An Encyclopedia of the History of Technology part 37 potx

PART TWO: POWER AND ENGINEERING 342 Sir George Cayley was the first to build an engine of Henry Wood’s type in 1807. Two other cycles that have been employed in external combustion engines are one due to John Ericsson in 1826 and another named after Robert Stirling, who patented an engine based on the cycle in 1816. Engines based on the Wood, Ericsson and Stirling cycles were used quite extensively in the nineteenth century because they did not need a steam boiler, which required space and involved the danger of explosion. Although external combustion engines were extinct by the early years of this century, engines based on the Stirling cycle have experienced a substantial revival of interest beginning in the late 1960s. Stirling engine The ideal Stirling cycle involves constant temperature heat addition and removal, and constant volume heat addition and removal. It has an efficiency equal to that of the Carnot cycle operating between the hot and cold reservoirs of the Stirling cycle, but without the need to accommodate the very large changes in volume of the working substance that are characteristic of the Carnot cycle. A particular feature of the Stirling engine is the use of two cylinders and two pistons. One of these, the displacer piston, serves to transfer the working substance between the hot and cold reservoirs, while the other, the power piston, is connected to the surroundings by some appropriate means (mechanical or electrical). The practical Stirling engine differs from the thermodynamic ideal by replacing the hot and cold reservoirs attached to the displacer cylinder by heat exchangers located in the transfer connection between the two ends of the displacer cylinder. A regenerator in the transfer connection is also added to improve the thermodynamic efficiency. APPENDIX Heat engines and thermodynamics A heat engine is a fixed mass of material, e.g. air, water or steam, called the working substance, that undergoes a series of processes in such a way that it converts heat into work. The processes are arranged so that the working substance returns to its original state, i.e., it has undergone a cycle. The cycle is also characteristic of a practical heat engine and in that case it is repeated as often and as rapidly as the operator desires or is practical. STEAM AND INTERNAL COMBUSTION ENGINES 343 The history of the heat engine is closely associated with the first and second laws of thermodynamics. The first law states that energy can be neither created nor destroyed so, in theory, all the chemical energy in the coal burnt to produce steam could produce work in a steam engine. The engine would then be said to be 100 per cent efficient. However, this is not observed in practice and Sadi Carnot showed that it was impossible. Carnot also showed that if the heat was supplied to the heat engine at a constant temperature T h (K or °R) and rejected at a constant temperature T c (lower than T h ), then the efficiency ( η ) of the heat engine would be η =1-(T c /T h ) . The corresponding heat engine cycle is called the Carnot cycle. This result is important in the theory of thermal prime movers because it indicates that the efficiency of a heat engine using the Carnot cycle (and, by implication, any other cycle) is increased if T c is decreased, T h is increased, or both. In practice the minimum available value for T c is 22°C (72 ° F), so the goal of all heat engine designers is to increase T h and this is the common thread that runs through the history of thermal power production. However, this ambition has usually been frustrated by the inability to obtain materials that can stand the elevated temperatures and pressures of the working substance. The Carnot cycle is said to be an ideal cycle because all its processes are reversible, that is, they occur infinitely slowly without friction, without fluid turbulence, and heat exchange employs minute temperature differences. Practical heat engines that are intended to work on a close approximation to the Carnot cycle have impractical dimensions, so ideal cycles have been defined that conform more nearly to the characteristics of the practical prime mover. A particularly important group of such cycles are the air standard cycles, which use air as the working substance. These are the ideal cycles associated with the internal combustion engine and the gas turbine. Reciprocating steam engines The reciprocating steam engine comprises two essential parts: the cylinder and the valve or steam chest. The cylinder has two ports at each end. These are opened and closed while the piston moves from one end of the cylinder to the other end, and back again, with the cycle of events repeated at each revolution of the crankshaft. The piston, which fits in the cylinder, is a circular disc with grooves around its circumference, which hold spring rings in position (Figure 5.24). The latter are free to expand outward and, thereby, fit the cylinder so tightly that steam cannot leak past the piston. The piston is secured to the piston rod. On both end faces of the cylinder there are a number of studs that pass through corresponding holes in