Compression Ratio Versus Overall Pressure Ratio
Compression ratio and overall pressure ratio are interrelated as follows:
| Compression ratio | 2:1 | 3:1 | 5:1 | 10:1 | 15:1 | 20:1 | 25:1 | 35:1 |
|---|---|---|---|---|---|---|---|---|
| Pressure ratio | 2.64:1 | 4.66:1 | 9.52:1 | 25.12:1 | 44.31:1 | 66.29:1 | 90.60:1 | 145.11:1 |
The reason for this difference is that compression ratio is defined via the volume reduction:
- ,
while pressure ratio is defined as the pressure increase:
- .
In calculating the pressure ratio, we assume that an adiabatic compression is carried out (i.e. that no heat energy is supplied to the gas being compressed, and that any temperature rise is solely due to the compression). We also assume that air is a perfect gas. With those two assumptions we can define the relationship between change of volume and change of pressure as follows:
where is the ratio of specific heats for air (approximately 1.4). The values in the table above are derived using this formula. Note that in reality the ratio of specific heats changes with temperature and that significant deviations from adiabatic behavior will occur.
Read more about this topic: Compression Ratio
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