The ideal gas law represents the first-order approximation to the P-T-V relationship of
a gas mixture when 
and 
. At high gas pressures, secondary effects such as finite molecular volume
and intermolecular attractive forces become important and a real-gas equation of state must
be used to properly include these real-gas effects. The Ansys Chemkin gas library adopts
the cubic equation of state to address the real-gas effects associated with high-pressure
combustion applications.
The cubic equations of state are widely adopted by combustion and CFD communities due to their simplicity. The general form of the cubic equation of state can be expressed as
 | (2–58) |
In the above equation, 
is the molecular volume in cm3/mole. Five
different models of the cubic equation of state are available in Ansys Chemkin: van
der Waals, Redlich-Kwong, Soave, Aungier, and Peng-Robinson. The model parameters
a,b, c, u, and w
are functions of critical temperature 
, critical pressure 
, and the Pitzer acentric factor 
; and their expressions are given in Table 2.1: Parameters of the cubic equation of state models
.
Table 2.1: Parameters of the cubic equation of state models
| Cubic EOS | u | w | a | b | c |
| van der Waals | 0 | 0 |
|
| 0 |
| Redlich-Kwong | 1 | 0 |
|
| 0 |
| Soave-Redlich- Kwong | 1 | 0 |
|
| 0 |
| Aungier-Redlich- Kwong [12] | 1 | 0 |
|
|
|
| Peng-Robinson | 2 | -1 |
|
| 0 |
