7
$\begingroup$

How do the RPMs stay constant in a gas turbine engine? For example in car with piston engine, when you keep pressing the accelerator the rpms keep picking up, to keep the rpms constant you must let off the throttle, if you keep the throttle floored you will reach peak powers rpm and even go higher rpm if engines max rpm isnt limited at peak power. My question is that how do gas turbine engines can stay at peak power rpm while still throttle is fully applied? Shouldnt it keep increasing in rpms like car engine?

$\endgroup$
3
  • 5
    $\begingroup$ "in car with piston engine, when you keep pressing the accelerator the rpms keep picking up" -> when I do that in my car in highest gear on the Autobahn, the RPMs will eventually stop increasing before reaching peak power and I've reached my top speed. I think your premise is somewhat flawed, probably by always driving your car well below top speed. $\endgroup$
    – Bianfable
    Feb 25, 2023 at 15:17
  • 1
    $\begingroup$ @Bianfable In autobahn its because majority of cars top gear are overdrive. you will reach your top speed way before you reach peak power in top gear because rpms are so low. If you are on 1st and 2nd gear etc you will easily reach peak power rpm and keep going up from there still until you hit the rev limiter. My cars engine makes peak power at 6000 rpm but if i keep pressing accelerator it easily goes to 6500 rpm which is the limit. I added the constant speed prop tag because i thought maybe the answer would be because they are similar but i really dont know $\endgroup$ Feb 25, 2023 at 16:05
  • $\begingroup$ Modern engines use en.wikipedia.org/wiki/FADEC. Here's a video explaining the implemtation that's quite good. nescacademy.nasa.gov/video/135553bc3b7b4171b7c54ee0578489211d $\endgroup$
    – Phil Sweet
    Feb 26, 2023 at 16:15

5 Answers 5

10
$\begingroup$

The reason that car piston engines and turbine engines seem to behave differently is that they're used in different ways.

If ran at full throttle with no load, both types of engine would accelerate to their maximum RPM (limited either by internal friction or by a fuel control unit). However, if you place either type of engine under load, they will both accelerate more slowly, and it will take much longer to reach that point.

Airplane engines and car engines are used differently. During takeoff, the engine is usually accelerated to full throttle as soon as the takeoff roll starts, and is left there until the airplane has sufficient altitude. This is plenty of time for it to reach its maximum RPM, and stay there. On the other hand, cars have a much more restrictive speed limit than planes (relative to what their engines can do), so the driver usually takes their foot off the accelerator long before the engine has enough time to reach its RPM limit.

This is easily seen by looking at the behavior of piston airplane engines. When I take off in a rented Cessna 172 with a piston engine, I advance the throttle to max, and the engine rapidly climbs to its maximum RPM and stays there. It doesn't continue to accelerate, even though the "accelerator" is floored. (And it doesn't have a fuel control unit, or indeed any kind of computerization at all.)

$\endgroup$
4
  • $\begingroup$ Yeah but why does it stay there? For example in car if you floor it in 1st gear you will soon reach the max rpm and hit the rev limiter. if rev limiter didnt exist it would keep revving even more and damage the engine. The ecu just cuts off fuel to prevent overrevving but this also means its also no longer making as much power because fuel delivery is now decreased. It will just bounce off the limiter. Do plane engines also limit fuel delivery to keep it at steady rpm? If so, isnt that basically decreasing their power output because now there is less fuel? Sorry my question is a bit confusing $\endgroup$ Feb 25, 2023 at 20:53
  • 7
    $\begingroup$ @TimothyScherer It stays at that RPM because the load on the engine precisely balances the power output. As RPM increases, the wind resistance of the propeller increases. Eventually, you reach a point where the engine can't produce enough power to overcome the resistance, and the RPM stabilizes at that point. A car in first gear presents a very low load to the engine, which would be roughly equivalent to a piston plane engine with no propeller attached. In those situations, the plane engine would behave very much like the car-in-first-gear engine: if you have the throttle up, $\endgroup$ Feb 25, 2023 at 21:36
  • 1
    $\begingroup$ it would over-rev until it either hits the rev limiter (if it has one) or until it suffers damage (if it doesn't have one). On the other hand, if it does have the propeller attached, it behaves analogous to a car in high gear, where the engine will eventually reach a balance point between the load and power before it hits the limiter. $\endgroup$ Feb 25, 2023 at 21:37
  • 2
    $\begingroup$ Also, yes, the fact that there's less fuel at max RPM limits the power output of the engine, whether it's a turbine, aviation piston, or car piston. This is, in fact, why it limits the amount of fuel, because having less power is preferable to blowing up the engine, and thus having no power. $\endgroup$ Feb 25, 2023 at 21:37
6
$\begingroup$

In a car, you virtually-always operate with the engine far below its power-limited RPM; this is usually not the case in a jet-engined airplane.

