Napier Deltic

The Napier Deltic engine is a British opposed-piston valveless, supercharged uniflow scavenged, two-stroke Diesel engine used in marine and locomotive applications, designed and produced by D. Napier & Son. Unusually, the cylinders were disposed in a three bank triangle, with a crankshaft at each corner of the triangle.

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 and the locomotives produced by English Electric using these engines, including its demonstrator locomotive named DELTIC and the production version for British Railways, which designated these as (TOPS) Class 55.

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

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 their highly flammable fuel made them vulnerable to fire, unlike the opposing 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 an aviation Diesel design known as the Culverin after licensing versions of the Junkers Jumo 204. The Culverin was an opposed-piston two-stroke design. Instead of each cylinder having a single piston and being closed at one end with a cylinder head, the Jumo-based design used an elongated cylinder containing two pistons moving in opposite directions towards the centre. This negates the need for a heavy cylinder head, as the opposing piston filled this role. On the downside, the layout required separate crankshafts on each end of the engine that must be coupled through gearing or shafts. The primary advantages of the design were uniflow breathing and a rather "flat" engine, originally intended to be buried in the wings of large aircraft.

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