Topics: Thermodynamics, Heat engine, Energy Pages: 4 (972 words) Published: March 14, 2013
5 Gas Cycles

Heat engine or thermal engine is a closed system (no mass crosses its boundaries)that exchanges only heat and work with its surrounding and that operates in cycles.

Elements of a thermodynamic heat engine with a fluid as the working substance:

1. A working substance, matter that receives heat, rejects heat, and does work;

2. A source of heat (also called a hot body, a heat reservoir, or just source), from which the working substance receives heat;

3. A heat sink (also called a receiver, a cold body, or just sink), to which the working substance can reject heat; and

4. An engine, wherein the working substance may do work or have work done on it.

A thermodynamic cycle occurs when the working fluid of a system experiences a number of processes that eventually return the fluid to its initial state.

Cycle work and Thermal Efficiency

QA = heat added


QR = heat rejected

QRW = net work

Available energy is that part of that the part of the heat that was converted into mechanical work.

Unavailable energy is the remainder of the heat that had to be rejected into the receiver (sink).

The Second Law of Thermodynamics

All energy received as heat by a heat-engine cycle cannot be converted into mechanical work.

Work of a Cycle

a) W = ∑Q

W = QA + (-QR)(Algebraic sum)

W = QA - QR(Arithmetic difference)

b) The net work of a cycle is the algebraic sum of the works done by the individual processes.

W = ∑W

W = W1-2 + W2-3 + W3-4 + . . .

The Carnot Cycle

The Carnot Cycle is the most efficient cycle conceivable. There are other ideal cycles as efficient as the Carnot cycle, but none more so; such a perfect cycle forms a standard of comparison for actual engines and...
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