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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.

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 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 slow 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.

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 toward the center. 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 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 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, having three and six banks respectively.

One of the interesting features of this engine was the clever 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 scavenge.

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 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 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 exhaust gas, and good volumetric efficiency for the fresh air charge. The arrangement suffered from the disadvantage that the two pistons contributed unequally to 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: they were half the size of the original engines.

Proving successful, the Deltic diesel engine became a common 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.

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 the United States Navy. PTF Nasty class boats served in the Vietnam War, largely for covert operations.

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

• 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.