PART TWO: POWER AND ENGINEERING 302 Figure 5.9 STEAM AND INTERNAL COMBUSTION ENGINES 303 Figure 5.9: Vertical section of the 1300MW, 3600rpm cross-compound turbine built by Brown, Boveri & Co. and installed at the Gavin station of the American Electric Power Co. in 1974. Steam inlet conditions: 241bar (3500psig), 538°C (1000°F) with single reheat to 538°C (1000°F). This is the largest steam turbine in the world supplied by a fossil-fuel fired steam generator. Although this turbine is described as a cross-compound (introduced c.1914) because sections of the machine are divided to drive two separate loads, it also incorporates tandem compound features (introduced c.1913) because the high pressure (upper left) and intermediate pressure (lower left) sections are coupled to the low pressure sections (on the right). All sections are double flow (introduced c.1906) with steam supply at the centre point of the section. The steam flows in opposite axial directions balancing any thrust on the rotors from the stage pressure drops. The high pressure section is supplied with steam from the steam generator. This section then exhausts, through the reheater, to the intermediate pressure section, which in turn exhausts to the four low pressure sections. Double shell construction (introduced 1937) is used in all sections. The increasing inlet temperatures and pressure shown in Figure 5.10 are a consequence of the desire to increase the efficiency of the thermodynamic cycle of which the steam turbine is a part. The result has been a considerable improvement in cycle efficiency over the years as indicated by the decreasing plant heat rate. Reliability is as important as efficiency, and the continual advances in steam conditions and power output have required a corresponding effort in metallurgy, material behaviour and inspection, blade and disc vibration, and fluid mechanics. The fact that reliability has been maintained with the continuing need to increase the turbine operating parameters is a monument to the engineers who have made it possible. INTERNAL COMBUSTION ENGINES The reciprocating internal combustion engine was the second type of thermal prime mover, after the reciprocating steam engine, to be developed, with the first practical example, the Lenoir engine, being built in 1860. There were three incentives to replacing the steam engine: (a) elimination of the boiler and condenser, and the need for good water; (b) the increasing availability of suitable fuels: coal-gas and petroleum derivatives; (c) the potentially higher thermal efficiency of the internal combustion engine, as indicated by thermodynamics, resulting from the higher allowable maximum temperatures compared to other prime movers. Three fuels, gas, petrol (gasoline) and oil are in normal use in the internal combustion engine. In addition, there have been, and there are, efforts to use coal as a fuel (although contrary to myth, Rudolf Diesel did not originally intend this for his engine). Engines can, therefore, be classified by the fuel they employ. Alternatively, engines may be categorized by the method used to ignite the fuel; PART TWO: POWER AND ENGINEERING 304 a hot surface, a spark or compression of the working substance. The modern tendency appears to favour the last method. Accordingly, the following nomenclature has been adopted here: the spark ignition engine describes essentially the modern petrol engine and its forebears; the compression ignition engine refers to what is now commonly known as the Diesel engine (which is not the machine originally invented by Diesel). Two additional classes, described further below, are the gas engine and the ‘hot bulb’ engine. Proposals and developments before 1860 The development of the internal combustion engine began in the seventeenth century with attempts by Christian Huygens to harness the energy released in Figure 5.10: Historical trend of the performance parameters of steam turbines in electric power generation service 1890–1985. The plotted curves represent the ‘average’ or ‘typical’ machine. The progressive increase in inlet steam temperature is particularly dependent on the materials used in the rotors and casings. Periods of application of different materials are roughly as follows: cast iron: 1883–1913; cast steel (ferritic): 1913–37; alloy steel (chrome ferritic): 1930 to date; alloy steel (austenetic): 1947–76. To convert the heat rate values to efficiency multiply by 2.93×10 -4 and invert. The curves are based on data in K.Baumann, Journal of the Institution of Electrical Engineers, vol. 48 (1912), pp. 768–877; vol. 59 (1921) pp. 565–663, and The Metropolitan-Vickers Gazette, vol. 12 (1930), pp. 212–20, and the annual power plant surveys appearing under various titles in Power between 1931 and 1985. STEAM AND INTERNAL COMBUSTION ENGINES 305 a cannon on firing. This was done by exploding a charge of gunpowder at the base of a vertical cylinder, closed at its lower end; the expanding products of combustion raised a free-piston fitted in the cylinder. If some of the products of combustion were released when the piston reached its point of maximum travel, then the residual gases would, on cooling and contracting, produce a pressure less than atmospheric (a vacuum) in the cylinder below the piston. The application of this vacuum to power production was the same