Question

What are the differences between air-standard analysis and cold air-standard analysis? What is the tradeoff in using air-standard analysis vs. using cold air-standard analysis?

Answer 1

Air Standard Assumptions
In power engines, energy is provided by burning fuel within the system boundaries, i.e.,
internal combustion engines. The following assumptions are commonly known as the air-
standard assumptions:
1- The working fluid is air, which continuously circulates in a closed loop (cycle).
Air is considered as ideal gas.
2- All the processes in (ideal) power cycles are internally reversible.
3- Combustion process is modeled by a heat-addition process from an external
source.
4- The exhaust process is modeled by a heat-rejection process that restores the
working fluid (air) at its initial state.

1. The working fluid is perfect gas.

2. There is no change in mass of the working medium.

3. All the process that constitutes the cycle is reversible.

4. Heat is assumed to be supplied from a constant high temperature source and not from chemical reaction during the cycle.

5. There are no heat losses.

6. The working medium has constant specific heats throughout the cycle.
Cp = 1.005 kJ /kg K ,Cv = 0.718 kJ /kg K ,? = 1.4

The actual gas power cycles are very complex, to reduce their analysis to a manageable level without significantly deviating from the actual cycles; air standard assumptions are introduced. To simplify the analysis to even easier, cold-air standard assumptions are utilized.
Assuming constant specific heats, (@25°C) for air, is called cold-air-standard
assumption.