Toward a unified understanding of the benefits of boost and lean burn in hydrogen and natural gas fuelled SI engines

Whilst boost is commonly used to increase the power and efficiency of gas-fuelled reciprocating engines, the combustion physics that can help realise these efficiency benefits have received little attention. This study therefore investigates the effect of boost on the performance of a variable compression ratio, octane rating engine operating on different blends of natural gas and hydrogen. While hydrogen addition consistently increases the propensity for engine knock, all fuel blends demonstrate increased net and gross engine efficiencies, reduced unburnt fuel emissions and maintained or even reduced specific NOx emissions by synergistically increasing boost and running at the leaner conditions that boost enables. Combustion analyses then suggest that this is because boosting reduces the impact of turbulent flame stretch, as manifest by reductions in the product of the premixture’s Karlovitz flame stretch factor and Lewis number, tending to suppress flame quenching and breakup. Whilst this more unified theoretical treatment needs further investigation, it demonstrates the importance of the Lewis number in this problem. It also suggests that increased boost may reduce the Karlovitz flame stretch factor, enabling stabler and more complete combustion at leaner conditions as a potential pathway for enhancing engine performance.