Some questions about big jet engines

Factual Questions
1

I am very familiar with the innards of automotive/truck turbochargers, and I’m wondering how they differ from the turbofans on big commercial aircraft in a couple of details.

On a turbocharger, the castings for the compressor and turbine housings are very thick and rigid relative to their size. As a result, tight clearances between the compressor/turbine wheels and their respective housing can be maintained, on the order of maybe a couple hundredths of an inch. Tight clearances are best because they limit the seepage of high-pressure gas around the edge of the impeller to the low-pressure side, maximizing efficiency.

OTOH, the housings of big turbofans like the ones on the Dreamliner appear to be made of silicone rubber, judging from how much they wobble and wiggle during landings and turbulence. My guess is that a concern for weight limits how rigid the ducting around the fan can be, which in turn will limit how tight the clearances can be between the blade tips and the housing. So…how big are those clearances?

Second issue: maintenance. Turbochargers hardly need any; if you’re vigilant about cool-down each time you shut the engine off (to avoid coking the spindle bearings), it should last for thousands of hours. OTOH, they’re mechanically simple and the exhaust gas upstream of the turbine isn’t at terribly high pressure, since the engine itself has already extracted the bulk of the heat. Not so for a gas turbine engine, in which the turbine blades (the first stage, at least) are exposed to hot exhaust gas at something close to the peak combustion temperature. So…how much maintenance do big jet engines need? Generally speaking, what sorts of replacements/inspections/adjustments get done, and roughly how often?

2

I couldn’t find the actual clearances, but it is clearly a matter of considerable research and innovation to keep them minimal. Not least in the materials and methods of construction.

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&uact=8&ved=0CD4QFjAGahUKEwi7yJm3iJrJAhVHOhQKHUetB0U&url=https%3A%2F%2Fsuppliers.rolls-royce.com%2FGSPWeb%2FShowProperty%3FnodePath%3D%2FBEA%2520Repository%2FGlobal%2520Supplier%2520Portal%2FSection%2520DocLink%2520Lists%2FStandards%2520and%2520specifications%2FCSSIS%2520(North%2520America)%2FColumn%25201%2FSection%25205%2FDocuments%2FRolls-Royce%2520Specification%2520Index%2F%2Ffile&usg=AFQjCNGziNbvmU9wU8mWc2y2IBbb6gzL1g

3

Whoops…that should read, “…and the exhaust gas upstream of the turbine isn’t at terribly high temperature…” :smack:

4

Not as big as they may appear from a distance - the engine, and the fan housing specifically, is tied to the nacelle and they move pretty much together. Operating clearances vary by model but on a wide-body engine they are multiple millimeters. Fan clearances are not as critical as core airfoil clearances, since they are much smaller percentages of the flow area (a smaller percentage of total flow can leak around the ends instead of producing useful delta-P), and pressure ratios aren’t that high anyway. A high-bypass turbofan produces most of its thrust by moving sheer volumes of air with the fan, not so much by raising its pressure.

That’s a complex question. An exterior visual inspection is usually supposed to be done before every flight, but a removal and full overhaul may occur only once in the lift of the airliner - I think the record is over 25,000 operating hours “on wing”. There are intermediate-level inspections, such as with a borescope to see what’s happening inside, at intermediate intervals.

5

I’m sure someone will come along and answer the specific questions you have, but it might be worth clarifying that the fan in a modern high-bypass turbofan isn’t a compressor and is just a ducted fan that provides the majority of the engine thrust – a traditional zero-bypass turbojet has no such component at all. So I doubt that its clearances are particularly critical since it’s not trying to contain engine back pressure like the compressor stages further back. There’s even an experimental type of engine called a propfan where the fan isn’t ducted at all.

