A spark ignited Otto cycle type engine have a fixed compression ratio which is supplied with a mixture of air and fuel having an octane number lower than that which would normally be appropriate for the compression ratio. The engine operates in a range below a predetermined high load without spontaneous combustion of the fuel-air mixture but will be susceptible to spontaneous combustion when operated above the predetermined high load. The engine includes means for controlling combustion of the fuel-air mixture when operating above the predetermined high load in a manner to decrease the efficiency of the engine and to prevent spontaneous combustion which would otherwise occur.