January 27, 2017, ©. Leeham Co: In the last Corner, we began looking at the in-service operation of a Turbofan. We covered how thrust and fuel consumption varies in the different phases of an airliner’s mission.
Now we will dig a little deeper into how a mission will stress the engine’s different parts.
With this knowledge, we will later look at how operators make sure their engines are safe and in good operational condition over the 20 years life of an aircraft.
In the last Corner, we showed data from a simulated Airbus A320 misson with a CFM56-5B4 engine. The professional simulation software GasTurb tabulated what happens inside the engine during Take-Off, Climb and Cruise.
In Figure 2, we have expanded the table to show more in-depth what is happening inside the engine. It’s about 10% of the simulation output from GasTurb, but the additional data is for engine designers, not for us who want a general understanding what is going on.
It might seems like an lot of data, but we take it in steps and discuss each row and what it shows.
At the top blue section, we have as before our key mission data. We added the bleed air the aircraft needs for the air conditioning and to keep the cabin pressure. A single-aisle aircraft like the A320 consumes around 1lb/s an engine, taken from the engine’s compressor as described before. We have also 67hp power off-take to drive the aircraft’s hydraulic pumps and electrical generators mounted on the engine’s peripheral gearbox.
Both these off-takes drain the engine of energy and often the air supply is shut off for take-off so the engines can focus on delivering thrust.
The next additions are Engine mass flow (lb/s) and Specific thrust (ft/s). Specific thrust is another name for the air mass’ overspeed relative to ambient air. We can see the air is traveling on average at 1020ft/s at Take-Off. This is close to the speed of sound at 1126ft/s on a standard day. Turbulence created by high airspeed is the primary source of noise from Jet engines.
As engines are made with higher bypass ratios, the mass throughput increases and the specific thrust decreases for the same thrust. This makes high bypass engines less noisy.
Next, we have temperatures in the engine. Just before the air enters the combustor, it’s compressed to 28 times the value at the inlet. This increases the temperature to close to 600°C (T3, Figure 1). The last stages of the compressor must therefore be made with high temperature resistant Nickel alloys as discussed before.
The turbines are the power generators in the engine. The more fuel which is injected in the combustor, the higher the entry temperature to the turbines and the more energy there is in the gas that drives the turbines.
T41 is the temperature entering the first turbine rotor. 1,300°C is a lower value than the high pressure turbine can sustain in a CFM56-5B (around 1,400°C is max). The 5B4 version is a “derated” version of the strongest variant, the 5B3 at 32klbf.
The engine control computer, the FADEC, cuts of fuel supply earlier on the B4 version than the B3. Therefore the B4 runs cooler. The result is the engine does not spin to maximum allowed RPM on the low and high speed shafts during max thrust periods (5,200 and 15,200 are the maximum allowed RPM on 5B3). This subjects the rotors and blades to less mechanical stress. Combined with lower temperatures, this results in an engine that can stay longer on wing before needing an overhaul.
It’s difficult to measure the temperatures around the combustor and high pressure turbine. Thus, engines are often controlled via the temperature around the Low Pressure Turbine (LPT), T45, or even further back like T49.5 (last LPT stage) for the CFM56. The temperature range is here around 1000°C and it’s called the Exhaust Gas Temperature, EGT.
When an engine deteriorates, the FADEC will compensate deteriorated performance by injecting more fuel to restore performance. This raises the EGT. Gradually it will come close to a limit which triggers a scheduled overhaul.
Figure 2 shows that the engine endures highest mechanical and thermal stress during Take-Off power at V2, when ambient temperature is 15° higher than normal ISA temperature. The net thrust is only 22klbf, but the engine is working hard. It’s developing 30klbf Gross thrust and working with the highest temperatures and RPMs during the mission.
The forward speed of M0.25 is taking away 8klbf of thrust and the high ambient temperatures is thinning the air, so mass flow goes down. The engine therefore increases RPM to increase specific thrust to compensate. This increases temperatures through the engine.
The V2 Take-Off thrust needed when one engine goes inoperative is often the dimensioning case for mechanical and thermal margins in the engine.