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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.
The Merlin I had a one-piece block and crankcase. It had a separate 'ramp head' ('semi-penthouse'), whose exhaust valves were in line with the cylinder and the inlet valves were at a 45-degree angle. The ramp head was developed to optimise 'squish' turbulence to delay the onset of detonation and pre-ignition. The development was carried out using a single-cylinder test engine, followed by testing of a modified Goshawk engine. However, the expected benefits were not realised when the ramp head was applied to the Merlin, and other problems arose. Nevertheless, 172 Merlin I engines were ordered, being needed to power the Fairey Battle, while work proceded to introduce a more conventional flat head arrangement, wherein the valves are parallel to the cylinder. This arrangement was adopted for the Merlin II. The cylinder blocks (jackets) were now separated from the block, and they incorporated the cylinder heads. At the same time, design work started on separate blocks and heads.
Part of the impetus for the move to separating the blocks and crankcases was to reduce the cost and time of repair in the event of major failure of reciprocating failure, which could necessitate replacement of the whole crankcase/block unit.
The first production standard single-piece block Merlin II was installed in a Hawker Horsley, and it completed 100 flight hours in 6.5 days in the summer of 1937.
First widely delivered as the 1,030 hp Merlin II in 1938, production was quickly stepped up.
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, and despite continuous increases in power output, 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.
Essential to the continuous increase in Merlin power output was the the increase in boost pressure and the development of increased fuel octane ratings. 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, initially 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.
Originally each cylinder block was made in one piece. Problems arose with progressive increases in output, and two-piece blocks were introduced.
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 required the use of large quantities of lead anti-knocking agent.
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 Beatrice Shilling, an engineer at the Royal Aircraft Establishment, Farnborough), 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 Rolls-Royce development of the US Bendix-Stromburg carburettor which injected fuel at 5 psi through a nozzle direct into the eye of 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.
By injecting fuel into the supercharger rather than into the inlet ports utilised the latent heat effect of evaporation which improved the compression ratio by about 7%.
In 1940 the British Air Purchasing Commission specified and ordered a new fighter, the P51 Mustang from North American Aviation, powered by the Allison V12 engine. In April 1942, Ronnie Harker, a Rolls-Royce service liaison pilot, took the opportunity to test the plane, and was impressed by its aerodynamic properties. He proposed that it could be improved by installation of the R-R Merlin 61. R-R Hucknall embarked on trial installation, while work was also put in hand in the USA to install a Packard-built Merlin. The first flight took place from Hucknall on 13 October 1942. In the USA, the first Packard Merlin-powered example started trials several weeks later.