Napier Deltic

The Legendary Napier Deltic – Super Diesel Triangle Engine

In Engines, Videos by Paul ShillitoLeave a Comment


As I did the video for the Rolls Rolls Crecy engine a short while back it reminded me about another equally eccentric British engine that not only made the Crecy look simple by comparison, it was the most powerful high-speed diesel engine for its weight and size at the time and unlike the Crecy it was put into production powering fast attack Motor Torpedo Boats, minesweepers, locomotives, power generators and even the worlds most powerful fire engine tender.

So I thought that you also might enjoy finding out about the engineering masterpiece that is the 18 cylinder, 2-stroke three bank triangle Napier Deltic.

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Much like the Crecy, the story of the Napier Deltic starts in the mid 1940s in the depths of world war 2.

The British admiralty was looking for a replacement engine for their motor torpedo boats which up until then had been powered usually by petrol aero engines adapted for naval use. The problem was that the highly inflammable petrol that powered them made them vulnerable to fire compared to their German counterparts, the E-boats which were diesel-powered.

However, diesels of this era were large and heavy and couldn’t compete for power with petrol engines of similar size and weight.

Now, as we now from the Crecy, work had been done on high-speed diesel’s before the war by several companies including Rolls Royce but some of the most advanced were built by the German aero company Junkers who had developed an H-6 diesel, the Jumo 204 which dated back to 1929. This was originally intended for use in larger aircraft like bombers & airliners and for a long time was the only diesel aero engine in use.

This 2-stroke H-6 design with two crankshafts, one at either end had no heavy cylinder head, instead, it used sets of opposing pistons in an elongated cylinder block to compress the fuel between the pistons.  The inlet and exhaust ports were offset in the middle of the engine block and the upper piston ran 11 degrees behind the lower. Although this made it not as smooth as a true opposed style engine, it gave better scavenging than a typical two-stroke design and could run as cleanly as a four-stroke but with much less mechanical complexity.

As the engine was relatively flat it was thought that it could be buried in the wing of large aircraft but problems with the oil flow meant that it had to be mounted vertically in an engine nacelle.

In 1933, the British company D. Napier & Son, an established engine builder licensed the Jumo 204 design to expand their engine range and made their version of it called the Culverin.

Napier thought that the horizontally opposed two-stroke design would make a safe fuel-efficient engine for the newly expanding air travel sector. However, it created very little interest and just 7 were built with zero sales so Napier halted work on the engine.

This is where in 1942 the British Admiralty came onto the scene looking for a high-speed high power diesel. It knew that Junkers had developed a multi crankshaft engine in the Jumo 223, a 24 cylinder with 4 banks of 6 pistons and 4 crankshafts in a rhombus shape, a square balanced on one corner. This was designed to produce 2500 hp and thinking was the Culverin could be the basis of a similar large powerful diesel engine.

Just 6 Jumo 223’s were ever produced which eventually fell into the hands of the Soviets after the war.

Meanwhile back in England, starting in 1947 Napier took the Culverin and created an 18 cylinder engine which was three Culverins joined together in a triangle. This created the E130 three-cylinder test engine to validate the concept. Applying this to a full engine looked like three V engines merged together but with no cylinder heads if looked at from an end-on view.

The engine was called the Napier Deltic after its triangular shape which is similar to the Greek letter Delta but stood on one corner and the “tic” from English Electric who had taken over Napier in 1942.

Now while the Jumo 223 was the inspiration for the Deltic, none of its design was carried over to the Deltic, in fact, Junkers had tried to make a triangle engine but had problems in getting the pistons to phase correctly with each other, so they dropped it and carried on with the square 4 bank engine which became the 223.

At Napier, they also had the same problems but a suggestion from a senior draftsman to make one of the three crankshafts rotate in the opposite direction fixed the piston phasing problem.

The three crankshafts are feed via phasing gears into a single output shaft and two engine sizes were available, a 9 or 18 cylinder. The normal 18 cylinder version was rated at 1650hp and a turbocharged one at 3700hp.

Because the engine ran much faster than a typical big diesel, it idled at 700 rpm the typical speed of a normal large diesel flat out and could reach 1500 rpm at max power. The phasing gears also gave it a unique buzzing or droning sound that increased with engine speed but once up to full speed it became really very loud and needed large silencers to keep it quite-ish. The 18 cylinder non-turbo units also used a mechanically driven blower to ensure the gases were drawn out of the cylinders making it much more efficient.

Now, this was designed for use in MTB’s or Motor Torpedo Boats, so by 1952, six were available for endurance trails. One of the first things they did was to use a captured German E-boat, this was because the Mercedes-Benz engines used were about the same power output. They replaced 2 out of the 3 original engines with the Deltics and the difference was startling. The Deltics were half the size and weighed one fifth the weight of the Mercedes engines.