In both a piston-engined car and a jet-engined airplane, if you apply a certain amount of power (by pressing the gas pedal a certain distance in a car, and by moving the throttle lever through a certain angle in a jet), the engine will accelerate until it reaches an RPM where the power available at that throttle position is just sufficient to keep the engine spinning at that RPM (i.e., the amount of power needed to counter the forces trying to slow the engine down is equal to the amount of power the engine is currently generating). Once it reaches this power-limited RPM, the engine will then operate at a constant RPM until either the throttle setting is changed (altering the amount of power available from the engine) or the forces resisting the engine's rotation change (altering the amount of power needed to counter these forces).

You can see this on a car, too, not just on an airplane. If you sit in a car with its engine idling, and then lightly press the gas pedal, deflecting it by a small amount, and hold the pedal at this deflection, the engine (and car) will accelerate up to a certain RPM and then level off at that RPM, just like how moving a jet's throttle lever to a certain position and leaving it there will cause the plane's jet engine(s) to accelerate to a certain RPM and stay there.

The difference between cars and airplanes in this regard is that most automobile drivers virtually never use maximum throttle for more than a second or two, if that. This is because, if you floor the gas pedal on a car and then hold it floored, the engine, and the car, will accelerate until the car is far above the speed limits on most roads before the engine stabilizes at its power-limited RPM. The steady-state throttle setting that corresponds to the car moving at a speed that doesn't earn its driver a speeding ticket is a fairly-small fraction of the maximum throttle setting, and it's fairly-hard to hold a specific fairly-small gas-pedal deflection constant with one's foot (especially since car gas pedals, unlike airplane throttle levers, automatically return to the idle position when released, requiring one to apply a constant fairly-small amount of pressure to the pedal in order to hold a constant fairly-small pedal deflection); it's much easier to allow the engine's power (and its RPM, and the car's speed) to decay somewhat, and then, once the power/RPM/speed's fallen past a certain level, push the gas pedal down to deliver a brief pulse of higher power to reaccelerate the engine (and the car) - a pulse that is brief enough that the engine never reaches its power-limited RPM before you let off the gas pedal again.

In contrast, maximum throttle is frequently used for up to several minutes at a time when flying jets (as it provides the best margins of safety for takeoffs and go-arounds), leaving plenty of time for the engine to stabilize at its power-limited RPM. Additionally, with jets, even when using less-than-full power, there are two major factors that make picking a certain throttle setting and allowing the engine to stabilize at its power-limited RPM for that throttle setting a much-more-practical course of action in most flight regimes than providing periodic throttle pulses (as in a car):

  • If you move an airplane's throttle lever to a certain position and then release it, it stays at that position (instead of trying to return to idle, as with a car's gas pedal), making it much easier to keep the throttle in the same position for long enough for the engine's RPM to stabilize.
  • Gas-turbine engines have much slower throttle response than the piston engines found in most cars; due to the massive rotational inertia of a jet engine's large, heavy compressor and turbine rotors, it takes much longer to speed up or slow down when the throttle setting is changed. This slow throttle response makes supplying periodic brief pulses of power much less practical than with a piston-engined car.

What all this means:

  • In a piston-engined car, when you apply high throttle, you generally release the gas pedal long before the engine stabilizes at its power-limited RPM.
  • In contrast, in a jet-engined airplane, when you apply high throttle, you generally keep the throttle lever at this throttle setting for way more than enough time for the engine to stabilize at its power-limited RPM.
$\endgroup$
2
$\begingroup$

ALL these engines will runaway, if load is abruptly removed

What keeps RPM at sensible numbers is that usable power (load) is kept balanced with the fuel supplied.

There are a number of X-factors depending on the application.

