Simanaitis Says

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BRISTOL’S HERCULES rivals the Rolls-Royce Merlin as the most important British aircraft engine of World War II. The Merlin was a liquid-cooled V-12 with conventional poppet valves, four per cylinder, two intake, two exhaust. The Hercules was decidedly different: an air-cooled 14-cylinder two-row radial with sleeve valves for inlet and exhaust. Talk about contrasting technology!


At left, Rolls-Royce Merlin liquid-cooled V-12. At right, Bristol Hercules air-cooled 14-cylinder two-row radial. These and other images from Allied Aircraft Piston Engines of World War II: History and Development of Frontline Aircraft Piston Engines Produced by Great Britain and the united (Reference) 1st (first) Edition by White, Graham [1995], Society of Automotive Engineers.

As Graham White observes in his Allied Aircraft Piston Engines of World War II, “In actuality, the engine does not care how the mixture gets into the cylinder or how the products of combustion are removed, provided these events take place at the correct time and the cylinder is completely sealed during the compression and power strokes.”

The idea of a sleeve valve is to surround a cylinder with a ported sleeve, its movement designed to align appropriate inlets to the cylinder’s intake and exhaust passages. The Knight automobile had one type of sleeve-valve engine, each cylinder with a pair of concentric sleeves that reciprocated to perform their intake and exhaust port alignment. A good many cars used this Silent Knight design under license. Double-sleeve designs were renowned for trailing an oil mist, formed through lubricating the reciprocating concentricity of their sleeves.

The Burt-McCollum design, the other type of sleeve valve, has a single sleeve for each cylinder with combined reciprocation and rotation to achieve its alignments of ports.


A Bristol illustration of single-sleeve-valve operation. Given that I’ve devoted rather less than a thousand words on its description, this video may also prove helpful.

In a sense, the Burt-McCollum approach replaces the lubricating difficulties of the Knight double-sleeve with complexities of combined reciprocation/rotation. And complex it was.


Bristol sleeve-valve drive had 56 gears providing actuation of its 14 sleeves.

Why not use conventional poppet valves? In 1925, the legendary Harry Ricardo (Sir Harry from 1948) built two test engines, one poppet-valve, the other sleeve-valve. In his classic book The High-Speed Internal-Combustion Engine (first published in 1931; Graham White cites its 1968 edition), Ricardo wrote that the sleeve-valve version had higher power, lower fuel consumption and lighter weight than its poppet-valve counterpart.

Roy Fedden, Sir Roy from 1942, was designer of Bristol’s sleeve-valve aero engines. He recognized the volumetric efficiency of multiple valves for intake and exhaust and that sleeve valves offered much simpler design of a multi-valve cylinder head. Being without fiery hot exhaust valves, such a head optimized whatever octane fuel was used. The Hercules, introduced in 1936, had a “five-valve” head, with three intake inlets and two exhaust outlets per cylinder.

Bristol met a challenge in making the sleeve-valve engine feasible for wartime production and field use. Hitherto, cylinder/sleeve clearance had required individual matching which precluded interchangeability of parts. One of several breakthroughs in materials and manufacturing was the use of a dull, undressed grinding wheel in finishing the sleeve’s outside diameter. This key step was discovered by accident when an operator mistakenly used the wrong grinding wheel.

The Hercules displaced 38.7 liters, typical of the era’s large aero engines. (A Wright Cyclone was 29.9 liters; its Duplex-Cyclone, 54.9 liters.) The first Hercules variant produced 1375 hp at 2400 rpm. By war’s end, its successors were developing 2500 hp, with around 65,000 Hercules engines produced.

Many British bombers were powered by the Hercules, among them variants of the Avro Lancaster, Bristol Beaufighter, Handley Page Halifax, Short Stirling and Vickers Wellington.


At left, a Vickers Wellington powered by a pair of Hercules sleeve-valve engines. At right, a Bristol Beaufighter, also Hercules-powered. By the way, the Bristol Brabazon was powered by eight (!?) Centaurus engines, 53.6-liter 18-cylinder offspring of the Hercules.

The Beaufighter has another claim to fame: England had land-based radar even before the 1940 Battle of Britain. The Beaufighter was the first of its aircraft to be fitted with (top secret) airborne-interception radar from November 1940.


The Hercules XVI, as rendered by James “Jimmy” Clark for Flight magazine. Image from Classic World War II Aircraft Cutaways, by Bill Gunston in association with The Aeroplane and Flight, Osprey Aerospace, 1995.

Bill Gunston notes in his Classic World War II Aircraft Cutaways, “These 14-cylinder sleeve-valve radial engines were in my view second only to the Merlin in importance among wartime British engines. Unlike the strident Merlin, the “Herc” just murmured with a deep booming sound suggestive of awesome power.” ds

© Dennis Simanaitis,, 2015

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