The Junkers Jumo Fo3 and 204 were licensed to Napier & Son, who built a small number as the Napier Culverin just prior to the war. Late in the war, they mounted three Culverins in a triangle layout to produce the Napier Deltic, which was for some time one of the most powerful and compact diesel engines in the world.

In the early years of jet development, during WWII, it became clear that the jet’s fuel efficiency was well below that of the reciprocating engine. The low continuous temperature tolerated by the combustion chambers, under 1200°K, was to blame. Piston engines tolerate a peak combustion temperature of some 4800°K, because this high temperature is sustained only for one or two milliseconds. The thermodynamic efficiency of an ideal engine is given simply by 100(1-Te/Tp) in percent, where Te is the exhaust temperature in Kelvin, and Tp is the peak combustion temperature. Thus, a jet could easily triple its fuel efficiency by using a diesel engine to inject the kerosene fuel and combust the fuel/air mixture. The best gasoline fuel (130 avgas) at the time was unsuitable as it tolerated a maximum compression ratio of about 10:1, whereas ordinary diesel engines could attain 18:1, and 22:1 with Ricardo indirect combustion chambers.

In 1945 the Air Ministry asked for proposals for a new 6,000 horsepower (4,500 kW) 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’s 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 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 1-(Tx/Tp), where Tx is the exhaust temperature (any absolute scale) and Tp is the peak combustion temperature. Jet engines have relatively low-temperature combustion systems which produce a Tp of no more than about 1000 kelvin, much less than the typical 5000 kelvin 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 horsepower (3,000 kW), and it was much heavier than a pure jet of the same power.

The Napier Nomad was 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 the mechanically independent stages, and the Nomad II, using the turbo-compound principle, coupled the two parts to drive a single propeller. The Nomad II had the lowest specific fuel consumption figures seen up to that time.

The initial Nomad design (E.125) or Nomad 3 was almost two engines in one. One was a turbo-supercharged two-stroke diesel, having some resemblance to half of a Napier Sabre. 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.

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. 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 horsepower (2,200 kW) and 320 lbf (1.4 kN) thrust. It had a specific fuel consumption (sfc) of 0.36 lb/(hp·h) (0.22 kg/(kW·h)). In contrast, a very efficient Pratt & Whitney R-1830 petrol radial engine consumes 0.49 lb/hp.h at cruise settings. However, in practice the jet engine is still preferable since it is much smaller and lighter, and operates at much higher altitudes and speeds. The jet thus consumes about the same amount of fuel for a given trip distance, due to its much shorter transit time at higher altitudes.

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 hydraulic clutch; 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. Overall about 1,000 lb (450 kg) was taken off the weight. The wet liners of the cylinders of the Nomad I were changed for dry liners. 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.

A further development, the Nomad Nm.7, of 3,500 shp (2,600 kW) was announced in 1953.

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. By this time civilian jets such as the Boeing 707 were nearing completion, and the Nomad was never seriously considered by any aircraft manufacturer.

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

Specifications:
Nomad II
Type: Twelve-cylinder, two-stroke valveless diesel engine compounded with three-stage turbine driving both crankshaft and axial compressor.
Bore: 6.00 inches (152 mm)
Stroke: 7.375 inches (187.3 mm)
Displacement: 2,502 cu.in (41.1 lt)
Length: 119 inches (3,000 mm)
Width: 56.25 inches (1,429 mm)
Height: 40 inches (1,000 mm)
Dry weight: 3,580 pounds (1,620 kg)
Valvetrain: Piston ported two-stroke
Supercharger: Napier Naiad turboshaft and gas generator, maximum boost pressure 89 psi
Turbocharger: Engine exhaust gases ducted in to Naiad turbine section
Fuel type: Diesel oil or kerosene or wide-cut petrol or “other fuels”
Cooling system: Liquid-cooled
Power output: 3,150 ehp (2,344 kW) max take-off at 89 psi (610 kPa)(208″Hg)(6.9Atm) boost including 320 lbf residual thrust from the turbine at 2,050 rpm (crankshaft) and 18,200 rpm (turbine)
Specific power: 1.25 ehp/cu.in (57.0 kW/lt)
Compression ratio: 8.1 (cylinder ratio), 31.5:1 (combined pressure ratio)
Specific fuel consumption: 0.345 lb/(ehp·h) (0.210 kg/(kW·h))(combined unit) at 11,000 ft and 300 knots
Power-to-weight ratio: 0.88 ehp/lb (1.44 kW/kg)

