WWII Fighter Engine Competition

However, his confidence was not borne out. Before reaching the coast the raiders were intercepted by Spitfires which would follow every move of the Fw190s and outperform them in speed and climb. At least five of the German aircraft went down in the Channel and the rest returned to France without bombing their target.

They had been intercepted by 91 Squadron flying the new Spitfire XII, an aeroplane produced rapidly to meet the threat of the low-level Fw190. The first Spitfire XIIs were Spitfire V conversions with a modified tail and clipped wings and powered by the Rolls-Royce Griffon VI, an engine rated at 15lb boost for low-altitude operations. The Griffon was similar in design to the Merlin, which had powered all Spitfires up to that time, but its capacity of 36. 7 litres was more than 25% greater.

The Griffon was equipped with a single-stage two-speed supercharger and produced 1,840hp at around 2,500ft and 1,640hp at 11,000ft. It endowed the relatively light aircraft with great acceleration, speed and rate of climb. These features, coupled with the Spitfire’s classic manoeuvrability, made the new version an effective answer to the low-level raider. It was a good example of how Vickers Supermarine and Rolls-Royce combined to meet an enemy challenge.

In 1940 the brunt of the fighting in the Battle of Britain had been borne by the more numerous Hurricanes, but it was evident that the Spitfire had greater development potential. The Spitfire was therefore continually modified and updated to meet the demands of the Royal Air Force right through to the end of the war.

During the Battle the Spitfire I was powered by the Merlin III engine with an output of 1,030hp at 16,000ft. At that time the Spitfire was fairly evenly matched with the Me109E. However, by the autumn of 1940 Me109Es with uprated Daimler Benz DB601 engines showed a performance edge above 20,000ft. The Merlin XII in the Spitfire II with its higher-geared Merlin I type supercharger gave some improvement but not enough particularly as the Luftwaffe responded in 1941 with more effective Me109F.

Vickers and Rolls-Royce answered with the Spitfire V, powered by the Merlin 45 or 46. This engine had a new single-stage single-speed supercharger. It gave 10-12% more power at its full-throttle height of about 21,000ft than previous engines gave at its full-throttle height of 16,000n. Full-throttle height is the altitude at which the engine reaches full throttle at combat rpm and is normally the point at which it develops its maximum power. Overall the Spitfire V was comparable with the 109F. However, by the end of 1941 a new German fighter, the Fw190, was making its mark. Powered by the BMW801 air-cooled radial of 42 Iltres, the Fw190 was faster and had a greater rate of climb than the Spitfire, whose only advantage lay in its ability to turn.

For some time Rolls Royce had been working on a two-speed two-stage supercharger with a large liquid-cooled intercooler. It was aimed at giving the Merlin better performance to put the Spitfire ahead of the opposition. What emerged was the Merlin 61, with two-piece cylinder blocks- to cope with the very high boosts envisaged as development continued – and other reliability features. The first production engine was ready for despatch from Derby on Christmas Day 1941. Initially, it produced more than 1,300hp at 23,000ft, using 15lb boost; 1,000hp was available at around 30,000ft.

A new Spitfire, the Mk Vill, was planned for this engine, but the urgency was such that Vickers Supermarine and Rolls Royce undertook to install it in a number of Spitfire Vs. The modified aircraft, with a four-bladed propeller, was known as the Spitfire IX. By July 1942 64 Squadron was equipped and operational. Although the Mk IX was regarded as an interim type it proved so successful that several thousand were built, compared with about 1,650 Spitfire Vills.

The Spitfire ix had a maximum speed of 410mph and was very effective, particularly around 25,000ft. However, a further new rating was devised to provide more power at lower levels. The efficiency of the supercharger was improved by increasing the diameter of the first-stage rotor. The drive gear ratios were reduced to lower full throttle heights, but an 18lb boost rating was used for combat. Engines to this standard, known as the Merlin 66, came into squadron service in the second half of 1943.

The Merlin 66 also had a Bendix low pressure injection carburettor which was unaffected by “g” variations. Earlier Merlins did not have this feature; during the Battle of Britain the Me109s, powered by DB601 engines with fuel injection, could dive without encountering negative “g” effects and thus had the advantage over the Spitfires. With its new rating and the Bendix carburettor the Merlin 66 was a most successful fighter engine; it was installed in the North American Mustang as well as in Spitfires. The engine gave close to 1,750hp at 5,000ft and more than 1,600hp at 16,500ft. This provided the F and LF versions of the Spitfire VIII and IX with a wide range of fighting altitudes, making them a match for the Fw190 and Me109G.

In 1944 150-grade fuel became available and this allowed 25lb boost at 3,000rpm. It gave the Merlin 66 more than 2,000hp “down. on the grass” and 1,860hp at 10,000ft, making the Spitfire even more formidable.

