Mr. K. M. Joshi Assi. Proffesor, MED SSAS Institute of … and... · Deviation of Actual Cycles...

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Mr. K. M. Joshi

Assi. Proffesor, MED

SSAS Institute of Technology, Surat

Hoisting equipmentsAtkinson cycle

Atkinson cycle is an ideal cycle for Otto engine exhausting toa gas turbine. In this cycle the isentropic expansion (3-4) of anOtto cycle (1-2-3-4) is further allowed to proceed to the lowestcycle pressure so as to increase the work output. With thismodification the cycle is known as Atkinson cycle.

The cycle is shown on p-v and T-s diagrams in Processesinvolved are:

Process 1-2: Reversible adiabatic compression (v1 to v2);

Process 2-3: Cons. volume heat addition;

Process 3-4: Reversible adiabatic expansion (v3 to v4).

Process 4-1: Cons. pressure heat rejection.

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Hoisting equipmentsBrayton Cycle (Simple Gas Turbine Cycle)

The Brayton cycle is a theoretical cycle for simple gasturbine. This cycle consists of two isentropic and twoconstant pressure processes.

The cycle is similar to the Diesel cycle in compressionand heat addition. The isentropic expansion of theDiesel cycle is further extended followed by constantpressure heat rejection.

Deviation of Actual Cycles from Air Standard Cycles:

The actual Otto and Diesel engines show marked deviations form theair-standard cycles described above. The main differences between theactual and theoretical cycles are as follows.

(a) Compression and expansion are not friction less adiabatic processes.A Certain amount of friction is always present and there isconsiderable heat transfer between the gases and cylinder wall.

(b) Combustion does not occur either at constant volume or at constantpressure.

(c) The thermodynamics properties of the gases after combustion aredifferent than those of the fuel-air mixture before combustion…

(d) The combustion may be incomplete.

(e) The specific heats of the working fluid are not constant butincreases with temperature.

(f) The cylinder pressure during exhaust process is higher thanthe atmosphere. As a result, more work has to be done by thepiston on the gases to expel them out of the cylinder, than workdone by the gases on the piston during the intake stroke.

This difference in work, called pumping work isrepresented by the pumping loop.

Note that this work is negative and represents loss of workcalled pumping loss.

For Isentropic process : 1-2,

For constant volume process : 2-3,

Now, Heat addition at constant volume = Heat addition at constant pressure

For isentropic expansion process : 4-5,

For constant pressure process : 3-4,

Mr. K. M. Joshi Assi. Proffesor, MED SSAS Institute of … and... · Deviation of Actual Cycles...

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