at the maximum power, whereas all five injection pressure ranges were allowed at the maximum torque. In hydrogen engine experiments, backfire occurred as an excess air ratio decreased. Backfire in a particular cylinder caused knock and pre-ignition in the subsequent firing-order cylinders; therefore, the engine load was drastically reduced. The backfire was suppressed by increasing the excess air ratio, but pre-ignition and knock occurred sequentially while delaying the hydrogen start of injection (SOI). Pre-ignition occurring in one cylinder did not affect the combustion processes in the other cylinders; therefore, the engine could be operated stably. Appropriately high excess air ratios to suppress backfire, early hydrogen SOIs to suppress pre-ignition and knock, and low hydrogen injection pressures within an actual injection duration limit were beneficial for achieving high full-load performance.