Endoscopic imaging of split-injected gas jet developments in a multi-cylinder hydrogen low-pressure direct-injection spark-ignition engine

Split injection is widely used in conventional spark ignition engines to control mixture formation. To utilise split injection in a hydrogen direct injection engine, it is important to understand gas jet development and its variations with injection timing. This is because prolonged duration of gaseous fuel direct injection is challenged by complex jet-flow interaction involving tumble flow, which could be more significant than the short-injected liquid fuel. The ambient air pressure and density during the gas injection also changes depending on the injection timing, adding more complexity. This study performs endoscopic high-speed imaging of gas jet laser shadowgraph in an inline four-cylinder low-pressure direct-injection spark ignition (H2LPDI) engine. Due to safety concern and its known similarity in macroscopic jet developments, helium was used as an alternative gas to hydrogen. The results showed that the gas jet development changes greatly with the split injection timing selected with respect to the intake valve closure (IVC). For pre-IVC split injection, the first jet and second jet exhibited a very similar jet structure with statistically identical spreading angle and mixture centroid because both injections occurred at low air density conditions and before a tumble flow structure developed. However, the second injection showed a lower penetration rate and jet mixing rate, indicating disturbance caused by the first injection. For post-IVC split injection, the second jet showed a narrower spreading angle due to lifted lower jet boundary, suggesting a strong influence of jet-tumble interactions. As the split injection was executed after the IVC, the tumble flow became strong and well defined to cause significantly accelerated jet penetration for the first injection but hindered the second jet penetration. Indeed, the mixture centroid position was lifted for the second jet evidencing the significant tumble flow influence.