The same version of the two-stage supercharger was applied to various Merlin marks with gear ratios to improve altitude performance. Power output was 1,400hp at 25,000 and 700hp at 40,000ft. A special Spitfire VII reached 47,000, and Alex Henshaw, who was chief test pilot at the Castle Bromwich Spitfire factory, says that he could reach 46,000ft in production Spitfire IXs.

As plans for the invasion of Europe developed the RAF sought ways of using the many existing Spitfire Vs in a close-support role. By cropping the supercharger rotor on Merlin 45s and 46s it was possible to get 18lb boost at a low level. Engines so modified were installed in aircraft with clipped wings, and gave a major improvement in performance at low level.

Before the invasion, these aircraft were used in other roles, including support for bombers at medium altitude. At these heights, the engines were badly down on power and could not perform adequately. The aircraft and engines, which had seen a lot of hard usage, were said to be “clipped, cropped and clapped” and were very unpopular with the pilots. It was not a long-lived modification.

Griffon developments continued, and after a run of 100 Spitfire Xlls the Spitfire XIV with the Griffon 65 engine came into service. Following Merlin practice this engine had a two speed two stage supercharger and intercooler, but drew air through a triple choke Bendix carburettor. At 18Ib boost and 2,750rpm, it gave 2,000hp at 7,000ft and 1,800hp at 21,000ft. This put the aeroplane into the 450mph category, making it and its variants the fastest Spitfires to see war service. Although outpaced by the Me262 jet fighter the Spitfire XIV was the first Allied fighter to claim the shooting down of an Me262 in a high-altitude encounter.

It is interesting to compare German and British engine practice. The Daimler-Benz engine of 33. 8 litres had a hydraulically-driven single-stage supercharger, allowing its speed to be varied. This benefited engine power at lower altitudes because in a climb the engine could be run with the throttle wide open and with supercharger speed increasing. British engines had a mechanically driven supercharger, and hence a fixed supercharger speed, and the throttle was opened progressively on the climb to full throttle height.

The BMW801 of 42 litres had a mechanically driven single-stage two-speed supercharger. Both engines operated at modest boost pressures of around 1. 3-1. 35 atmospheres (for comparative purposes atmospheres are absolute while boost on Rolls-Royce engines is in pounds per square inch above atmospheric pressure).

The DB engines were upgraded not by making major increases in boost pressure but by stepping up engine speed and using bigger valve overlaps and changes in compression ratio in line with automotive practice at the time. The BMW801 had improvements in its supercharger drive gear to provide a higher compression ratio with the use of 95-octane instead of 87-octane fuel. Later there were aerodynamic improvements to the supercharger and an rpm increase, providing a boost of 1. 62 atmospheres with increased supercharger speed.

Rolls Royce practice on both the Merlin and the Griffon was to leave basic engine parameters unchanged and to raise power output by higher boost pressures.

By the end of the war more than 20,000 Spitfires had been produced, of which about 90% were Merlin powered. However, the major improvements in performance on the Merlin were also of benefit to other aircraft.

Fitting a Merlin in the Mustang turned it into a top-class fighter, with a range that enabled it to escort 8-17 bombers on daylight raids deep into Germany. More than 12,000 Mustangs, powered by Packard-built Merlins, were produced for the United States Army Air Forces (USAAF) and the Royal Air Force, and they were highly regarded by both Services.

The brilliant de Havilland Mosquito, successful in so many roles, made extensive use of the single stage two speed Merlin XX series engines throughout its production. In 1942 a special lightweight Mosquito with two Merlin 61s reached 42,000ft while still climbing at 500ft/min. It also had a speed of 420mph.

One interesting development was the equipping of a number of Mosquito night fighters with nitrous oxide injection. The power boost thus achieved from their Merlin 25 engines gave a speed increase of 45-50mph for six minutes at 28,000ft. This eased the task of successful interception of high-flying Me41O fighter bombers.

After the war the piston engine faded rapidly with the introduction of gas turbines, but it is salutary to look back and see how the development of piston engines made major advances possible in aircraft performance.

The Merlin doubled its power output and altitude ratings almost entirely by increasing boost pressure and leaving other parameters unchanged. At the same time overhaul life and average life in service progressively improved. Yet this was not a limited run of special engines: more than 160,000 Merlins were built in 52 marks; they ultimately powered 19 types of aircraft which saw war service.

NEXT ISSUE: We analyse the German point of view in developing their respective V12 inline Daimler Benz and BMW Radial engines.

Alec Harvey-Bailey recently retired from Rolls-Royce after 42 years’ of service. His wide experience ranges from the Merlin and the last days of the Kestrel to the customer launch of the RB211 in North America, for which he was responsible as executive vice-president of Rolls Royce Aero Engines Inc. In 1978 he became managing director of the Arab British Engine Company, the joint venture set up to start aero engine manufacture in the Arab world. (Courtesy of Rolls Royce Magazine. )