Bjorn’s Corner: The Engine challenge

 

By Bjorn Fehrm

By Bjorn Fehrm

October 21, 2016, ©. Leeham Co: In our Corners on East bloc aeronautical industries, we could see that the hardest part to master in a new civilian airliner is the engine.

Both new airliners from Russia and China (Irkut MC-21 and COMAC C919) start their lives with Western engines.

Why is this so? What are the challenges that make engines harder to create than aircraft?

leap-1c-engine

LEAP-1C which will be standard engine on COMAC C919. Source: COMAC.

We will spend several Corners on the main reasons that airliner engines are harder to do than aircraft.

The technology challenge

A modern high-by-pass turbofan poses a combination of technical challenges to its designers that are exceeding in many ways those presented to civil airliner designers.

Aerodynamics

For starters, the aerodynamics are more complex. An airliner design presents subsonic and transonic aerodynamic problems. These are relatively straight-forward to model in CFD (Computational Fluid Dynamics) and to measure in windtunnels.

A modern high by-pass turbofan mixes subsonic, transonic and supersonic aerodynamics on a single fan/compressor blade. And the blades are running close to hard objects affecting the airstream, such as a fan case or within a stator structure for the compressor.

Add to that, that the blades are so closely spaced that their aerodynamics interact. It’s like flying a Baron von Richthofen Fokker Dr. I triplane through the sound barrier and cater for the interacting shock waves from each wing. As they are staggered, the interaction is not symmetrical.

Materials

Secondly the material stresses are higher. Not only shall a turbine blade endure 1,700 °C/3100°F but it shall do it at 1,7 MPa/250 Psi pressure and at a centrifugal force of 40,000N/ 9,000lbf for a 0.1kg turbine blade.

It shall be considered that the melting point of the metals in the engine is below these temperatures, let alone the point of plastic deformation. Hence it’s all about exotic alloys, sophisticated cooling schemes and a very special mechanical design.

Seals

But the challenges don’t stop there. The pressure rise over the compressor stages are in the order of two times. This means the air will find every little passage way to flow back. Efficient rotating seals must be invented, seals that can withstand rotating at 10-20,000 rpm in the harsh conditions for 10 years and 20,000 missions.

Seals must also be designed to keep oil where it should be, in the bearing areas. If oil seeps into adjacent hot areas, the catastrophe is programmed, as seen for the Qantas Airways Airbus A380 climbing out from Singapore or the Bombardier CSeries ground testing its engine.

Control

Finally, is must all function in harmony, from the coldest day (-40°C) to the hottest (+50°C), from M0.00 to M0.85 and from idle to full thrust. Today, all this is controlled by a redundant set of control computers in the so called FADEC (Full Authority Digital Engine Control).

The challenge for the control is not to make the control program. It’s to find measurement methods for the parameters that one need for the control. There is no way to reliably measure the perhaps most critical parameter in the engine, the inlet temperature of the first turbine stage after the combustor.

Sensors that operate at 1,700C and 1,700 000 Pa for years and measure the temperature on an accuracy of degrees, in a vibrating environment with aggressive gases passing by at M0.5 are not available.

Therefore, control of the engine is done via indirect sensors like EGT (Exhaust Gas Temperature sensor) which sits at the end of the engine. Part of the development of the engine is to establish the reliable relationship between the parameter that you can measure and the one you wanted to measure.

A look in more detail

In subsequent Corners, we will look at these problem areas in more detail. We will do that as a journey through a typical high by-pass turbofan.

We will use the GasTurb engine modeling program to give us the typical data for a modern engine and we will point out the technological challenges the different engine parts pose.

We will also cover what is the present state of the art for the area and what the engine manufacturers are working on for future engine generations.

18 Comments on “Bjorn’s Corner: The Engine challenge

  1. A nice introduction to the challenges. Nicely done as usual Bjorn. You could do a whole corner on FADEC’s alone which I suspect u might. I would look forward to it.

