Napier Nomad

The Napier Nomad is a British diesel aircraft engine designed and built by Napier & Son in 1949. They combined a piston engine with a turbine to recover energy from the exhaust and thereby improve fuel economy. Two versions were tested, the complex Nomad I which used two propellers, each driven by mechanically independent stages, and the Nomad II, using the turbo-compound principle which coupled the two parts to drive a single propeller. The Nomad II had the lowest specific fuel consumption figures seen up to that time.^[1] Despite this the Nomad project was cancelled in 1955 having spent £5.1 million on development, as most interest had passed to turboprop designs.^[2]

In 1945 the Air Ministry asked for proposals for a new 6,000 hp (4.5 MW) class engine with good fuel economy. Curtiss-Wright was designing an engine of this sort of power known as the turbo-compound engine, but Sir Harry Ricardo, one of Britain's great engine designers, suggested that the most economical combination would be a similar design using a diesel two-stroke in place of the Curtiss petrol engine.

Before World War II Napier had licensed the Junkers Jumo 204 diesel design to set up production in the UK as the Napier Culverin, but the onset of the war made the Sabre all-important and work on the Culverin was stopped. In response to the Air Ministry's 1945 requirements Napier dusted off this work, combining two enlarged Culverins into an H-block similar to the Sabre, resulting in a massive 75 litre design. Markets for an engine of this size seemed limited, however, so instead they reverted to the original Sabre-like horizontally opposed 12 cylinder design, and the result was the Nomad.

The objective of the design was to produce a civilian power plant with far superior fuel efficiency to the emerging jet engine. Thermal efficiency is given by ${\displaystyle 1-T_{e}/T_{p}}$, where T_e is the exhaust temperature in kelvins and T_p is the peak combustion temperature. Jet engines have relatively low-temperature combustion systems which produce a T_p of no more than about 1,000 K, much less than the typical 5,000 K of a reciprocating engine, and so jets have very poor thermal efficiency. The Nomad design focused on replacing the low-temperature combustion chambers of the jet engine with highly efficient Diesel combustion chambers. In practice, it was much too difficult to couple the diesel power output back into the turbine cycle. The maximum practical power of the Nomad was 4,000 hp (3.0 MW), and it was much heavier than a pure jet of the same power. By this time civilian jets such as the Boeing 707 were nearing completion, and the Nomad was never seriously considered by any aircraft manufacturer.

The initial Nomad design (E.125) or Nomad 1 was incredibly complex, almost two engines in one. One was a turbo-supercharged two-stroke diesel, having some resemblance to half of a Napier Sabre's H-24. Mounted below this were the rotating parts of a turboprop engine, based on the Naiad design, the output of which drove the front propeller of a contra-rotating pair. To achieve higher boost, the crankshaft drove a centrifugal supercharger, which also provided the scavenging needed for starting the engine from rest. During take-off additional fuel was injected into the rear turbine stage for more power, and turned off once the aircraft was cruising.^[3]

The compressor and turbine assemblies of the Nomad were tested during 1948, and the complete unit was run in October 1949. The prototype was installed in the nose of an Avro Lincoln heavy bomber for testing: it first flew in 1950 and appeared at the Farnborough Air Display on 10 September 1951.^[4] In total the Nomad I ran for just over 1,000 hours, and proved to be rather temperamental, but when running properly it could produce 3,000 hp (2.2 MW) and 320 lbf (1.4 kN) thrust. It had a specific fuel consumption (sfc) of 0.36 lb/(hp⋅h) (220 g/kWh).

The prototype Nomad I is on display at the National Museum of Flight at East Fortune Airfield in Scotland.

Even before the Nomad I was running, its successor, the Nomad II (E.145) Nomad 6, had already been designed. In this version an extra stage was added to the axial compressor/supercharger, eliminating the separate centrifugal part and the intercooler. The turbine (which also received an additional stage) was now only used to drive the compressor, and feed back any excess power to the main shaft using a Beier variable-ratio gear;^[5] the separate propeller from the turbine was deleted, just as the whole of the "afterburner" system with its valves etc. So the system was now like a combination of a mechanical supercharger, and a turbocharger without any need for bypass. The result was smaller and considerably simpler: a single engine driving a single propeller.^[6] Overall weight reduction was 1,000 lb (450 kg). The wet liners of the cylinders of the Nomad I were changed for dry liners.^[7] Later development added water injection, increasing the take-off rating to 3,570 ehp at 2,050 rpm (3,476 shp plus 230 lbf (1.0 kN) residual thrust).^[8]

While the Nomad II was undergoing testing, a prototype Avro Shackleton was lent to Napier as a testbed. The engine proved bulky, like the Nomad I before it, and in the meantime several dummy engines were used on the Shackleton for various tests.

