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power stroke, but had a complicated mechanism to provide a shorter admission/compression run and a longer expansion/exhaust run). The modern Atkinson cycle, used in most of the hybrid electric cars since 1997, is just a 4-stroke Otto cycle with highly-retarded closure of the intake valve; i.e. in its ideal p-V sketch in Fig. 17.8, in the admission stroke the piston goes from 0 to 4; in the second stroke, there is some partial air evacuation at constant pressure (from 4 to 1) followed by compression from 1 to 2; in the third stroke, there is the heat release; and the fourth stroke is the expansion until ambient pressure (3 to 4). Note that there is some fresh-air charge rejected (from 4 to 1), lowering the compression ratio to about r=8, but leaving a larger expansion ratio, about r=12; instead of pushing this fresh-air back into the intake manifold, it can be directed to the exhaust manifold (opening the exhaust valve instead of leaving open the intake valve), with the advantage of some cooling on valves and walls; the engine must be of direct-injection type. Engines following the Atkinson cycle yield better fuel efficiency than Otto engines, but have smaller specific engine power, and consequently lower torque (but this is not a problem with its companion electric motor in hybrid cars). VAPOUR POWER CYCLES Rankine cycle Most large electricity generating plants (central power stations), and very large ship engines, use water vapour (steam) as working fluid, following some variation of the basic Rankine cycle (named after the Scottish inventor William Rankine, that in 1859 wrote the first book on Thermodynamics), the only vapour power cycle in practical use since 1840 until in 1984 Alexander Kalina patented in the USA the cycle named after him. The heat source for the boiler is usually the combustion products of a fuel (mainly coal) and air, or the primary refrigerant of a nuclear reactor, and the heat sink in the condenser is usually a water loop, open like in a river, or closed like in a cooling tower (as explained in Chapter 8). Thomas Newcomen is credited with the invention of the steam engine in 1705 for the purpose of driving the pumps used in clearing groundwater from mine shafts. Although the work-producing element was initially reciprocating cylinder-piston devices, in 1882 Gustav de Laval introduced the vapour turbine that has taken over. Fig. 17.9. Carnot cycle within the two-phase region, basic Rankine cycle in the T-s and p-V diagram, and sketch of a vapour plant. The four processes in a simple Rankine cycle are: isentropic compression of the liquid from 1 to 2 (Fig. 17.9), isobaric heating of the liquid, boiling and super-heating the vapour (from 2 to 3), isentropic expansion from 3 to 4, and isobaric heat rejection until full condensation of the vapour. The Rankine cycle is less efficient than the Carnot cycle (Fig. 17.9), but it is more practical since the compression is not in the two-phase region (see Chapter 6) and only requires a small work, and the expansion is mainly in the gaseous phase (high-speed droplets erode turbine blades). Water is not the ideal working substance because it changes phase at relatively low temperatures (below the critical point at 647 K), generating a lot of entropy in the heat transfer from typical high-temperatures heat-sources: 1000 K in nuclear reactors up to 2000 K in conventional combustion plants. Nevertheless, water is practically thePDF Image | THERMAL MACHINES AND HEAT ENGINES
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