you are forgetting the merlin
In the early 1930s, Rolls started planning for the future of its aero engine development programmes, and eventually settled on having two basic designs. The 700 horsepower (500 kW) Rolls-Royce Peregrine was an updated, supercharged development of their existing V-12, 22 L Rolls-Royce Kestrel which had been used with great success in a number of 1930s designs. Two Peregrines bolted together on a common crankshaft into an X-24 layout would create the 1,700 hp (1,300 kW) 44 L Rolls-Royce Vulture, for use in larger planes like bombers. There was also the possibility that the famous 36 L 'R' engine from the Supermarine racing planes could be developed into a 1,500 hp (1,100 kW) class engine of its own, itself a development of the Rolls-Royce Buzzard, a scaled up Kestrel.
However, this plan left a large gap between 700 and 1,500 hp (500 and 1,100 kW). To fill the gap work was started on a new 1,100 hp (820 kW) 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 (480 km/h). Fortunately, 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 (770 kW) 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 (valves parallel to the cylinder) was adopted for the Merlin II.
Early Merlins were considered to be rather unreliable, but Rolls soon introduced a superb 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 with no problems.
As it turned out, the Peregrine saw use in only two aircraft, the Westland Whirlwind and the Gloster F9/37. Although the Peregrine appeared to be a satisfactory design, it was never allowed to mature; Rolls-Royce's priority was troubleshooting the Merlin. The Vulture was fitted to the Hawker Tornado and Avro Manchester, but proved unreliable owing to failures of the crankshaft to connecting-rod bearing caused by lubrication problems. With the Merlin itself soon pushing into the 1,500 hp (1,100 kW) range, the Peregrine and Vulture were both cancelled in 1943.
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 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 (750 kW) from its 27 L displacement compared to 1,100 hp (820 kW) from the 34 L Daimler-Benz DB 601. From June 1940 small quantities of 100 octane fuel, imported from the U.S, 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 the otherwise similar engine delivered 1,300 hp (970 kW). 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 Is, IIs, and IIIs. 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 (1,200 kW) in common versions, and as much as 2,070 hp (1,544 kW) 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 tetraethyl lead--so much, in fact, 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.
 Carburettor Developments
The Merlin's lack of direct fuel injection meant 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 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 Bendix-Stromburg carburettor 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 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.
 Other uses for the engine
A non-supercharged version of the Merlin using more steel and iron components was produced for use in tanks. This engine, the Rolls-Royce Meteor, in turn led to the smaller Rolls-Royce Meteorite.
In 1938 Rolls Royce started work on modifying some Merlins which were later to be used in British MTBs, MGBs, and RAF Air-Sea Rescue Launches. For these the superchargers were modified single-stage units and the engine was re-engineered for use in a marine environment.
A Spanish-built version of the Messerschmitt Bf 109G-2, the Hispano Aviacion HA 1112M1L Buchon, was built with the Rolls-Royce Merlin 500/45 engine of 1,600 hp, with four-bladed propeller, in the Hispano Aviacion factory in Seville- a fitting powerplant for the last-produced version of the famous Messerschmitt fighter, as the Bf-109 V1 prototype aircraft had been powered by the Rolls-Royce Kestrel V-12 engine in 1935.
 Packard's legacy
The Merlin was considered to be so important to the war effort, negotiations soon started to establish an alternative production line outside the UK. Rolls-Royce had checked out a number of North American automobile manufacturers, in order to select one to build the Merlin in the USA or Canada, and Packard Motor Car Company's attention to high quality and engineering impressed the parent British company so much, Packard was selected to build the Merlin. Agreement was reached in September 1940, and the first Packard-built engine, designated V-1650-1, ran in August 1941.
The first American production of the Merlin was the Packard Merlin 28 (Mark XX). This engine was a single stage, two speed supercharger. As the Merlin 28, it was used for the Lancaster bomber. The USAAF version of this engine was used in the P-40Fs. The initial Packard modifications were done on this engine by changing the main bearings from a copper lead alloy to a silver lead combination and featured indium plating. This had been developed by General Motors' Pontiac Division to prevent corrosion which was possible with lubricating oils that were used at that time. The bearing coating also improved break-in and load carrying ability of the surface. British engineering staff assigned to Packard were astonished at the suggestion but after tear down inspections on rigidly tested engines were convinced the new design offered a decided improvement.
