Active fuel management is a technology developed by General Motors, which is used to improve engine efficiency in the times when the engine is operating under loads which are considerably less than the rated load, as shown in this link. This technology exploits the property of piston engines where some of the cylinders are turned OFF while only few of them work, and hence improving the overall efficiency of the engine. Can a similar technique be employed on an aircraft's engines, where some of the turbine stages (Compressor or Turbine) are switched off in order to improve the performance efficiency of the engine?
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asked Jul 6, 2015 at 7:23
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8$\begingroup$ I don't think your analogy works. A car is powered by multiple identical, indivisible units (cylinders) and the power management system turns some of those off to conserve fuel. A plane is also powered by multiple identical, indivisible units (engines) so the analogous control mechanism would be to turn off some of those units (shut down an engine or two), not to somehow turn off parts of the units. $\endgroup$– David RicherbyCommented Jul 6, 2015 at 12:21
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5$\begingroup$ First of all, a compressor stage is not turbine's equivalent of a cylinder. This would be the combustor (aka flame holder). $\endgroup$– Agent_LCommented Jul 6, 2015 at 14:41
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1$\begingroup$ The second issue is that reciprocating engine's cylinder uses very specific amount of fuel per cycle. Use too much or not enough and you get burn problems. In a car you also have very tight coupling of engine to the wheels: speed of X requires Y rpm which means Z fuel consumption (gearbox helps a bit, but just a bit). You can't dial down the power by reducing to any rpm you like. This problem does not exists on an aircraft: almost every rpm works at any speed (because jet exhaust is always faster than IAS). You can simply dial down the RPM, no need of partial shutdown of the engine. $\endgroup$– Agent_LCommented Jul 6, 2015 at 14:52
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5 Answers
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You cannot "switch off" a stage of a turbine engine.
As seen elsewhere jet engines are a single block mechanically locked together: all the stages (compressor AND turbine) connected to the same shaft. If the shaft turns, all connected stages turn with it.
You can have designs with multiple shafts, but this increases the complexity (design and maintenance) heavily.
Even if we assume for a moment that you could have 1 shaft per stage, stopping one of these would be generally a bad idea. For acoustic reasons the number of blades on the stationary part of the stage and those on the rotary part must be prime to each other (example: 37 blades on the stationary side and 39 on the rotary one); this means that when the rotary part stops you cannot align all the blades with the stationary ones, creating drag and losses within the engine, thus reducing the efficiency.
How would efficiency of a Jet Engine change, if we assume for a second that its a possibility, to stop a few stages(turn them into free instead of locked)?
You would still decrease the efficiency: compressor stages are the ones actually pulling the aircraft forward (you would then produce less thrust*) and the turbine stages are the ones extracting energy from the exahust.
*: you could even risk having the flame going upstream if you stop too many compressor stages and not having enough pressure at the combustion chamber inlet.
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$\begingroup$ Lets say that we have an engine where the stages are attached to the shaft by some mechanism (which is a black box). This mechanism has the ability to turn to free and locked state. How would efficiency of a Jet Engine change, if we assume for a second that its a possibility, to stop a few stages(turn them into free instead of locked)? $\endgroup$ Commented Jul 6, 2015 at 7:36
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5$\begingroup$ Stopping individual cylinders in Active Fuel Management reduces the fuel burn, because otherwise a volume of air & fuel is needed for each revolution. A jet engine doesn't have this fixed displacement problem; the fuel flow can just be reduced (within limits). Perhaps you could let some turbine or compressor discs in the engine freewheel by adding extra clutches, but I suspect that would be far too complex mechanically, as well as add drag inside the engine... $\endgroup$– AndyCommented Jul 6, 2015 at 9:54
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8$\begingroup$ More to the point. Additional stages increase Carnot efficiency by more closely approximating adiabatic expansion. The ideal jet engine has infinite stages (obviously not ideal from a manufacturing perspective), as at that limit, we get adiabatic expansion. There is no equivalent in an ICE, as ICEs do not have staged PV cycles. $\endgroup$– AronCommented Jul 6, 2015 at 14:43
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Can it be done?
Yes
Has it been done?
Yes
Wait, what?! Where what how?
The Pratt and Whitney J58 was designed to allow partial shutdown of the compressor. It does this by placing a distinctive spike in front of the compressor assembly.
However the purpose of the transition to ramjet that is distinctive of the J58 was to allow it to avoid overheating the turbine during supersonic flight, where the increased inlet velocity translated to additional heating of the air.