On an equivalent power basis, the Nomad II had an SFC of 0.327 lb/(hp⋅h) (199 g/kWh) at 25,000 ft (7,600 m) cruise altitude.^[9]

A further development, the Nomad Nm.7, of 3,467 shp (2,585 kW) was announced in 1953.^[10]

Before cancellation of the programme in 1955, Napier proposed a further development designated as the E.173 Nomad III. The design was intended to add fuel injection in the exhaust manifold and an air-to-water aftercooler between the compressor and the cylinders. A projected “wet” take-off rating of 4,500 ehp (3,356 kW) at 2,050 rpm was reported (4,412 shp (3,290 kW) plus 230 lbf (1.021 kN) residual thrust), with an estimated weight of 3,750 lb (1,701 kg), about 170 lb (77 kg) more than the Nomad II. No example of a finished Nomad III engine was built.^[8]

By 1954, interest in the Nomad was waning, and after the only project, the Avro Type 719 Shackleton IV, based on it was cancelled, work on the engine was ended in April 1955, after an expenditure of £5.1 million. The design was also considered for the Canadair Argus, a similar maritime patrol aircraft being designed for the Royal Canadian Air Force. This design turned to the Wright R-3350, the design that the Nomad was intending to best.

A Nomad II is on display at the Steven F. Udvar-Hazy Center in Virginia.^[11]

• Airspeed Ambassador (planned only)^[12]
• Avro Lincoln (test bed only)
• Avro Shackleton (planned only)
• Bristol Britannia (planned only)^[13]
• Canadair CP-107 Argus (planned only)

Data from Flight (1954)^[14]

• Type: 12-cylinder, two-stroke, loop-scavenged, valveless diesel engine, turbo-compounded to the crankshaft via Beier variable-ratio gear^[8]
• Bore: 6.00 in (152 mm)
• Stroke: 7.375 in (187.3 mm)
• Displacement: 2,502 in³ (41.0 L)
• Length: 119.25 in (3,029 mm)^[8]
• Width: 56.25 in (1,429 mm)
• Height: 40 in (1,000 mm)
• Dry weight: 3,580 lb (1,620 kg)

• Valvetrain: Piston ported two-stroke
• Supercharger: 12-stage axial compressor (derived from Napier Naiad designs), maximum boost pressure 89 psi (610 kPa)^[8]
• Turbocharger: Three-stage exhaust-driven axial turbine; no bypass, excess power compounded to the crankshaft^[8]
• Fuel type: Diesel oil or kerosene or wide-cut petrol or "other fuels"
• Cooling system: Liquid-cooled

• Power output: 3,135 hp (2,338 kW) take-off (equivalent) at 89 psi (610 kPa) boost and 2,050 rpm (crankshaft); includes 250 lbf (1.1 kN) residual thrust; turbine shaft 18,200 rpm^[8]
• Specific power: 1.25 hp/in³ (57 kW/L)
• Compression ratio: 8.1 (cylinder ratio), 8.25:1 (compressor pressure ratio)^[8]
• Specific fuel consumption: 0.326 lb/(ehp·h) (0.198 kg/(kW·h)) at 22,250 ft (6,782 m), 2,027 ehp (1,511 kW) at 1,750 rpm^[8]
• Power-to-weight ratio: 0.88 hp/lb (1.45 kW/kg)

• Type: Exhaust-driven turbine and axial compressor (derived from Napier Naiad designs)
• Length:
• Diameter:
• Dry weight:

• Compressor: 12-stage axial flow
• Turbine: 3-stage axial flow

• Maximum thrust: 250 lbf (1.1 kN) residual at 18,200 rpm
• Overall pressure ratio: 8.25:1 (compressor pressure ratio)
• Air mass flow: 13.0 lb/s (5.9 kg/s)

Comparable engines

• Dobrynin VD-4K
• Rolls-Royce Crecy
• Wright R-3350-91 Duplex-Cyclone

Related lists

• List of aircraft engines

• Lumsden, Alec (2003). British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing. pp. 170–172. ISBN 1-85310-294-6.
• https://www.thegrowler.org.uk/avroshackleton/the-nomad-proposal.htm – The Nomad Proposal, research by Chris Ashworth – it was amongst the original sources of the article according to the history
• "Compound diesel engine design analyzed". Aeronautical Engineering. Aviation Week. Vol. 60, no. 20. 17 May 1954. pp. 30–32, 34. ISSN 0005-2175.
• Anderton, David A. (6 April 1953). "New Nomad's fuel appetite is small". Aeronautical Engineering. Aviation Week. Vol. 58, no. 14. pp. 30–32, 37–38, 41. ISSN 0005-2175.
• Anderton, David A. (8 October 1951). "Weak production handcuffs British air". Farnborough Verdict. Aviation Week. Vol. 55, no. 15. pp. 12–15. ISSN 0005-2175.

• Aircraft Engine Historical Society photo gallery of Napier piston engines
• D. Napier and Son Limited (25 April 1952). "Napier Nomad" (PDF). Flight. p. 35. Archived from the original on 28 March 2015.
• "NOMAD NNm6: Napiers' new compounded engine" (PDF). Flight. 20 March 1953. p. 348. Archived from the original on 18 May 2014.
• "Napier, Luton..." Flight. 1 April 1961. pp. 760–761. Archived from the original on 5 March 2016.