6

Engine maintenance schedules vary between engine types, operators, and brands. A lot of airlines contract out their engine maintenance to either a third party specialist or even the engine manufacturer themselves, they all offer Service Plans based on projected flight hours. Here’s one an example:

http://www51.honeywell.com/aero/common/documents/myaerospacecatalog-documents/BA_MSP-documents/TFE731_MSP.pdf

Some airlines handle it in-house, and provide the service for other customers as well:

7

The relatively small size of turbochargers and modern manufacturing techniques allow those tight clearances without incurring high costs. But car and truck turbos (at least for passenger vehicles) are not all that efficient. Many of them have no diffuser and simply accelerate air. For practical reasons they inlet efficiency may be very low also. On top of that they use radial compressors limiting the potential compression ratio and by their shape they’ll be much more rigid. So your general observation is correct, it will be much more difficult to maintain such close tolerances in large aircraft engines. The tight spacing between their axial compressor stages is incredible now with some taking advantage of miniature shock waves formed by over accelerating air to increase the compression ratio. Frankly I’m surprised jet aircraft engines don’t blow apart in midair more often, we’re talking about an incredibly high level of engineering involved.

8

the comparison to a turbocharger both is and is not really valid; while a turbocharger is indeed a gas turbine, it relies on a completely separate engine to be its gas generator. as for what you’re seeing “wobble” around in flight, it’s the entire engine nacelle. the fan case is quite rigid since it has to contain a blade separation.

9

For info on jet engine, take a look at the Youtube videos from this guy, AgentJayZ

He works mostly on older engines, but still very informative.

10

Jet engines need maintenance a few times less often than ICE but the maintenance is more involved. The turbines are made of fancy stuff like superalloys so they can withstand the heat and pressure.

11

Another consideration in a turbocharger is speed. Some engines turbos turn at over 100,000 RPM inorder tokeep them small.

12

As said above, and **TriPolar **in particular …

There’s very little in common nowadays between a modern centrifugal flow turbocharger and a modern axial flow jet engine.

Blade clearance tolerances in the inner compressor and turbine are on the order of 2 thousandths of an inch. Really amazing materials are needed to have low enough thermal expansion & low enough strain under centrifugal stress. Blades are also hollow and full of cooling passages that are pure voodoo. The idea that 1500F air is used as a cooling fluid gives you some appreciation for how far outside normal experience all this stuff is. Shadetree mechanic’s “common sense” is wholly inapplicable.

And the whole thing has to run with vibration amplitudes in the tenths of thousandths to keep the tolerances tight. As Snnipe70E says, the good nes is RPMs are much less. Modern fans generally turn in the 3000 - 4000 RPM range while internal spools are more like 6000 to 10,000. The new P&W geared fan engines are slowing the fan speed down to 1500-2000RPM.

As noted above, the only part you can see from the outside is the fan. Which has a very, very low compression ratio. It’s a volume machine not a pressure machine. Typically there’s a rub strip on the inside of the nacelle made of a phenolic fiberglass sorta stuff. Tip clearances there are on the order of 10-15 thousandths of an inch. I.e. thicker than aluminum kitchen foil, but not much.

The whole engine as a unit bounces around a lot as the wing & pylon flex. From the cabin inflight you may also see some hoop flex in the outer cowling. All of which is just large panels of thin sheet aluminum with a hinge line along one edge & clamps every so often along the other three edges.

Internally the whole shebang is insanely rigid.

13

As to maintenance …

Modern engines are insanely reliable, going a whole decade of 16-hours-a- day use before being removed for overhaul. The internal engine health monitoring system will be recording hundreds of parameters continuously and reporting anomalies back to the operator’s HQ.

Oil levels are checked before each flight & replenished as needed, typically every couple of days. As well the fan and exhaust end are inspected before each flight looking for impact damage, clogged probes, funky soot patterns, etc.

Oil samples are analyzed every week or so for contaminants. Oil filters / strainers are replaced every couple of months. Engines will get borescope inspections every year or so when the airplane is otherwise laid up for periodic airframe inspection & repair.

In the old days ignitor plugs used to get slowly consumed, much like spark plugs on a 1960s Ford. Nowadays they last many years. But beyond oil they’re about the only other consumable on the engine.

Beyond the obvious main feature of the air path and its turbomachinery, a modern engine has a buttload of other attached stuff. Valves pull air off the engine case at various points and meter it as to pressure, temperature, or volume. There are electrical generator drive transmissions, electrical generators, fuel pumps, fuel strainers, fuel control systems, hydraulic pumps, anti-icing systems, fuel heaters, oil coolers, oil pumps, etc. And for each of these there’s a control system, a servo mechanism to convert electrical or subtle pressure signals into motive power, instrument or idiot light connections, and in some cases a lot of computer smarts.

