Hesselman Engine: History, Principle, and Marine Use

The Hesselman engine is a hybrid internal-combustion design developed by Swedish engineer Jonas Hesselman around 1925. It injects fuel directly into the cylinder under low pressure and ignites it with a spark plug, operating at compression ratios of 7 to 9:1, far below the 14 to 20:1 of a contemporary diesel. The engine starts on petrol and switches to cheap heavy fuels once warm, bridging the commercial gap between the spark-ignition petrol engine and the compression-ignition diesel during the inter-war decades. Volvo, Scania-Vabis, and AB Atlas Diesel all produced Hesselman-cycle engines for trucks, buses, and small marine craft from the late 1920s through 1947, when the maturing high-pressure diesel rendered the design obsolete.

The Hesselman engine occupies a specific and often misread position in engine history. It isn’t a failed curiosity or an engineering dead end. It is a pragmatic inter-war solution to a real commercial problem: cheap fuel was available, but the injection equipment needed to exploit it reliably in a full diesel was expensive, failure-prone, and difficult to service in remote fishing harbors or rural truck depots. Hesselman solved that problem with a simpler injection system backed by a spark plug, and the result worked well enough to sell in substantial commercial numbers for two decades.

For the earlier generation of low-compression oil engines that preceded Hesselman’s work, see the Hornsby-Akroyd hot-bulb engine, whose surface-ignition principle sat at the same junction between heavy oil and reliable ignition thirty years earlier. For the Scandinavian marine hot-bulb type that competed directly with Hesselman in harbor craft through the 1930s, see Bolinder hot-bulb marine engines. For the full diesel that ultimately displaced both, see marine diesel engine.

Jonas Hesselman: engineer at AB Atlas Diesel

Jonas Hesselman was born in Sweden in 1877 and trained as a mechanical engineer in the Swedish engineering tradition that produced several of the early twentieth century’s most capable engine designers. He joined AB Atlas Diesel in Stockholm in the early 1900s, when Atlas was building stationary and marine oil engines derived from the Diesel license the company had acquired. Atlas Diesel engines of this period were slow-speed, large-bore machines, reliable in stationary and large-vessel service but not well suited to vehicles or small craft.

Through the 1910s and early 1920s, Hesselman worked on the refinement of medium-speed and high-speed diesel designs. His central technical frustration was the state of fuel injection. By the early 1920s, Rudolf Diesel’s original air-blast injection system, in which a high-pressure air charge atomized fuel into the cylinder, was being replaced by solid mechanical injection using jerk pumps and precision nozzles. But those jerk-pump systems in the early 1920s were still expensive to manufacture, sensitive to contamination, and difficult to calibrate in the field. Fuel filters adequate to protect the close-tolerance plunger-and-barrel pairs were not yet standardized.

Hesselman’s response was to step back from high-pressure injection entirely. If a spark plug could be relied upon for ignition, the injection system could be simplified to a low-pressure mechanical pump delivering fuel through a comparatively crude nozzle. The fuel did not need to atomize under its own injection pressure to achieve self-ignition: the spark would fire the charge regardless, as long as a combustible mixture was present in the cylinder at the moment of ignition.

The first practical Hesselman engine ran around 1925 at the Atlas Diesel workshops in Stockholm, and within two years the design had attracted commercial interest from truck manufacturers. Hesselman continued working on engine design at Atlas Diesel until retirement, and died in 1957. His specific contribution to engine technology, the deliberate combination of direct injection with spark ignition at low compression, was patented in Sweden and several other countries through the late 1920s.

The AB Atlas Diesel context

AB Atlas Diesel had been founded in Stockholm in 1873 as Atlas Mekaniska Verkstäder and had diversified into diesel engines after acquiring a Diesel license in 1898. By the 1920s it was one of Sweden’s leading engine manufacturers, producing the Atlas-Polar series of marine oil engines and competing with Bolinder and Skandiaverken for the Scandinavian fishing-vessel market. The company was later renamed AB Atlas Copco after the 1956 merger that created the modern Atlas Copco group.

