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The Deltic Locomotive

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The British Rail Class 55 locomotive was one of the great pieces of British Engineering. It combined elegant styling with ground breaking technology.

The Engine

At the heart of the Class 55 was the Napier Deltic Engine. Originally commissioned by the Royal Navy in 1944, it had its roots in the German Aircraft industry.

That Sinking Feeling

Most of the world's navies had fleets of small attack boats. These motor torpedo boats were fast and manoeuvrable, and could be used for patrols or attacking enemy shipping. Germany had diesel powered E-boats and the Royal Navy had petrol-powered MTBs. While the petrol engine was probably lighter and more powerful, they had the minor problem of petrol vapour exploding on ignition. British diesel engines were big, slow and heavy, totally unsuitable for the boats. The Admiralty asked Napier and Sons, who had been working on copies of a German fast-revving diesel before the war, to come up with a design.

The Opposed Diesel Engine

In your average diesel engine, a piston compresses the air/fuel mixture to a temperature at which it explodes. The sequence of Intake, Compression, Injection and Exhaust is a four-stroke cycle. In an opposed engine1 there are two pistons per cylinder. One piston covers the inlet valve, one covers the exhaust.

The two pistons are geared slightly out of phase, so the inlet is uncovered; fuel comes in and is compressed in between both pistons. As it expands, the exhaust is uncovered first allowing the gases to escape. In essence this is a two-stroke engine and can be revved much faster.

Another advantage is that the engine is flat and could in theory be mounted inside a plane's wing if necessary. The Junkers Jumo 205 was an engine of around 600 horsepower and was used in some bombers and seaplanes. As the war progressed, more powerful variations were built, culminating in the Jumo 224 which generated 4,500 horsepower.

Napier's version of this was called the Culverin.

The Deltic Engine

The Culverin was not powerful enough for the Royal Navy. Not constrained by the space requirements of a plane, they tried to make a bigger engine by bolting Culverins together.

If you look at a V-8 or flat six car engine, they basically consist of two banks of cylinders joined together along a single crankshaft. Whether the engine is in Vee form or a boxer just depends on the angle between the blocks: anything between 0 and 180 degrees is a Vee, 180 degrees is a boxer. Taking it a step further, Volkswagen have made W-12 engines by bolting two V6s together.

Since they have crankshafts at both ends, the ideal arrangement for opposed engines would have two cylinders on each crankshaft. The Deltic arrangement was a triangle with three crankshafts at the corners and the cylinders as the sides of the triangle. Napier's big breakthrough was realising that one crankshaft had to be geared in reverse to allow the engine to function.

The Deltic engine could have three, six, nine, 12, or more cylinders in blocks of three. The first 18-cylinder engines were being made by 1950.

One big problem with the Deltic was its complexity. The engine was designed to be easily removed and replaced so that the whole engine could be returned to base rather than be repaired in place. Some components were also designed for easy replacement. The other concern was the stress that the pistons would be under. The two-stroke Deltic engine idles at 700rpm2, a speed most big four-stroke engines would not reach even at maximum. As well as generating much more power than a four-stroke, this also meant that the components were under much more stress and generated much more heat. Napier had included some interesting innovations such as oil cooled pistons that helped avoid the overheating problem associated with two-stroke diesels.

Like most big diesel engines, it was turbocharged.

When the navy tested an 18-cylinder engine in an old German E-boat they were pleased with the compact design's performance. The Deltic had similar power (around 3100 horsepower) to the original engine but was only half the size. Engines were commissioned and went on to power small military boats for many of the world's navies.

The engine had a capacity of 88 litres (5384, was 317cm (125 inches) long, 176cm (69 inches) wide and 239cm (94 inches) tall. It weighed in at well over five tons.


The English Electric Company had bought Napier and wanted to use the company's Deltic engine to power a railway locomotive. 1955 saw the DP1 prototype introduced. This diesel-electric3 locomotive was built at the Dick Kerr works in Preston.

The loco was designed to look vaguely like a large American locomotive. With sleek high sides and a set back cab, the DP1 was an impressive and elegant sight. It ran two 18-cylinder Deltic engines rated at 1650hp (they were restricted to reduce stress) powering 12 wheels4, via electric motors.

