Power cycles mean those cycles which gives us net motion or electricity from heat supplied from some source in one form or the other. All power cycles are heat engine cycles. These operate on one cycle or the other. The efficiency depends on the manner the various processes are carried out. The most efficient will consist of only reversible processes which in actual practice cannot be achieved. Thus we are far away from realty.  In gas power cycles, gas is the working substance (normally air is assumed to be working substance and cycle is assumed to be a closed cycle, In realty no cycle is a closed cycle because of fresh input and exhaust.).

It is assumed that air behaves as an ideal gas. All those processes which make up the cycle are assumed to be internally reversible (no losses of any kind, no friction, no heat transfer, no pressure losses etc.). The combustion process of the gas cycle is replaced by a heat-addition process from an outside source. The exhaust process is replaced by a heat-rejection process in order to bring the working fluid to its initial state.

Gas Power Cycles

The various gas power cycles are Carnot Cycle (Maximum Efficiency Cycle), Otto Cycle (In a Spark Ignition Engine, petrol engine), Diesel Cycle (In a Compression ignition Engine, diesel engine) and Brayton Cycle (In a Gas-Turbine). While doing the analysis of gas power cycles, P-v diagrams are used and all the gas laws are applicable in full. T-s diagrams are used to compare two cycles to know which one is more efficient. Carnot cycle is the most efficient cycle and consists of two reversible isothermal processes and two reversible adiabatic processes. However Carnot cycle cannot be achieved with gas as a working substance. It has been achieved to a larger extent in a vapor power cycle known as Rankine cycle applied to steam engines and steam turbines. Gas power cycle has been used for Otto cycle, Diesel Cycle and Brayton cycle.

Internal Combustion Engines

All the internal combustion cycles consists of suction stroke, compression stroke, power stroke and exhaust stroke. Thermal efficiency of all heat cycles is defined as (Heat supplied-Heat rejected)/Heat supplied. It has been well established by analysis that thermal efficiency increases with an increase in the average temperature at which heat is supplied to the system or efficiency decreases with the decrease in the average temperature at which heat is rejected from the system. Vapor power cycle has been applied only in steam engines under the name of Rankine cycle. It is modified Carnot cycle.