Hesselman’s position inside Atlas Diesel gave him manufacturing access and a direct commercial route to market. Atlas licensed the Hesselman design to other manufacturers rather than trying to serve all segments internally. Volvo took the truck and bus license, Scania-Vabis followed, and Atlas itself produced marine and stationary units.

Working principle and cycle position

The compression ratio difference

The most important number in understanding the Hesselman engine is the compression ratio. A contemporary petrol engine of the 1920s and 1930s compressed its air-fuel mixture at ratios of 4:1 to 6:1, limited by the knock resistance of available petrol. A contemporary diesel, by contrast, needed ratios of 14:1 to 20:1 to raise air temperature to the 250 to 350 degrees Celsius needed for autoignition of fuel oil.

A Hesselman engine ran at 7 to 9:1. This was high enough to produce a charge temperature around 150 to 200 degrees Celsius at top dead center, which is below the autoignition point of fuel oil but well above ambient. The spark plug fired into this warm, compressed air-fuel mixture, and combustion proceeded as a pressure-controlled burn, not an explosive detonation. The result was smoother pressure rise than in a diesel, with lower peak cylinder pressures: typically 40 to 60 bar, versus 80 to 120 bar in a contemporary medium-speed diesel.

Lower peak pressure meant simpler construction. The cylinder head, connecting rod, bearings, and block could be lighter and cheaper. A Hesselman engine of equivalent output cost less to manufacture than a contemporary diesel, and this cost difference was the commercial argument for the design.

The fuel injection system

Fuel injection in a Hesselman engine used a low-pressure mechanical pump, typically operating at 100 to 200 bar, delivering fuel through a single-hole or multi-hole nozzle directly into the cylinder or pre-chamber. This is mechanically similar to a diesel injector but at a fraction of the pressure: contemporary jerk-pump diesels of the 1930s operated at 300 to 600 bar, rising to 800 bar and beyond in later designs.

The low injection pressure meant that atomization was coarser than in a full diesel. This was acceptable because ignition didn’t depend on the fuel breaking into fine droplets that could self-ignite: the spark plug handled ignition regardless of droplet size. Once the charge was burning, the combustion quality was good enough for acceptable fuel economy.

The injection was timed to occur near the end of the compression stroke, approximately 10 to 20 degrees before top dead center, matching diesel injection timing. The spark plug fired shortly after fuel delivery began, at or slightly before top dead center.

Starting and fuel changeover

Cold starting was where the Hesselman engine showed its clearest advantage over both the hot-bulb semi-diesel and the full diesel of the period. The starting procedure was identical to a petrol engine: a petrol carburettor or auxiliary petrol injection provided the starting fuel, the throttle and choke were set, and the engine was cranked by hand or by electric starter. Petrol has a wide flammability range and a low autoignition temperature, so it ignited reliably from the spark plug even at low cranking speeds and cold cylinder temperatures.

Once the engine reached operating temperature, typically two to five minutes of petrol running, the operator opened a fuel changeover valve that diverted delivery from the petrol supply to the heavy fuel tank. The carburetor was closed, and the engine ran on direct injection of gas oil, kerosene, or light fuel oil. This changeover could be done while the engine was running, without stopping.

The changeover valve design was one of the more failure-prone elements of Hesselman marine installations. A valve that stuck partially open delivered a mixture of petrol and heavy fuel simultaneously, producing incomplete combustion, black smoke, and fouled spark plugs. Operator training emphasized that the valve had to be fully open in one position or the other, never intermediate.

Comparing the three cycles

The Hesselman engine sat squarely between the two established types. It didn’t match the full diesel’s thermal efficiency or its ability to burn the heaviest residual fuel oils, but it was cheaper to build, easier to start in cold weather, and could run on fuels that were three to five times cheaper than petrol at 1930s European prices.

