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Magical Merlin, the aero engine that won the war


ELECTRIC vehicle zealots look away now – because today we’re marking the 90th anniversary of one of the greatest internal combustion engines, the Rolls-Royce Merlin.  

On October 15, 1933, the iconic 27-litre V12 that would help the Allies win the Second World War by powering aircraft such as the Spitfire, Hurricane, Lancaster and Mosquito was first run on a test bed at the Rolls factory in Derby.  

Let me emphasise that I’m not an expert either in engineering or aviation, so I won’t go into technical details – there are numerous excellent book and website sources about the Merlin for that. It’s just that, in an age when we’re ordered almost daily to ‘celebrate’ so many ridiculous things, I think this is one that’s truly worth paying homage to.  

The Merlin began life as aero engine PV-12, the designation given it by Rolls-Royce. PV stood for Private Venture, indicating that it was a project with no Government funding, and 12 stood for 12-cylinder. It was being developed because Rolls realised the growing aviation market needed a more powerful engine than the firm’s 21-litre Kestrel.  

Fortunately, as Europe slid towards war, the PV-12 turned out to be a perfect fit for two new fighter aircraft – the Supermarine Spitfire and the Hawker Hurricane. The Air Ministry placed orders for the PV-12 and Rolls dubbed it the Merlin, in keeping with the firm’s tradition of naming its four-stroke engines after birds of prey. (The merlin is a small falcon; others in the series were Eagle, Kestrel, Peregrine, Griffon and Buzzard.)  

The first versions of the Merlin achieved around 880 horsepower. It was constantly modified and improved before and throughout the war years, and by late 1945 it was putting out more than 2,000 hp.  

It first proved its worth during the Battle of Britain in 1940, when the RAF’s 360mph Spitfires and 340mph Hurricanes held their own against the Luftwaffe’s formidable Messerschmitt 109 fighters. There’s still fierce debate over which British plane was the better, but neither would have been contenders without the Merlin.  

In the early days, the engine was not without its problems. The most serious was that because it was carburettor-fed rather than having fuel injection, it would be starved of petrol when a Spitfire or Hurricane went into a high-speed dive, as happened frequently during dogfights.  

This hitch was solved by Miss Beatrice ‘Tilly’ Shilling, an engineer at the Royal Aircraft Establishment in Farnborough. She invented a small thimble-shaped device which could easily be fitted into the fuel system to prevent engine cut-out. In those days of political incorrectness, it became known as ‘Tilly’s orifice’ or ‘Tilly’s diaphragm.’  

As demand for the Merlin grew, Rolls-Royce opened massive factories in Crewe, Glasgow and Manchester. The engine was also built under licence in the US by the Packard Motor Car company of Detroit. By the war’s end, a total of 150,000 Merlins had been produced worldwide.  

As well as powering Britain’s fighters, the Merlin was most notably used for the four-engined Lancaster bomber, stalwart of the RAF’s night-time campaign against Germany. More Merlins were made for Lancasters than for any other plane.  

The engine also powered the De Havilland Mosquito, the remarkable plywood-built light bomber that could reach almost 400mph, outpacing any petrol-engined German fighter.   

Perhaps most significantly for the later course of the air war, in 1943 the North American Mustang P-51 fighter-bomber was equipped with Packard-built Merlins. This hugely improved the plane’s range and performance, enabling it safely to escort daylight US bomber fleets to and from their targets in Germany, fending off Luftwaffe marauders. From then on, the Americans were dominant in the air battle.  

A version of the Merlin called the Meteor was used to power the Cromwell tank, which gave sterling service following the D-Day landings in 1944. The engine was also used in some fast patrol boats. In the modern era, one zany petrolhead even put a Merlin in a car he called The Beast. 

Today, the much-loved Merlin continues to be used in the aircraft of the RAF’s Battle of Britain Memorial Flight and in many restored Spitfires and Hurricanes in private hands. There’s a superb documentary here about how Rolls-Royce manufactured the engine.  

There’s little doubt that the Merlin was the most significant piston aero engine in history. On its 90th birthday, amid the crazed clamour for Net Zero, long may it keep flying the flag for the internal combustion engine.  

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Henry Getley
Henry Getley
Henry Getley is a freelance journalist.

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