By 1932 it was becoming apparent to Rolls-Royce that their best-selling engine, the 21.2 litre 745HP Kestrel was coming towards the end of its development life. A decision was made by Sir Henry Royce to develop a new engine using some of the experience of the Schneider Trophy winning 'R' engine. It retained the V12 configuration and geared supercharger of its predecessors, but was of 27 litres swept volume. It was anticipated initially that this engine would be able to reliably deliver around 1000HP. The engine was known initially as PV12 (Private venture, development initially entirely funded by Rolls-Royce). When in October 1933 the Air Ministry agreed to finance the development, it was named Merlin (Rolls-Royce piston engines were named after birds of prey, jets after rivers).

Ironically, in view of its later reputation of extreme reliability, the early development of the engine was plagued with problems, with gear train failures and persistent failures of the water jackets (the cooling mixture eventually became 30% glycol as antifreeze in water at +18PSI pressure). Eventually in July 1934 the Merlin passed its type testing and was rated at 790HP at 12 000 feet at 2500 RPM. The Merlin B was tried with a ramp head to the cylinder which had improved fuel mixing and flame propagation in Rolls auto engines, and in February 1935 delivered 950HP at 11000 feet equivalent. Based on their experiences, Rolls-Royce decided to make the crankcase and cylinder blocks as 3 separate castings, with bolt-on ramp heads to the cylinders. This engine was the Merlin C. By this time the promise of a low-profile aero engine of 1000HP had persuaded both RJ Mitchell and Sydney Camm to base their designs around this new untried engine. This engine still had problems, but after some modifications as Merlins E and F, a Merlin E passed a civil 50 hour Type Test at 955HP (maximum 1045HP). As an emergency measure it was decided to scale up the parallel cylinder head as used in the Kestrel to the larger engine (Merlin G). This engine easily passed its Type Test a month before the F (now released as Merlin I) and was then designated Merlin II. This engine weighed 1335lb and was rated at a maximum power of 1030HP at 3000RPM at 16250 feet, and ran on 87 octane fuel. It is worthy of note that in 1937 an attempt was made to break the World Landplane Speed Record, using a highly modified Spitfire I and a specially strengthened Merlin II. This engine actually generated 2160HP, and showed the potential for development of the engine. Most of the modifications developed for this engine eventually found their way into production Merlins. The Merlin III was adapted for the use of a constant-speed propeller and a constant-speed unit.
A variant of this engine with a higher supercharger gearing (providing up to 12.5lb boost) and a Coffman cartridge starter was termed the XII and marked the difference between the Spitfire I and II.
In 1935, after problems with supercharger gearing, Rolls-Royce decided to take out a license for the Farman 2-speed drive. The advantage of the 2-speed supercharger was that it could be run at low speed, using little energy, in the thick lower altitudes, while being available to enrich the air supply at altitude. There are supercharged engines providing zero extra boost at sea level being flown today. The first of these engines with a 2-speed supercharger was the Merlin X. This added significantly to the length of the engine.

In 1939 a decision was made to focus on 100 octane fuel for aero engines. This fuel permitted higher boost pressures and temperatures without detonation, and allowed the use of +12lb boost rather than the previous limit of +6lb.

The next major development of the Merlin came from Sir Stanley Hooker. It was realised in the development of the Merlin for the World Speed attempt that the efficiency of the Merlin supercharger was relatively poor. Hooker, a mathematician by trade, examined the supercharger from first principles, and markedly improved its efficiency. He also improved the flow characteristics of the air inlet, which improved the output power at altitude. Although the original installation was elongated even further, it was found that by turning the carburetor around the length was similar to the original installation. This new engine was the Merlin XX, and allowed power to be maintained at much higher altitudes (1175HP at 20 000 feet compared to 1160HP at 13 500 feet for the Merlin II). The single-speed supercharger Merlin 45 incorporated many of these modifications, and this engine, fitted to the Spitfire airframe, became the Mark V Spitfire.

Some of these engines were modified for low-altitude power, since most of the air combat was taking place around 6 000 feet. In these, the supercharger impellers were shortened, and the speed of the constant-speed unit increased. This gave a maximum power height of around 6 000 feet, and increased speed by around 22 mph at this height. If coaxed to higher altitudes, however, the engine suffered badly.

