A Hesselman engine is basically a spark-ignition engine, converted to run on heavier petroleum products such as oil, kerosene or diesel oil. The fuel is injected into the combustion chamber using an injection pump. Because of the engine low compression the fuel must be ignited by a spark plug, unlike the diesel engine, where the fuel ignites spontaneously by the heat generated by the high compression ratio. Hesselman engines were often started on gasoline and then switched to diesel or kerosene when at working temperature. Clearing the heavy oil for the next start was achieved by switching back to gasoline before stopping. It is the first spark ignition engine with direct injection of fuel into the cylinder.