When you hear “WWII aviation engine” and “Rolls Royce” chances are, you immediately think of the venerable Merlin engine. However, there was another engine on the Rolls Royce drawing boards, that had the potential to surpass the Merlin’s success story. So, the question is, if it’s so great and full of potential, why haven’t you ever heard of the Rolls Royce Crecy engine before? Well, that can be explained by a single word: Jets.
Unfortunately, the engine’s timeline doomed it to obscurity. Development started in the midst of WWII in 1941 but wasn’t completed until after the war ended. And the reason the experimental program was canceled had nothing to do with its inherent capabilities, but rather because the development of the jet engine took priority. The Crecy’s potential was never even given a chance to prove itself inside of a Supermarine Spitfire cowling, as it was originally intended.
Crecy: The Most Advanced Two-Stroke Aero Engine Ever
While the title might sound like hyperbole, that is how modern aviation experts describe the Crecy engine. Let’s look at what has everyone so enamored with the engine. It’s a 26.1-liter (1593.4 cubic inch) 90-degree V12 engine. It was a two-stroke, sleeve-valve design that featured direct injection. It had a 5.100-inch bore and a 6.500-inch stroke, with a modest compression ratio of 7.0:1. That was augmented by 15psi from a supercharger (which we need to talk more about later, as it was a really neat design), and allowed the engine to make almost 1,800 horsepower in the testing that was completed.
Using reciprocating sleeve valves, the Crecy valves we open-ended allowing for much greater exhaust flow out of the engine; like, a lot more exhaust flow. A neat feature on paper, was that the high-flowing two-stroke exhaust design would allow the engine’s exhaust to provide additional forward thrust to the aircraft, on the order of an additional 30 percent over the propeller. Unlike traditional two-stroke engines, the Crecy used a traditional oiling system. This was thanks to the supercharger forcing the intake charge into the cylinder. The direct injection system allowed a much leaner air-fuel ratio to be achieved, and ignition was handled by dual spark plugs.
Getting into the Crecy supercharger system, things got really interesting. The first test engines had adjustable compressor blade angles. This was done to reduce the drag of the blades through the air when high boost wasn’t required. This reduction of drag allowed for increased fuel efficiency and economy while cruising, with the ability to switch to an aggressive air-moving profile when maximum power was needed.
Then, further getting creative was adding an exhaust turbine to the system. No, this didn’t make a turbocharger — at least not in the way we think of turbochargers nowadays. The exhaust turbine instead drove the accessory drive to free up parasite loss from the crankshaft, while the compressor section was still directly driven by the crankshaft. While the principle proved to be sound, the implementation was unsuccessful as mechanical issues plagued the experimental power recovery systems.
Too Little, Too Late
Unfortunately, while all of these novel features of the Rolls Royce Crecy were quite impressive, it never really got a chance to prove itself. The engine was never flight tested and all of the massive horsepower numbers produced in testing were either calculations or extrapolations. After the engine’s highest recorded output in late 1944 of 1,798 horsepower, it was calculated that the exhaust turbine system would have resulted in a total output of 2,500 horsepower. Similarly, single-cylinder test numbers were then multiplied by 12 to claim a theoretical horsepower output of over 5,000 horsepower.
With the end of the second world war and jet engine technology rapidly advancing, the Crecy engine was a victim of timing and circumstance. While its experimental technologies were, and still are, very interesting, it wasn’t enough to keep up with rapidly approaching jet age, and the Rolls Royce Crecy engines were destined to become nothing more than an interesting footnote in aviation history.