However this does not increase the efficiency of the jet engine at all.
Overall you actually want as many stages as possible for subsonic flight. This is because multiple stages allow the gas to compress/expand more adiabatically, leading to better Carnot efficiency.
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$\begingroup$ Very interesting - I thought the sr71 spikes were only for reducing airflow through the engine. $\endgroup$ Commented Jul 6, 2015 at 16:00
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$\begingroup$ +1. I thought I knew the answer (it does not make sense) but I learned it is possible, and SR71 did that, for reasons linked. Thank you! $\endgroup$ Commented Jul 6, 2015 at 22:08
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1$\begingroup$ I can hardly agree. the PW-J58, as shown in the wiki page you linked, simply reduced the airflow to the turbojet duct. In addition
The gas generator RPM was kept at 100% to maintain maximum thrust., hardly "switched off". $\endgroup$– FedericoCommented Jul 7, 2015 at 5:34
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Short answer: No.
Longer answer: Jet engines are designed with multiple compressor and turbine discs mechanically attached to a single shaft, called a spool. Usually, there are 2 (or sometimes 3) of these spools within the engine, due to the need of the high pressure compressor and the main intake fan to spin at different speeds. Each spool spins freely and is driven by the compressed air in the engine core being pushed through the turbine stages attached to that spool.
Diagram of a Turbofan Jet Engine
Source: Wikipedia
There is no way to "shut down" a particular spool. As long as the pressure in the engine core is higher than the pressure behind the nozzle, the turbines will spin and drive the spool. If you detach either one of the turbine stages or one of the compressor stages from a spool such that it 'freewheels,' the turbine discs will likely overspeed due to decreased load. Additionally, an undriven compressor stage would experience compressor stall, which is essentially an aerodynamic stall of the blades of the compressor stage.
Finally, this is entirely unneeded for a couple of reasons:
First, aside from the problems it would cause, there would be nothing to gain from shutting down a compressor stage. As Andy correctly mentioned in a comment, jet engines do not have a constant fuel burn per revolution, unlike piston engines. If a jet engine doesn't need to produce as much power, it can simply inject less fuel in the combustion chamber. As the pressure in the combustion chamber drops, the spools will spin down accordingly on their own (due to less pressure driving the turbine stages.)
Second, the power output profiles of jet aircraft operation are nothing like those of a car. For a car, high engine power is usually used only in brief spurts for acceleration and cruise uses a small fraction of that amount of power. In a jet airplane, on the other hand, the engines will be operated at a rather high percentage of their maximum RPMs for most of the flight. This is because additional power allows them to fly higher, which actually increases efficiency. See the answers to the previous question How far can a 777 fly with just one engine at altitude? Unlike a car, significantly reducing the power during cruise flight actually reduces the fuel efficiency of an airliner.
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$\begingroup$ "In a jet airplane, on the other hand, the engines will be operated near their maximum performance for most of the flight." That is certainly not true in civil aviation. The thrust in cruise is typically around 25% of the maximum takeoff thrust. I think the answer in link on "flying on one engine" seems somewhat confused. $\endgroup$ Commented Jul 6, 2015 at 15:18
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$\begingroup$ @alephzero Thrust or RPMs? Thrust will be lower for the same RPMs due to less dense air (as that answer mentions.) I'll edit my answer to make clear that I mean RPMs here, not thrust. $\endgroup$– reirabCommented Jul 6, 2015 at 15:24
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If you extend the idea of "switch off" a bit, this is already done in several different ways.
Surveillance aircraft may be designed to loiter over their target area with some engines shut down completely. That is not just a matter of shutting down the engines - for example if you are going to loiter at high altitude for several hours at temperatures below -50C, you are likely to need special systems to heat the oil in the shut-down engines, to avoid damage when you restart them.
Many jet engines contain variable stator vanes in the compressors, which are automatically adjusted to improve the efficiency under different working conditions.
In multi-shaft engines, the relative speeds of the different shafts changes depending on the working conditions. That is more efficient that forcing all the compressor and turbine stages to run at the same speed on a single shaft. In a multi-shaft turbofan, the bypass ratio (i.e. the fraction of the air being compressed in the core engine and used for combustion, rather than just being blown by the fan to generate thrust) will also change.
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Fuel efficiency is not improved by shutting off stages of a compressor or turbine. Fuel efficiency is a function of overall pressure ratio, reduce this and efficiency is reduced.
