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Several studies have been performed to investigate the effects of using hydrogen in spark ignition (SI) engines. One general conclusion that emerged was that stoichiometric operation of premixed charge hydrogen engines features increased losses compared to other fuels such as methane. Most studies attribute this higher loss to increased rates of heat transfer from the working fluid to the combustion chamber walls. Indeed, heat flux measurements during combustion and expansion recorded much higher values for hydrogen compared to methane stoichiometric operation. With regard to fluid properties, using the same net heat release equation as for gasoline engines results in an over prediction of heat losses to the combustion chamber walls. Also, the variation of specific heats ratio greatly influences calculated values for the rate of heat release. Therefore, a more detailed analysis of heat losses is required when comparing hydrogen to other fuels.