the cylinder covers so that the latter can be secured by nuts screwed on to the studs. The rear cover has an opening for the piston rod that is sealed by a stuffing box filled with packing held in place by the gland. PART TWO: POWER AND ENGINEERING 344 A space is always left between the piston and the cylinder covers when the piston is at either end of the cylinder to ensure that the piston does not strike the cylinder cover. The piston moves to and fro with a reciprocating motion and this linear motion is converted into a circular one by a connecting rod, which is joined to the piston rod at the crosshead. The latter slides on the guides or slidebars. The connecting rod big-end is located at a distance, equal to half the piston stroke, from the centre of the crank shaft. The valve or steam chest has a plain flat surface machined parallel with the axis of the cylinder in which there are three ports, the outer ones connected to the corresponding ports in the cylinder, and the middle one the exhaust. Steam Figure 5.24: The reciprocating engine (steam or internal combustion). (a) Crosshead engine (double acting). Key: A cylinder head; B cylinder wall; C piston ring; D piston; E piston rod; F gland; G crosshead guides; H crosshead; I connecting rod; J crank-pin; K crank; L crank shaft. (b) Trunk piston engine (single acting). Key: A cylinder head; B cylinder wall; C piston rings; D gudgeon pin; E piston; F connecting rod; G crankpin; H crank; I crankshaft. STEAM AND INTERNAL COMBUSTION ENGINES 345 is supplied from the boiler through the pipe. The slide valve moves back and forth on the machined surface and opens and closes the ports to admit and release steam in accordance with the cycle of piston movements. This is ensured by driving the valve by a crank, with the two connected through a more or less complicated system of linkages known as the valve gear. (Modern steam engines, particularly when working with high temperature steam, used piston valves. Other important types are the Corliss valve, the poppet valve and the drop valve.) It is important to note that the valve does not admit steam throughout the piston stroke, but only for a short period at the beginning. The pressure and energy of the steam decreases as the piston completes its stroke and, in an ideal engine with no friction, or other losses, the energy given up by the steam appears as a moving force (work) at the piston rod. The steam is then said to be used expansively. The same effect could be produced by admitting steam to the cylinder throughout its stroke, but such non-expansive working would not be as efficient as expansive working. This is clarified in Figure 5.25. Figure 5.25: Effect of cut-off on the performance of the reciprocating steam engine. In the diagram each square equals one unit. Adapted with permission from C. S. Lake and A. Reidinger, ‘Locomotive Valves and Valve Gears’ (Percival Marshall, London, n.d.). PART TWO: POWER AND ENGINEERING 346 Steam turbines The steam turbine converts the internal energy of the steam into rotary motion by accelerating it to a high velocity in a specially shaped stationary passage called a nozzle. The steam leaving the nozzle is then directed on to a row of blades or buckets attached to a rotating wheel (see Figure 5.26). The flow cross-section of the blade passages is specially designed to change the direction of motion of the steam, and, in the reaction steam turbine, the pressure of the steam. Because the linear speed of the blade increases with the radius, warped twisted blades were introduced in the 1930s. This ensures that the steam is incident on the blade at all radii with the minimum of losses due to turbulence and friction. The form of the stationary nozzle that accelerates the steam and directs it on to the rotating blades depends on the desired speed of the steam at the nozzle exit. De Laval discovered in 1888 that for very high steam speeds the nozzle Figure 5.26: Principle of the steam turbine. Steam passes through the stationary nozzle and is directed as a high velocity jet onto the blades attached to the periphery of the rotating wheel. The steam experiences a drop in pressure as it flows through the moving blades. The blades are so shaped that the steam, which is flowing axially in this portion of the turbine, is turned (see the enlargement). The change in steam direction and pressure (if employed) as the steam passes through the moving blades imparts a force in the tangential direction to the wheel that causes it to turn. Reproduced with permission from W.G.Scaife, ‘The Parsons Steam Turbine’, in Scientific American, vol. 152, no. 4 (1985), pp. 132–9. STEAM AND INTERNAL COMBUSTION ENGINES 347 must have a converging-diverging form. The form of this nozzle is contrary to expectation, in that it might be assumed that the continually decreasing cross- section of a purely converging nozzle would be required to accelerate the steam. However, the paradox arises from our everyday experience with incompressible fluids, particularly water. Steam is a compressible fluid, that is, its density depends on its pressure, whereas the density of water is, for practical purposes, independent of pressure. A fluid, as