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.

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.)

engine cycle

engine cycleBrayton cycle.i. The cycle of events that must be accomplished in the transformation of chemical energy into mechanical energy. The two most common cycles are the Otto cycle, which describes the events of a reciprocating engine, and the Brayton cycle, which describes the transformation taking place in a gas turbine engine. See also Otto cycle and Brayton cycle.
ii. A specific activity in an engine such as from one start to the switch-off of the engine, one advance and retard of the engine, etc. For ease of record keeping within airlines, cycles are recorded as one takeoff and landing. The recording of cycles such as the times the engine has been started and switched off and the times the throttle has been advanced and retarded require special recording procedures for maintenance runs.