The mechanical side of this accessory stuff is the major source of in-service unplanned maintenance events. Valves get sticky, pumps leak, connections leak, mechanical feedback systems get out of adjustment, etc.

14

I get the impression that modern engines, and indeed a large fraction of the plane itself is maintained on an as-needed basis, based upon trending analysis of continuously measured parameters. So much so that RR at least, have a 7x24 engineering centre that tracks all their modern engines in flight. In principle they can have a part, or a maintenance crew ready to meet the plane on the basis of this data. Trending is important as it is often not an absolute parameter value that indicates an issue, but a change in value.

Modern swept wing planes are explicitly designed to have the engines on pylons, and to have them nod up and down. This was worked out in the late 50’s and (I think) first seen on the B-47, but quickly became mainstream on the KC-135/707. The point is that the wing is flexible, and under just the right conditions an oscillatory yaw/roll can develop that, if allowed to get out of control, will roll the plane. It comes about through flexing of the wing causing the plane to roll, and the system starts to feed back. A dynamic yaw dampener is part of the solution, the other is to mass load the flexible wing. A nice heavy engine on the end of a pylon is just the answer. The nodding of the engine is it playing its part in reducing the natural frequency of the wing, and thus stabilising the system.

(There is a joke about poles on the left side of the plane here. Engineers will have heard it.)

15

“Dutch Roll” is what I think you’re referring to?

16

ETA: @ Francis Vaughan

Quite right. For both reliability and for max economy you want these things in top tune all day every day. So the idea of doing teardowns every few months just to see what’s inside and look for problems is very much 1960s practice, not current practice. The onboard instrumentation can pretty quickly detect the signature of, say, a clogged cooling passage or sticking trim valve long before the issue becomes overt enough to stop a flight.

There’s also a strong outer feedback loop in place. By instrumenting the entire fleet the manufacturer has excellent insight into which parts are holding up as designed and which need to be improved. High time engines are blazing a statistical path that newer engines are expected to follow. Anybody wandering off the main population gets looked at real closely.

Using engines as (expensive!!) bobweights is also legit engineering. But look up [whirl mode vibration] for ways in which that can go badly awry. Using a 2D design model for a 3D plus rotational/gyro forces reality can be a simplification too far.

17

@jz78817: Dutch roll is a different phenomenon and will occur with swept wings of even infinite rigidity. It’s a roll/yaw coupling driven by the airstream-relative yaw angle of each wing changing in opposite direction after any yaw displacement.

**Francis **is talking about structural divergence where wing flex in spanwise torsion amounts to uncontrolled and uncoordinated roll inputs on the two sides separately. Which if unchecked by sufficient stiffness can roll the airplane, or diverge and rip the wings apart. Even straight wings can have this issue, but swept tapered ones suffer from it a lot more. As a first approximation it’s a fight between aspect ratio and torsional stiffness.

18

Mostly, yes. Some components, mainly rotating parts and airfoils, are limited by fatigue life and are replaced after a certain number of hours or cycles (flights) regardless of their apparent condition.

So do all the manufacturers. Military operators typically do it themselves.

19

For those unaware here’s the explanation. I’ve heard the joke, can’t say I really understand the technical part.

20

The earliest aircraft jet engines used radial compressors and turbines, but designers quickly worked out how to make axial designs work, and these package a bit better. There are still some turboshaft and APU engines that use radial compressors.

Another difference between a turbocharger and a jet engine is that the turbine on a jet engine must operate at a lower pressure drop than the pressure increase of the compressor.

Because the recip engine that the turbocharger is attached to is a positive displacement machine, a turbocharger can operate fine when exhaust manifold pressure exceeds intake manifold pressure…that condition would not work in a jet engine. In practice turbochargers that operate in this regime do exist, but it is not the most desirable condition. This happens mostly on diesels, because the lower turbine inlet temperature leads to lower velocity/volumetric flow.