The Hesselman engine is a hybrid internal-combustion design developed by the Swedish engineer Jonas Hesselman in 1925, sitting between the spark-ignition petrol engine and the compression-ignition diesel engine. The engine starts on petrol with electrical spark ignition, then runs on lower-grade fuels including kerosene, gas oil, vegetable oils, and even crude oil under direct injection but with continued spark ignition. Hesselman engines were used extensively in trucks, buses, and small marine craft from the late 1920s through the 1940s but were displaced by the maturing high-speed diesel after the Second World War. The design retains historical and engineering interest as a pragmatic transitional technology that allowed access to cheaper fuels before high-pressure direct-injection diesels became commercially robust.
Jonas Hesselman and origin of the design
Jonas Hesselman (1877 to 1957) worked for AB Atlas Diesel in Stockholm from the early 1900s and was a senior engineer there throughout the development of large stationary and marine diesels in the Atlas-Polar lineage. By the early 1920s he had become convinced that the high-pressure injection equipment available at the time was insufficiently reliable for medium-speed and high-speed applications, and that a simpler hybrid combining a low-pressure injection pump with electrical ignition would deliver most of the diesel’s fuel-cost advantage without the durability and starting problems then plaguing direct-injection diesels.
The first practical Hesselman engine entered service in 1925, and the technology was widely licensed through the late 1920s and 1930s. Volvo built Hesselman-cycle truck engines from 1932, with the FB and LV series carrying them into commercial production. Scania-Vabis followed in the same period, and the engines were exported widely.
Working principle
A Hesselman engine compresses air at compression ratios of around 7 to 9 to 1 (well below the 14 to 18 to 1 typical of contemporary diesels) and injects fuel near the end of the compression stroke through a low-pressure mechanical injection pump. Combustion is initiated by an electrical spark plug rather than by the heat of compression. The result is an engine that:
- Starts reliably from cold on petrol with throttle and choke control similar to a spark-ignition engine.
- Once warm, accepts a wide range of low-grade liquid fuels including kerosene, gas oil, light fuel oil, and vegetable oils.
- Operates at moderate cylinder pressures, allowing simpler block and bearing design than a full diesel.
- Achieves fuel consumption approximately 20 to 25 per cent better than a contemporary petrol engine but worse than a true diesel.
The compromise was attractive in the inter-war years when high-pressure injectors and pumps were expensive and prone to wear, electrical ignition was already mature, and low-grade fuels were significantly cheaper than petrol or kerosene fit for spark ignition.
Marine applications
Hesselman-cycle engines saw moderate marine use in the 1930s and 1940s, principally in fishing vessels, harbour craft, and small inland craft along the Swedish coast and in the Baltic. The fuel flexibility was particularly valuable to operators with intermittent access to diesel fuel oil during wartime supply disruption. Several manufacturers, including AB Atlas Diesel, AB Bolinder, and June-Munktell, produced small marine Hesselman engines in the 1 to 50 brake horsepower range. The engines were notable in cold-climate fishing operations where reliable cold starts on petrol were a meaningful operational advantage over starting a hot-bulb semi-diesel.
A Hesselman engine in marine service was typically configured for fixed-pitch propeller drive at moderate shaft speeds (600 to 1,200 revolutions per minute), with hand-cranked or electric starting on petrol followed by a fuel changeover valve to gas oil or kerosene once the engine reached operating temperature. The fuel-changeover practice required careful operator training and a clean fuel changeover valve, since incomplete switchover was a common cause of running issues and exhaust smoke.
Decline and historical significance
The development of robust high-pressure jerk-pump injection systems by Robert Bosch and CAV in the late 1920s and 1930s, and the maturation of high-speed diesel engines from MAN, Daimler-Benz, Junkers, Mercedes, and others, eroded the Hesselman engine’s commercial niche. By the late 1940s most Hesselman truck and marine production had ceased, with surviving examples being either replaced by full diesels during major refits or kept running by enthusiasts. Hesselman truck engines remained in production at Volvo until 1947.
The engine remains historically significant for several reasons. First, it demonstrated that the conceptual gap between Otto-cycle and Diesel-cycle engines could be bridged commercially, presaging modern direct-injection spark-ignition (DISI) and compressed-charge compression-ignition (CCCI) research. Second, it provided fuel flexibility decades before that became a regulatory and market priority. Third, Hesselman-cycle engines preserved engineering capability in Sweden during a period when the country was building its own marine and automotive industries, contributing engineers and manufacturing experience that fed directly into the post-war success of Volvo Penta, Scania, and Atlas Copco.
Modern relevance
The Hesselman cycle is occasionally referenced in contemporary engineering literature in the context of dual-fuel and methanol-diesel hybrid concepts, where spark-assisted ignition of low-cetane fuels remains an active research area. Modern marine methanol pilot-ignited engines, such as Wärtsilä’s pilot-injection methanol concepts and MAN ES dual-fuel methanol engines, are conceptually distant descendants of Hesselman’s hybrid approach, even though they use modern common-rail injection and are physically very different machines.
Several Hesselman engines survive in operating condition in Swedish maritime museums and in privately preserved fishing boats. The engine is a legitimate piece of marine engineering history rather than a mainstream production technology of the present day.