The hot-bulb semi-diesel and why Hesselman was different

To place the Hesselman accurately, it’s worth contrasting it with the hot-bulb semi-diesel, the other dominant low-compression oil engine of the same period. The Bolinder, Skandia, and other Scandinavian hot-bulb engines that powered fishing vessels from the 1900s through the 1940s also used low compression ratios, typically 3:1 to 5:1, and also ran on heavy oil. But their ignition mechanism was completely different.

A hot-bulb engine relied on a small external vaporizing chamber, the bulb, preheated to redness before starting with a blowlamp. The bulb retained heat from each firing cycle and provided the surface temperature needed to ignite the next fuel charge. There were no spark plugs. Ignition was purely thermal, from the hot metal surface. This made hot-bulb engines extremely simple and almost maintenance-free in terms of ignition system components: no magneto, no distributor, no spark plugs to foul or replace.

The Hesselman’s spark plug was both its advantage and its limitation. In cold weather, the Hesselman could start without preheating any external component, giving it a genuine operational edge over the hot-bulb type in winter Baltic conditions. But spark plugs of the 1930s fouled readily when running on gas oil or heavier fuels, because the combustion was not always clean enough to keep the electrode faces clear. Plug fouling was the most common maintenance issue in Hesselman marine installations, and a fouled plug in a single-cylinder fishing vessel engine meant the engine stopped.

The Hornsby-Akroyd hot-bulb engine patented in 1890 had established the principle of low-compression surface-ignition heavy-oil engines three decades before Hesselman’s work, and the Bolinder hot-bulb marine engines that dominated the Baltic fishing trade had refined that principle into a highly practical form. Hesselman was not trying to replace hot-bulb engines on the same basis: he was offering a different trade-off, better cold starts at the cost of an electrical ignition system, and the ability to run on a wider range of fuel qualities.

Commercial production: Volvo, Scania-Vabis, and Atlas

Volvo

The most commercially successful Hesselman-cycle vehicles were Volvo’s heavy trucks. Volvo introduced its first Hesselman-engined trucks in 1932, using the LV series (Lastbil, “truck” in Swedish) with Hesselman engines designated initially as the “Hesselman motor” in Volvo’s own sales materials. The LV100 series and subsequent variants carried Hesselman engines rated between 60 and 100 horsepower through most of the 1930s.

The appeal to Swedish truck operators was direct: Sweden in the 1930s was a country where petrol and high-quality diesel fuel were imported and taxed, while domestically available gas oil and kerosene were cheaper. A fleet operator running Volvo trucks on gas oil instead of petrol cut fuel costs by roughly half at Swedish 1930s prices. The Hesselman engine converted that price advantage into a commercially viable powertrain without requiring the high-precision diesel injection equipment that was still expensive and difficult to service outside major cities.

Volvo’s engineers made several modifications to the basic Hesselman concept for truck service. They improved the lubrication system to handle the heavier carbon deposits that accumulated from gas oil combustion at low compression. They also refined the fuel changeover mechanism to reduce the risk of mixed-fuel operation. Volvo continued producing Hesselman-cycle trucks until 1947, when their own high-speed diesel development program, the TD (turbocharged diesel) series, produced an engine that made the hybrid approach unnecessary. Approximately 8,000 Volvo trucks left the factory with Hesselman engines between 1932 and 1947.

Scania-Vabis

Scania-Vabis, which later became Scania AB, adopted the Hesselman design for its bus and heavy vehicle range in the same period. Scania-Vabis bus engines of the late 1930s used Hesselman-cycle six-cylinder units in the 90 to 120 horsepower range, serving Swedish municipal bus routes where fuel cost was a significant operating expense.

Scania-Vabis buses with Hesselman engines operated in Stockholm, Gothenburg, and several other Swedish cities through the late 1930s and the wartime years. During the Second World War, when Sweden’s neutrality didn’t protect it from fuel supply disruptions, the Hesselman engine’s ability to run on whatever petroleum distillate was available proved its worth in city bus operation.

