Napier Deltic

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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 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 Class 55.

[edit] History

An animated schematic of a Deltic engine. The inlet is coloured green and the outlet purple. Notice the bottom shaft contra-rotating with respect to the other two; the lag between the exhaust and inlet ports opening; and that ignition occurs when the pistons are not at equal positions in the cylinder.

The Deltic story began in 1944 when the British Admiralty commissioned Napier to design a diesel engine for their Motor Torpedo Boats and other lightweight, high speed craft. Hitherto in the Royal Navy, such boats had been driven by gasoline engines but this fuel is obviously highly flammable and made them very vulnerable to fire, putting them at a disadvantage compared to their German opponents, the diesel-powered E-boats.

Up to this point, diesel engines were characterised by their poor power-to-weight ratio and their slow speed. However, before the war Napier had been working on various 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.

The original Napier Culverin was an opposed piston design, meaning that instead of each cylinder having a single piston and being closed at one end with a cylinder head, the elongated cylinder contained two pistons driving in opposite directions toward the middle. This led to a fairly "flat" engine that was originally intended to be buried in the wings of large aircraft. The Admiralty required a much more powerful engine, however, so for the added power Napier took three of their original Culverins and "bolted them together".

The result was an inverted triangle, the cylinder banks forming the sides, and tipped by three crankshafts, one at each corner of the triangle. The crankshafts were connected together with phasing gears to drive one output shaft. Various models of Deltic engine could be produced with varying numbers of such three-cylinder banks, though nine and eighteen cylinders were the most common models, being of three and six banks respectively.

Perhaps one of the more interesting features of this engine was the clever way the crankshaft phasing was arranged to allow for exhaust port lead and inlet port lag in the gas-flow of the engine. These engines are called 'uniflow' designs because the flow of gas into and out of the cylinder is one-way, and is assisted by some mild supercharging to improve cylinder scavenge.

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

Also of interest was that earlier attempts at designing such an engine had failed because it appeared to be impossible to arrange the pistons to move in the correct manner, for all three cylinders in one delta. Napier realised that this problem could be solved by gearing the crankshafts such 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.

However, crankshaft connecting-rod journals were so arranged that the exhaust piston 'led' the inlet piston by 20 degrees of crankshaft rotation. This meant that in any one cylinder, the exhaust piston reached its top-dead-centre position before the inlet piston in the same cylinder, and that the exhaust piston was on its way down the cylinder before the inlet piston reached its top-dead-centre position.

This clever arrangement 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 both to good scavenging of the cylinder of combustion gas residues, and good volumetric efficiency for the fresh air charge. However, the arrangement suffered from the disadvantage that the two pistons did not contribute equally to the power output.

The first Deltic unit was produced in 1950, and 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 there: they were half the size of the original engines.

Proving successful, the Deltic diesel engine became a commonly used powerplant in small, fast naval craft. The Royal Navy used them first in the Dark Class Fast Attack Craft, and subsequently in a number of other smaller attack and minesweeper classes. Napier Deltic engines are still in service in Hunt Class Mine Countermeasure Vessels.

In addition, Deltic diesels served in MTBs and PT Boats built for other navies; particularly notable being the Norwegian built Tjeld (Nasty) class, which were also sold to Germany, Greece, and even the United States Navy. PTF Nasty class boats served in the Vietnam War, largely for covert operations.

While the Deltic engine was certainly successful and very powerful for its size and weight, it was also a highly-strung unit, requiring a lot of care and maintenance. This was facilitated by a policy of maintaining by unit replacement rather than repair in place - rather ahead of its time. Deltic engines were also easily removed should they break down, generally being sent back to the manufacturer for repair.

[edit] External links

• Deltic technical details.
• The Deltic Preservation Society.
• Hunt Class deltic powered Mine Countermeasure Vessel.
• Deltic Animations 3-D animations of the piston motion in the Deltic engine.