as that in the steam engine (see p. 273). The free-piston engine is not convenient for producing mechanical power because on the upward (expansion) stroke the piston has to be disconnected from the output shaft. An atmospheric engine with a conventional permanent connection to its load was first demonstrated by William Cecil in 1820. This was a hydrogen fuelled, single acting, atmospheric engine that drove its load through a rocking beam (like Newcomen’s steam engine: see p. 275) and crank. It was spark ignited, and Cecil designed an automatic device for metering and mixing the air-fuel mixture. Samuel Brown, a Londoner, constructed between 1823 and 1833 the first commercially successful internal combustion engine. A vacuum was produced in a constant volume container by burning coal-gas and cooling the products of combustion with a water spray (as in Watt’s steam engine: see p. 275). The vacuum was then used in a separate power cylinder, fitted with a piston. A free-piston engine was designed and built, between 1854 and 1864, by the Italians Eugenio Barsanti and Felice Mattucci. This was a return to the original concept of Huygens, but the fuel was now coal-gas with spark ignition. For various reasons it was not a commercial success. 1860–1880: the early gas engine The first internal combustion engine that could be said to provide a reliable and continuous source of power was the gas engine (using coal-gas) introduced in France in 1860 by Etienne Lenoir. The air standard Lenoir cycle is shown in Figure 5.11 (a). It was a double-acting, two-stroke cycle engine (see below) that used slide valves to control the admission and exhaust processes. It was very popular, being made in sizes between one half and six brake horsepower, but the thermal efficiency was low (5 per cent). The most important contribution to the identification of the principles that should be followed in the design of internal combustion engines was made by Alphonse Beau de Rochas and given in a French patent filed in 1862. In addition, Beau de Rochas advocated the use of a four-stroke cycle (see Figure 5.12 (a)) for maximum efficiency, rather than the two-stroke cycle (see Figure 5.12 (b)) that was more popular at that time. PART TWO: POWER AND ENGINEERING 306 Figure 5.11 (a) The air-standard Lenoir cycle plotted on coordinates of pressure (P) and volume (V) for a double acting engine showing suction and exhaust strokes. In the actual engine an air and coal-gas mixture is drawn into the cylinder (1)–(3). When the piston reaches mid-stroke the mixture is ignited, followed by a steep rise in pressure (1)–(2). The pressure decreases (2)–(3) as the piston continues its stroke. The exhaust stroke is (3)–(3). This cycle is sometimes known as the non- compression cycle because the maximum pressure at point (2) is obtained from heat addition (by combustion in the practical engine) not from motion of the piston. T M =maximum temperature at the conclusion of the heat addition (combustion) process, T o =temperature at the conclusion of the suction stroke (ideally equal to the exhaust/admission temperature T F ). STEAM AND INTERNAL COMBUSTION ENGINES 307 (b) The air-standard Otto cycle plotted on coordinates of pressure (P) and volume (V). Exhaust and suction strokes are not shown. The working substance (air-fuel mixture in the actual engine) is compressed adiabatically (no heat transfer) and reversibly (with no friction or other dissipative effects) from (1)–(2). Heat is added at constant volume (2)–(3) (by combustion of the air-fuel mixture in the actual engine). Reversible and adiabatic expansion (3)–(4) follows. From (4)–(1) the working substance is cooled at constant volume (in the actual engine the products of combustion are discharged to the ambient at P1). The free-piston internal combustion engine reappeared in 1867 when N.A. Otto and Eugen Langen demonstrated one at the Paris Exhibition. Its fuel consumption was less than half that of the Lenoir engine. Although this engine sold widely, it was heavy and noisy, and in 1876, Otto produced an engine working on the four-stroke cycle of Beau de Rochas. Otto based his engine on an air standard cycle (Figure 5.11 (b)) that is nowadays identified by his name, which is applicable to both two- and four-stroke cycle engines. An important feature of Otto’s four-stroke engine was its incorporation of the concept of the stratified charge, which is applied in some modern engines to minimize the production of undesirable pollutants in the exhaust gases (see p. 319). His objective was to provide smooth operation by eliminating combustion ‘knock’, or detonation (see below). In the stratified charge engine the air and coal-gas mixture was introduced into the cylinder in such a way that it was lean near the piston with increasing richness toward the ignition source (a gas pilot-flame). This was accomplished by a special valve gear that first admitted air and then admitted the gas when the piston was about half-way through the induction stroke. The Otto engine, called by its manufacturers the Otto Silent Engine (compared to the very noisy Otto and Langen engine), was a landmark in the history of internal combustion engines, because it incorporated all the essential features of the modern internal combustion engine. Four-stroke and two-stroke cycle engines The cycles forming the