Effect of injection position on in-cylinder airflow motion and combustion in a diesel-ignited hydrogen free-piston engine

With the environment deteriorating increasingly, the exploration of clean energy has become extremely urgent. Therefore, this study aims to deeply analyze the variation laws of in-cylinder airflow dynamics and combustion characteristics of hydrogen-fueled free-piston engines by adjusting the diesel injection positions. In view of the special structure of the engine, a new coupling model integrating dynamics and thermodynamics principles was constructed, and iterative algorithms were used to carry out computational analysis. The results show that the diesel pilot ignition technology can change the in-cylinder airflow movement and promote fuel mixing. Meanwhile, increasing the distance from the injection position to the top dead center (TDC) has a positive effect on the in-cylinder turbulent kinetic energy, combustion, and working efficiency, and also helps to reduce Soot emissions. When the distance is 2.9mm, the maximum in-cylinder turbulent kinetic energy reaches 33m 2 /s 2 , which is approximately 43.5% higher than that at 0mm. The thermal efficiency and mean indicated pressure are 43.6% and 0.64MPa, increasing by 0.4% and 1.6% respectively compared with those at 0mm. From the perspective of heat transfer, the time and amount of heat transferred by FPE for heat transfer will also increase, resulting in greater heat transfer loss. When the injection position is 2.9mm, the heat transfer loss is approximately 12% higher than that at 0mm. Therefore, appropriately increasing the distance from the injection position to the TDC is beneficial to improving engine performance and reducing heat loss.