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An operation with a lean air-fuel mixture enables smaller cogeneration gas engines to operate at both high efficiency and low NOx emissions. Conventionally, the combustion process is induced through spark ignition. However, its small reactive mixture volume sets limits on increasing the air-fuel ratio, as a higher dilution reduces mixture inflammability as well as flame propagation speed. In addition, the spark plug durability is limited due to electrode wear, particularly through spark erosion, causing high maintenance costs. The ignition by means of a hot surface has great potential to extend the frequency of servicing intervals as well as to improve the trade-off between engine efficiency and NOx emissions. Compared to conventional spark ignition, ignition by means of a hot surface is achieved by accelerated combustion. The latter is produced by an increased initial reactive mixture volume.