Napier Deltic - History and Design

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 with 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 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 either end of the engine, and some form of gearing to take off power and combine it back into a single shaft. The primary advantage of the design was that it led to a rather 'flat' engine, intended to be buried in the wings of large aircraft.

The Admiralty required a much more powerful engine, and knew about Junkers' designs for multi-crankshaft engines of straight six and diamond-form. The Admiralty felt that these would be a reasonable starting point for the larger design which it required. The result was a triangle, the cylinder banks forming the sides, and tipped by three crankshafts, one at each apex. The crankshafts were connected with phasing gears to drive one output shaft. In this arrangement, there were six banks of pistons driving three crankshafts, the same as three separate V-engines of the same overall size. 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 feature of the engine was the way that 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 port order is In/Out/In/Out/In/Out going around the triangular ring (i.e. the inlet and outlet manifold arrangements have C₃ rotational symmetry).

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, and this was the problem which caused Junkers Motorenbau to leave behind work on the delta-form while continuing to prototype a diamond-form four-crankshaft 24-cylinder Junkers Jumo 223. Mr. Herbert Penwarden of the Admiralty Engineering Laboratory solved this problem by suggesting that one crankshaft needed to revolve anti-clockwise to achieve the correct piston-phasing, so Napier designers produced the necessary gearing in order that one of them rotated in the opposite direction to the other two.

Being 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 'fork and blade' connecting rods, the latter an 'inlet' piston used to open and close the inlet port, and the former 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 also led to the even, buzzing exhaust note of the Deltic, with a charge ignition every 20 degrees of crankshaft revolution, and a lack of torsional vibration, ideal for use in mine-hunting vessels.

Although the engine was cylinder-ported and required no poppet valves, each bank had a camshaft, driven at crankshaft speed. This was used solely to drive the fuel injection pumps, each cylinder having its own injector and pump, driven by its own cam lobe.