April 14, 2017, ©. Leeham Co: We’ve been talking engines on Fridays since October 2016. The Corners covered several areas, from technologies to operations.
And we could go on and dig deeper. But we will move on.
Before we go, we sum up what we have learned in the 24 Corners around airliner Turbofans.
We started with the technology challenges an aircraft Turbofan presents. We asked: Why is it harder to do an engine than an aircraft? That this is true is clear. United Aircraft and COMAC are creating new single aisle and wide-body aircraft with own technology. But all aircraft use Western engines. Point proven.
Why so hard? Because engines present a multitude of technological challenges. They have complicated aerodynamics and require advanced material technology to master the heat and the challenging sealing problems they present.
Even experienced Western manufacturers are fighting to get it right, as witnessed by Pratt & Whitney’s combustor and sealing problems at the introduction of the GTF Turbofan for the Airbus A320neo.
Before we looked at the engine’s different parts we went through the engine cycle. It’s not unlike our normal car engine, but it’s a continuous cycle rather than a discrete one.
The similarities are larger than one thinks. For example, the car engine’s compression ratios and the turbofans OPR are very similar.
Figure 2 shows the typical car engine compression ratios (volume ratios, with Petrol engines at ~10 and Diesels at ~16). When we convert these to pressure ratios, we are right where the modern turbofans operate.
We also described the basic thrust equation:
Thrust = air mass flow * air over-speed relative to surrounding air
The lower the air over-speed, the higher the efficiency, but also the lower the thrust. So, if we design with low over-speed, we need lots of air mass flow.
This creates problems for the turbofan’s fan section. It grows large, as it shall give a low acceleration to a large air mass. The large size creates weight problems, as the fan is large and surrounding casing must be built strong. It must contain a break-off blade. Advanced techniques like hollow/laminated metal or CFRP (Carbon Fiber Reinforced Polymers) blades are used to master the weight problems.
Compressors are all about pressure ratio gain per stage, while keeping the process stable when the rotating speed varies between full RPM to idle. When the RPM and blade speed decreases, the blades approach stall. Bleed and variable stator vanes come to help.
The combustor is about a stable flame. Therefore, we diffuse the air to M0.2 from the typical M0.5. The flame must be stable even when contamination particles enter the flame. If not, it will eat the combustor liner. The combustor and fuel nozzle are also about low emissions. NOx is the most challenging emission, as it increases with increased OPR.
Turbines are easier to get stable than compressors because the air is flowing to lower pressures (stall is less probable). But it’s very hot. The gas temperature is way hotter than the melting point of even the most exotic high temperature alloy. It’s time for advanced cooling and coatings to do their job, keeping the rotational stresses from stretching the blades.
Finally, the nozzle accelerates the air to the correct speed before leaving the engine (figure 3). If we fly at M0.85 and the air is leaving the engine at M0.5, we have created a brake instead of an engine.
Then we covered how engines are used. We gradually created the table in Figure 3. It shows the most stressful moments for the engine as the take-off and the following V2 phase at ISA +15°C ambient conditions.
The critical Stator Outlet Temp T41 (second red row) is at its highest at these mission points (over 1,300°C). To lower the stress, airlines apply de-rate when they don’t need full thrust. This means less thrust is demanded from the engines during take-off and the temperatures go down.
The engines are kept fit for purpose despite the stressful life by advanced surveillance techniques. Operational data is continuously fed to the airline’s maintenance department and often directly to the engine OEM. By continuous trend watch of critical parameters and visual inspections with Boroscopes, the engines can be maintained “On condition.”
This means they are only taken off wing for overhaul visits, when some parameter like EGT (Exhaust Gas Temperature) runs too high during take-off or some other parameter says “Stop, time for a shop visit.”
Shop visits run from hundred thousands of dollars into the millions, depending on what needs restoration. For large engines, the cost for shop visits pass $10m. To keep these costs under control, the overhaul market develops OEM independent shops when enough engines are out of OEM warranty and initial power-by-the-hour agreements.
We now know how the engines are kept fit for purpose. It can be time to understand how the rest of the aircraft is kept top-notch as the next step.