A gas turbine is a type of combustion engine that can convert fuel into energy. How does this work, and how did this manner of producing energy come about? Read on to learn about how gas turbines evolved, and what role they play in our lives now…
The Basic Principles
Gas turbines use the possibility of transforming the internal energy of a fluid (associated with its temperature or pressure) into kinetic energy (associated with its velocity), and vice-versa.
In gas turbines these transformations are used to exchange energy in the form of mechanical work and heat with a steady stream of gas – usually air.
The air ingested by the gas turbine is passed through a compressor, which raises its temperature and pressure. More energy is added to the air emerging from the compressor by burning fuel in the combustor. The air exiting the combustor carries a huge amount of energy. Part of this energy is recovered through a turbine to drive the compressor and the engine auxiliary equipment (Figure 1). The remaining energy is available to form a propulsive jet, which can be used to push forward an aircraft, like in a jet engine, or to drive a turbine to make power available on a shaft, like in a ship or in a power station.
Modern gas turbines are immensely powerful: the fan of each of the engines powering a long haul aircraft like a Boeing 777 use 65 MW of power (Figure 2). This is roughly equivalent to 1000 family cars. They are also very efficient: a long-haul flight uses the same amount of fuel per passenger per mile as a motorway car journey. But it takes place at nearly the speed of sound!
The first recognizably modern gas turbine was described in a US patent application in 1899. The first gas turbine able to produce power was built in 1903 in Norway. The first work on the theory of gas turbines was published by A.A. Griffith, at the Imperial College of Science, Medicine and Technology (now Imperial College London) in 1920.
World War II
In 1931 a little know RAF flight officer, Frank Whittle (later Sir Frank), patented a gas turbine design where the energy produced by the gas turbine was used to form a jet that could propel an aircraft. The new engine was more suited than piston engines for high altitude and high-speed flight.
World War II started soon after. While Whittle struggled to secure resources to develop his idea, both allied and enemy powers started their own gas turbine program. In Germany, Hans Pabst von Ohain flew the first jet powered aircraft in 1939. The first US jet engine flew in 1940. Whittle’s first jet engine flew in 1941.
Rover, the car company, was given the task to produce Whittle’s engine. Rover’s management and Whittle were barely talking to each other by 1942 and Ernest Hives (later Lord Hives), the general manager of Rolls-Royce, persuaded Rover’s managing director Maurice Wilks to transfer the whole operation to Rolls-Royce, in exchange for a tank engine factory near Nottingham. Soon the UK had the world’s first operational jet engine, the Rolls-Royce Welland, followed by the Derwent (Figure 3) and the Nene. Two of the engineers involved, Adrian Lombard and Stanley (later Sir Stanley) Hooker from Rolls-Royce, went on to lead the design of most UK civil and military engines of the following thirty years. Hooker was an Imperial College graduate and had come to Oxford to study for a DPhil at Brasenose College.
The struggles to develop his idea in wartime with modest resources took a heavy toll on Whittle. He left the RAF in 1948 and emigrated to the US in 1977 where died of lung cancer in 1996. He met Von Ohain in 1966 in the US and the two men became good friends despite Whittle’s early suspicions.
The Cold War
Whittle’s designs were based on centrifugal compressor, where the air moves radially outwards and must turn to enter the combustor. Compressors of this type have favourable characteristics for small mass flow rates, but they become less efficient when the machine is scaled up to power a bigger aircraft. In the years after World War II all the major engine programs started using axial compressors where the air flows in a roughly straight path, like in the Rolls-Royce Avon shown in Figure 5. The push towards bigger and more powerful engines came from the demands of a new phase in the world history: the Cold War.
Early jet engines were used mainly for small fighter planes, because poor fuel economy made them unsuitable for larger aircraft. Improvements in aerodynamics and materials meant that soon jet engines became suitable for large aircraft, such as heavy bombers. Jet-powered nuclear bombers became one of the three legs of the principle of nuclear deterrence during the Cold War: the other two were nuclear-powered submarines and intercontinental ballistic missiles (ICBMs).
The need to penetrate enemy territory with relative impunity led to the development of faster and faster bombers – and fighter aircraft in response. As the power required of the jet engines increased, so did the amount of work done by the compressor. It soon became apparent that the compressor could be operated stably over a wide range of conditions only if it could be split into two sections, rotating at different speeds and each driven by a separate section of the turbine. This marked the introduction of multi-spool engines, like the Pratt & Whitney J57 or the Bristol Olympus, the engine of the Concorde.
Commercial aviation and the bypass engine
The fuel economy of a jet engine can be improved by reducing the speed of the jet it produces and by increasing its mass flow rate. An effective way of doing this is by passing only a small fraction of the air ingested by the engine through its core and compressing the rest through a compressor of large capacity but low pressure ratio. The ratio between the mass flow rate ingested by the engine and the mass flow rate passed through the core is called bypass ratio.
Griffith had first proposed this layout in 1947. An engine based on this idea first ran in 1952 and evolved into the Rolls-Royce Conway (Figure 6). The Conway and its competitor the Pratt & Whitney JT8D powered the Boeing 707 and the DC8, which opened the age of mass air travel. They also powered many military aircraft of the time.
At the end of the 1960s all major airframers were contemplating wide-body commercial aircrafts capable of carrying 200 or more passengers over distances in excess of 5000 nautical miles to satisfy the increasing demand for air travel. Engine manufacturers responded by proposing new engines with high bypass ratios. The challenges of developing such an engine are formidable and require vast investments, immense skills and a strong leadership. Adrian Lombard, who had overseen the development of Rolls-Royce engines since immediately after the war, passed away suddenly in 1967. Four years later Rolls-Royce declared bankruptcy on February 4th, 1971 because of the cost overruns associated to its first high-bypass engine, the RB211 (Figure 7). The news shared the front page of the New York Times with coverage of the successful landing of Apollo 14. The Lockheed L-1011, for which the RB211 was intended, almost bankrupted Lockheed in turn. The RB211 went on to become a successful and reliable engine under the stewardship of Stanley Hooker, who had left Rolls-Royce for Bristol in 1963. Rolls-Royce – minus the original car business – returned into private ownership soon after and grew to secure about half of the large engine civil market.
What the future holds
The multi-spool turbofan layout that has dominated the skies for the last forty years has reached its limit: it is no longer possible to drive a large and powerful fan with a reasonably sized turbine.
In order to overcome this problem, Pratt & Whitney invested considerable resources in developing a geared turbofan, the PW 1100 (Figure 8). In this engine the fan and the turbine driving it can spin at different speeds. The added weight of the gear box (which has to handle about 40MW in a device the size of a medium frying pan) is compensated by the increased efficiency and reduced weight of the turbine.
All major manufacturers are investing in their own designs for geared fans.
All Rolls-Royce credited images have been provided with the expressed permission of Rolls-Royce PLC. St John’s Inspire Programme are grateful to Rolls-Royce PLC for their support in providing the images in Figure 3,5,6,7 and granting permission to publish them.
Luca Di Mare