AB Atlas Diesel and marine production

AB Atlas Diesel produced marine Hesselman engines in smaller numbers than Volvo produced truck units, but the marine installations were technically interesting because they pushed the design into territory where fuel quality and operating conditions were less controlled than in a factory-maintained truck fleet.

Atlas Diesel marine Hesselman engines in the 1 to 50 brake horsepower range were installed in fishing vessels, harbor launches, and small coasters along the Swedish west coast and in the Baltic Sea. The engines were typically four-stroke, single- or multi-cylinder, running at 600 to 1,200 revolutions per minute, and driving fixed-pitch propellers through direct shaft or simple gearbox arrangements.

A marine Hesselman installation typically included a small petrol tank of 10 to 20 liters for starting, a main heavy fuel tank of 100 to 500 liters for running, the changeover valve, a low-pressure injection pump, and a conventional coil-and-battery or magneto ignition system. The added complexity compared to a hot-bulb semi-diesel was real: the Bolinder required no electrical system beyond an optional lighting dynamo, while the Hesselman needed a functional battery and ignition circuit at all times.

Marine applications in detail

Fishing vessels and harbor craft

The primary marine market for Hesselman engines in Sweden was the fishing industry operating in the Skagerrak, Kattegat, and southern Baltic, where vessel sizes of 10 to 30 gross tons were typical and engine powers of 10 to 50 brake horsepower were standard. These were family-operated vessels where reliability, fuel cost, and ease of maintenance by the skipper himself were the dominant criteria.

The Hesselman engine competed in this market against the hot-bulb semi-diesel types from Bolinder and Skandia, and against small full diesels that were beginning to appear in the late 1930s from manufacturers including Yanmar (Japan), Deutz (Germany), and Volvo Penta’s own growing diesel range. The Hesselman won market share primarily through cold-start reliability. A Norwegian or Swedish fisherman departing at 04:00 in February needed an engine that started without a blowlamp heating ritual. The hot-bulb semi-diesel required 10 to 20 minutes of preheating before it would start in winter temperatures. A Hesselman engine started in the same time as a petrol engine: crank, throttle, run.

During the Second World War, the practical fuel flexibility of the Hesselman gave it additional value. Sweden maintained its neutrality but supply chains for specific petroleum products were disrupted. A Hesselman-engined fishing vessel could run on whatever gas oil or kerosene was available at a given port, whereas a full diesel might reject fuel that was too light or too contaminated, and a hot-bulb engine needed time and heat that a distress situation didn’t always permit.

Small cargo and coasting trade

Several Hesselman-engined small cargo vessels operated in Swedish coastal trade during the 1930s. These were typically motor coasters of 50 to 200 gross tons carrying timber, farm produce, and general cargo along the Swedish coast and across to Finnish and Estonian ports. The engine power range was 50 to 150 brake horsepower for this segment, and Hesselman engines of this size were less common than in the fishing sector because full medium-speed diesels from Atlas Diesel’s own Atlas-Polar range were commercially available and increasingly competitive on reliability grounds.

The Hesselman engine was not generally used in vessels above roughly 200 gross tons or above 200 brake horsepower. At those scales, the investment in a proper medium-speed diesel installation was warranted, and the operational advantages of the hybrid approach were outweighed by the superior fuel economy and longer overhaul intervals of a true diesel.

Cold-climate limitations in marine service

Marine service exposed the Hesselman engine’s limitations more sharply than road vehicle service. Spark plug fouling was the dominant maintenance issue. In a truck, the engine ran at relatively consistent load and speed on a smooth road, and combustion conditions remained stable. In a fishing vessel, the engine speed and load varied constantly with sea conditions, propeller slip, and the variable drag of fishing gear. Partial-load operation, which occurred often in harbor maneuvering and slow trawling, produced incomplete combustion and accelerated plug fouling.

Marine Hesselman installations typically replaced spark plugs every 150 to 250 hours of operation, compared to 500 to 1,000 hours in truck service. A Baltic trawler fishing six days per week might change plugs every three to four weeks. This was acceptable maintenance by the standards of the period, but it was a cost and operational inconvenience that a hot-bulb engine completely avoided.

