Numerical evaluation of pilot-ignition technology used for a hydrogen fuelled free piston engine generator

The spark-ignited free-piston hydrogen engine is characterized with relatively slow combustion and low indicated thermal efficiency. This study explored the pilot-ignition technology to the free-piston hydrogen engine and presented a research to evaluate its applicability. An advanced full-cycle operation model which couples with dynamic, combustion and gas exchange was established to iteratively simulate the effect of pilot-ignition technology on a free piston hydrogen engine. The simulated performances were compared with a corresponding spark-ignition situation. Results indicate that compared with the spark-ignition, the pilot-ignition technology can speed the hydrogen fuel combustion in the free-piston engine and enhance the level of isochoric heat release around top dead center, resulting in more indicated work and higher indicated thermal efficiency. The hydrogen engine adopting the pilot-ignition technology features with faster cycle, larger compression ratio, higher generating power, and slightly more NO emission than the spark-ignition situation. Moreover, the pilot-ignition technology also brings about longer ignition delay and shorter rapid combustion period for the hydrogen engine. However, the released heat in rapid combustion period is significantly more than the spark-ignition situation, which makes that the hydrogen engine operates with larger peak combustion pressure and temperature, and the peak values appear earlier and last shorter.