  2. This was a nice summary. As an ex-aero engine engineer I think this sums it up nicely.

    I am often baffled why there are so many airplane nerds, but relatively few engine ones when the engine is where it happens.

    /mneja

      • It is easier to talk about seat pitch than to discuss a p-v diagram.

        Fuselage design dating back to the 367-80 era is still in mass production powered by the latest engine designs – well, not the bigger ones due to short legs.

  3. Excellent introduction! Perhaps worth noting that on older engines the EGT was the temperature at entry to the exhaust nozzle. With modern engines the actual temperature is measured in between the HP turbine and the LP turbine and was frequently called ITT for inter turbine temperature. The turbine inlet temperature can then be deduced from the temp drop required to drive the HP compressor and the ITT. The actual temperature at the hot nozzle exhaust will be much lower than the EGT, because of the large temp drop needed to power the high bypass ratio fan. Due to tolerance buildup individual engines will have slightly varying EGT, but will have a specified EGT margin which will deeriorate in service until the engine must be removed for maintenance.

    • ‘because of the large temp drop needed to power the high bypass ratio fan”?
      Where does the heat go for that temperature drop ? I get that the temperature is increased by burning fuel to the compressed air which raises its energy level and that energy is then taken out by powering the turbine sections as rotational energy. The turbine blades have to be internally cooled which takes away some heat, but only some appears to be hot exhaust ?

      • The heat is transferred into volume or pressure increase. That increase drives the engine. Just like in diesel engine. The gas is ignited by the compression but than the explosion is expanded and transferred into work.

        Thermodynamics is not an easy business. Check P-V-diagram at wikipedia for more insight.

      • A pressure drop at different inlet temperatures produces different amount of work “h”. Hence the hotter the mix of combusions gases and cooling air at constant pressure the more work you can get out of the turbine. Just follow the consatnt pressure curves in the h-s diagram. There is a limit in turbine inlet temp/pressure when jonisation and other catalytic reactions starts stealing energy.

  4. Thank you Bjorn, I particularly loved the

    “It’s like flying a Baron von Richthofen Fokker Dr. I triplane through the sound barrier and cater for the interacting shock waves from each wing. As they are staggered, the interaction is not symmetrical.”

    I too will follow the rest with a great deal of interest.

  5. I am not an gas turbine engineer, but I always wonder if the shorter engines (RR 3 shaft designs?) were stiffer and therefore had better seal and bearing wear characteristics? It’s one thing to have to deal with big temperature differentials and changes, but to have to deal with the engine’s structure twisting and distorting too as the plane manoeuvres has got to be harder.

    Does that show up in servicing costs at all?

    • They do. But the three spool design is very cramped so other compromises need to be made. The resulting design is more complex than a two spool design and is more difficult to maintain, hence mx costs are a little higher.

      But the increased stiffness due to the reduced length is very welcome as is the lower weight. Check Jane’s for weight numbers for GE90-94B and Trent 800 for example.

      • Thats the first time I have heard a 3 spool described as ‘more crowded’ compared to 2 spool. They are all high thrust jet engines, everything is there for a reason, its always crowded.

        • RR makes 3-spool engines with more parts, lots comes from good practice over the years like non-torque carrying extra shafts under bearing races and very solid fire protection. GE’s early CF6-series detoriated performance more than RR 3-spool competing engines before the mighty GE90, but now GE engines are pretty perfect symmetrically built with little performance degradation. RR must pull the string harder nowdays to compete against the GE90’s, GE9X’s and similar built LEAP-X’s and pay thru their noses for all replacement parts in their Trent power by the hour programs. One can call the Trent700 a good 3-spool evolution of the CF6-80C2 as RR was involed in designing the -80C2 and GE did design work on the RB211-535. Now we’ll see if history repeat itself for the Boeing MoM aircraft.

  6. Rolls-Royce says it does and I have had no two shaft manufacturers saying otherwise. They key is, how much does one gain?

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