Turbofan engines work on the principals of compression, combustion, and expansion. The incoming air passes through a series of compressor stages (blades and vanes), where the air is compressed. As the air velocity decreases, the pressure and temperature of the air increases.
The compressed air is mixed with pressurized fuel, and ignited in the combustion chamber. The hot gasses expand, and pass through a series of turbine stages (blades and vanes), before exiting through the exhaust. During the process, the exit velocity becomes greater than the free-stream velocity, which generates thrust and propels the engine (and the aircraft) forward.
Engine Pressure Ratio (EPR)
One of the measures of the engine performance is the amount of thrust it produces. The amount of thrust is directly proportional to the engine pressure ratio of the air entering the engine to the air exiting the engine.
Pressure sensing probes are installed at the engine inlet, as well as the turbine exhaust. A differential pressure transducer calculates the ratio between the two pressure readings, before transmitting the data to the cockpit monitors (ECAM for Airbus, and EICAS for Boeing).
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 inlet pressure. An integrated signal conditioner calculates the IEPR, and transmits the data to the cockpit monitors.
Based on the position of the thrust lever, directed by the pilots, the resulting EPR or IEPR is displayed. The higher the operational thrust, the greater the internal air pressure.
Exhaust Gas Temperature (EGT)
Another measure of jet engine performance is the EGT, sometimes referred to as the Turbine Outlet Temperature (TOT). The EGT is a measure of the temperature leaving the exhaust of the turbine. Multiple thermocouple probes are installed at the exhaust of the turbine (usually past the high-pressure turbine), and the data is transmitted to the cockpit.
In order to determine the on-wing time and the health of the engine, the EGT Margin is calculated. It is the difference between the incurred takeoff EGT and the Redline (maximum limit) EGT. Newer engines have a higher EGT Margin compared to engines that have been on wing for longer periods of time. As the EGT Margin decreases, the specific fuel consumption (SFC) of the engine increases.
As the number of cycles increase on an engine, the thermal and centrifugal stresses cause wear on the turbine blades. The gap between the tips of the blades and the turbine cases begin to grow. As a result, the engine runs at a relatively hotter temperature with a higher operating EGT.
Photo: David Monniaux via Wikimedia Commons
One of the common reasons for an inefficient engine is the notching on the high-pressure turbine (HPT) blades. The CFM International states that a single notch, of approximately 0.03 inches (0.075 cm), on the tip of an HPT blade of a CFM56 engine, can reduce the EGT by 10 degrees Celsius. As a result, the SFC of the engine increases by roughly 0.7%.
One way to minimize time between scheduled engine removals is to configure takeoffs at reduced thrust ratings, where possible. The EGT Margin is a key performance parameter that the MRO service providers aim to minimize during engine overhauls.
What are your thoughts on the various parameters of the jet engine performance? Tell us in the comments section.