Abstract
Hydrogen has been investigated as a clean alternative to fossil fuel. However, hydrogen combustion has unique characteristics those at influence the thermal efficiency of internal combustion engines. Namely, hydrogen has a higher flame propagation velocity and a shorter quenching distance as compared with to hydrocarbons. The combustion characteristics of hydrogen supposedly influence the cooling loss and the degree of constant volume. The cooling loss and the degree of constant volume are two major factors influencing the thermal efficiency of internal combustion engines. A rapid change in temperature and composition of in-cylinder gas associated with the high burning velocity largely influences specific heats of working substance. Therefore, a treatment of specific heats supposedly affects the analysis of indicator diagram in hydrogen combustion.
This research analyzed factors influencing the thermal efficiency of a hydrogen-fuelled SI engine by using measured indicator diagrams. The analysis was especially focused on the cooling loss ratio and the degree of constant volume. The cooling loss ratio was quantitatively evaluated by using analyses of indicator diagram and exhaust gas composition, and the degree of constant volume cooling and the degree of constant volume burning were also evaluated by a newly proposed method from indicator diagram. In addition, this research investigated the influence of treatments of specific heats on calculated rate of heat release in hydrogen combustion.
The main results derived from the analysis are as follows: The cooling loss ratio is quite high and essential to improve thermal efficiency in hydrogen combustion. In addition to the high cooling loss ratio, the degree of constant volume cooling is also high in hydrogen combustion. The effect of treatments of specific heats is small in the degree of constant volume but large in the cooling loss ratio.