Rolls Royce Crecy – The Most Advanced Piston Aero Engine Never Made.

We are all aware of the Rolls Royce Merlin engine, the driving force behind so many of the most important aircraft of world war two like the Spitfire, Mustang, Hurricane, Mosquito and Lancaster to name just a few.

But behind the scenes, another Roll Royce engine was poised to take what had been done with the Merlin and move it one significantly with a radical design that not only made the Merlin look simple by comparison but it was much more powerful and for its 26-litre capacity it had the highest specific output of any piston aero engine then or since.

This is the story of the Rolls Royce Crecy, a revolutionary two stroke aero engine and what it might have become if it had been just a few years earlier.

The story of the Crecy starts in the mid-1930s, this was at a time when Germany was rearming and the build-up of its Luftwaffe was causing concern amongst the British government & military.

The plan at the time was to have aircraft patrolling along the coast at altitude on the lookout but there was a limited number of them available and they could only patrol for as long as they had fuel.

As trials of the early British radar progressed it showed that there would be an opportunity for a high-speed sprint engine for an inceptor fighter that would be able to take off and climb quickly to confront any foreign aircraft to augment regular patrols.  

In 1935 this was suggested by Henry Tizard, later of the Tizard mission the US,  at a meeting of the Aeronautical Research Committee or ARC of which he was the chairman. He asked the question that if such an engine could be made, could they disregard the fuel and oil consumption to gain very high performance for short flights only.

At the time aero engines were big but not that efficient. The specific power of the Pratt and Whitney twin wasp for example which was one of the most common engines at the time was 0.66hp per cubic inch and just making them bigger added power but it didn’t fix the power to weight ratio, so new engine designs and materials would be needed to make lighter more powerful engines. On both sides of the Atlantic there was a race to produce a hyper engine that had a specific power of 1hp per cubic inch.

Tizard was a good friend of Harry Ricardo, now regarded as one of foremost engine designers of the 20^th century. Ricardo had been working on a sleeve valve, 2 stroke compression ignition diesel aero engine that could equal the output of a conventional petrol aero engine for several years.

A couple of months after the meeting of the ARC, in a letter to Tizard from Ricardo, he suggested that his diesel engine could be converted to a spark ignition to produce an extremely high output engine with a small frontal area, this engine would become the Crecy and be developed by Rolls Royce and Ricardo’s company.

As Tizard was overseeing all engine development through ARC, he also was encouraging the likes of Haye Constant and Frank Whittle as they worked on the early turbojet engines, so the development of both jet engines and advanced piston engines was seen a the best approach to deal with the future threat from the Luftwaffe.

Ricardo thought that the more traditional poppet valves engines, like the Merlin with two round valves in the cylinder head, would be unlikely to make more than about 1500 hp because there is a limit as to how big the valves could be and thus how quickly the fuel-air mix and exhaust could be got in and out of the engine.

He had already published a study on sleeve valve engines in 1927 and thought these should be the next generation as they offered greater volumetric efficiency.

Now you may be forgiven in thinking what is a sleeve valve engine?. Well, they are a very different design that became popular in luxury cars around the 1920s.

Instead of the two-valve cylinder head design which we think of as normal now, the piston goes up and down in a sleeve that has holes cut in to it which work as the inlet and exhaust valves, a bit like a two-stroke engine but with a difference.

This difference was that the sleeve itself then moves up and down and from side to side to line up with inlet and exhaust ports as the engine goes through the four-stroke cycle. This sleeve allows for multiple larger ports than would be possible with the valves in the cylinder head design and consequently the engine can breathe much better. It also made the engine less susceptible to pre-ignition or detonation due to the poor quality of the fuel at the time.

But there was downside to these engines, the gearing mechanism to make the sleeve move up, down and side to side in time with the piston makes them much more complicated. It not too bad in a single-cylinder engine but in a V12 or rotary it there are lots of extra cranks, shafts and gears involved.

Sleeve valve engines were used in other aero engines during the war. Frank Halford who worked in Ricardo’s London office from 1919 -1921 went on to Napier to design the 24 cylinder Sabre sleeve-valve engine which went on to become one of the most powerful engines of the war and was used in the Hawker Typhoon, while Ricardo worked with the Bristol Aeroplane Company to design a range of rotary sleeve valve engines for the Bristol Beaufighter.

The difference between those and the Rolls-Royce Crecy was that the Crecy was a two-stroke with petrol injection and a unique variable thrust supercharger.

The simplicity of the two-stroke design combined with the sleeve valves could be a game changer with power outs double that of similar four-stroke engines.

Ricardo’s company did most of the development work on single-cylinder and v-twin engines, the V-twins being of particular use with the sleeve valves. This was normal to work out the best design before building multi-cylinder engines like a V-12. One of the unique features Ricardo created was a cone-shaped cylinder head with a bulbous upper chamber with twin spark plugs and a central fuel injector at the top.