The fuel changeover valve was the second chronic problem in marine service. Saltwater ingress, vibration, and infrequent operation of the valve caused seizing and leaking. A stuck changeover valve meant either running on petrol permanently, which was expensive, or being unable to start, since the petrol supply was needed for cold starting.

Comparison with Bolinder semi-diesels in Baltic fishing fleets

The Bolinder hot-bulb marine engines dominated the Baltic fishing industry from the 1890s through the 1940s and represented the primary competitor to the Hesselman in the small marine sector. The comparison was not simply technical: it was economic and practical.

A Bolinder engine of the 1930s had a decades-long track record in the Baltic, a network of service agents throughout Sweden, Norway, Denmark, and Finland, and a reputation for running 10,000 to 15,000 hours between major overhauls. The simplicity of hot-bulb ignition, no electrical system, no spark plugs, no timing to adjust, was a profound operational advantage in remote harbors where spare parts were scarce and the skipper was his own mechanic.

The Hesselman’s cold-start advantage was genuine but not decisive for most operators. Baltic fishermen had learned to preheat their Bolinder engines the previous night, and a well-maintained hot-bulb engine in good condition could be started in under ten minutes by an experienced hand. The Hesselman’s advantage was clearest in the coldest months and for less experienced crews.

Market evidence from the period suggests the Hesselman took modest share in the Swedish small-vessel market, perhaps 15 to 25 percent of new engine installations in the 10 to 50 horsepower range through the 1930s, while Bolinder and Skandia retained the majority. The Hesselman never achieved the Bolinder’s dominance in Norway or Denmark, where the established hot-bulb networks and lower diesel fuel availability tilted the economics more strongly toward the established type.

Decline: the maturing high-speed diesel

Robert Bosch and the jerk-pump breakthrough

The commercial case for the Hesselman engine rested entirely on the unreliability and cost of contemporary diesel injection equipment. That case began dissolving in the early 1930s when Robert Bosch GmbH in Stuttgart brought its PE-type in-line injection pump to commercial production.

The Bosch PE pump, developed through the late 1920s and refined after 1931, used hardened steel plunger-and-barrel pairs manufactured to tolerances of a few microns. Bosch invested heavily in the precision grinding and honing technology needed to produce these pairs in volume at acceptable cost, and established a testing protocol that validated each pump’s delivery characteristics before shipment. The result was a pump that delivered repeatable fuel quantities across the full speed and load range of a diesel engine, maintained those delivery characteristics over tens of thousands of hours of operation, and could be serviced by trained technicians at Bosch’s growing dealer network.

CAV (Charles Atkins and Vernon) in the United Kingdom developed a parallel injection pump system in the same period, competing directly with Bosch for the British and Commonwealth market. Simms Motor Units also contributed to British diesel injection development. By the mid-1930s, a small diesel engine equipped with a Bosch or CAV pump could start reliably in cold weather with a decompression-lever technique, run on standard gas oil available at any fuel merchant, and go 1,000 hours between injection pump service.

At that point, the Hesselman engine’s remaining advantage was narrow. It still had somewhat simpler injection equipment and a more familiar electrical starting system for operators trained on petrol engines. But the fuel changeover complication, the spark plug maintenance, and the slightly worse fuel economy than a true diesel at equivalent power output made it harder to justify commercially.

Wartime acceleration and post-war obsolescence

The Second World War paradoxically extended the Hesselman engine’s useful life. Sweden’s fuel supply was disrupted by the conflict surrounding its neutrality, and fuel flexibility had real operational value between 1939 and 1945. Swedish manufacturers continued producing Hesselman-cycle trucks and marine engines through the war because the alternative, diesel, required petroleum products that were not always guaranteed.

