PART TWO: POWER AND ENGINEERING 332 Early development At the time (about 1900 to 1910) that engineers started seriously to consider the gas turbine as a candidate prime mover, neither the necessary understanding of turbo-machinery fluid mechanics nor satisfactory materials were available. Nevertheless, the advantages of the gas turbine were such that a number of machines were constructed and operated, notably by Armengaud and Lemale (1903–6), Holzwarth (1908–33), Karavodine (1908), and Stolze (1900–4). The Armengaud and Lemale gas turbine, and that due to Stolze, used the constant pressure heat addition cycle, while the Holzwarth and Karavodine turbines used the constant volume heat addition cycle. A significant feature of these machines was the use of water cooling of the expansion turbine in order to decrease the amount of air that would otherwise have been required in order to control its inlet temperature. Holzwarth’s was the most successful of the early gas turbines and a number of versions were built between 1908 and 1933 under his supervision. The final model, a 5000kW machine, was built by the Brown Boveri Company and placed in service in a steelworks at Hamborn, Germany, in 1933. This experience apparently demonstrated to Brown Boveri the superiority of turbines based on the constant pressure combustion cycle and led to their involvement in the development of this type; the company was to become one of the major contributors to gas turbine technology. 1933–1946: beginnings of the modern gas turbine There was a substantial hiatus in gas turbine activity following the early attempts to construct the machines described above, while efforts were made to develop high efficiency compressors with an adequate compression ratio (at least 4:1). In addition, during this period, as a consequence of their work with Holzwarth’s Hamborn gas turbine, the Brown Boveri Company took an important step toward the development of a practical gas turbine based on the constant pressure heat addition cycle. It was noted that in the Hamborn machine, because of the high velocity and pressure of the combustion gases, the heat transfer rates in the combustion chambers and turbine inlet nozzles were extremely high. This suggested that it might be possible to construct a boiler with a large output per unit of heat transfer surface. The gas (expansion) turbine became an essential element of the device by employing the products of combustion leaving the boiler to drive the compressor, which, in turn, supplied the high pressure, high velocity air to the boiler. Brown Boveri called this supercharged boiler the Velox boiler: it sold in substantial numbers between 1933 and 1965. The supercharging principle, and hence the gas turbine, found a further application in 1936 when it was used in the Houdry catalytic STEAM AND INTERNAL COMBUSTION ENGINES 333 converters being installed at the Marcus Hook, Pennsylvania, refinery of the Sun Oil Company. While these applications of the gas turbine did not involve the production of useful amounts of work, the knowledge gained by Brown Boveri in the development of the Velox boiler, and their association with the Houdry process, placed it in a strong position to construct a power-producing gas turbine. This came about in 1939 when the first commercial gas turbine-driven electric generator was placed in service in Neuchâtel, Switzerland. The set was for emergency standby purposes and it was only expected to run on an intermittent basis, an ideal application for a new and untried machine. During the time that Brown Boveri were developing their land-based gas turbine, Frank Whittle, a British air force officer, was working on aircraft gas turbines. (A comparable or even larger effort was under way in Germany at this time, led by Hans von Ohain, to develop an aircraft gas turbine. However, its influence on gas turbine history appears to have been slight, probably because of Germany’s defeat in the Second World War and the consequent dominance of Anglo-American technology.) Whittle’s work, which was to revolutionize aircraft propulsion and gas turbine technology in general, was a consequence of his recognition that the thermal efficiency of the gas turbine increases with altitude, which is the opposite of the behaviour of the reciprocating internal combustion engine. In 1930, Whittle obtained a patent on an aircraft gas turbine jet propulsion engine. He was unable at that time to interest the British government in supporting the effort to develop a working engine, but eventually, in 1936, he raised some private capital. At this point the British air force recognized the potential in Whittle’s ideas and released him from his regular duties so that he could devote all his time to the development of the gas turbine engine. Whittle commenced work, with the help of the British Thomson-Houston Company, on his first engine (see Figure 5.19 (a)) in 1936. It employed a double-sided centrifugal compressor, a combustion system, and a single stage impulse expansion turbine with an output of about 3000hp. The compression ratio was 4:1, much higher that the 2:1 available in contemporary centrifugal compressors. Even more remarkable was the combustion system, which involved volumetric heat release rates far greater than previously experienced in a continuous flow combustor. The engine first ran on 12 April 1937. In the summer of 1939 the British government placed an order for an engine, called W1, that would be tested in an aircraft built specially for the purpose. It first flew on 14 May 1941. (It was not the first jet propelled aircraft to fly: a German Heinkel HeS3 engine first flew in August 1939.) This engine, which is shown in Figure 5.19 (b), was, together with its successor the W2, a remarkable technical achievement. A modified version of the latter engine, the W2B, became the basis of the American