Napier Nomad
| Nomad | |
|---|---|
 |
|
| Napier Nomad II | |
| Type | Turbo-compound aero-engine |
| National origin | United Kingdom |
| Manufacturer | D. Napier & Son |
| First run | October 1949 |
| Major applications | Avro Lincoln (test bed only) Avro Shackleton (planned only) Britannia Maritime Reconnaissance (planned only) Airspeed Ambassador (BEA) (planned only) |
| Program cost | £5.1 million |
The Napier Nomad was a British diesel aircraft engine designed and built by D. Napier & Son in 1949. They combined a piston engine with a turbine to recover energy from the exhaust and thereby improve fuel efficiency. 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. Despite this the Nomad project was cancelled in 1955 having spent £5.1 million on development, as most interest had passed to turboprop designs.
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 kelvins, and Tp is the peak combustion temperature. Thus, an airplane could easily triple its fuel efficiency by using a diesel engine to inject the kerosene fuel and combust the fuel/air mixture instead of a jet engine.
The Nomad compound engine achieved a specific fuel consumption of 0.345 lbs per horsepower-hour. 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 consumes about the same amount of fuel for a given trip distance, due to its much shorter transit time at higher altitudes.
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.
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