But the war also accelerated diesel development. Military demand for reliable, efficient diesel engines in vehicles, submarines, and aircraft drove investment in injection equipment, combustion chamber design, and cold-start technology on a scale that no peacetime commercial program could match. Glow plugs for diesel cold starting, which circulate current through a coil to preheat the prechamber or intake air before cranking, became commercially reliable by the late 1940s. The single most important cold-weather advantage of the Hesselman engine over the diesel was eliminated.

Volvo stopped Hesselman truck production in 1947. By then, Volvo’s own TD-series diesel engines were ready for commercial service, and the hybrid approach had served its purpose. No major commercial manufacturer produced Hesselman-cycle engines after 1950. The type had existed as a mainstream commercial product for roughly 25 years, from 1925 to 1950.

Historical significance

The ignition spectrum from 1890 to 1950

The Hesselman engine sits inside a 60-year sequence of ignition systems that ran from the Hornsby-Akroyd hot-bulb surface-ignition engine of 1890 through to the fully mature high-speed diesel of the late 1940s. That sequence was not linear and not driven by a single inventor’s vision: it was a market-driven search for the cheapest and most reliable way to extract energy from heavy petroleum fuels.

The Hornsby-Akroyd hot-bulb engine, described in British Patent No. 7146 of 1890, established that heavy oil could be ignited by surface contact with a preheated metal surface at low compression. The engine required no spark, no electrical system, and no high-pressure injection. It ran at 3:1 to 5:1 compression. It was the first commercially successful heavy-oil engine in the world, predating Diesel’s engine by two years.

Rudolf Diesel’s compression-ignition engine of 1897 pushed compression ratios to 35:1 in his original concept, though practical commercial engines settled at 14:1 to 18:1. Diesel required precise injection but no ignition system at all. The engine was thermally superior to everything else but mechanically demanding.

Between these two extremes, the inter-war period produced several hybrid approaches. The Hesselman was the most commercially significant: direct injection plus spark ignition at moderate compression. But it wasn’t alone. Lanova cell engines, Prechamber diesels from MAN, Ricardo Comet head designs, and various semi-diesel types all explored different balances between injection pressure, combustion chamber geometry, and ignition energy. The Hesselman’s specific solution, low-pressure injection plus retained spark ignition, was the one that best suited the Scandinavian commercial conditions of the 1930s.

Presaging direct-injection spark-ignition research

The Hesselman engine is occasionally cited in modern engine research literature as a historical antecedent of direct-injection spark-ignition (DISI) concepts, which became a major focus of gasoline engine development from the 1990s onward. Mitsubishi’s GDI (Gasoline Direct Injection) engine of 1996, for example, sprayed petrol directly into the cylinder at moderate pressure and ignited it with a spark plug, operating in a stratified-charge mode at partial load and a homogeneous mode at full load. The thermodynamic structure is not identical to a Hesselman, but the core idea, direct cylinder injection plus retained spark ignition, is the same family.

Modern research into lean-burn spark-ignition engines, compressed-charge combustion, and dual-fuel methanol-diesel engines also recalls the Hesselman’s hybrid position between purely Otto-cycle and purely Diesel-cycle operation. Wärtsilä’s pilot-injection methanol concepts and MAN Energy Solutions’ dual-fuel methanol engines use a small pilot diesel injection to ignite a methanol charge; the structural analogy to Hesselman’s use of a spark to ignite a directly injected gas-oil charge is genuine, even if the modern implementations use common-rail injection at pressures hundreds of times higher and are physically different machines.

The significance is not that anyone is building Hesselman engines today, but that the problem Hesselman addressed, how to ignite a non-self-igniting fuel delivered by direct injection at low compression, is still an active engineering problem in the 2020s, now applied to methanol, ammonia, and hydrogen.

Sweden’s engineering contribution

The Hesselman engine is also significant for what it reveals about Sweden’s engineering culture in the early twentieth century. AB Atlas Diesel, Bolinder, Skandiaverken, and later Volvo and Scania were all companies that developed original engine technology rather than simply licensing and manufacturing others’ designs. The Swedish engineering tradition in this period produced a remarkable cluster of engine types: the Atlas-Polar marine diesel, the Bolinder hot-bulb semi-diesel, the Hesselman hybrid, and eventually the Volvo and Scania diesel families that dominated European trucking in the post-war decades.