version of the Whittle engine, the IA, which was built by the General Electric Company at Lynn, Massachusetts. PART TWO: POWER AND ENGINEERING 334 Meanwhile, the success of Brown Boveri with the gas turbine for its Velox boiler stimulated the interest of other engineers, notably in the Large Steam Turbine group of the General Electric Company in Schenectady, New York. This interest by GE was natural, both because of the company’s extensive experience with steam turbines and because it had a long-standing interest, dating from 1903, in the gas turbine and, since 1918, in the gas turbine- driven supercharger for reciprocating internal combustion engines. In the late 1930s design studies for a locomotive gas turbine 3355kW (4500hp) were initiated by GE. Before work on this had proceeded very far the engineers concerned became involved in the American wartime effort to develop an aircraft gas turbine, stimulated by information conveyed secretly in 1941 by the British to the Americans. Consequently, a major engineering programme involving the Westinghouse Electric Company, General Electric and various Figure 5.19: Early examples of Whittle gas turbine engines. (a) First model of the Whittle engine which was operated between April and August 1937. A single combustion chamber was located in the air duct (chain dotted) carrying the air from the compressor to the expansion turbine. Note the water-cooled turbine disc. (b) W1 engine, the first flight engine, which was operated between January 1940 and May 1941. Ten separate combustion chambers were used. It is not clear if water cooling was used during flight. Reproduced with permission from F.Whittle ‘The Early History of the Whittle Jet Propulsion Gas Turbine’, Proceedings of the Institution of Mechanical Engineers, vol. 152 (1945), pp. 419–35. STEAM AND INTERNAL COMBUSTION ENGINES 335 aircraft manufacturers was initiated. This led to the production by Westinghouse (1941–3) of the Type 19 gas turbine, and by GE of two engines, one a jet engine, the TG180 (1943–6) and the other a turboprop engine, the TG100 (1941–5). The two GE engines are of considerable historical significance because they were, apart from the German engines, probably the first engines using axial compressors to fly, as well as being the direct forerunners of a long line of GE aircraft and land-based gas turbines. Post-war development of individual types The successful demonstration of the aircraft gas turbines by Whittle in the mid-1940s turned the thoughts of engineers to other applications of this prime mover. Its coming had been long awaited and, as hindsight shows, unreasonable expectations were raised. Numerous applications were proposed which, when tried, were found to be inappropriate for the gas turbine. Furthermore, many engineers appeared to forget that the development time for such a radically new prime mover could be expected to be lengthy. Probably the most important factor constraining the application of the gas turbine has been fuel; it is just not possible for the gas turbine, as was first thought, to use any fuel. These early opinions about fuel were undoubtedly influenced by the experience in the 1920s and 1930s with fuels for gasoline engines, where the problem of engine knock was a serious constraint on both automotive and aviation applications of the spark ignition engine (see p. 316), and the struggle to overcome it made a strong impression on engineers with an interest in prime movers. Since the gas turbine introduces the fuel into the cycle after compression, knock is absent. However, this difficulty has been replaced by the effects of corrosion and ash on the performance of the expansion turbine blades (buckets). It would appear that gaseous fuels are the most desirable, particularly natural gas. Liquid fuels have to be refined and corrosive constituents removed, or their activity suppressed by additives. Solid fuels have not so far been applied successfully because of their tendency to foul the expansion turbine blades with ash. Historical trends show that the gas turbine is most useful in aircraft propulsion and electric power generation, as well as in gas pipeline compression. Aircraft Aircraft propulsion is by far the largest use of the gas turbine. This has come about because it has the following advantages compared to the piston engine: (a) the efficiency increases with height and speed; (b) the engine weight and PART TWO: POWER AND ENGINEERING 336 volume are smaller for a given power output; (c) lack of vibration; (d) greater reliability. Application of the gas turbine to aircraft propulsion was initially motivated by the desire to fly at heights (11,000m or 36,000ft and above) at which the piston engine, even with supercharging, could not produce adequate power because of the low air density. Whittle, as noted earlier, appears to have been one of the first to appreciate that the gas turbine could provide a suitable engine for high altitude flight. The aircraft gas turbine is applied in one of three forms, the turbojet, the turboprop, and the turbofan or ducted fan. This classification is based on the way in which the engine output is used, as illustrated in Figures 5.20 and 5.21. Although these different modes of using the gas turbine had been known for some time (at least since the mid-1930s, when Whittle described the turbofan engine, and work started in Britain on the turboprop), attention was concentrated initially on the turbojet engine, together with a somewhat lower level of interest in the turboprop engine. Following the Second World War there was a marked increase in the development effort applied to the turboprop engine, and particularly noteworthy aircraft that flew using this type of engine were the Vickers Viscount (1953), using four Rolls-Royce Dart engines, and the Lockheed Electra (1959) with four Allison T56 engines. However, since about 