How is the performance of aircraft engines measured?



Turbofans work on the principles of compression, combustion and expansion. Incoming air passes through a series of compressor stages (blades and blades), where the air is compressed. As air velocity decreases, air pressure and temperature increase.

Compressed air is mixed with pressurized fuel and ignited in the combustion chamber. The hot gases expand and pass through a series of turbine stages (blades and vanes) before exiting through the exhaust. During the process, the output velocity becomes greater than the freestream velocity, which generates thrust and propels the engine (and aircraft) forward.


Engine Pressure Ratio (EPR)

One of the measures of engine performance is the amount of thrust it produces. The amount of thrust is directly proportional to the engine pressure ratio between the air entering the engine and the air exiting the engine.

Pressure sensing probes are installed at the engine inlet, as well as at the turbine exhaust. A differential pressure sensor calculates the ratio between the two pressure readings, before transmitting the data to the cockpit monitors (ECAM for Airbus and EICAS for Boeing).

ECAM Jumbero via Flickr

In some engines, an integrated engine pressure ratio (IEPR) is measured for engine performance. The IEPR is the ratio of the sum of the hot stream exhaust pressure and the cold stream bypass duct pressure to the engine manifold pressure. An integrated signal conditioner calculates the IEPR and transmits the data to the cockpit monitors.

Depending on the position of the throttle, directed by the pilots, the resulting EPR or IEPR is displayed. The higher the operational thrust, the higher the internal air pressure.

Exhaust Gas Temperature (EGT)

Another measure of jet engine performance is EGT, sometimes referred to as turbine outlet temperature (TOT). EGT is a measure of the temperature exiting the turbine exhaust. Several thermocouple probes are installed at the turbine exhaust (usually after the high pressure turbine) and the data is transmitted to the cockpit.

CFM56 installed Bidgee vs Wikimedia Commons

In order to determine flight time and engine health, the EGT margin is calculated. This is the difference between the incurred take-off EGT and the Redline EGT (maximum limit). Newer engines have a higher EGT margin than engines that have been in flight for longer periods. As the EGT margin decreases, the specific fuel consumption (SFC) of the engine increases.

On-wing performance

As the number of cycles of an engine increases, thermal and centrifugal stresses cause the turbine blades to wear. The gap between the blade tips and the turbine casings begins to widen. As a result, the engine runs at a relatively hotter temperature with a higher operating EGT.

CFM56 David Monniaux via Wikimedia Commons

Photo: David Monniaux via Wikimedia Commons

One of the common reasons for an inefficient engine is notching on the high pressure turbine (HPT) blades. CFM International states that a single notch, approximately 0.03 inches (0.075 cm), on the tip of a HPT blade of a CFM56 engine, can reduce EGT by 10 degrees Celsius. As a result, the SFC of the motor increases by approximately 0.7%.

One way to minimize the time between scheduled engine pullbacks is to configure takeoffs at reduced thrust rates whenever possible. EGT margin is a key performance parameter that MRO service providers aim to minimize during engine overhauls.

What do you think of the various jet engine performance parameters? Tell us in the comments section.

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