Models for the convective heat transfer from the combustion gases to the walls inside a spark ignition engine are an important keystone in the simulation tools which are being developed to aid engine optimization. The existing models have, however, been cited to be inaccurate for hydrogen, one of the alternative fuels currently investigated. One possible explanation for this inaccuracy is that the models do not adequately capture the effect of the gas properties. These have never been varied in a wide range because air and ‘classical’ fossil fuels have similar values, but they are significantly different in the case of hydrogen. As a first step towards a fuel independent heat transfer model, we have investigated the effect of the gas properties on the heat flux in a spark ignition engine. The effect of the gas properties was decoupled from that of combustion, by injecting different inert gases (helium, argon, carbon dioxide) into the intake air flow of the engine under motored operation. This paper presents the results of the experiment, which was designed with DoE techniques. The paper shows that the three investigated effects (throttle position, compression ratio and gas) and the interaction between the throttle and the compression ratio are significant with a significance level of 1%. Both the individual and combined effects of the gas properties are investigated. The most remarkable effect observed in the data was that the dynamic viscosity influences the heat flux in two contrasting ways. At the one hand, it increases the heat flux by increasing the gas temperature, at the other hand, it reduces the heat flux through the convection coefficient. A preliminary test shows that modeling under motored operation could be based on classical concepts. However, some scatter occurs in the data which needs further investigation.