basis of the reciprocating internal combustion engine may be classed as either two-stroke or four-stroke (Figure 5.12), with the former having a power stroke every revolution and the latter on every second revolution. The four-stroke cycle is in practice (although not in theory) more efficient than the two-stroke cycle, but nineteenth-century engineers continued to be interested in the latter because of its theoretical advantages, and, more importantly, because it circumvented Otto’s patents on the four-stroke cycle. The first engine operating on the principle was built by Dugald Clerk in 1878. Experience showed that the power output of the two-stroke cycle engine was only about 30 per cent greater than that of the corresponding four-stroke PART TWO: POWER AND ENGINEERING 308 Figure 5.12 (a) Indicator diagram (actual pressure-volume diagram for one complete cycle) of a four-stroke cycle internal combustion engine. This is the cycle advocated by Beau de Rochas in his 1862 patent. The cycle events are as follows: (4)–(2) intake during an entire outward stroke of the piston; (2)–(S) compression during the following return stroke of the piston with ignition at point S; (S)–(3) increase in pressure due to combustion of the air-fuel mixture followed by expansion to point (3) on the outward piston stroke; (3)–(1) exhaust on the fourth, and last, stroke of the cycle. (b) Indicator diagram (actual pressure-volume diagram for one complete cycle) of a two-stroke internal combustion engine. The cycle of events is similar to STEAM AND INTERNAL COMBUSTION ENGINES 309 that in the four-stroke cycle except for the exhaust processes. Close to the end of the expansion stroke the exhaust ports (Ex.P.) open and the products of combustion are discharged from the cylinder into the exhaust manifold. Before the outward stroke of the piston is complete and before the exhaust ports close the inlet ports (I.P.) open. On the return stroke of the piston, after it passes bottom dead-centre (BDC), the inlet ports close followed by the exhaust ports. This shows the intake process in the two-stroke cycle differs from that in the four-stroke cycle in the following respects: (i) as the piston returns from BDC it is opposing the intake process; (ii) intake only occurs during part of the piston stroke which can inhibit the induction of the charge. For these reasons engines working on the two-stroke cycle are not self-aspirating and a compressor (called the scavenge pump) is required to force a fresh charge into the cylinder and to push the products of combustion out. Because there is a tendency for the incoming charge to flow straight out of the exhaust ports, two-stroke spark ignition engines, where the air is carburetted outside the cylinder, have a high fuel consumption compared to the corresponding four- stroke cycle engines. Reproduced with permission from D.J.Patterson and N.A.Heinen, Emissions from Combustion Engines and their Control (Ann Arbor, Ann Arbor Science Publishers, 1973). cycle engine, instead of being 100 per cent greater, as predicted by theory. This is because the two-stroke cycle engine is not self- exhausting (technically, self- aspirating, see Figure 5.12 (b)). The fresh incoming charge (air or air-fuel mixture) must drive out the residual gases, a process called scavenging. Incomplete scavenging of the burnt gases decreases the amount of fresh charge that can be introduced, so the power output of the engine is less than predicted. The scavenging process is assisted by ensuring that the pressure of the incoming fresh charge is slightly higher than the pressure of the burnt gases in the cylinder at the end of the expansion stroke (see Figure 5.12 (b)). A scavenge pump provides the necessary pressure increase in the engine inlet manifold. Various types of reciprocating and rotary pumps are used, and in some cases the under side of the piston in a single-acting engine (see Figure 17(b)). A supercharger can, in principle, also act as a scavenge pump, but because the efficiency of a two-stroke cycle engine is very sensitive to the design of its exhaust system, it is usual, particularly with an exhaust gas turbine driven supercharger (turbocharger), to provide a separate scavenge pump (see Figure 17(b)) The two-stroke cycle is used only by the lowest power (15kW or 20hp) spark ignition engines or by the highest power (7500kW or 10,000hp or higher) compression ignition engines. In both situations the two-stroke engine is used to provide the maximum power from an engine of minimum volume. This is accomplished in the spark-ignition engine with the very simplest techniques (crankcase compression), and in the large compression-ignition engine the most sophisticated methods are applied (see below). PART TWO: POWER AND ENGINEERING 310 1880–1900: different types of fuel By about 1880 the principles of the practical internal combustion engine were established. These early engines used coal-gas as their fuel, but this was inconvenient where the engine was to drive a vehicle, and, for stationary applications, because ready access to the gas mains was not available at all locations. Liquid fuels provided a solution to this problem, but satisfactory combustion required them to be vaporized before they were ignited. This was (and is) accomplished in one of three ways: (a) carburetion: engine induction