13 a main tank 12 for natural gas and an auxiliary tank 13 for gasoline the auxiliary tank 13 for gasoline is necessary since the number of gasoline filling stations at which natural gas can be tanked is still very small and it could be necessary to drive the vehicle temporarily with gasoline in order to be able to reach the next available natural gas fuelling station when the main tank 12 is empty the spark ignition engine in accordance with the invention is therefore driven with gasoline instead of with natural gas when in reserve operation however in the case of a high compression of approximately 13 1 for which the spark ignition engines in accordance with the invention are designed gasoline would combust with knocking and result in damage to the engine if the engine were to be supplied with the same maximum quantity of air fuel mixture in the case of gasoline operation as is suitable for operation with natural gas at the same engine speed in order to prevent this a metering system 14 of the spark ignition engine in accordance with the invention u2014 that normally serves to supply natural gas to the engine block 11 from the tank 12 and when the latter is empty fuel from tank 13 u2014 is designed to take account of this the type of fuel supplied is taken into consideration when metering the mixture quantity fed to the engine in each combustion cycle the metering system 14 comprises a valve arrangement a butterfly valve 16 and an electronic control circuit 17 the valve arrangement serves to permit entry of only one fuel at a time to a supply line 23 of the engine it includes a stop valve 15.