Engine Pressure Ratio?

Question

Could somebody please explain to me what EPR is, how does it apply, and what the number means? (1.0, 1.8?) Maybe how it realtes to a P&W TF-33 engine?

I mean relates.

Answer 1

The ratio of pressure in the front of the engine to the air exiting the back. EPR is used to set engine power.

There are two probes, the front one called PT2 and the back one is PT7. PT is Total pressure and the number is the engine station (2 being the front of the first stage commpressor/fan and 7 being just behind the last turbine stage). These pressures are brought into the EPR gauge and compared to produce a reading.

1.00 is no thrust and what is read when the engine is shut down.

Readings vary from engine to engine. The JT8D on the 727 had a typical takeoff setting of 2.00 to 2.02. The RB211 is like 1.55, even though it produces way more thrust.

Large fan engines seem to have lower EPRs since the extra turbines for the large fan lower the pressure in the exhaust. Also Rolls Royce uses IEPR (Integrated Engine Pressure Ratio) that take pressure readings from the fan discharge as well.

The numbers will be aircraft/engine specific. Fuel flow is also used to determine engine power settings.

N1 is used on certain highly automated engines (only the CFM56 and the TFE 731 as far as I know) because it doesn't account for air density changes (100% N1 at sea level is very different that 100% N1 at 40,000 feet).

Answer 2

Engine Pressure Ratio

Answer 3

Another excellent answer Steve and others.

So all I have is some questions for the other answers.

Frank, which airplane has N1 as the High pressure compressor? All multi-spool engines that I have ever worked on or with, N1 is always the low pressure section. Specifically the aircraft I fly now is equipped with TF-33-100As and N1 is the low pressure compressor. Who gave you your ATP? lol. Also, how do you "overboost" a turbine engine? There is no maximum EPR limit. You could overspeed the engine, or overtemp it. That term "overboost" comes from turbocharged engines.

Depending on what regime of flight we are in, we use EPR, N1 and even Fuel Flow for power setting.

Answer 4

Jet engines have pressue sensors mounted at various places inside. As air travels through the compressor stages of an engine, its pressure increases. Take the measurements from the inlet and exhaust and divide them to obtain the pressue ratio. A gauge displays the ratio to the pilots.

On aircraft with older engines, the pilot would set the power of the engine by setting a specific pressure ratio.

Modern engines are set differently - by the use of % N1. N1 is the rotational speed of the compressor, so 98% N1 means the compressor is rotating at 98% of its design maximum.

Answer 5

EPR is the ratio of the total pressure of the exhaust flow (static + dynamic) to the same pressure in the engine inlet for a gas turbine. It is proportional to the thrust and therefore a good indicator of engine output.

1.0 is essentially zero thrust.
1.8 is fairly high thrust, depending on the engine. Don't have any specifics for the TF-33, but you will likely find it in the flight manual.

Air Florida flight 90 crashed at DCA in January of 1982 due in part to having an iced-over inlet pressure probe which caused an EPR reading of 2.04 when the actual EPR was only about 1.70.

See: http://www.airdisaster.com/special/special-af90.shtml

Answer 6

EPR is the ratio between exhaust pressure (jet blast) and inlet (static) pressure on a turbo-jet or turbo-fan engine. This tells the pilot how much power the engines are generating. EPR= exhaust pressure divided by inlet pressure.

The higher the EPR the higher the engine thrust.

EPR is used to avoid over-boosting an engine (like the P&W TF-33) and to set Takeoff and Go-around power if needed.

Not all turbines use EPR, some like the Rolls Royce Spey used P7 pressure (exhaust only).

Turbo-Props use torque to meassure engine power.

Newer Turbo-Fan engines use N1 or the percentage of rotation of a (high-pressure) compressor stage.

Answer 7

This Site Might Help You.

RE:
Engine Pressure Ratio?
Could somebody please explain to me what EPR is, how does it apply, and what the number means? (1.0, 1.8?) Maybe how it realtes to a P&W TF-33 engine?

Answer 8

Compression ration. The air mixture inside the cylinder is compressed 8.5 times. This is accomplished by the movement of the piston from its lowest position to its highest position thus "squashing" the air. Eric