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The Rolls-Royce Crecy was a two-stroke 90-degree V12 liquid cooled aero engine of 26.1 litres capacity, featuring sleeve valves and direct petrol injection. Single cylinder development began in 1937 under project engineer Harry Wood using a test unit designed by Sir Harry Ricardo. Although originally conceived as a compression ignition engine by the Ricardo company, by the time Rolls-Royce started serious development, the decision had been taken to revert to a more conventional spark-ignition layout, although still retaining petrol-injection.

The first complete V12 engine was built in 1941, designed by a team under Wood with Eddie Gass as the Chief Designer and in bench-testing it produced 1400 hp. There were problems with vibration and the cooling of the pistons and sleeves. The firing angle was 30 degrees and 15 lbf/in² (100 kPa) supercharger boost was typical. Bore was 5.1", stroke 6.5", compression ratio 7:1 and weight 820 kg. The thrust produced by the two-stroke exhaust was estimated as being equivalent to an additional 30% increase in power at the propeller on top of the rated power of the engine, and was exceptionally loud.

Unlike most two-stroke engines, supercharging or turbocharging was used rather than crankcase compression to force the charge into the cylinder. Stratified charge was used where the fuel was injected into a bulb-like extension of the combustion chamber where the twin spark plugs ignited the rich mixture. Operable air-fuel ratios of from 15 to 23 were available to govern the power produced between maximum and 60%. The lean mixtures reduced detonation allowing higher compression ratios or supercharger boost. Supercharger throttling was used as well to achieve idling. The supercharger throttles were novel vortex types, varying the effective angle of attack of the impeller blades from 60 degrees to 30. This reduced the power required to drive the supercharger when throttled and hence fuel consumption at cruising power.

The sleeve valves were open ended rather than sealing in a junk head - the open end uncovered the exhaust ports high in the cylinder wall at the bottom of the sleeves' stroke leaving the ports cut in the sleeve to handle the incoming charge only. They had a stroke of 30% of the piston and were 15 degrees in advance.

It was named after the Battle of Crécy, battles being the chosen theme for Rolls Royce two-stroke aero engines. There were however no subsequent Rolls Royce engines of this type, and rivers were used for jet engine names.

Sir Henry Tizard was a proponent of the engine as Chairman of the Aeronautical Research Council. The power of the engine being interesting in its own right, but also the exhaust thrust at high speed and altitude making it a useful stop gap between engines such as the Rolls-Royce Merlin and anticipated jet engines.

Only six complete examples were built when the research was terminated in December 1945. An additional eight vee twins were built. Serial numbers were even, Rolls-Royce practice being to have even numbers for clock-wise rotating engines when viewed from the front. Crecy number 10 achieved 2500 hp on 21 December 1944. Subsequently single cylinder tests achieved the equivalent of 5000 bhp for the complete engine.

General characteristics

• Type: 12-cylinder supercharged liquid-cooled 2-stroke Vee aircraft piston engine
• Bore: 5.1 in (130 mm)
• Stroke: 6.5 in (165 mm)
• Displacement: 1,593 in³ (26.1 L)
• Dry weight: 1,807 lb (820 kg)

Components

• Valvetrain: Sleeve valve
• Supercharger: Gear-driven centrifugal type supercharger with variable angle of attack of the impeller blades providing up to 15 psi (100 kPa) of boost.
• Fuel system: Direct fuel injection
• Cooling system: Liquid-cooled

Performance

• Power output: 2,500 hp (1,865 kW)
• Specific power: 1.57 hp/in³ (71.5 kW/L)
• Compression ratio: 7:1
• Power-to-weight ratio: 1.38 hp/lb (2.27 kW/kg)

• The Rolls-Royce Crecy, A. Nahum, R.W. Forester-Pegg, D. Birch, Rolls-Royce Heritage Trust ISBN 1-872922-05-8