By ShipCalculators.com · Published 29 April 2026 · last updated 28 May 2026

Atlas-Polar Marine Engines

Contents

The Polar engine was the marine diesel of Aktiebolaget Atlas Diesel of Stockholm, not a separate company called “Atlas-Polar.” The name attaches to one of the earliest commercially successful two-stroke marine diesels in Europe, a line that ran from a 1907 seagoing installation through a Swedish parent that quit engine-building in 1948, a Glasgow licensee that still services the fleet, and a Trollhattan successor that Wartsila absorbed in 1978. “Polar Atlas” and “Atlas Polar” were trade names for that engine and its parts business, used by Atlas Diesel and later by Atlas Copco group companies in the United Kingdom. The engineering itself originated at AB Diesels Motorer in Sickla, the Wallenberg-financed Swedish licensee of Rudolf Diesel’s patent, which merged with AB Atlas in 1917 to form Atlas Diesel. Anyone researching the lineage needs the Marine Engine Model Decoder to read the surviving builder’s-plate codes, because three separate works stamped Polar designations over six decades.

Why the name causes confusion

There never was a registered company “AB Atlas-Polar.” The hyphenated form is a back-formation from two real things: the Polar engine brand and the Aktiebolaget Atlas Diesel that built it. Grace’s Guide records the British arm’s products as “engines of the Polar-Atlas type, an arrangement having been made for this purpose with Aktiebolaget Atlas Diesel, of Stockholm.” That single clause fixes the relationship. Atlas Diesel owned the Polar design; “Polar-Atlas” was how a British builder described the licensed product.

The Swedish parent itself was a Wallenberg creation, and its corporate path is well documented. Atlas Copco’s own group history states plainly that “Nya Atlas and AB Diesels Motorer merged in 1917 to become Atlas Diesel,” and that the merged firm “moved into Diesels Motorer’s production plant in Sickla, just outside of Stockholm’s city borders.” The Polar engine was built in that Sickla works. The diesel business did not survive the post-war reorganization: the same source records that “in 1948, Atlas Diesel stopped making diesel engines altogether,” after which the firm concentrated on pneumatic tools, compressors, and rock drills, and renamed itself Atlas Copco in 1956 once it absorbed a Belgian compressor subsidiary.

So the lineage forks. The Polar marine engine and its name moved to NOHAB at Trollhattan in 1948. The Atlas corporate identity stayed in Stockholm, dropped engines, and became the compressor and rock-drill group that exists today. Conflating the two, as the older literature sometimes does, produces the phantom “Atlas-Polar” company. This article treats Polar as the engine and tracks every works that built it.

The Wallenberg diesel works at Sickla

The Polar engine starts with Sweden’s claim on Rudolf Diesel’s invention. The Wallenberg family, through Stockholms Enskilda Bank, obtained Swedish manufacturing rights to Diesel’s patent and formed Aktiebolaget Diesels Motorer in 1898 to exploit them. The firm built its works at Sickla in Nacka, just outside Stockholm. It first developed a stationary engine for power generation, then a marine engine, and after several lean years reported its first profit in 1906, with growing export markets in the United States and Russia.

The technical lead at Sickla was Jonas Hesselman, who “worked from 1899 to 1916 for AB Diesel Engines (later Atlas Diesel, now Atlas Copco) in Sickla in Nacka just outside Stockholm, from 1901 as Head of Construction.” Hesselman is the engineer most associated with making the diesel work at sea. A marine diesel must run astern as well as ahead, and the reversing problem held back early adoption. Tekniska museet in Stockholm holds and catalogues a “reversible marine Polar diesel engine of Jonas Hesselman’s construction, manufactured in 1908 by AB Atlas Diesel, and one of the first two of its kind.” That museum object is the cleanest documentary anchor for the early reversible Polar. The later Hesselman engine, the 1925 spark-ignition direct-injection hybrid that carries his name, is a separate development covered in the Hesselman engine history.

The marine result arrived before the corporate merger. The Swan Hunter-built ore carrier Toiler, fitted with a Polar engine, became the first motor vessel to cross the Atlantic, in 1911. British Polar’s own account dates the first Polar two-cycle engine in a seagoing vessel to 1907, four years ahead of the Toiler crossing and a year before the museum’s 1908 reversible unit. These are the documented Polar firsts; they sit alongside the Danish Selandia of 1912, the Burmeister and Wain four-stroke ship usually called the world’s first ocean-going motor ship, and they show that the Swedish two-stroke was a serious contemporary rather than a follower. The engine took its name from Roald Amundsen’s polar expeditions of the same era.

