WO Bentley's aero engines

Walter Owen (WO) Bentley was into aero engines and railway locomotives long before he gave his name to cars. The Bentley BR2 was described by aviation authority Bill Gunston as the pinnacle of the rotary aircraft engine and the Sopwith Snipe and Camel, which used it, as the best Allied fighter aircraft. Nearly 500 Snipes were built in 1918, a total of 1,567 were delivered to the Royal Air Force, as well as a ground attack version, the Sopwith Salamander. Snipes with Bentley rotaries remained in service until November 1926.

Known first as the Admiralty Rotary (AR1), then BR for Bentley Rotary it used similar valvegear to the troublesome French Clerget it replaced, but little else, and WO was upset by allegations that his engine was no more than an imitation. “These (claims) originated from people who glanced only at the cam mechanism, which was the first thing they saw and, for ease of production, was the only similar feature. The crankcase, crankshaft, method of securing the cylinders as well as their heads were all fundamentally different.”


Commander Wilfred Briggs RN took charge of relations between aircraft engine manufacturers and the Royal Navy during the First World War. He created a department for the Royal Naval Air Service (RNAS), and in 1915 appointed Lieutenant WO Bentley RNVR to its Technical Board looking into troubles with rotary engines. Rotaries, in which the cylinders spun round the crankshaft, had been developed by the Société des Moteurs Gnome in 1908. Carefully made in nickel steel they were expensive, but they were also light, and since the entire engine spun like a flywheel behind the propeller they ran smoothly. The cylinders of a rotary, to which the propeller was attached, rotated around a fixed crankshaft so the leading edge of the cylinder barrel was invariably better cooled than the trailing edge. As a result the bores went oval, the piston rings broke and the engines suffered catastrophic seizures. Gwynnes Pumps of Chiswick, which was making them was unhappy with WO’s suggestions to deal with the difficulties so Briggs suggested trying them out on a single sample cylinder with encouraging results. Gwynnes was worried about making changes, in case they compromised its licensing agreement with Clerget, and turned them down.

Other drawbacks included overhauls after only 20 hours and heavy oil consumption. Designed by Pierre Clerget in 1911, his Chiswick-made 9B had a better cylinder than the Gnome with deep cooling fins, and took mixture to the inlet valves through pipes instead of through the crankcase. It had dual ignition and was effective in Sopwith Camel and Nieuport fighters, notwithstanding the gyroscopic effects of the rotary engine. Each cost £907 10/-, but suffered failures of the obturator ring, an artillery term for the sealing ring of a gun breech, describing a flexible piston ring of thin bronze or light alloy, providing a seal when the 0.06in (1.5mm) thick cylinder wall distorted. The flimsy cylinders were stressed to the utmost as the tips reached 150mph (241kph) whirling round. The turbulent airstream cooled cylinders asymmetrically and failures were frequent. WO Bentley’s modifications increased power from 110bhp (82kW) to 130bhp (96.9kW) and improved reliability for a new version of the engine, which went into production as the AR (for Admiralty Rotary) and later BR (Bentley Rotary) made by Humber and Daimler. Bentley specified aluminium air-cooled cylinders with shrunk-in iron liners, redesigned the steel cylinder heads, secured them by four long bolts and to the Admirals’ delight made the engine for £605. The stroke was increased from the Clerget’s 6.7in (170.2mm), bringing the capacity to 1,055cu in (17,288cc). Some 150bhp (112kW) @ 1250rpm could be sustained for 100 hours between overhauls. Bentley made further improvements, then in April 1917 three prototypes of a fresh design were ordered, weighing only 93lb (42kg) more, yet with bigger cylinders and giving 200bhp (149kW) @ 1300rpm. Some 1,500 a month were planned, at £880 still well below the price of the troublesome Clerget. The new engine was designated BR2, and although it used similar valvegear, was a complete reincarnation of the old French engine, and the most powerful rotary in service.

Walter Owen Bentley (1888-1971)

INTRODUCTION BR1 1917; BR2 1917-1920s. weight 475lb (215.5kg) .
ENGINE 9-cylinders, radial rotary; 5.5in (140mm) x 7.1in (180mm), 1522cu.in (24,961cc); compr 5.3:1; 238bhp (177.5kW) @ 1300rpm; 9.56bhp (7.13kW)/l.
ENGINE STRUCTURE 2 overhead valves per cylinder operated by pushrods from epicyclic gears at front of crankcase; finned aluminium cylinders with cast iron liners, screwed into crankcase; forged steel cylinder heads; carburettor, dual ignition, fuel system; circular main bearing, master connecting rod on ball bearings, and eight slave rods wrist-pinned; air-cooled.
PERFORMANCE maximum speed 121mph (195.7kph) in Sopwith Snipe.
PRICE £880.
PRODUCTION WO claimed 30,000 were ordered. BR1 production started June 1917, Humber delivered 600, Vickers 523, total 1123. BR2 first ran October 1917, Crossley delivered 83, Daimler 1415, Gwynnes 82, Humber 391, Ruston & Proctor 596, total 2567

From: The Complete Bentley, by Eric Dymock