The working fluid of heat engines is subjected to a series of changes (in temperature, volume, pressure, etc.), known as thermodynamic air standard cycles, through which the energy absorbed as heat is converted into mechanical work. Internal combustion engines do not operate on a thermodynamic cycle as it involves an open system. However, it is often possible to analyze the open cycle by imagining one or more processes that would bring the working fluid at the exit conditions back to the condition of the starting point. The actual gas power cycles are rather complex.
To reduce the analysis to a manageable level, the following approximations, commonly known as air standard assumptions, are made:
- The working medium is a perfect gas.
- There is no change in the mass of the working medium.
- All the processes that constitute the cycle are reversible.
- Heat is supplied from a constant high-temperature source.
- Some heat is assumed to be rejected to a constant low-temperature sink during the cycle.
- There are no undesired heat losses from the system to the surroundings.
- The working medium has constant specific heats throughout the cycle.
- The physical properties (cp, cv, μ, γ) of the working fluid are constant.
Another assumption that is often utilized to simplify the analysis, even more, is that air has constant specific heats whose values are determined at room temperature (25°C). When the assumption is utilized, the air standard assumptions are called cold air standard assumptions. A cycle for which air standard assumptions are applicable is frequently referred to as an air standard cycle.
Throughout the study, the heat supplied and heat rejected denoted by Qs and Qr, respectively. The total work output is expressed as Wnet. The mean effective pressures are referred as pm. Mass flow rate of working fluid is ‘m’, and ‘p, v, T’ are the pressure, specific volume, and absolute temperature of the working medium with subscripts of the points in the cycle. Compression ratio (r) is the ratio of volume during the compression process, and pressure ratio is the ratio of pressures.
Mean effective pressure (pm) is defined as that hypothetical constant pressure acting on the piston during its expansion stroke producing the same work output like that from the actual cycle:
This can be determined using indicator diagram as
Important thermodynamic air standard cycles are listed below.