A Combustion CFD study on the Pre-chamber Jet Combustion Technology for Large Motorcycle Gasoline Engine

The objective of this study is to enhance the full-load output and improve the part-load thermal efficiency of a gasoline spark-ignition engine for large motorcycles. To achieve these goals, it is essential to increase the combustion speed and mitigate knocking. Therefore, a passive-type pre-chamber jet combustion system was applied. In the specification study, a three-dimensional combustion simulation incorporating detailed chemical kinetics was used to analyze the combustion mechanism, including knocking detection. For full-load conditions, a passive-type pre-chamber jet combustion was applied. Increasing the compression ratio promoted the inflow of air-fuel mixtures into the pre-chamber during the compression stroke and reduced residual gases. This intensified pre-chamber combustion and extended the jet penetration length. Consequently, the pre-chamber jet combustion system accelerated combustion by increasing turbulent kinetic energy in the main chamber through jets ejected from the pre-chamber. By increasing the compression ratio by 2.0, the full-load output improved by 4.3% compared to conventional SI combustion. Under part-load conditions, the passive-type pre-chamber jet combustion system faced challenges, such as reduced jet temperature due to increased residual gas in the pre-chamber, heat loss at the orifices, and excessive initial jet penetration, which prevented ignition in the main chamber. To address these issues, a dual-plug pre-chamber jet combustion system was implemented, where main chamber side-plug ignition was followed by pre-chamber ignition. The pre-chamber jet 2plug combustion system enhanced jet ignition through flame interaction in the main chamber, resulting in increased combustion speed. Furthermore, relocating the main chamber side plug to a position between the exhaust valves closer to the bore center improved combustion speed and mitigated knocking. As a result, the pre-chamber jet 2plug combustion system, with the side plug located between the exhaust valves, improved part-load thermal efficiency by 2.1 percentage points compared to conventional SI combustion.