Numerical Investigation of Engine Speed and Fuel Composition Effects on Convective Heat Transfer in a Spark Ignition Engine Fueled with Methane-Hydrogen Blends

One general conclusion that emerged from studies of spark ignition (SI) engines fuelled with hydrogen, was that stoichiometric operation is associated with high heat losses compared to other gaseous fuels such as methane. A confirmation of higher heat transfer rates was obtained through heat flux measurements, that were found to be much higher for hydrogen compared to methane stoichiometric fuelling. Given that quasi-dimensional models offer an acceptable compromise between accuracy and computational requirements, this study employed three zone simulation in order to study the effects of fuel composition and engine speed on heat transfer rates in a premixed charge SI engine. After an initial validation based on in-cylinder pressure measurements performed on a research engine, a comprehensive numerical study was undertaken in order to evaluate the effects of crank angle rotational velocity and fuel composition. Engine speed was swept in a wide range, that covers the specific case of automotive and stationary energy production applications; combined use of methane and hydrogen (dual fuelling) was considered, ranging from one pure fuel to the other. Stoichiometric operation was simulated for all cases, given that this is the closest situation to real-world applications, for which closed-loop air-fuel ratio control is employed. The results of predicted heat flux suggest that hydrogen addition in its blend with methane increases laminar flame speed (and therefore shortens combustion development), but also results in higher heat losses due to augmented convective heat transfer. Both effects were found to be less evident as engine speed was increased. These findings can be used for adapting engine control strategies when using methane-hydrogen blends in premixed charge SI engines, in order to optimize efficiency and reduce environmental impact.