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Note: This is a sub-section of Rolls-Royce
In the early 1930s work was started on a new 1,100 hp class design as the PV-12 – PV for 'private venture' as the company received no government money for work on the project. The PV-12 first flew on the front of a Hawker Hart biplane in 1935, using the new evaporative cooling system then in vogue. The cooling system proved unreliable, and when supplies of ethylene glycol (Prestone) from the US became available, the engine was changed to the conventional liquid cooling system instead.
In 1936, the Air Ministry had a requirement for a new fighter aircraft with airspeeds that would eventually have to be over 300 mph. Two designs had been developed entirely as private venture exercises: the Hawker Hurricane and Supermarine Spitfire. Both were designed around the PV-12 instead of the Kestrel, and were the only British modern fighters to have been so developed. Production contracts for both aircraft were let in 1936. The PV-12 was instantly catapulted to the top of the supply chain and became the Merlin.
First widely delivered as the 1,030 hp Merlin II in 1938, production was quickly stepped up. The Merlin I had a 'ramp head' where the inlet valves were at a 45-degree angle to the cylinder. This was not a success and only 172 were made before the conventional flat head arrangement wherein the valves are parallel to the cylinder was adopted for the Merlin II.
Early Merlins were considered to be rather unreliable, but Rolls soon introduced a reliability-improvement programme to improve matters. This consisted of taking random engines from the end of assembly line and running them continuously at full power until they failed. Each was then dismantled to find out which part had failed, and that part was redesigned to be stronger.
After two years of this, the Merlin had matured into one of the most reliable aero engines in the world, and could be run at full power for eight-hour bombing missions.
By the end of its production run, over 150,000 Merlin engines had been built. It was supplanted in service by the Rolls-Royce Griffon which was a development of the R engine.
Most of the upgrades to the Merlin power output were the result of ever-increasing octane ratings in the aviation fuel available from the US, and ever more efficient supercharger designs. At the start of the war the engine ran on the then-standard 87 octane aviation spirit and could supply just over 1,000 hp from its 27 litre displacement compared to 1,100 hp from the 34 litre Daimler-Benz DB 601.
From June 1940 small quantities of 100 octane fuel, imported from the US, became available and the Merlin IIIs were found to be capable of running on it.
The next major version was the XX which ran on 100 octane fuel. This allowed it to be run at higher manifold pressures, which were achieved by increasing the "boost" from the centrifugal type supercharger. The result was that the otherwise similar engine delivered 1,300 hp. Another improvement made to the XX and future Merlin variants was a redesign of the cooling system to work using a 70/30% water/glycol mix rather than the 100% glycol of the Merlin I, II and III series. This allowed the engines to run some 70 degrees C cooler, substantially improving engine life and reliability. This also removed a potential fire hazard from Merlin powered aircraft, as pure ethylene-glycol is a flammable liquid.
The process continued, with later versions running on further-increased octane ratings, delivering higher and higher power ratings. By the end of the war the "little" engine was delivering over 1,600 hp in common versions, and as much as 2,070 hp in the Merlin 130/131 versions used on the de Havilland Hornet. The Merlin was running on 150 Octane fuel by the time it was used in the Lancaster bomber. This high octane rating was achieved by large quantities of lead anti-knocking agent, so much in fact, that the engine cowlings around the exhaust outlets were usually heavily stained with it. It had to be regularly removed for aerodynamic, not to mention weight, reasons.
The Merlin's lack of direct fuel injection meant that both Spitfires and Hurricanes were, unlike the contemporary Bf-109E, unable to nose down into a deep dive. This meant the Luftwaffe fighters could 'bunt' into a high-power dive to escape attack, leaving the pursuing aircraft spluttering behind as its fuel was forced by negative 'g' out of the carburettor. RAF fighter pilots soon learned to 'half-roll' their aircraft before diving to pursue their opponents. The use of un-injected carburettors was calculated to give a higher specific power output, due to the lower temperature, and hence the greater density, of the fuel/air mixture, compared to injected systems.
"Miss Shilling's orifice" (invented in March 1941 by an engineer at the Royal Aircraft Establishment, Farnborough, named Beatrice Shilling), a holed diaphragm fitted across the float chambers, went some way towards curing the fuel starvation in a dive.
Further improvements were introduced throughout the Merlins: 1943 saw the introduction of a Bendix-Stromburg carburettor which injected fuel at 5 psi through a nozzle direct into the supercharger and was fitted to the Merlins 66, 70, 76, 77, and 85.
The final development was an SU injection carburettor which injected fuel into the supercharger using a fuel pump driven as a function of crankshaft speed and engine pressures, which was fitted to the 100 series Merlins. Production of the Griffon-engined Spitfire Mk. XII had begun the year before.