eans 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.
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.