The development of high-altitude bombers required the development of an engine with a higher full-throttle height. Rather than move to turbochargers, Hooker suggested adding two superchargers in series. Since a high altitude supercharger of the right size had already been developed, the output from the Rolls-Royce Vulture supercharger was simply fed into the supercharger of a Merlin 46. The only modification required was the incorporation of a cooling stage after the two supercharger stages to prevent premature fuel detonation during compression in the cylinders. The new engine, the Merlin 60, had a full-throttle height of nearly 30 000 feet. A redesign changed the supercharger gearing and introduced a 2-piece cylinder block to produce the Merlin 61. This engine produced spectacular effects when fitted to a Spitfire. Although intended for the Mark VIII, it was possible to fit it to the Mark V airframe, and this became the Spitfire Mark IX/XVI series. The extra cooling necessary became evident by the enlarged radiator under the left wing.

As the specific power from the engines increased, the focus of much of the design was strengthening. An empirical approach was to run an engine at high power until something broke, then strengthen or redesign that part and carry on. The consequence of these developments was the Merlin 130 with a low level power of 2030HP, and an elevation of the height at which 1000HP was available from 16 000 to 36 000 feet. In late 1944 a Merlin was run for 15 minutes at 2640HP! After the difficult beginnings the ability of the Merlin to withstand abuse became a watchword. Few engines tolerate full power loads for any great period, but there are examples on record of Lancaster pilots losing one of their Merlins shortly after takeoff, but simply continuing the mission with all the remaining throttles pushed to the stops. The engines rarely failed.

The 500, 600, and 700 series Merlins were mostly post-War developments for civilian transports. In these the focus was not on absolute power but on component life. In total the production run on the Merlin was 168 040.

Carburettor design

One of the great problems as discerned by pilots was the tendency for the carburetted engine to cut out under negative 'g'. Luftwaffe pilots learned to escape by simply pushing the nose of their aircraft down into a dive, as their fuel- injected engines did not cut out under these circumstances. By 1941 Miss Tilly Shilling in Farnborough had developed a partial cure for the problem. A diaphragm across the float chambers with a calibrated hole allowed negative 'g' manouvres, and was fitted as standard from March 1941. Sustained zero 'g' manouvres were not sorted out until somewhat later. In 1942 an anti-g version of the SU carburetor was fitted to single and two-stage Merlins. 1943 saw the introduction of the Bendix-Stromburg carburetor which injected fuel at 5psi through a nozzle direct into the supercharger and was fitted to the Merlins 66, 70, 76, 77, and 85. The final development was the SU injection carburetor which injected fuel into the supercharger using a fuel pump driven as a fuction of crankshaft speed and engine pressures, which was fitted to the 100 series Merlins.

The first Griffon was built in 1934, and was effectively a derated engine of "R"-type, as was used in the Schneider- Trophy winning Supermarine S6 aircraft. As such it was a V12 liquid-cooled engine of 37 litres swept volume. Much of the design philosophy for the Griffon was to keep the overall dimensions close to those of the Merlins, to allow interchangeability. It is unfortunate that this engine was never developed to the degree that the Merlin was, and never exceeded it in overall power development. The major differences for the pilot was a less-smoothly running engine and one that rotated the propellor the opposite way to the Merlin. The standard anti- torque actions applied at takeoff power also needed to be the opposite way round to prevent a rapid departure off the side of the runway! By June 1940 the Griffon II was rated at 1720HP with 1495HP at 14 500 feet. The First Griffon-powered production Spitfires were the Mk XII with a 1815HP Griffon VI. All Griffons had a Coffman cartridge starter.

By 1943 the two-speed two stage Griffon 60 series were introduced. The Griffon 65 was rated at 2035HP at 7000 feet, while the Griffon 66 included a blower for cabin pressurisation in the Spitfire PRXIX. Griffon 72 and 74 were produced for the Firefly at 2245HP at 9250 feet, while the Griffon 83 to 88 had gear trains for contra-rotating propellers. The ultimate Griffon was the Griffon 101 with a three-speed supercharger which was rather temperamental, but also pushed the Spiteful at 494 MPH, and was never put into production. The last military Griffons flying were the Griffon 58 of the Shackleton, designed to deliver 2455HP at low level with water/methanol injection into the supercharger and 25lb boost, driving contra-rotating propellers. On the retirement of the Shackletons, some of the engines have been converted for driving the Griffon-engined PRXIX of the BBMF. Even more interestingly, an ex-Shackleton Griffon with the contra-rotating prop mechanism retained is being fitted to a PRXIX in the USA.