This bulbous chamber allowed the fuel-air charge to pre-combust in a rich mixture around the spark plugs before spreading to the rest of the weaker mixture in the remainder of the cylinder.  This allowed it run on lower octane fuel or at higher boost pressures without detonation, something which held back a lot of the other engine designs during the war until fuel octanes of 130 to 150 were available later on.

As the Crecy was a two-stroke, when the V-12 engine was running at its full speed of 3000 rpm there would be 36,000 exhaust pulses per minute, twice that of a four-stroke engines like the Merlin, so it really would scream much like the German Junkers Ju87 Stuka dive bombers which used two wind-driven sirens known as the Jericho Trumpets to create that characteristic scream that would scare the living daylights out of those on the ground, the Crecy’s exhaust could do that all by its self.

There was another advantage to the sleeve valve in that the opening of the ports could done with an eccentric gear operation so that it opened very much faster than that of a typical overhead valve driven by a cam.   

The effect of this almost instant valve opening was to make the exhaust gas exit much faster, which combined with the doubled frequency of cylinder firing gave the Crecy up to 350 lbs of thrust just from the exhausts. The Merlin engines also used the exhaust thrust but were only able to achieve 150 lbs of thrust. At 300 mph, each lb of thrust was equal to 1hp through an 80% efficient propeller.

This explosive exhaust and doubled frequency also made it extremely loud. It’s a pity there is no footage or audio recording of it when it was being tested as it is said that when it was running on the test stand it could be heard over 5 miles away and when they first started testing it, it triggered the air raid sirens 15 miles away.

On December 21^st 1944 Crecy No 10, a V12 26.1 litre engine achieved 1798 bhp from a normally aspirated engine running on 100 octane fuel. When the calculated blower power was added it would be 2350 bhp but once corrected for the test stand losses it was about 2500 bhp.

If you compare that to the most powerful Merlin derivative, the Griffen which was 37 litres, 11 litres bigger than the Crecy and running on 150 octane fuel with a two stage supercharger to the Crecy’s single blower, that produced 2220 bhp.

If you compare the specific output of the two, the Crecy was 1.77 hp per cubic inch to the Griffen’s 0.91 hp per cubic inch, easily beating the 1 hp per cubic inch goal of a hyper engine.

Tizard, talking in 1946 said that the research had shown that the two-stroke sleeve valve engine was capable of nearly 200 hp per litre, a figure which is only beaten today by the highest performance race engines running at much higher rpm’s. As the Crecy was a 26.1 litre V-12, its fully realised output could have been 5000 hp, double that of the best performing four-stoke engines.

So what happened to such a powerful and important engine?. Well, basically it arrived just too late and had been a low priority throughout the war with the design and manufacturing talent being assigned to the more important tasks such as the building and improving the Merlins and their derivatives because they were used in so many of the RAF aircraft and others during the war.

Because of its extra complexity, it was unreliable and broke down many times but it has to be remembered that it was still under development and combined a lot of different technologies and newly developed metals in one engine, something that hadn’t been done before.

It was due to be tested in a Spitfire but it was found that the spitfire’s airframe was not strong enough to handle the power. The P51 Mustang was thought to be a more suitable match for the engine but no further action was taken, so instead, it was to be flight-tested in a Hawker Henley but when no engine became available and the Hawker test aircraft stood idle until the end of the war.

By 1945 came it became clear that the jet engine would be the future and resources were moved towards that and away from the Crecy. Because of it high fuel efficiency, there were plans to use it in civilian transport after the war but again the development of the jet put paid to those.

Just six full-size V-12 Cracys were built and eight V-twin test engines. The V-12s ran for total 1,060 hours and the V-twins ran for 8,600 hours but all of them only ever ran on the test stand.

Crecy No 12 was to use a shaft-driven exhaust gas turbine blower, which was a 1/2 scale WR-1 turbojet as a power recovery device to make use of the exhaust thrust as well as a variable trust mechanical supercharger which would have made it more fuel-efficient for larger transport aircraft. The back pressure created from the turbine also worked to increase the power of the two-stroke, whereas on a four stoke it lost power.   

But by then, Rolls Royce decided that it would be just too much effort to bring the Crecy up the reliability of the Merlin as the market for high power piston engines waned with the end of the war and the jet engine took off, so the Crecy became just interesting footnote in the history of aviation. What happened to the six Crecy engines is unknown, but to date, none have been found complete or in pieces.

In a letter to Ricardo, the Crecy project engineer Harry Wood said that if it had been given another six months of development it would have not only much more powerful but reliable as well.

It said that if Whittle and Von Hain,  the designers of the first jet engines had been born ten years later the Crecy could have been the fill-in between the Merlin and the jet engine.

In the race to create the best aero engine for the war, it wasn’t just the Crecy that lost out, it was the piston engine itself, jets just did a better job which has been proved by history but it was only through the dogged determination of a few highly-skilled engineers like Frank Whittle and Hans von Hain that this came about.

Paul Shillito

Creator and presenter of Curious Droid Youtube channel and website www.curious-droid.com.