# Is overall pressure ratio same as compressor pressure ratio for a gas turbine engine?

• For a typical gas turbine engine, turbofan in particular, is the compressor pressure ratio same as the overall pressure ratio?
• For modelling purposes can we use the same value of overall pressure ratio for compressor pressure ratio too?
• If we do so, will we get accurate results for performance?

## 2 Answers

Modern turbofan engines typically have several stages of compression, the overall pressure ratio is the ratio of the highest pressure to the intake pressure. But the pressure ratios of each compressor can be a different.

The fan and booster add a bit of compression, followed by an optional intermediate pressure compressor and finally the high pressure compressor.

Because of these multiple shafts and therefor having some parts of the compressors not working on their design point you probably won't get accurate results by simulating a single stage compressor.

• The only 3-spool engines I know are the large Rolls Royce engines. All GE and I believe P&W are dual-spool and don't have an intermediate compressor and turbine. Mar 18 '19 at 15:06
• That is true, I mixed up the booster and intermediate compressor there.
– Jan
Mar 19 '19 at 17:29

The overall compression ratio is known as OPR.

Wikipedia states the definition very well:

overall pressure ratio, or overall compression ratio, is the ratio of the stagnation pressure as measured at the front and rear of the compressor of a gas turbine engine

But, should read … as measured at the front of the first compression process and the exit of the last compression process…

The compression ratio of a compression process (where mechanical work is used to increase the pressure) is known by PR, generally using a prefix. Compressor Pressure Ratio, CPR, or Fan Pressure Ratio FPR.

The overall pressure ratio is thus calculated by multiplying the individual ratios.

$$OPR= FPR \times IPR \times CPR$$

For a typical gas turbine engine, turbofan in particular, is the compressor pressure ratio same as the overall pressure ratio?

No, but the OPR can be the PR in simple gas turbine configurations known as turbojet engines if there is a single compressor. These types of engines are uncommon these days. Today’s engines (turbofan engines) have some sort of a bypass raio to increase the efficiency of the cycle (large bypass for commercial/transport flight, lower for fighter aircraft or business jet engines).

Increasing the OPR is done with increasing turbine inlet temperature (TIT) to allow higher efficiencies while enough fuel can be added to the cycle to get enough power to propel the aircraft (either through the fan, accelerating much air by little, or accelerate few air by a large amount).

For modelling purposes can we use the same value of overall pressure ratio for compressor pressure ratio too?

No, in assessing cycle calculations for gas turbine designs (design exploration studies) to come we generally work with the OPR using constraints on the intermediate and high pressure compressors. We generally choose a FPR and an OPR and calculate the needed HPC pressure ratio.

Dr. N. Cumpsty has done some excellent work in this respect.

Turbofan generations are typically characterized by FPR and bypass ratio, OPR and TIT. In order for your calculations to make sense you need to pick/calculate/use the correct values. Increasing bypass ratio and decreasing FPR cannot be unlimited done without altering the OPR and TIT