The DP1 was initially going to be called Enterprise, but everybody referred to it as Deltic, so the name stuck. Deltic was made to look more American by being painted bright blue, having a large single headlight built into the top of its nose and having three curved chevrons painted there too. Its teardrop-shaped windows added to the impression of a fast locomotive. The curved roof, nose and lower sides led to the locomotive being surprisingly aerodynamic.

One of the Deltic's most famous characteristics was its sound. The sounds of the fast revving cylinders and phasing gears were like nothing else on the railway. At high speeds, the drone of the engine led to it and its brethren being nicknamed 'Lancaster Bombers'. Its other famous characteristic was that it released two massive clouds of smoke as it set off. Two-stroke engines drink their way through oil, and when idling, a fair amount of oil collects in the exhaust drum. When starting off, this oil was expelled half-burnt as blue smoke.

Deltic weighed over 100 tons5 and was almost 70 feet (21 meters) long. It could reach 100 mph and was the pinnacle of diesel locomotive design. It was originally planned for use on the Midland region; however, plans to electrify it meant that DP1 was introduced to the Eastern Region to replace some of the Gresley Pacific6 steam trains. The Gresley steam locos were some of the most sophisticated and elegant examples of steam locomotives in the world. The express locomotives, which included the A4 class, of which Mallard was a member, could easily top 100mph.

As well as offering the speed to be able to replace the steam trains, Deltic was probably the only locomotive elegant enough to replace the A4. Besides, the locomotive could generate a cloud of smoke that even a big steam engine would be proud of.

Deltic was withdrawn from service in 1961 after a power plant failure.

BR Class 55

Pleased with Deltic, British Rail ordered 22 further units. These, numbered D9000 to D9021, were meant to replace over 40 A4 locomotives. The locomotives were introduced in 1961 and painted in two-tone green. The locomotives based in Gateshead and Edinburgh were named after regiments of the British Army and the ones in Finsbury Park, London, were named after racehorses.

The locomotives were built at the English Electric Vulcan Foundry works in Newton Le Willows.

These served along the East Coast Mainline, from London to Edinburgh via York and Newcastle. They also pulled the legendary Flying Scotsman train. Their reign on the line lasted only until the 1970s when HSTs were introduced onto the East Coast Main Line. Deltics were moved onto secondary services; however, it was too expensive for BR to maintain such a small fleet of non-standard locos, especially as they were really too powerful for the duties they were assigned. Deltics were withdrawn from the end of the 1970s, the last serving in 1981.

Most Deltics were repainted the standard BR blue with a yellow nose. Most engines had either nameplates or headcode boxes7 on their nose.

Despite the complexity and the newness of the design, the Deltic was as reliable as a normal four-stroke diesel. Because the engine was made for the quick replacement of parts, the Class 55s racked up miles very quickly.

Looking back, the Class 55 should be judged as a success. It was an outstanding application of a remarkable piece of technology. At 100 tons and with over 3300 horsepower, the locomotive had a quite exceptional power to weight ratio for a large unit. The impressive performance was combined with decent reliability and a design that was one of the best combinations of form and function in the post-steam era.


Due to its elegant style and unique engineering it developed a large following. Deltic and six of the 22 class 55s have found their way into the hands of museums and enthusiasts. The class 55s have been repainted with their traditional two-tone green livery.

Deltic was donated to the Science Museum in London, and is currently at a National Railway Museum exhibition at Shildon in County Durham.

The Deltic Preservation Society owns three: D9009 (55 009) Alycidon, D9015 (55 015) Tulyar and D9019 (55 019) Royal Highland Fusilier. Deltic 9000 Fund8 bought D9000 (55 022) Royal Scots Grey for preservation. They also bought Gordon Highlander for spares, but like all good railway buffs, they couldn't face dismantling it, so rebuilt it for use instead. The National Railway Museum owns D9002 (55 002) The King's Own Yorkshire Light Infantry.

