17 July 2015, ©. Leeham Co: It is summer in south of Europe and we have had over 30°C/86°F for weeks. It makes one realize the conditions where the engines have to work over their flat rating point in the Middle East.
Aircraft engines are a bit fidgety. They don’t like temperature although they are made to sustain that their hottest parts, the nozzle and first turbine after the combustor, gets scalded to 1700°C/3,092°F or more.
Go down to the very back end of the engine and we come to where the key engine parameter, EGT (Exhaust Gas Temperature), is measured. It determines a lot of things, among them the time the engine stays on wing. Things are typically 700°C/1,832°F cooler here and this is where a reliable temperature measurement probe can be placed. Based on its values, the total health of the engine’s core is determined. It is also a key input whether the engine shall be throttled back in a hot take-off like in the Middle East.
Whereas directly after the combustor we would talk degrees and tens of degrees in terms of engine parameters, at the end of the turbines we talk about fraction of degrees. These fractions determine whether the engine will continue to be used or it has to be taken off for service.
We talk about the EGT deterioration rate for the engine.
Imagine that at EGT redline, let’s say 1,000°C (engine EGT is given as Celsius world-wide), it is finished. The engine can no longer be started and asked to perform yet another take-off. It will be reported as used up and ready for service, often a hot part performance restoration overhaul. It will bring back most of the EGT margin that we had on the engine when new. Typically a new engine has a margin to the point where it can’t stay on wing anymore of around 50°C for a long haul engine and 100°C for a short haul engine.
The engine’s wear each time it is cycled through a take-off (which is the real stress of an engine) is determined to a fraction of a degree. Turbofans typically wear fast the first 1,000-2,000 take-offs after service or new, chewing up 20 of the degrees they had as EGT margin, then they settle down, chipping along at 0.5°C per 100 take-offs. This means we have typically 6,000 cycles before a long haul engine is due, and 16,000 cycles or more before our short haul engine has reached red-line EGT.
Those that work with airline engines knows things are a bit more complicated than this but “grosso modo” this describes the life of an engine, small or large, short haul or long haul. I thought it might be fun to know that it is all down to fraction of degrees in this thing which is running at least twice the melting point of most of its internals.
The EGT also determines if the engine will deliver full power at a hot take-off. Normally engines keep their performance up until a certain external temperature, then the engine computer start to throttle back the fuel it injects in the combustor and we are past the flat rating point or the corner point. This is typically the 30°C/86° F we experience now, meaning most airlines don’t get full take-off thrust from Nice Airport these days.
Engines can be given “thrust bumps.” This means that for a very limited amount of time, typically minutes to allow a take-off to be performed, the engine computer can allow a higher corner point than normal. Let’s say it starts to throttle back the engine at 40°C instead.
This does not come for free. It increases the EGT wear rate, so instead of the normal 0.5 degree per 100 cycles we might see closer to one degree. That would half the time on wing of the engine and this is why Tim Clark, president and CEO of Emirates Airline, says “we don’t do thrust bumps;” he wants his engines on the wing for the full 5,000-7,000 hours or whatever is standard from the engine OEM.
The engine EGT margin is also what gets squeezed when one specifies a stronger CFM56 or IAE V2500 for one’s 737 or A320, a so called “throttle-push” engine variant. For a “throttle-push” variant the OEM simply modifies the parameters in the engine computer to inject more fuel in the combustor, thereby spinning the engine faster. As it is a rotating air pump it will push more air out the back at higher speed, voilà more thrust!
But once again nothing is for free in this world. Example: a CFM56-7B27 for Boeing’s 737-800 or -900 with 27,000 lb thrust can have an EGT margin when new of around 50°C. Specify the 26,000 lb variant instead and you get 75°C or the 24,000 lb variant (more for the -800) and you have a nice 100°C to chip away at. The deterioration as described is still the same, “grosso modo,” 20 degrees initial wear for the first 2,000 cycles and then five degrees per thousand. So after the first 2,000 cycles, you have another 6,000 on the stronger engine or 16,000 on the smaller. In reality it might a little bit different but it makes one understand why higher thrust is not always better; rather specify what we need but not more.
Other things getting hot
It is not only aircraft turbofans that are fidgety. Solar Impulse 2 is grounded for nine months at Hawaii because its batteries got too hot. The amount of solar energy that gets captured by its solar cell-laden wings and fuselage varies and it has therefore a large pile of high performance batteries to store energy when sunny and use when not so sunny. It will also use energy from the batteries for power peaks like take-offs or climbs.
When taking-off and climbing from Nagoya on the way to Hawaii, these batteries were working hard and got too hot. They thereby destroyed themselves so they now have to be replaced. At the same time the Solar Impulse team realized they had not calculated right when it came to how to cool and use the batteries. They will figure out how to modify that and then repair Solar Impulse 2 before they fly again. It takes nine months but this is the way progress works. Pioneers take the leap and learn, then leap again. Kudos to those that dare the leap.