air passed over or through the fuel in a carburettor, which was independently invented in 1885 by Wilhelm Maybach and Karl Benz; (b) hot bulb engine: spraying the fuel on to a hot surface and passing the engine induction air over it (see below); (c) compression ignition: spray the fuel into the cylinder, relying for evaporation on the hot gases produced by compression of the air in the cylinder (the Diesel engine, see p. 311). The application of the first method was limited between 1880 and 1900 because the necessary low volatility fuels (flash point -12°C to -10°C; 10°F to 14°F) were hazardous, resulting in legislation that restricted their use. This made hot bulb engines with fuels (e.g. kerosene) of high flash point (above 23°C or 75°F) the most practical form of liquid-fuelled engine until the relevant legislation was changed. The ‘hot bulb’ engine The first liquid fuel engine was constructed by G.B.Brayton in 1872 in Boston, Massachusetts. It used a carburettor, and the fuel-air mixture, which was compressed before admission to the engine cylinder, was ignited by a flame. The next liquid fuel engine was built by W.D.Priestman of Hull, Yorkshire, in about 1885. This operated on the four-stroke Otto cycle and employed an external, exhaust gas heated vaporizer (flame heated for starting) into which the fuel was sprayed. The induction air passed through the vaporiser and the resulting mixture was ignited in the cylinder by an electric spark. Thermal efficiency was about 13 per cent (specific fuel consumption about 1lb mass/ hphr; 0.61kg/kWhr). Herbert Akroyd Stuart was the first (1890) to invent an engine, operating on the four-stroke Otto cycle, that made no use of an ignition source (spark or flame) and is, therefore, clearly related to the modern compression ignition engine. The vaporizer, or ‘hot bulb’, into which the fuel was sprayed was mounted on the cylinder head and connected to the cylinder by a narrow passage. It was heated either by hot cylinder cooling water, or by the exhaust gases (an external flame was used for starting). The induction air was drawn into the cylinder, and compressed, through the narrow connecting passage, STEAM AND INTERNAL COMBUSTION ENGINES 311 into the vaporizer, where ignition occurred when a combustible fuel-air mixture was attained. This was self-ignition resulting from contact between the mixture and the hot walls of the vaporizer, and should not be confused with ignition due to the high air temperatures encountered in the compression ignition engine. The fuel consumption of Akroyd Stuart’s engine was comparable to that of Priestman’s, but it avoided the spark ignition (unreliable in those days) of the latter. The hot bulb engine lasted in various forms until the late 1920s (often being called a semi-diesel, no doubt for advertising purposes) even though it was not as efficient as the compression ignition engine. It had the advantage of simplicity because it did not require the air compressor used in the early compression ignition engines, since the fuel was injected mechanically (so- called solid injection) near the beginning of the compression stroke, at a much lower pressure than the injection pressure of the compression ignition engine. The petrol engine The application of the internal combustion engine to transport requires, besides a liquid fuel, a high power-weight ratio, which in turn requires an engine operating at a high speed. Increasing the speed from 170rpm to 800rpm should reduce the engine weight by about 80 per cent. This was the objective of Gottlieb Daimler and Wilhelm Maybach. Almost at the same time in 1886, Karl Benz and Daimler and Maybach produced single cylinder Otto cycle engines, using petrol and a carburattor, operating on a four-stroke cycle, that was light enough for use in the automobile. One important difference between the engines was the method of ignition. Daimler and Maybach employed the hot-tube igniter; Benz used spark ignition provided by a battery and an induction coil (the coil contacts were opened and closed independently of the engine speed, producing sparks at a steady rate). Ignition and carburation underwent significant advances before the close of the century. The float-fed spray carburettor was introduced by Maybach in 1893 on the Daimler engine. In 1899, Daimler engines were converted from the hazardous flame-heated hot tube to Priestman’s electric spark ignition with the spark generated by the low tension magneto invented by Robert Bosch. The Diesel engine A most important advance in internal combustion engine design was made by Rudolf Diesel with his invention of the compression ignition engine, so called because the fuel is introduced directly into the cylinder, in the form of a finely . (Ex.P.) open and the products of combustion are discharged from the cylinder into the exhaust manifold. Before the outward stroke of the piston is complete and before the exhaust ports close the inlet. than the pressure of the burnt gases in the cylinder at the end of the expansion stroke (see Figure 5.12 (b)). A scavenge pump provides the necessary pressure increase in the engine inlet manifold sections. The increasing inlet temperatures and pressure shown in Figure 5.10 are a consequence of the desire to increase the efficiency of the thermodynamic cycle of which the steam turbine is a part.