Engine Cycle

engine cycle [′en·jən ‚sī·kəl]

(thermodynamics)

Any series of thermodynamic phases constituting a cycle for the conversion of heat into work; examples are the Otto cycle, Stirling cycle, and Diesel cycle.

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Cycle, Engine 

a sequence of processes that regularly recur in an internal or external combustion engine and form the basis of the engine’s operation. A distinction is made between thermo-dynamic and actual cycles. In an actual cycle, as opposed to a thermodynamic cycle, allowances are made for heat losses, hydrodynamic losses, and so forth.

An engine cycle may be plotted as a closed curve in coordinates of the volume and pressure (V, p) or the entropy and temperature (S, T) of the working fluid. The area bounded by the contour is proportional to the work done. Figure 1 gives examples of thermodynamic cycles that are prototypes of the actual cycles of a carburetor engine (Figure 1,a) and a diesel engine (Figure l,b and l,c). The cycle of the carburetor engine consists of compression of the working fluid (adiabatic curve ac), addition of the heat Q₁ (isochor cz), incomplete expansion (adiabatic curve zb), and rejection of the heat Q₂ (isochor ba). The diesel cycle comprises compression of the working medium (adiabatic curve ac), addition of the heat Q₁ (isobar cz), incomplete expansion of the working fluid (adiabatic curve zb), and rejection of the heat Q₂ (isochor ba); or it may consist of compression (isotherm ac), addition of the heat Q’₁ (isochor cz’) and Q” (isobar zz’), incomplete expansion (adiabatic curve zb), and rejection of the heat Q₂ (isochor ba).

The gas-turbine cycle (Figure 1, d) with constant-pressure heat addition and complete expansion involves compression of the

Figure 1. Thermodynamic engine cycles: (a) carburetor engine, (b) and (c) diesel engine, and (d) gas-turbine engine

working fluid (adiabatic curve ac), addition of the heat Q₁ (isobar cz), expansion of the working medium (adiabatic curve zb), and release of the heat Q₂ (isobar ba). A cycle with constant-volume addition of the heat Q₁ is also possible.

Steam engines and turbines operate on the Rankine cycle. (See alsoCARNOT CYCLE and CYCLE.)