In a decarbonized society, adapting internal combustion engines (ICEs) to various carbon-free fuels is crucial for their sustainable use. One such application is gas heat pumps (GHPs), which currently rely on hydrocarbon-based fuels. However, a transition to decarbonized and low carbon fuels, such as hydrogen and synthetic methane, is essential for achieving carbon neutrality. Hydrogen is a carbon-free fuel, but its tendency to ignite with extremely low energy and a wide flammability range, making it highly reactive. While these properties enable efficient combustion, they also increase the likelihood of abnormal combustion, such as backfire and knocking, when used in ICEs. Furthermore, hydrogen’s low volumetric energy density has been reported to result in reduced engine output compared to hydrocarbon fuels. On the other hand, methane, the simplest alkane, has a volumetric energy density approximately three times higher than that of hydrogen and exhibits lower combustibility than