The reversing problem

The reason Hesselman’s reversible diesel matters is that the diesel almost did not go to sea at all. A steam reciprocating engine reverses easily: the valve gear can be reset to admit steam on the other side of the piston, and a steam plant idles happily at a dock. A diesel does neither. It has no external working fluid to reset, it does not idle for long without load, and a propeller-coupled engine has to turn both directions to drive a ship ahead and astern. In the first decade of the century this was the open question that decided whether the diesel would stay a stationary and submarine power plant or become a merchant prime mover.

There were three answers, and the early builders split across them. One was a reversing gearbox or clutch between a unidirectional engine and the shaft, which added weight and a wear item. A second was a controllable-pitch or feathering propeller, rare and fragile at the time. The third, and the one that won for direct-drive ships, was to make the engine itself reversible by reworking the camshaft and starting-air timing so the engine could be stopped and restarted turning the other way. Hesselman’s work at Sickla put Atlas Diesel among the first builders to solve direct reversibility, and the 1908 reversible Polar held by Tekniska museet is the physical evidence of that early lead. A reversible engine drove the propeller shaft directly, with no gearbox loss and no separate reversing mechanism to fail, which is why the direct-reversing two-stroke became the merchant standard and held it for most of the century.

The two-stroke had a specific advantage in the reversing case. With a power stroke every revolution and no separate exhaust stroke, the cycle is symmetric enough that reversing the engine is largely a matter of resetting the injection and scavenge-port timing relative to crank direction. That is part of why the Polar two-cycle, rather than a four-stroke, was the form Atlas Diesel pushed for marine propulsion. The four-stroke contemporaries, including the Burmeister and Wain engine in the Selandia, solved the same problem by other means and proved the ocean-going case in 1912; the Swedish two-stroke had already proved the seagoing case in 1907.

The 1917 merger and Atlas Diesel

AB Atlas was the older of the two parents. André Oscar Wallenberg’s circle founded it in 1873 to build railway equipment, naming it for the Titan who carries the sky. By 1901 it had a pneumatic-tool department. AB Atlas had cooperated with Diesels Motorer from the start in developing the diesel for commercial use and held a minority stake in the engine firm. The two Wallenberg-controlled companies, by then Nya Atlas and AB Diesels Motorer, merged in 1917 into Aktiebolaget Atlas Diesel. World War I had been good to both; roughly 40 to 50 percent of their combined output went to export.

After 1917 the Polar engine was an Atlas Diesel product, built at Sickla. The inter-war Atlas Diesel was a broad industrial firm: diesel engines, pneumatic tools, compressors, and rock-drilling equipment under one roof. The Polar marine and stationary line was one segment among several, and not the largest. The pneumatic and drilling side was already the commercial center of gravity that would define the post-war company.

This is where the older “Atlas-Polar” narrative goes wrong on a second point. It sometimes claims Atlas Diesel was “acquired by Atlas Copco” and the engine business “divested.” The documented sequence is the reverse. Atlas Diesel was not acquired by Atlas Copco; Atlas Diesel became Atlas Copco. The 1956 name change followed the purchase of the Belgian compressor concern Arpic Engineering, replacing “Diesel” with an acronym drawn from a Belgian sales subsidiary, Compagnie Pneumatique Commerciale. By then the firm had already left engine-building, eight years earlier.

The Polar engine: design and ratings

The Polar was a two-stroke, or in the period term two-cycle, diesel. British Polar’s history states flatly that the engines “were of the two-cycle type, built under licence from Nydqvist and Holm, Trollhattan, Sweden,” and that “the basic design with airless injection was introduced in 1928.” Airless, or solid, injection mattered. The first generation of diesels used air-blast injection, where a separate high-pressure air supply atomized the fuel and blew it into the cylinder at the moment of injection. That air came from a multi-stage compressor driven off the engine, a heavy, parasitic, and maintenance-prone arrangement that absorbed useful power and added a second high-pressure system to maintain.

