Effects of Various Bore–Stroke Ratios on Hydrogen Direct Injection Spark Ignition Engines With Variable Valve Timing Under Low-Load Conditions

This study investigates the effects of various bore–stroke (S/B) ratios on the combustion characteristics, energy fractions, and performance of a hydrogen direct injection spark ignition engine equipped with a variable valve timing (VVT) system under low-load conditions. The experiments were conducted at S/B ratios of 1.0, 1.2, and 1.47 while maintaining a fixed displacement volume and compression ratio. The energy budget analysis focused on heat transfer loss, combustion loss, and exhaust loss to determine their effects on gross work. The results showed that as the S/B ratio increased, heat transfer loss increased due to enhanced piston speed and in-cylinder mixing, resulting in faster combustion. Combustion loss was highest at an S/B ratio 1.0 due to longer combustion duration. In contrast, exhaust loss did not show a clear trend with varying S/B ratios. The effects of fuel injection timing and excess air ratio on engine performance and emissions were investigated. The findings of this study suggest that optimizing the S/B ratio, fuel injection timing, and excess air ratio can significantly improve the thermal efficiency and emission characteristics of hydrogen engines, providing practical insights for the design and development of future hydrogen engine technologies.