Turbine section
Type: Gas generator based on Napier Naiad
Compressor: 12-stage axial flow compressor
Turbine: 3-stage axial flow
Maximum thrust: 320 lbf residual at 18,200 rpm
Overall pressure ratio: 8.25:1

Early in the First World War Napier were contracted to build aero engines to designs from other companies: initially a Royal Aircraft Factory model and then Sunbeams. Both proved to be rather unreliable, and in 1916 Napier decided to design their own instead. Reasoning that the key design criteria were high power, light weight, and low frontal area, the engine was laid out with its 12 cylinders in what they called a “broad arrow”—three banks of four cylinders sharing a common crankcase. This suggested the design’s first name, the Triple-Four. These designs are sometimes referred to as a W-block, although that designation applies more correctly to an engine in which a common crankcase is shared by not merely three but in fact four rows of cylinders (since a “W” is made of four lines or bars). The engine was also advanced in form, the heads using four valves per cylinder with twin overhead camshafts on each bank of cylinders and a single block being milled from aluminium instead of the more common separate-cylinder steel construction used on almost all other designs.

Under A. J. Rowledge, the design of the newly renamed Lion was completed in 1917, and the first hand-built prototypes ran later that year. It was fitted to a de Havilland built DH.9 in early 1918, proving to have many cooling problems. In addition the milled block turned out to be difficult to build with any accuracy and they reverted to separate cylinders, although they remained aluminium. Both of these problems were worked out by the middle of the year and the engine entered production in June 1918. The first Lion I versions delivered 450 hp (335 kW) from their 24 litres. As the most powerful engine available (particularly after a turbocharger became an option in 1922), the Lion went on to be a huge commercial success. Through the years between the wars Napier manufactured little else. Between the wars it powered over 160 different types of aircraft.

In highly-tuned racing versions the engine could reach 1,300 hp (970 kW), and it was used to break many world records: height, air speed, and distance in aircraft, boats, delivering 1,375 hp (1,025 kW) in a highly tuned Lion for a water speed record of 100 mph (160 km/h) in 1933. In land speed records, Lion engines powered many of Sir Malcolm Campbell’s record breakers including a record of over 250 mph (400 km/h) in 1932 and John Cobb’s 394 mph (634 km/h) Railton Mobil Special in 1947 – a record that came well after the Lion had passed its prime and stood until the 1960s. The record had been held by British drivers for 32 years. Lions powered successful entrants in air racing, the Schneider Cup, in 1922 and 1927, but were then dropped by Supermarine in favour of a new engine from Rolls-Royce, the Rolls-Royce R which had been especially designed for racing. It was a Lion that powered the Supermarine S5 that won the 1927 Schneider Trophy for seaplane speed racing.

It was still being fitted to planes in 1929 and it was still being put into boats in 1933.

During the 1930s a new generation of much larger and more powerful engines started to appear, and the Lion was clearly past its prime. Gradually, they fell further and further behind. By the time the Bristol Hercules and the Rolls-Royce Merlin arrived in the late 1930s, the Lion was too small and old-fashioned.

A marine version of the Lion, unsurprisingly called the Sea Lion, was used to power high speed air-sea rescue launches operated by the RAF.

Another adaptation for the Lion aero engine was propeller-driven motor sleighs, which were used for high-speed transport and SAR duties on sea ice by the Finnish Air Force and Navy.