They soon became a common power plant for small fast naval craft like the Dark Class attack vessels and due to the aluminium construction and as such low magnetic signature and low engine vibration they were also used for the Royal Navy’s Ton and Hunt class Minesweepers. The Deltics were also used in the Norwegian Nasty Class MTBs which were sold to Germany, Greece and the US Navy who used them in the Vietnam war for covert actions, utilising the low humming sound at tick over they made so as not to sound like a typical engine.

Now while they may well have been smaller than a similar powered conventional four-stroke diesel engines it was hardly a small engine which can be seen here with a technician working on the phasing gear case for a comparative size and it also shows the phasing gears combining the output of the three crankshafts to the single output shaft in the centre.

In 1956 a turbo compound deltic was planned for naval use and a prototype built. This used the Deltic as a gas generator inside a gas turbine for a hoped 6,000 hp but as engineers who knew the project had predicted, it had a conrod failure at 5,600 hp. As gas turbine technology improved, the compound Deltic was dropped in favour of pure gas turbine engines even though it would have been more efficient.

In the late 1950s and early 1960s, the UK was transitioning from Steam to Diesel power for its railways and the Deltic was seen as a potential power unit for high-speed locomotives. A prototype loco called the DELTIC after its engine ran from 1955 to 1960 covering 450,000 miles on the east and west coast mainlines.

From this In 1959 British rail created the Class 23 and then in 1961-62 the Class 55 locomotives, though they became known as the baby Deltic and Deltic respectively.

In 1961-62, 22 Deltics entered service and replaced 55 steam locos including the Pacific and A4 class like the Mallard, the fastest steam engine in the world. This was possible because they were not only more powerful but they didn’t need the preparation, fueling, firing and cleaning between runs that the steam engines did.

The Class 23 Baby Deltics used a single turbocharged 9 cylinder rated at 1100hp and the Class 55s used two mechanically blown 18 cylinder engines rated at 1680hp each giving it a total power output of 3300 hp and making the Class 55 one of the most powerful and fastest diesel locomotives in the world at the time. Using two lower tuned engines in instead of one high performance one was done to extend the working life between major rebuilds.

The Class 55’s could pull 13 coaches from London to Edinburgh on the east coast main line at an average speed of around 100mph, with up to 114 mph on flat sections and 125mph on downhill ones.

Although a “Super Deltic” Class 51 was proposed with two turbocharged 18 cylinders engines, it was never made but if it had it would have had up to 4600 hp making it the most powerful locomotive in the world.

In total, 44 Deltic engines were ordered plus 13 spares, the Deltic locos were the first in BR’s history to be designed for high availability so if there was a fault with an engine they would swap the whole engine in a day for a spare one and return the loco to service rather than fix the engine fault in situ which could days or longer depending upon spares availability.

The Deltic locos were phased out of high-speed mainline duties when the Intercity 125 trains were introduced in the late 1970s, though they continued in service until 1982 when they were finally withdrawn and most were scrapped.

But not all were cut up,  you can still experience the Deltics locomotives as there a still six in various stages of operation and you can even hire some of them if you want for use on private and some mainlines routes

Now, Probably the most unusual use for a Deltic engine was in the New York Fire Departments Super Pumper. This was the central pump that was part of a five truck system which included the super pumper, the tender, and three satellite tenders.

The idea of a land-based fireboat or super pumper was first proposed by William Gibbs, a naval architect in 1910 but there were no engines powerful and small enough to be mounted on a truck.

It wasn’t until 1962 that his idea could be put into practice with the use of a Naper-Deltic T18-37C  with 2220hp to drive the DeLaval six-stage centrifugal pump but it would take a massive fire on Staten Island in 1963 before the fire dept finally commissioned Mack Trucks to build Gibb’s super pumper for a price of $875,000.

The super pumper was designed to supply enough water to deal with massive fires like that of Staten Island and pump water up to skyscraper heights. It could take water from eight fire hydrants at once or even draw it from the sea or a river and pump 10,000 gallons per minute at low low pressure and 8,800 at 350 psi. The 8 inch barreled monitor could propel a water stream up to 600ft in any direction including straight up vertically into the air at 10,000 gallons per minute. At full power, the Deltic used 135 gallons of diesel fuel per hour.

In 1967 at a fire in a postal annexe, it supplied the huge monitor as well as to the three other satellite tenders, two ladder towers plus multiple other hand lines.

The super pumper was used from 1965 to 82 and responded to over 2,200 calls including the biggest fires across the city’s five boroughs and even stepping in to take over from a failed pumping station for the city water supply.

The end came when the city’s finances collapsed in the late 70s and the cost of running the super pumper became an issue, that and the power of the water stream was so strong that it was capable of collapsing walls and taking the roofs of building so it was gradually used less and less. The pumping unit was saved by Jimmie Dobson who took it to his toy and fire truck museum in Bay City, Michigan.

So I hope you enjoyed the video and if you did then please subscribe, hit that bell notification, like and share. And as we come to the end I’d also like to say a big thank you to all of our wonderful Patreons out there for the ongoing support.

Paul Shillito
Creator and presenter of Curious Droid Youtube channel and website

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