1960 the turboprop has been mainly applied in helicopter engines and in the smaller pressurized aircraft used by feeder airlines. The most important type of aircraft gas turbine, because of its high propulsion efficiency, is now the turbofan engine. A high propulsion efficiency is ensured if the jet speed and aircraft forward speed are about equal. However, under these conditions the engine has a very low thrust (which is proportional to the difference between the two speeds). But, because the thrust is also proportional to the mass rate of flow of air through the engine, it is possible to have both a high thrust and high propulsive efficiency by designing the engine to operate with a high air mass flow rate. In principle, the turboprop could provide the required characteristics because the effective mass flow (m) handled by the engine can be controlled by suitably sizing the propeller. Unfortunately, at the time this approach to improving aircraft engine performance was under consideration (the early 1940s), compressibility effects at the tips of the propeller at speeds exceeding 560km/hr (350mph) greatly degraded its performance. In view of this limitation, it occurred to Whittle that very short propeller blades, fan blades, arranged in an enclosure, so that the incoming air was decelerated to avoid compressibility effects, would be satisfactory. The fan would be driven by the expansion turbine and the engine would be called a ducted fan, or by-pass engine. Preliminary experiments with ducted fan engines were carried out by Whittle and others, but these were premature because many other details of STEAM AND INTERNAL COMBUSTION ENGINES 337 Figure 5.20: Turbojet and turboprop aircraft engines. (a) Single shaft turbojet. (b) Twin spool turbojet, (c) Direct connected turboprop, (d) Free-turbine turboprop. Reproduced with permission from R.H.Schlotel, ‘The Aircraft Applications’ in H.R.Cox (ed.), Gas Turbine Principles and Practice (Van Nostrand, New York, 1955), pp. 22–1 to 22–120. Figure 5.21: By-pass and fan turbojet aircraft engines, (a) By-pass engine (twin spool), (b) Rear fan engine (twin spool). Reproduced with permission from R.H.Schlotel, ‘The Aircraft Applications’ in H.R.Cox (ed.), Gas Turbine Principles and Practice (Van Nostrand, New York, (1955), pp. 22–1 to 22–120. PART TWO: POWER AND ENGINEERING 338 the turbojet engine had to be perfected before serious attention could be given to the by pass concept. Consequently, this type of engine did not begin to supersede the pure jet until the late 1950s. As a consequence of difficulties experienced with the rear fan engine, interest shifted in the early 1960s to the forward fan engine (see Figure 5.21). This type of engine is a standard item of equipment on the wide-body (‘jumbo’) jet aircraft that were introduced into airline service in the mid-1970s. In fact, its high thrust, particularly at take-off, and good fuel consumption (propulsive efficiency) can be said to be essential to the operation of this type of aircraft. The history of the progress in the performance of aircraft gas turbine engines between 1939 and 1985 is shown in Figure 5.22. Electric power generation The application of the gas turbine to the generation of electric power represents the largest use of this type of prime mover after aircraft propulsion. Figure 5.22: The historical trend of aircraft gas turbine engine performance parameters 1940–85. The plots are based on data in O.E.Lancaster, ‘Aviation Gas Turbines’, in R.T. Sawyer (ed.), Gas Turbine Engineering Handbook, vol. II, (Gas Turbine Publications, Stamford, Conn., 1976), pp. 211–32 and Jane’s All the World’s Aircraft (Jane’s Publishing Co., London, 1969–85). STEAM AND INTERNAL COMBUSTION ENGINES 339 This historic trend of the application of the gas turbine to electric power generation is illustrated by the growth in the cumulative number installed in the United States from one in 1949 to about 1600 units by 1985. As noted above, the gas turbine was first applied to electric power generation in 1939 when a 4000kW stand-by plant was installed at Neuchâtel. After the war a number of gas turbine-driven electric generating plants were built. Because the gas turbine at that time had a significantly lower thermal efficiency (17 per cent) than the steam power plant (30 per cent), complicated cycles involving various stages of compression and expansion with heat recovery from the exhaust were devised. This approach was eventually abandoned because the plant reliability and cost were adversely affected by its complexity, and because the performance of machines based on the simple cycle improved as a result of the development of materials and cooling methods that allowed the inlet temperature to the expansion turbine to be increased. In addition there was a steady decline in fuel costs in the 1950s and 1960s. However the increase in fuel costs from 1973 reawakened interest in more complex plants. The gas turbine is currently most widely used in electricity generation to meet peak loads, because: (a) it starts quickly (full load can be attained within a few minutes and in no more than about 30 minutes); (b) it can be remotely started with automatic equipment; (c) first, installation, and maintenance costs are low; (d) the short duty-cycle associated with peak load operation means that the relatively low efficiency and relatively high fuel costs of the gas turbine are inconsequential to the overall economics. Until about 1965 the interest in gas turbines by the power companies was not very great. However, a number of events, particularly the power failure in the north-eastern United States that occurred in 1965, which indicated the need for generating equipment that could be started without outside power, stimulated interest in this type of prime mover. Other factors were delays in the completion of nuclear power plants, unexpected