it passes through a nozzle, experiences a decrease in pressure, so, if it is steam, its density decreases. At first the density change is small and the velocity increase, induced by the drop in pressure, is large. Since, for a constant mass rate of flow at all points along the nozzle, the cross-sectional area is inversely proportional to these two quantities, a decrease in nozzle cross-section is required. However, at a point where the so-called critical pressure is reached in the nozzle, the density starts to decrease much more rapidly with the decrease in pressure along the nozzle, and, since the steam velocity is still increasing, the cross- sectional area has to increase in order to accommodate the same mass rate of flow. Hence, if the nozzle operates with an exit pressure less than the critical pressure, it has the initial converging form followed by the diverging sections. ACKNOWLEDGEMENTS The author would like to thank Ian McNeil for the invitation to prepare this chapter, and for his continued encouragement and patience during its preparation. At the Rensselaer Polytechnic Institute: Mary Ellen Frank and Susan Harris for assistance with the typing, the staff of the Folsom Library for obtaining numerous papers and for facilitating the preparation of illustrations, Professors F.F.Ling and M.Lai, chairman and acting chairman of the Department of Mechanical Engineering, Aeronautical Engineering and Mechanics. FURTHER READING Steam engines Buchanan, R.A. and Watkins, G. The industrial archaeology of the stationary steam engine (Allen Lane, London, 1976) Dickinson, H.W. A short history of the steam engine, 2nd edn (Cass, London, 1963) Hunter, L.C. A history of industrial power in the United States 1780–1930. Volume two: steam power (University of Virginia Press for the Hagley Museum, Charlottesville, VA, 1985) Matschoss, C. Die Entwicklung der Dampf-Maschine, two vols. (Julius Springer, Berlin, 1908) PART TWO: POWER AND ENGINEERING 348 Rolt, L.T.C. and Allen, J.S. The steam engine of Thomas Newcomen, (Moorland, Hartington and Science History Publications, New York, NY, 1977) Thurston, R.H. A history of the growth of the steam engine, Centennial Edition (Cornell University Press, Ithaca, NY, 1939) Steam turbines Harris, F.R. ‘The Parsons centenary—A hundred years of steam turbines’, Proceedings of the Institution of Mechanical Engineers, vol. 198, no. 9 (1984), pp. 183–224 Internal combustion engines Büchi, A.J. Exhaust turbocharging of internal combustion engines, Monograph no. 1 (published under the auspices of the Journal of the Franklin Institute, Lancaster, PA, 1953) Cummins, C.L. Jr. Internal fire (Carnot Press, Lake Oswego, Oregon, 1976) Delesalle, J. ‘Les Facteurs du progrés des diesel’, Entropie, vol. 21, no. 122 (1985), pp. 33–9 Hardy, A.C. History of motorshipping (Whitehall Technical Press, London, 1955) Jones, J. ‘The position and development of the gas engine’, Proceedings of the Institution of Mechanical Engineers, vol. 151 (1944), pp. 32–53 Mondt, J.R. ‘An historical overview of emission-control techniques for spark-ignition engines’, Report No. GMR-4228 (General Motors Research Laboratories, Warren, MI, 1982) Norbye, J.P. The Wankel engine: design, development, applications (Chilton Book Co., Radnor, Pa., 1971) Pattenden, R.F.S. ‘Diesel engine research and development’, Chartered Mechanical Engineer, vol. 9, January (1962), pp. 4–12 Ricardo, H.R. ‘Diesel engines’, Journal of the Royal Society of Arts, vol. 80 (1932), pp. 250– 62, 267–80 —— The high-speed internal combustion engine, 4th edn (Blackie, London, 1953) Taylor, C.F. ‘Aircraft propulsion: a review of the evolution of aircraft power plants’, Smithsonian report for 1962 (Smithsonian Institution, Washington, DC, 1962), pp. 245–98 Gas turbines Baxter, A.D. ‘Air flow jet engines’, in O.E.Lancaster, (ed.), Jet propulsion engines, Vol. XII, High Speed Aerodynamics and Jet Propulsion (Princeton University Press, Princeton, NJ, 1959), pp. 29–53 Cox, H.R. ‘British aircraft gas turbines’, Journal of the Aeronautical Sciences, vol. 13, (1946), pp. 53–87 Denning, R.M. and Jordan, T. ‘The aircraft gas turbine—status and prospects’, in Gas turbines—Status and prospects, (ME Publications, New York, 1976), pp. 17–26 Keller, C. and Frutschi, H. ‘Closed cycle plants—Conventional and nuclear-design, application operations’, in Gas turbine engineering handbook, Vol. II, (Gas Turbine Publishers, Stamford, CT, 1976), pp. 265–83 STEAM AND INTERNAL COMBUSTION ENGINES 349 Meyer, A. ‘The combustion gas turbine: its history, development and prospects’, Proceedings of the Institution of Mechanical Engineers, vol. 141, (1939), pp. 197–222 Moss, S.A. ‘Gas turbines and turbosuperchargers’, Transactions of the American Society of Mechanical Engineers, vol. 66, (1944), pp. 351–71 Whittle, F. ‘The early history of the Whittle jet propulsion gas turbine’, Proceedings of the Institution of Mechanical Engineers, vol. 152, (1945), pp. 419–35 External combustion engines West, C.D. Principles and application of Stirling engines (Van Nostrand Reinhold, New York, 1986) 350 6 ELECTRICITY BRIAN BOWERS STATIC ELECTRICITY The attractive power of lodestone, a mineral containing magnetic iron