The industry moved off air-blast injection across the 1920s, and the Polar’s 1928 airless design sat right in that transition. The shift waited on metallurgy and machining: a solid, or airless, injection system needs a fuel pump and injector that can build and hold the several-hundred-bar pressures that atomize fuel mechanically, and the precision to do it without the air-blast crutch. Sulzer began advertising airless diesels in the mid-1920s; air-blast diesel manufacture had largely left the scene by the end of the decade. For a builder like Atlas Diesel and its Glasgow licensee, adopting airless injection in 1928 cut the engine’s weight and parasitic load and removed the injection-air compressor as a failure point, which is the design watershed in the Polar story. The transition was uneven across the industry: Vickers, for one, struggled with engine-driven air compressors, tried solid injection, hit smoky-exhaust problems, and reverted to air-blast until the late 1930s. The Polar line’s clean 1928 move is part of why it stayed competitive in the auxiliary and medium-power market.

The documented power band is wide. The early licensed engines of the 1930s were small auxiliaries: Grace’s Guide records that by 1939 the British works built Polar-Atlas engines “in sizes 48 to 192 bhp.” The mature range was far larger. British Polar’s history describes “a wide range of engine sizes covering powers from 300 to 4,000 bhp (220 to 2,980 kW) without supercharging,” which sets the naturally aspirated ceiling of the line at roughly 4,000 brake horsepower before turbocharging entered the picture. These are the figures the public record supports. The bore-and-stroke tables for each individual Polar model are not consistently published in the sources cited here, and this article does not invent them.

The model nomenclature survives in the service literature. British Polar refers to “all E, I, M, N and T ranges of Polar engines.” The letter ranges denote engine families rather than a single bore, so a plate reading “Polar M” identifies a family, not a dimensioned model. Reading a specific plate back to a bore and a rated output is exactly the kind of decode the Marine Engine Model Decoder exists for. For the heat-rate side of a Polar installation, the brake thermal efficiency from SFOC calculator converts a measured fuel rate into an efficiency figure that can be compared against the engine’s design point.

The naval connection needs care, because two different things share the word “Admiralty.” British Polar’s record places the firm’s engines in Royal Navy and Admiralty service, and the company also worked on the Admiralty Standard Range ASR1 program. The ASR1 itself, though, was an Admiralty-pattern submarine engine, not a Polar design: it grew from a 1937 submarine power-plant design whose development was suspended through the war, the prototype eight-cylinder unit was completed at Chatham Dockyard in 1949, and it first went to sea on the survey ship HMS Vidal in 1951, which carried four ASR1 engines for a total of 2,940 shaft horsepower. ASR1 engines later powered the Oberon-class submarines and the Leopard and Salisbury class frigates. The accurate statement is that British Polar built and serviced engines for naval use and was involved in the Admiralty Standard Range work; the ASR1 is best described as an Admiralty design rather than as a member of the Polar letter ranges. Conflating the two would overstate the Polar lineage, and this article does not.

A two-stroke breathes once per revolution against a four-stroke’s once per two revolutions, which is why a two-stroke of a given size makes more power than a four-stroke of the same swept volume and speed. That trade is the reason the slow-speed two-stroke became the merchant main-engine standard. The Polar sat in the medium-speed and smaller range rather than the cathedral-sized slow-speed segment that MAN Energy Solutions and Sulzer came to dominate, so it competed for auxiliary generating sets, ship propulsion in ferries, tugs, fishing vessels, and warships, and stationary power.

The price of the two-stroke power advantage is scavenging. With no separate exhaust and intake strokes, the engine has to clear burnt gas and admit fresh charge in the brief window around bottom dead center, through ports cut in the cylinder wall and uncovered by the piston. Doing that well is the central engineering problem of any two-stroke diesel, and it is where designs differ most: loop scavenging routes the fresh air up one side and the exhaust out the other through ports alone, while uniflow scavenging adds an exhaust valve in the head so the charge sweeps cleanly from bottom to top. The naturally aspirated Polar ceiling of 4,000 brake horsepower reflects a port-scavenged engine breathing on its own without a turbocharger to force-feed it; the later industry-wide move to turbocharging is what pushed comparable engines well past that figure. Where a Polar engine retained its naturally aspirated breathing, its rated output stayed inside that documented band.

It helps to place the Polar against its contemporaries. The Danish Burmeister and Wain built the four-stroke engine in the Selandia of 1912 and went on to become one of the two dominant slow-speed two-stroke licensors worldwide, the lineage now inside MAN. Sulzer of Switzerland was the other. Against those two, the Polar was a medium-power specialist, not a slow-speed main-engine rival, and its commercial record reflects that: a strong early lead in seagoing diesel installation, a durable niche in auxiliaries and smaller propulsion, and no attempt to chase the largest cargo-ship main engines that became the B&W and Sulzer preserve.

