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radiation, and so on, like in the steam engine); it faded away in the 19th century against the steam engine (which was more practical), and later against the gas engines (that are more powerful), but it has got considerable attention lately as a non-polluting engine (if driven by solar energy or waste heat). Recall that combustion is just a mean to get high temperatures; any thermodynamic cycle (Otto, Diesel, Brayton...) can be made to run on solar energy, waste heat, etc. However, all gas power cycles require a large temperature difference in practice (e.g. concentrated solar power), and the only cycles to run with temperature differences of less than 100 K are the vapour cycles (see ORC, below). The working substance in a Stirling engine may be just air, or better helium or hydrogen (to have the best thermal conductivity), and the four processes ideally followed are (see Fig. 17.8): from 1 to 2, isothermal compression with heat rejection to the environment; from 2 to 3, constant-volume compression by heat input from a heat regenerator (a porous media with a steady longitudinal temperature-ramp; the fluid enters cold at 2, and exits hot at 3); from 3 to 4, isothermal expansion with heat input from the hot source; for 4 to 1, constant-volume heat rejection to the regenerator (the gas now flows contrary to the 2-3 process). The regenerator divides the working gas in two regions: a hot one and a cold one, with a linear temperature variation between both ends (the porous matrix must be thermally insulating, to keep this gradient, but with a large contact area to enhance heat transfer with the gas). A Stirling engine may run without regenerator, but then the efficiency is very low. An auxiliary piston, the displacer, forces the gas at constant total-volume to move through the regenerator; the displacer has to overcome the pressure loss of the gas through the regenerator, but does not pressurises the gas, whose pressure rise is due to heating and not to pushing. The displacer is cinematically driven by the main crank-shaft mechanism, or dynamically driven by resonance. Fig. 17.8 The ideal Stirling, Ericson, Atkinson, and Lenoir cycles. Similar to the Stirling cycle is the Ericson cycle, where heat regeneration is isobaric instead of isometric as in the Stirling one (the regenerative Brayton cycle with infinitesimal multistage compressions and expansions approaches to the Ericson cycle). Notice that there is nothing essential to the four-process engines, and a three-process cycle (see Fig. 17.8), named after J.J.E. Lenoir, a pioneer of two-stroke engines in the 19th century, is used to model the operation of a pulse-reactor. Energy and exergy efficiencies of 36% and 50% respectively have been reached with prototype Stirling engines of up to 10 kW, the main problems being the regenerator loss of efficiency at high speeds (>30 Hz, i.e. >1800 rpm), the radiant heat loses at high temperature (>1000 K), incomplete exchange of gas between the hot and cold zones, and leaking at high pressure (>5 MPa). An old cycle that has gained interest lately is the Atkinson cycle (Fig. 17.8). The original Atkinson cycle was proposed in 1887 by James Atkinson as a modification of the Otto cycle with a prolongedPDF Image | THERMAL MACHINES AND HEAT ENGINES
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