What is Air standard cycle?
1] The air standard cycle is a thermodynamic power cycle that converts heat energy into mechanical power by considering air as a working fluid with constant specific heats (Cp & Cv).
2] In this cycle, the air is the only working fluid that goes through changes in its thermodynamic properties to convert heat into mechanical power.
3] In this cycle, there is no internal energy generation by means of burning any fuel inside of the engine.
4] The heat is extremely added by means of a heat exchanger & also it is rejected by using heat exchange to the sink.
5] It is consists of a non-flow process. It means that a certain amount of air is continuously performing successive cycles. No intake or exhaust of air is permitted into the cycle.
6] Hence it works as a closed cycle with no mass transfer. Working fluid air is considered an ideal gas with constant specific heat.
7] Following are examples of air standard cycle:-
- Carnot cycle
- Otto cycle or Petrol cycle
- Diesel cycle
- Air standard dual cycle
- Brayton cycle etc.
Air standard cycle assumptions:
The Air standard cycle assumptions are as follows:-
- Air is considered as working fluid.
- Air has constant specific heat.
- Air behave as ideal gas.
- All processes in cycle as internally reversible.
- For air Change in K.E.= 0, Change in P.E.= 0
- No internal heat generation.
- heat is added & rejected externally.
- The cycle is closed & processes are non-flow processes.
- There is no frictional losses.
Important formulae:
1] Thermal efficiency:-
Air standard cycle thermal efficiency is given by
`eta _{th}= frac{W_{Net}}{Q_{s}}`
Where
`W_{Net}` = Net work = `W_{\text{Turbine}}-W_{\text{compressor}}`
`Q_{s}` = Heat supplied
2] Net work ratio (`WR_{Net}`):-
It is given by,
`WR_{Net}=frac{W_{Net}}{W_{\text{Turbine}}}`
`WR_{Net}` = `frac{W_{\text{Turbine}}-W_{\text{compressor}}}{W_{\text{Turbine}}}`
`WR_{Net}=1-frac{W_{\text{compressor}}}{W_{\text{Turbine}}}`
3] Back work ratio:-
It is given by,
`WR_{Back}=frac{W_{\text{compressor}}}{W_{\text{Turbine}}}`
Difference between Air standard cycle and Actual cycle:
| Sr. No. | Air standard cycle | Actual cycle |
|---|---|---|
| 1 | Air is working fluid. | A mixture of air and fuel is a working fluid. |
| 2 | Air behaves as the ideal gas. | Working fluid does not always behave like an ideal gas. |
| 3 | Specific heat of air is constant. | Specific heats are variable with respect to temperature. |
| 4 | Processes are non-flow processes. | Working fluid flows continuously. |
| 5 | No internal heat generation. | Generation of energy by burning of fuel with air. |
| 6 | Heat added externally. | Energy is generated inside of the engine. |
| 7 | No frictional losses. | Frictional losses are present. |
| 8 | This cycle is completely closed. | Because of the inlet and exhaust process, the cycle is not closed. |