Hesselman himself exemplifies the productive relationship between academic technical training and industrial practice that characterized Swedish engineering institutions in the late nineteenth and early twentieth centuries. The Tekniska museet (National Museum of Science and Technology) in Stockholm holds records related to this period of Swedish engine development, including materials from the AB Atlas Diesel collection.

Limitations of the Hesselman design

The Hesselman engine’s limitations were not theoretical: they were practical problems that operators encountered in daily use.

Spark plug fouling was the most frequent. Running on gas oil at low compression produced carbon deposits on the plug electrodes faster than petrol operation at the same compression ratio. In marine service, partial-load operation during harbor maneuvering and slow trawling accelerated fouling. Plugs required inspection every 100 to 150 hours in heavy marine service and replacement every 150 to 250 hours, a maintenance burden with no equivalent in a hot-bulb semi-diesel.

Fuel changeover reliability depended on the valve design and maintenance. A partially open or stuck valve delivered a combustible mixture of petrol and gas oil simultaneously, which fouled plugs faster and produced erratic power delivery. In marine installations, saltwater ingress and vibration degraded the valve seat over time. Annual inspection and reconditioning of the changeover valve was standard practice.

The injection pump, though lower-pressure than a diesel pump, still required cleaner fuel than a hot-bulb engine. Particulate contamination above roughly 20 microns could cause nozzle blockage. Baltic fishing operations didn’t always have access to well-settled or filtered fuel, and nozzle maintenance was a recurring issue in remote-harbor service.

The engine’s fuel economy was its other structural limitation. Running on gas oil at 7 to 9:1 compression, a Hesselman engine achieved a brake specific fuel consumption roughly 20 to 25 percent better than a petrol engine of equal output, but 10 to 15 percent worse than a well-maintained full diesel of the same period. The gap widened as diesel injection technology improved through the 1930s, because improving Hesselman efficiency required increasing compression ratio, which eventually brought it into diesel territory and removed the injection cost advantage.

There was also a weight and size disadvantage relative to the hot-bulb semi-diesel for the smallest power outputs. A single-cylinder Bolinder of 5 to 10 horsepower was a mechanically simple, lightweight unit with minimal maintenance. A Hesselman of equivalent power carried the added weight and complexity of a battery, ignition coil or magneto, distributor, spark plugs, and injection pump. For the smallest vessels, this complexity had no compensating advantage.

Surviving examples and museum record

Several Hesselman engines survive in operating condition in Scandinavian technical museums and in privately preserved vessels. The Sjöhistoriska museet (National Maritime Museum of Sweden) in Stockholm holds records of Swedish marine engine production from the early twentieth century, and the Tekniska museet holds industrial records from the Atlas Diesel period.

Volvo has preserved examples of its Hesselman-cycle truck engines from the LV series, and the Volvo Museum in Gothenburg includes the Hesselman period in its coverage of Swedish truck manufacturing history. Some privately owned Swedish classic trucks of the 1930s and 1940s, maintained by enthusiast clubs, retain their original Hesselman engines in running condition.

In the marine sector, some fishing vessels built in the 1930s on the Swedish west coast retain Hesselman engines, though the operational costs of maintaining spark ignition systems on engines that were designed to run on gas oil have led most surviving examples to be converted to modern diesel or petrol power.

See also

• Hornsby-Akroyd Hot-Bulb Engine
• Bolinder Hot-Bulb Marine Engines
• Marine Diesel Engine
• Two-Stroke Marine Diesel Engine Fundamentals
• Volvo Penta Marine Engines: History and Product Range
• Gotaverken: Swedish Marine Engines and Shipbuilding
• Atlas-Polar Marine Engines
• Heavy Fuel Oil

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