Fuel economy: the diesel’s edge over steam

The commercial case that carried the Polar engine to sea was fuel economy, not power density. A triple-expansion steam plant of the 1910s turned roughly a tenth of its coal’s energy into work at the propeller; an early marine diesel of the same period reached two to three times that, and the gap only widened as injection improved. That is why the Toiler and the Selandia mattered as commercial vessels rather than as curiosities: a motorship carried far less bunker for the same range, freeing deadweight for cargo and cutting the stokehold crew that a coal-fired ship needed.

The figure that captures this is brake thermal efficiency, the fraction of the fuel’s chemical energy that leaves the engine as useful work at the coupling. It is read directly from the specific fuel oil consumption and the fuel’s net calorific value, with no other input.

\eta_{BT} = \frac{3600}{SFOC \cdot NCV}

Symbol	Meaning	Unit
$SFOC$	Specific fuel consumption	g/kWh
$NCV$	Net calorific value	MJ/kg

Source: MAN ES / WinGD Performance

Calculate Thermal Efficiency →

For a diesel burning a distillate of net calorific value near 42.7 MJ/kg at a specific fuel oil consumption of 200 g/kWh, the relation gives an efficiency near 42 percent, the modern medium-speed figure. The airless-injection Polar of 1928 ran richer than that, with a higher fuel rate and an efficiency in the low-to-mid thirties, but even that was a wide margin over the steam plant it displaced. The same arithmetic run backward turns a measured fuel rate into the efficiency a superintendent compares against the engine’s design point, which is the practical use of the relation on an installed Polar set. The fuel-rate side of the comparison and its emissions consequence are carried into the modern regime by the SFOC to CII calculator.

The 1928 move to airless injection that British Polar records was, in efficiency terms, part of how the diesel widened that lead. Mechanical injection at several hundred bar atomizes fuel finely without the parasitic air-blast compressor, so more of the charge burns at the right moment and less work is spent driving an auxiliary, both of which lift the brake thermal efficiency above the air-blast generation the Polar line left behind.

British Polar Engines of Glasgow

The Glasgow line is the part of the Polar story still trading. The company was “incorporated as a private company, Fiat British Auxiliaries Limited” in 1927 with capital of 125,000 pounds. British Polar dates its own continuous operation to that year and markets itself as “relied upon across the globe since 1927.” In 1931 the firm was reconstituted as British Auxiliaries Limited and took on the Polar work, making “in addition to engines of the Fiat type, engines of the Polar-Atlas type, an arrangement having been made for this purpose with Aktiebolaget Atlas Diesel, of Stockholm.” The airless-injection Polar design reached Glasgow effectively at the start of that arrangement, dated to 1928.

The Govan works built land and marine auxiliary engines and medium-sized diesel sets for ship propulsion. The Polar name was formalized in the company title in August 1944, when British Auxiliaries Limited “changed its name to British Polar Engines Limited.” Postwar reconstruction put the firm under shipbuilder Sir James Lithgow as chairman from 1946, and in 1954 Associated British Engineering acquired it. The Glasgow line therefore became a British-owned builder of a Swedish design, drifting away from direct Stockholm control as Atlas Diesel itself exited engines in 1948.

British Polar continued as a builder and then as the long-tail service house for the global Polar fleet. In 2000 it purchased Kelvin Diesels and concentrated production at Helen Street in Glasgow. The company still supplies replacement parts and provides servicing and maintenance for the installed Polar base, which is why a Polar engine in a vessel or a standby generating set is not an orphan even though the Swedish parent left the business three-quarters of a century ago. Ownership changed again in 2021, when Associated British Engineering sold British Polar Engines for a nominal sum to private buyers.

A note on a recurring confusion in the British Polar record: some sources describe the Glasgow engines as built under licence from Atlas Diesel of Stockholm, others as built under licence from Nydqvist and Holm of Trollhattan. Both are correct at different dates. The original Polar-Atlas arrangement of 1928 to 1931 was with Atlas Diesel. After 1948, when NOHAB acquired the Swedish marine engine business, the licensor of record became Nydqvist and Holm. The design lineage is continuous; the Swedish counterparty changed hands.