While the 1990s privatisation of British Rail may not have been a triumph, it has allowed Deltics back onto the tracks. Under BR, privately-owned diesel locomotives were not allowed onto the network, however under Railtrack and National Rail, anybody can use it. This has led to the owners running private trains and excursions that have been Deltic-powered.

All six Class 55s have been certified for use on the rail network or are in the process of being converted.

The Not Quite Deltics

While Deltic and the Class 55s have attracted the attention, there are a few other locomotives that used similar technology or had a similar design.

Class 23 Baby Deltics

Ten class 23 locos were introduced in 1959 for working trains around London. Built in Doncaster, these Bo-Bo locos were powered by two 1100 hp 9-cylinder Deltics. Their years in service were not a success.

First off they were too heavy for their original routes so were moved onto the outer suburban services from Kings Cross and Moorgate towards Cambridge. They were then banned from Moorgate because like their big brothers, they generated a lot of smoke, which didn't go down well in the tunnels!

Then they just started breaking. Shafts out of the engine were breaking and damaging equipment, pistons were seizing and cylinder linings were cracking. English Electric refitted the engines with new parts in 1963 and the class re-entered service. Aside from cooling problems, the trains did reasonably well.

The trains looked like smaller Deltics; shorter bodies with much shorter snouts. After refurbishment they were repainted with two-tone green livery like their bigger brothers.

By the late 1960s, with BR looking to cut costs, an obvious target was the little fleet of non-standard Class 23s. The locos were withdrawn by 1971, with just one remaining in works service, before it was replaced in 1974, then cut up in 1977.

Deltic look-a-likes. Classes 37, 40 and the Peaks

Five classes of BR diesel locomotive looked vaguely like the Deltics. They all had the same snout and set-back cab, however they tended to lack the curves and simplicity of the Class 55.

Mechanically, these locomotives were conventional, with big four-stroke diesel engines. They were introduced from the late 1950s onwards. The largest of these, the 1Co-Co19 class 45 weighed in at over 135 tons but at 2500hp were far less powerful than the Deltic.

These are the classes that look similar to the Deltics.

  • Class 37
  • This was the smallest of the Deltic-alikes, but was still heavier than the Deltic. Of these, 309 were produced and were used to pull anything from freight to passenger trains. After refurbishment in the 1980s, many were still in service into the 21st century. Quite a few are being held by various companies as reserves, many have been preserved and some have been exported. If you see a locomotive that looks like a Deltic in service, is not green or pulling an excursion train, it will most likly be a Class 37.

  • Class 40
  • These were used on many of the express services on the West Coast and in East Anglia before being used on freight services in their later days. Weighing 132 tons and having 2000hp they could hit 90mph. 200 locos were produced and seven are in preservation. The most infamous of these locos was D326 which was involved in the Great Train Robbery.

  • The Peaks - Classes 44, 45 and 46
  • Under the original BR numbering scheme, the three classes were all lumped together. Under the newer numbering scheme, they were split into the three classes. These were slightly shorter than the class 40s, but were even heavier. There were ten class 44s, 127 class 45s and 56 class 46s. The Class 44 was the 75 mph prototype, the classes 45 and 46 can reach 90 mph. All of these were withdrawn from service in the 1980s. They got the nickname 'Peaks' because they tended to be named after mountain peaks. There are a few examples of each class in preservation.

TrainspottingDeltic Engine Animation
1This is nothing to do with a horizontally-opposed, or boxer, engine which is a way of arranging cylinders.2Revolutions per minute.3The power that Deltic produced was converted into electricity to drive electric traction motors.4It had a Co-Co wheel arrangement, which means that each end of the locomotive has three powered axles (A=1 powered axle, B=2, C=3 etc) and each axle has its own traction motor (the 'o' means individual motors).5It was originally 98 tonnes, but its weight increased over time.6Pacific referrers to the 4-6-2 wheel arrangement. The front two sets of wheels are unpowered, the next three were powered and there was a rear unpowered set of wheels.7Sets of discs used by BR to determine services.8Now called Deltic 9000 Locomotives Limited.9Each end has three powered axles and one non-powered axle to help spread the weight.

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