Napier Deltic Engine

Napier Deltic powered British Rail Class 55 Alycidon, at the National Railway Museum, York, UK

The term Deltic (meaning in the form of the Greek letter Delta) is used to refer to both the Deltic E.130 opposed-piston high-speed diesel engine designed and produced by D Napier & Son, and the locomotives produced by English Electric using these engines, including their demonstrator locomotive named DELTIC and the production version for British Railways, who designated these as (TOPS) Class 55.

A single half-sized turbocharged deltic power unit also featured in the English Electric built Type 2 loco, designated as the Class 23. Both locomotive and engine became better known as the "Baby Deltic".

History and design

The Deltic story began in 1943 when the British Admiralty set up a committee to develop a high-power, lightweight diesel engine for Motor Torpedo Boats. Hitherto in the Royal Navy, such boats had been driven by petrol engines but this fuel is highly flammable, making them vulnerable to fire, and at a disadvantage compared to the German diesel-powered E-boats.

Until this time, diesel engines had poor power-to-weight ratio and low speed. Before the war, Napier had been working on aviation diesel designs (licensed versions of the Junkers Jumo 204) and the Admiralty felt these would be a reasonable starting point for the larger design they required.

Animated diagram of Deltic engine layout

The original Napier Culverin was an opposed piston design. Instead of each cylinder having a single piston and being closed at one end with a cylinder head, the elongated cylinder contained two pistons moving in opposite directions towards the centre. This negates the need for a heavy cylinder head. This led to a rather "flat" engine, intended to be buried in the wings of large aircraft. The Admiralty required a much more powerful engine, so for the added power Napier took three of their original Culverins and "bolted them together".

The result was a triangle, the cylinder banks forming the sides, and tipped by three crankshafts, one at each corner. The crankshafts were connected with phasing gears to drive one output shaft. Various models of Deltic engine could be produced with varying numbers of cylinders, though nine and eighteen cylinders were the most common, having three and six cylinders per bank respectively. In 1946, the Admiralty placed a contract with the English Electric Company, parent of Napier, to develop this engine.

One of features of the design engine was the way the crankshaft phasing was arranged to allow for exhaust port lead and inlet port lag. These engines are called 'uniflow' designs because the flow of gas into and out of the cylinder is one way, assisted by mild supercharging to improve cylinder exhaust scavenging. The inlet/outlet valve order is In/Out/In/Out/In/Out going around the triangular ring (ie the inlet and outlet manifold arrangements have C3 rotational symmetry)

Napier Deltic engine at the National Railway Museum, York, UK

Earlier attempts at designing such an engine failed because of the difficulty in arranging the pistons to move in the correct manner, for all three cylinders in one delta. Napier solved this problem by gearing the crankshafts so that one of them rotated in the opposite direction to the other two.

In an opposed piston design with no inlet or exhaust valves, and no ability to vary the port positions, the Deltic design arranged each crankshaft to connect two adjacent pistons operating in different cylinders in the same plane, using forked connecting rods, one an 'inlet' piston used to open and close the inlet port, and the other an 'exhaust' piston in the adjacent cylinder to open and close the exhaust port.

Crankshaft connecting-rod journals were arranged so that each cylinder's exhaust piston 'led' its inlet piston by 20 degrees of crankshaft rotation. This allowed the exhaust port to be opened well before the inlet port, and allowed the inlet port to be closed after the exhaust port, which led to both good scavenging of exhaust gas, and good volumetric efficiency for the fresh air charge. It suffered from the disadvantage that the two pistons contributed unequally to power output.


Naval service

Development began in 1947 and the first Deltic unit was produced in 1950. By January 1952 six engines were available, enough for full development and endurance trials. An ex-German E-Boat, powered by three Mercedes-Benz diesel engines, was selected for these trials, since its power units were of approximately equal power to the new 18 cylinder Deltic engines. Two of the three Mercedes-Benz engines were replaced with Napier Deltics, the compactness of the Deltic being graphically illustrated: they were half the size of the original engines. The Deltic weighed one fifth of its contemporaries of equivalent power.

The Deltic-powered Hunt class Mine Countermeasure Vessel HMS Ledbury

Proving successful, Deltic diesel engines became a common powerplant in small and fast naval craft. The Royal Navy used them first in the Dark-class fast attack craft. Subsequently they were used in a number of other smaller attack craft. The low magnetic signature lent itself to use in mine countermeasure vessels and the Deltic was selected to power the Ton-class minesweeper. The Deltic engine is still in service in the Hunt-class. These versions are de-rated to reduce engine stress.

Deltic diesels served in MTBs and PT Boats built for other navies. Particularly notable Was the Norwegian Tjeld or Nasty class, which were also sold to Germany, Greece, and the United States Navy. Nasty-class boats served in the Vietnam War, largely for covert operations.

Smaller nine cylinder Deltic 9 engines were used as a marine engine, notably by minesweepers. The Ton-class were powered by a pair of Deltic 18s and used an additional Deltic 9 for power generation for their magnetic influence sweep. The Hunt class used three Deltic 9s, two for propulsion and again one for power generation, this time with a hydraulic power unit integrated as well.

Railway use

The 'deltic' engines were used in two types of British rail locomotive: classes 55 and 23, built in the 1960s - both earned the nom-de-plume or nickname from the type of engines - "Deltics" and "Baby Deltics" respectively.

The Class 55 used two deltic engines: mechanically blown 18 cylinder engines , the Class 23 used a single less powerful nine cylinder turbocharged T9-29 Deltic of 1,100 hp (820 kW).

Reliability in service

While the Deltic engine was successful and very powerful for its size and weight, it was a highly-strung unit, requiring careful maintenance. This led to a policy of unit replacement rather than repair in place. Deltic engines were easily removed upon breakdown, generally being sent back to the manufacturer for repair.

Turbo-compound Deltic

The E.185 or Compound Deltic turbo-compound variant was planned and a single protoype was built in 1956 and tested in 1957. This capitalised on Napier's experience with both the Nomad and their increasing involvement with gas turbines. It used the Deltic as the gas generator inside a gas turbine, with both a twelve-stage axial compressor and a three stage gas turbine. Unlike the Nomad, this turbine wasn't mechanically coupled to the crankshaft but merely drove the compressor. It was hoped to produce 6,000 horsepower, with fuel economy and power-to-weight ratio "second to none". Predictions by the engineers closely connected with it were that connecting rod failure would be the limit on this power, failing at around 5,300 bhp. On test it actually produced 5,600 bhp, before throwing a connecting rod through the crankcase just as predicted. Naval interest had waned by 1958 in favour of the pure gas turbine, despite its heavier fuel consumption, and no further development was carried out.