The Hesselman engine is a hybrid between a petrol engine and a Diesel engine introduced by Swedish engineer Jonas Hesselman in 1925. It represented the first use of direct gasoline injection on a spark-ignition engine used to power a road going vehicle. The Hesselman engine saw use in heavy trucks and buses in models produced in the 1920s and 1930s.
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's 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 for automotive use with direct injection of fuel into the cylinder.
Advantages and disadvantages
Hesselman engines could use heavier oils which were considerably cheaper than gasoline and were therefore more economical to operate for the vehicle owner. Contemporary tests also pointed to a slightly lower fuel consumption in comparison to gasoline engines of similar power.
A Hesselman engine compared to a diesel engine had smaller dimensions and hence lower weight. In the 1930s knowledge of metallurgy was less advanced, therefore diesel engines were heavy to cope with the high compression and pressure during combustion. Later, diesel engines were developed with better materials and Hesselman engines lost this advantage.
Hesselman engines had several disadvantages. Due to the low compression it was difficult to reach the working temperature. The result was an incomplete combustion. The incomplete combustion led to the spark plugs quickly fouling, but above all to what even in that time was termed "heavy smoke". In today's terms, this meant that the engines generated toxic exhausts on a scale that would be considered completely unacceptable.
Hesselman engines were produced by all three Swedish truck manufacturers of its day, Scania-Vabis, Tidaholms Bruk and Volvo, from the late 1920s. Scania-Vabis replaced Hesselman engines with proper Diesel engines from 1936, and Volvo from 1947.