Effect of Hydrogen Supplementation on Performance, Knock Behaviour, and Emissions of an SI Engine Operating at Lean Air-fuel Ratios

Abstract

There is contradictory literature about whether hydrogen (H2) supplementation may increase or decrease the likelihood of knock in spark ignition (SI) engines. Such contradictory literature may correlate to the fast laminar flame speed property of H2, which increases the burning rate. Faster combustion allows less time for end-gas autoignition to occur, but also increases the end-gas pressure and temperature, which may reduce the time required for autoignition. Firstly, via experiments, this thesis demonstrates that “knocking cycles are the cycles with faster burning rate when they are compared with normal cycles”. Then, via simulations, it is verified that faster combustion promotes knock. Secondly, the thesis shows that H2 supplementation practically promotes knock in an experimental manner. This implies that the knock promotion effects of the fast laminar flame speed and the lower heat capacity of H2 dominate the knock suppression effect of the high research octane number rating. Therefore, from the knock perspective, H2 is not an ideal supplement for gasoline SI engines. Thirdly, an alternative use of H2 supplementation has been studied, i.e. for idle condition. In this part, both performance and emissions are studied. In performance, it is demonstrated that H2 supplementation reduces the cycle-to-cycle variation by increasing the burning rate. Net indicated efficiency, as well as fuel economy, are significantly improved with H2 supplementation. In emissions, carbon dioxide, carbon monoxide, and unburned hydrocarbon are greatly reduced with H2 supplementation. On the other hand, due to the high adiabatic flame temperature of H2, nitrogen oxides (NOx) emissions are increased. However, this is less important as the idle NOx emissions are very small relative to the normal operation condition.