The real improvement Packard incorporated into the Merlin was adopting the Wright supercharger drive quill. This modification was designated the V-1650-3 and became known as the "high altitude" Merlin destined for the P-51. The two speed, two stage supercharger section of the -3 featured two separate impellers on the same shaft which were normally driven through a gear train at a speed of 6.391:1. A hydraulic gear change arrangement of oil operated clutches could be engaged by an electric solenoid to increase this ratio to 8.095:1 in high speed blower position. The high speed gear ratio of the impellers was not as great as the ratio used in the Allison but speed of the impeller alone was not the factor that increased the engine performance at altitude. The double staging of the compressed fuel/air mixture provided the boost pressure through a diffuser to the intake manifolds which increased the critical altitude of the power plant.
The ability of the supercharger to maintain a sea level atmosphere in the induction system to the cylinders allowed the Packard Merlin to develop 1,210 horsepower at 25,800 feet. The two stage impeller created extreme heating of the fuel/air mixture during the compression process and in order to prevent detonation of the compressed charge, it was necessary to cool the mixture prior to entry into the cylinders. This cooling was accomplished by the casting of an intercooler passage into the wheelcase housing between the first and second stage impellers.
Ethylene glycol coolant was circulated by a pump through this passage to carry off the excess heat generated by the impellers. Without the intercooler the temperature of the charge could be as high as 400°F (204°C). The intercooler in itself was not adequate to deal with the high temperature and an additional cooling fin and tube core was placed between the outlet of the blower and the induction manifold to the cylinders. This radiator was known as an aftercooler and served as a reservoir for the system. The glycol mixture used for the supercharger cooling was independent of the main engine cooling system and used a centrifugal pump driven by the engine to circulate the coolant through an aircraft radiator system at a rate of 30 gallons per minute.
This combined system reduced the charge temperature to suitable levels. The throttle valves in the updraft carburetor throat were controlled by an automatic boost control through the pilot's linkage to maintain the selected manifold pressure during changes in altitude. These valves were only partially open during ground and low level operation to prevent overboosting of the engine. As air density decreased with an increase in altitude, the throttle valves were moved to an open position by boost pressure corresponding to aircraft altitude. This system provided full power within engine boost limitations up to the critical altitude of 26,000 feet. This was the improvement Packard brought to the Merlin.
When the first of the Packard-built Merlins arrived in Britain, the engineers at Rolls-Royce stripped it down and were amazed to find the production-line built Packard engine, far from being as bad as they expected it to be for component tolerances, was actually better. Up until then, R-R Merlins were hand built, every face being finished off by hand, and this time-consuming process placed great strain on the production capability of the skilled workforce involved in the manufacture of these engines. The Packard engine changed many minds, although there were still some at R-R who remained unconvinced of the quality of the American engine, produced as it was by a largely unskilled and semi-skilled female workforce. In the end, the engine's performance removed any doubts about its quality and workmanship.
The Packard V-1650 so outperformed the Allison V-1710 it supplanted the Allison in the North American P-51 Mustang, which then became one of the best fighters of the war. It was also incorporated into some models of the Curtiss P-40, specifically the P-40F and P-40L. Packard Merlins powered Canadian-built Hurricane, Lancaster, and Mosquito aircraft, as well as UK-built Spitfires in the shape of the Mark XVI, otherwise the same as the Mark IX with its British-built Merlin.
Although it is not commonly known, Packard greatly improved the maintainability of the engine (by allowing easier use of interchangeable parts, rather than custom finished ones), and their changes were also incorporated in subsequent British production.
A common misconception is that Packard Merlin engines were used in American PT boats; the engine used was in fact a Packard V-12 engine, a modification of the Liberty L-12, totally unrelated to the Merlin. The possibility is that these engines were also used by British MTBs and MGBs.