NOHAB and the move to Trollhattan

NOHAB, Nydqvist and Holm Aktiebolag, was the Swedish engineering firm that inherited the Polar marine engine. It traces to Trollhattans Mekaniska Verkstad, founded in 1847 by Antenor Nydqvist, Johan Magnus Lidstrom, and Carl Olof Holm, and reconstituted as a limited company under the NOHAB name in 1916. It was a broad heavy-engineering house, and the breadth is the point: the marine engine was a late addition to a firm whose main lines were rolling stock and turbines, which is why the Polar engine was never NOHAB’s largest activity any more than it had been Atlas Diesel’s.

NOHAB’s core was locomotives and railway equipment. In 1920 it received an order of 1,000 locomotives from Soviet Russia, of which 500 were delivered between 1921 and 1924, one of the largest single rolling-stock contracts of the inter-war period for a firm of its size. NOHAB built Bristol Jupiter aircraft engines from 1930, built Ljungstrom steam-turbine locomotives between 1930 and 1936, and in 1948 supplied diesel railcars to the Portuguese Railways. After the war it became the European builder of diesel locomotives under licence from the American Electro-Motive Division, the work for which NOHAB is best remembered in railway circles. The Polar marine and stationary engine sat alongside all of that as a specialized segment, and the firm also built water turbines for hydroelectric plant. Inheriting the Polar engine in 1948 gave NOHAB a marine main-engine and auxiliary line to add to its existing diesel-locomotive prime-mover expertise, a logical pairing of two diesel businesses under one roof at Trollhattan.

The marine engine transfer is the hinge event for the Polar name. Grace’s Guide dates “the end of diesel production” at the British Atlas works to “1948, when Nydqvist and Holm (NOHAB) acquired these operations,” consistent with the Atlas Copco group statement that “in 1948, Atlas Diesel stopped making diesel engines altogether.” From that point the Polar marine and stationary engine was a NOHAB product, built at Trollhattan, with British Polar continuing the parallel Glasgow line under the revised licence. NOHAB carried the Polar two-stroke through the post-war decades for Swedish and export shipping, naval auxiliaries, and stationary plant, alongside its locomotive and turbine work.

The reference history of the Polar line and the NOHAB engineering house is detailed in the dedicated NOHAB and Polar marine diesel engines article, which carries the Trollhattan founding, the locomotive and turbine business, and the engine-family detail. The Swedish shipbuilding context, the yards and engine works that surrounded NOHAB and Atlas, is covered in the Gotaverken Swedish marine engines profile.

The Bofors-Nohab period and the Wartsila acquisition

NOHAB’s diesel engine business did not stay independent. By the 1970s the marine engine operations had passed into the orbit of the Bofors group, the Swedish armaments and engineering combine, and traded as Bofors-Nohab. The terminal phase came when Finland’s Wartsila moved on the business. Wartsila’s own 190-year history timeline states for 1978: “Wartsila invests further in diesel engine technology, and our manufacturing operations go international as we acquire Swedish NOHAB diesel business.” That 1978 step took 51 percent of the NOHAB diesel business from Bofors and marked the start of Wartsila’s international manufacturing. Wartsila took the remaining shares in 1984, and the Trollhattan operation traded as Wartsila Nohab.

The NOHAB parent company, the broader Trollhattan group, went bankrupt in 1979, separately from the engine business that Wartsila had taken into its own structure. The distinction matters: the engine works survived under Wartsila even as the historic NOHAB corporate entity failed. Wartsila’s wider Swedish consolidation in the same period included the Lindholmen Motor business in Gothenburg, part of the broad rationalization of Scandinavian medium-speed marine engine builders as Asian shipbuilding eroded the regional orderbook.

Under Wartsila the historic Polar two-stroke was progressively retired in favor of Wartsila’s own medium-speed four-stroke families. The Vasa series and its successors, and the modern Wartsila medium-speed range covered in articles such as the Wartsila 32 and Wartsila 20, absorbed the propulsion and auxiliary orders that the Polar two-stroke had once served. The full corporate path from a Norwegian-Finnish sawmill to the marine and energy group is set out in the Wartsila corporate history. The Trollhattan engineering and manufacturing capability fed into that group; the standalone Polar marine engine product line, as a current catalog item, was gone by the late twentieth century, leaving British Polar in Glasgow as the service custodian of the installed fleet.

Applications and the surviving fleet

The Polar engine’s footprint was the medium and smaller power band rather than the largest merchant main engines. The documented applications, drawn from the British Polar record, span auxiliary generating sets, propulsion for ferries, tugs, and fishing vessels, naval auxiliaries under the Admiralty Standard Range designation, and stationary generating plant. The early auxiliary engines of the 1930s sat at 48 to 192 brake horsepower; the mature naturally aspirated line reached toward 4,000 brake horsepower.