breakdowns of generating equipment and, in the United States, a sharp increase in the air conditioning load. Applications of the gas turbine to base load operation require that it be incorporated as part of a steam power plant. Such combined cycles, which can have overall thermal efficiencies as high as 40 per cent, are essential in this case because of the comparatively low efficiency and high fuel cost of the gas turbine. In these cycles heat is either extracted from the gas turbine exhaust in an economizer (last stage feedwater heater), or the gas turbine exhaust is used in a steam generator (this would include an economizer). Figure 5.23 shows the change in performance of gas turbines used in non- aviation applications, including electric power generation, from 1949 to 1985. The improvement in specific fuel consumption is particularly noteworthy and is indicative of the steady improvements in the performance of the individual components of the gas turbine. PART TWO: POWER AND ENGINEERING 340 Other applications In spite of optimistic statements made in the mid-1940s, the gas turbine has only slowly been adopted for industrial purposes. This appears to be partly a consequence of its apparent advantages not outweighing the proved reliability of other prime movers, and partly because the range of fuels that can be used in the gas turbine is much more restricted than was at first realized. The most important industrial applications have been to gas pipeline compression, and as power sources on off-shore oil and gas drilling rigs. The gas turbine was applied experimentally in a number of merchant ships, but it has shown itself to be much more useful as a propulsion unit for naval vessels: since 1969 the British navy has adopted a policy of using gas turbines as the only form of propulsion in all classes of major warships. The gas turbine was first applied to a railroad locomotive when Brown Boveri placed a 1640kW (2200hp) locomotive in service on the Swiss railways in 1941. The most extensive application was between 1948 and 1969 in the United States. However, it was found that fuel costs were higher than expected and that the increasing power output of contemporary diesel locomotives Figure 5.23: The historical trend of industrial gas turbines 1940–85. The plots are based on data in the annual power plant surveys appearing under various titles in Power between 1931 and 1985. STEAM AND INTERNAL COMBUSTION ENGINES 341 tended to reduce the advantage of the gas turbine in this application. The most important current use appears to be in special purpose, lightweight, permanently coupled, multiple unit trains. Immediately following the Second World War the gas turbine was enthusiastically promoted as a new automobile power plant. A demonstration model, Jet 1, was introduced by the Rover Company in Britain in 1950. Unfortunately, the apparent promise was never realized because of high first and operational costs. Particularly significant was the inability to devise a satisfactory heat exchanger, which was required for good fuel consumption, by exchanging heat between the leaving exhaust gases and the entering air. The development of the gas turbine as summarized in Figures 5.22 and 5.23 has been very rapid and has been a consequence of two factors, the experience provided by the development of the steam turbine between 1884 and 1935, and the demands of the aircraft industry for engines of increasing power and efficiency. The fluid mechanical and materials problems of the gas turbine could not have been solved without the foundation of knowledge generated by the development of the steam turbine; and the difficulty of the engineering problems presented by the gas turbine are so great that it is unlikely that it would have reached its current state of development without the impetus provided by the demands of the aircraft industry for engines of high power and good fuel economy. EXTERNAL COMBUSTION ENGINES The use of a substance that does not undergo the phase change associated with water when used in the steam engine appears to have originated with Henry Wood in 1759. He obtained a patent to use hot furnace gases in the engine cylinder, instead of steam, thereby avoiding the steam generation process with its associated inefficiencies. Such a device is commonly called a hot air engine, but might better be called an external combustion engine. It is appreciated that the question of nomenclature is difficult in this case. First, the hot air is not restricted to air as a working substance, but can use any gas that does not undergo a phase change in the course of the working cycle of the engine. Second, the nomenclature is applicable to the steam engine, because the combustion in the furnace of the steam engine is external to the engine. Third, the first example, the engine proposed by Wood, is not an external combustion engine, because the working substance is the combustion gases of the heat source. However, it is an external combustion engine in the sense that the process takes place outside the piston and cylinder. In spite of the unsatisfactory nomenclature, the term ‘external combustion engine’ serves to differentiate this engine from the steam engine and the internal combustion engine. . consumption is particularly noteworthy and is indicative of the steady improvements in the performance of the individual components of the gas turbine. PART TWO: POWER AND ENGINEERING 340 Other applications In. outweighing the proved reliability of other prime movers, and partly because the range of fuels that can be used in the gas turbine is much more restricted than was at first realized. The most important. Figures 5.22 and 5.23 has been very rapid and has been a consequence of two factors, the experience provided by the development of the steam turbine between 1884 and 1935, and the demands of the aircraft