oxide, was known to Lucretius and Pliny the Elder. The use of the magnetic compass for navigation began in medieval times. A letter written in 1269 by Peter Peregrinus gives instructions for making a compass, and he knew that it did not point to the true North Pole. Knowledge of static electricity is even older, dating back to the sixth century BC: Thales of Miletus is said to have been the first to observe that amber, when rubbed, can attract light bodies. The scientific study of electricity and magnetism began with William Gilbert. Born in Colchester and educated at Cambridge, Gilbert was a successful medical practitioner who became physician to Queen Elizabeth I in 1600. In that same year he also published his book De Magnete, which recorded his conclusions from many years’ spare-time work on electrostatics and magnetism, and, for the first time, drew a clear distinction between the two phenomena. In a very dangerous experiment the American statesman Benjamin Franklin showed that a kite flown in a thunderstorm became electrically charged. His German contemporary Georg Wilhelm Richmann was less fortunate: he was killed trying the same experiment at St Petersburg in 1753. Franklin also studied the discharge of electricity from objects of different shapes. He suggested protection of buildings by lightning conductors and, in the light of his discharge experiments, said that they should be pointed. The value of lightning conductors was not fully accepted at first. Some argued that they would attract lightning which would not have struck if the conductor had not been there. Some people preferred ball-ended lightning conductors to pointed ones. Among these was King George III, whose reasoning seems to have been that since Franklin was a republican his science must be suspect too. ELECTRICITY 351 The distinction between conductors and insulators; the fact that there were two forms of static electricity, later called ‘positive’ and ‘negative’; and the principle of storing electric charges in a Leyden jar—a capacitor formed by a glass jar coated with metal foil inside and out—were all worked out during the eighteenth century. Most electrical experiments at that time used electricity produced by frictional electric machines. There were many designs of such machines, but typically a globe or plate of glass or other non-conducting material was rotated while a cloth rubbed its surface. The discovery of the electric current, about 1800, did not end the story of static electricity. Two important machines of the nineteenth century were Armstrong’s hydro-electric machine and the Wimshurst machine. William Armstrong was a solicitor and amateur scientist who founded an engineering business in Newcastle upon Tyne. His attention was drawn to a strange effect noticed by an engine driver on a colliery railway. The driver experienced ‘a curious pricking sensation’ when he touched the steam valve on a leaking boiler. Armstrong found that the steam, issuing from a small hole, became electrically charged. He then built a machine with an iron boiler on glass legs and a hard-wood nozzle through which steam could escape. He found the steam was positively charged, and he then made a larger machine which was demonstrated in London producing sparks more than half a metre long. This ‘hydro-electric machine’, as he called it, established Armstrong’s scientific reputation. A War Office committee on mines suggested in 1857 that Armstrong’s machine, with its very high voltage output, could be used for detonating mines. In practice magneto-electric machines were soon available, and Armstrong’s machine never saw practical use. During the nineteenth century numerous machines were made which multiplied static electric charges by induction and collected them in Leyden jars or other capacitors. Best known was the Wimshurst machine, made in 1883 by James Wimshurst, a consulting engineer to the Board of Trade. CURRENT ELECTRICITY An electric current, as opposed to static charges, was first made readily available in 1800 as a result of the work of the Italian Alessandro Volta who later became Professor of Natural Philosophy at the University of Pavia. He was following up work done by his fellow-countryman Luigi Galvani, Professor of Anatomy at the University of Bologna. Galvani had been studying the effects of electric discharges from frictional machines on the muscles of dead frogs. In the course of this work he noticed that a muscle could be made to twitch with the aid of nothing more than two pieces of different metals. Galvani thought the source of the phenomenon he had . M.Lai, chairman and acting chairman of the Department of Mechanical Engineering, Aeronautical Engineering and Mechanics. FURTHER READING Steam engines Buchanan, R.A. and Watkins, G. The industrial. specially designed to change the direction of motion of the steam, and, in the reaction steam turbine, the pressure of the steam. Because the linear speed of the blade increases with the radius, warped twisted. accommodate the very large changes in volume of the working substance that are characteristic of the Carnot cycle. A particular feature of the Stirling engine is the use of two cylinders and two