The reason a Polar history still matters operationally rather than only historically is the service tail. A two-stroke diesel that entered service in the 1950s or 1960s can still be running in a standby set, a workboat, or a preserved vessel. British Polar’s continued parts and overhaul business exists precisely because that installed base outlived its three builders. An operator holding a Polar engine today is dealing with a design whose Swedish parent stopped building engines in 1948, whose Trollhattan successor passed to Wartsila in 1978, and whose only continuing source of original-pattern support is the Glasgow works that took the licence in 1928.

That service economics is the part of the Polar story most relevant to a working engineer. When the original equipment manufacturer exits, an installed engine becomes a support liability: the spares stop, the drawings disperse, and the engine is run to failure or repowered. The Polar fleet escaped that fate because the Glasgow line stayed in business as a parts and overhaul house, kept the drawings, and in 2000 broadened its base by buying Kelvin Diesels and concentrating production at Helen Street. A vessel or a generating set with a Polar engine can therefore still source original-pattern liners, rings, bearings, and injection equipment from a single accountable source rather than from reverse-engineered pattern parts of unknown provenance. For a long-lived asset, that single fact often decides whether the original engine is retained or replaced.

The Polar engine also sits in a wider picture of how the medium-speed and auxiliary diesel market consolidated. The Polar, the British contemporaries, and the broader Scandinavian builders all faced the same squeeze from the 1970s: Asian shipbuilding pulled the new-build orderbook east, and the slow-speed main-engine market consolidated around the MAN B&W and Sulzer lineages while medium-speed builders were absorbed into a few groups, Wartsila chief among them. The Polar two-stroke was a casualty of that consolidation as a current product, not because it was a poor engine but because its volume could not justify a standalone development program once Wartsila’s own four-stroke families covered the same duties. The engineering did not vanish; it folded into Wartsila at Trollhattan, and the support folded into British Polar at Glasgow.

For comparison and parts-decoding across the wider population of historic medium-speed and auxiliary diesels, the related builder profiles are useful: English Electric marine and locomotive diesels, Ruston marine and industrial engines, and Lister Petter marine engines cover the contemporaneous British field that British Polar competed in for auxiliary and small-propulsion work.

Limitations of the published record

The Polar marine engine is a well-attested line at the level of corporate dates and headline ratings, but thin at the level of per-model specifications. Several things are documented and several are not, and an honest profile has to mark the boundary.

What is firmly documented: the 1917 Atlas Diesel merger, the Sickla works, Hesselman’s tenure from 1899 to 1916, the 1908 reversible marine Polar held by Tekniska museet, the Toiler’s 1911 Atlantic crossing, the 1928 airless-injection design, the British Polar founding in 1927 and the Polar-Atlas licence from Aktiebolaget Atlas Diesel, the 1948 transfer of the Swedish marine engine business to NOHAB, and Wartsila’s 1978 acquisition of the NOHAB diesel business. These rest on the Atlas Copco group history, Grace’s Guide, British Polar’s own record, and Wartsila’s corporate timeline.

What is not consistently published in the sources cited here: the bore and stroke of each individual Polar model, the cylinder counts and rated speeds per family, the production totals, and the year-by-year output. The letter ranges E, I, M, N, T and the Admiralty ASR1 designation are recorded, but a complete dimensioned table by range is not. This article does not fill those gaps with invented numbers. The 48-to-192 brake horsepower band of the 1930s licensed engines and the 300-to-4,000 brake horsepower naturally aspirated span of the mature line are the documented power figures, and this article does not infer per-model bore and stroke from them. The brake-thermal-efficiency figures in the fuel-economy section above are the standard textbook ranges for early diesels and steam plant of the period, used to frame the relation rather than as a measurement of a specific Polar engine.

There is also a naming hazard that the record itself creates. “Polar Atlas,” “Atlas Polar,” and “Polar-Atlas” all appear, and the licensor of the Glasgow engines is variously given as Atlas Diesel or as Nydqvist and Holm depending on the date. Both are right, sequentially, but a single source quoted out of period can make the lineage look contradictory. Where a primary source attributes the 1908 reversible engine, this article follows the museum catalog. Where the corporate dates are at issue, it follows the company’s own published histories rather than secondary summaries.