Lion models:

Lion I
1918
450 bhp (340 kW) at 1,950 rpm
geared, also related IA and 1AY

Lion II
1919
Works No: E64
450 bhp (340 kW) at 2,000 rpm

Lion IIII
experimental geared
Gloster Gorcock

Lion V
470 bhp (350 kW) at 2,000 rpm
500 bhp (370 kW) at 2,250 rpm
VA had increased CR to 5.8
Mainstay engine of the RAF in the late 1920s, replaced by Lion XI

Lion VS
Works No: E79
Turbocharged, intercooled

Lion VIS
1927
Turbocharged
Application: Gloster Guan

Lion VII
1925
700 bhp (520 kW) (racing)
Application: Gloster III (Schneider Trophy entrant)
Supermarine S.4

Lion VIIA
1927
Works No: E86
900 bhp (670 kW) (racing)
Application: Golden Arrow
Blue Bird (1927)
Miss England I
Supermarine S.5
Gloster IV

Lion VIIB
1927
875 bhp (652 kW) (racing)
geared
Application: Supermarine S.5
Gloster IV

Lion VIID
1929
Works No: E91
1,350 bhp (1,010 kW) at 3,600 rpm (racing)
Supercharged, about 6-8 built
Application: Blue Bird (1931)
Fred H Stewarts Enterprise
Betty Carstairss Estelle V powerboat
Miss Britain III
Gloster VI (Schneider Trophy entrant)
Railton Special (John Cobb’s land speed record car)

Lion VIII
1927
direct drive
Gloster Gorcock

Lion XIA
1928
580 bhp (430 kW) at 2,585 rpm, 6:1 CR
RAF production model
Application: Napier-Railton

Lioness
Works No: E71
Inverted layout, for better visibility. At least some were built turbocharged, for racing.

Sea Lion
1933
500 and 600 bhp (370 and 450 kW)
Marine version of Lion XI
British Power Boat Company Type Two 63 ft HSL

Applications:
Aircraft
Alliance P.2 Seabird
Avro Bison
Blackburn Blackburn
Blackburn Dart
Blackburn Pellet
Blackburn Ripon
Blackburn Velos
Boulton Paul Atlantic
Boulton Paul Bodmin
Boulton Paul Bolton
English Electric Kingston flying boat (prototype)
Fairey III
Fairey Fawn
Fokker C.IV-W
Fokker C.V
Fokker D.C.I
Fokker D.XIII
Gloster Gorcock
Gloster Guan
Handley Page H.P.31 Harrow
Handley Page Hyderabad
Mitsubishi B1M
Parnall Pike
Parnall Possum
Parnall Puffin
Supermarine S.4
Supermarine S.5
Supermarine Seagull
Supermarine Southampton
Tarrant Tabor
Vickers Vernon
Vickers Valparaiso
Vickers Victoria
Vickers Virginia
Vickers Vixen
Westland Walrus

Other applications
British Power Boat Company Type Two 63 ft HSL
British Power Boat Company 60 ft 4 in

Specifications:
Lion II
Type: 12-cylinder water-cooled W-block (3 banks of 4 cylinders) aircraft piston engine
Bore: 5.5 in (139.7 mm)
Stroke: 5.125 in (130.17 mm)
Displacement: 1,461.6 cu.in (23.9 L)
Length: 57.5 in (1460 mm)
Width: 42.0 in (1067 mm)
Height: 43.5 in (1105 mm)
Dry weight: 960 lb (435 kg)
Valvetrain: Two intake and two exhaust valves per cylinder actuated via double overhead camshafts per cylinder block.
Cooling system: Water-cooled
Power output: 480 hp (358 kW) at 2,200 rpm at 5,000 ft
Specific power: 0.32 hp/cu.in (15.0 kW/L)
Compression ratio: 5.8:1
Power-to-weight ratio: 0.5 hp/lb (0.82 kW/kg)