A turbofan is going through thick air, which places considerable load on it. The fan is directly coupled to the N1 shaft. It would be extremely unlikely for the fan to unexpectedly start ingesting the vacuum of space, and that's what would be required for the load to suddenly be removed.

Now the N1 shaft could break and you bet that would cause an overspeed event. Similarly a helicopter or turboprop transmission could snap, or the prop could simply be feathered the wrong direction. In that case you have some sort of rev limiter, or a proper governor that changes fuel flow to try to reach and stay at the target RPM.

Most diesels and small engines have these governors built right in. You don't actually work the throttle on a bulldozer or lawnmower engine, you are telling the governor to seek a different target RPM. The fuel rack sticking wide-open is death to a diesel.

With gas cars, someone could jab the clutch or shift the automatic into neutral. In automobiles you have a simple rev limiter that knocks out the spark, though these days they like to knock out fuel also. You would never have that on an airplane, but absent a feathering mistake a runaway is unlikely due to that thick fluid the prop is always pushing. Same for a marine waterscrew.

With stationary turbine generators like the LM2500, the reactivity of the AC power grid is going to force it to the target RPM no matter what it does. The only option the turbine has is to push, or be dragged lol. However if it suffered a generator trip where the grid connection was abruptly removed, it will immediately overspeed unless controls cut off the fuel.

In fact, this was seen in The China Syndrome - it started with a generator trip due to a grid issue. To prevent the steam turbine from overspeeding, the turbine tripped (cut off steam), so suddenly there was nowhere for 2.5 gigawatts of thermal energy to go, so the reactor SCRAMmed to stop that at the source. Jack felt a shudder and the story begins.

$\endgroup$
2
$\begingroup$

Pretty early on in the development of jet engines they realized that it was necessary to limit fuel to prevent runaway RPM. For this reason RPM is governed by a Fuel Control Unit that meters fuel according to certain parameters such as RPM, throttle position, temperature, pressure, etc. Even at full throttle the FCU is set up to limit top RPM.

$\endgroup$
7
  • $\begingroup$ Thank you, this is what i was looking for. Other answer doesnt make any sense. One thing i am also wondering though, since you cut off fuel supply to limit rpm to not let it overspeed, isnt that also decreasing the maximum power/thrust? For example AGT-1500 gas turbine engine in m1 abrams tank makes 1500hp peak power but if the governor cuts the fuel to prevent it from overspeeding , would it not make less than 1500hp? Or is 1500hp the advertised hp after rpm settles with the governor cutting the fuel delivery? $\endgroup$ Feb 25, 2023 at 20:18
  • 1
    $\begingroup$ This answer isn't correct. The OP was asking about the difference between turbine engines and piston engines. Piston engines are also susceptible to runaway RPM if they have no load, and thus also have some kind of Fuel Control Unit that limits top RPM. $\endgroup$ Feb 25, 2023 at 20:29
  • 1
    $\begingroup$ @HiddenWindshield, the OP apparently disagrees with you. $\endgroup$ Feb 25, 2023 at 20:36
  • 1
    $\begingroup$ @HiddenWindshield Yeah the ecu just cuts off fuel when you hit the rev limiter. My question was basically how did gas turbines and jet engines etc also coped with this situation of overspeeding and Michael Halls answer provided the answer. $\endgroup$ Feb 25, 2023 at 20:44
  • 3
    $\begingroup$ @TimothyScherer I see. I may have misunderstood the question, then. Though, in my defense, if the question really was just "Do jet engines have FCUs?", then I would argue that it was poorly worded. $\endgroup$ Feb 25, 2023 at 20:51
1
$\begingroup$

With car engines, the connection between car speed and engine speed is stiff, and only changed by changing gears or operating the clutch. So a car engine need to change rpm often to adapt to car speed and load (idling, accelerating, going up- or downhill). If kept on full throttle, rpm rise to a point that depends on the gear, mechanical losses, air resistance and road steepness.

On contrast, a jet engine rpm is connected to the speed of the airflow going through the engine. Even if the plane is standing on the ground, full throttle will result in a high airflow, giving a far less stiff connection between plane speed and engine rpm. The rpm on full throttle will finally depend mostly on air density and intake pressure, which changes with height and airplane speed. But engine exhaust will always be faster than airspeed. So if full power is needed, full throttle can be applied and the resistance to the engine will not change that much while rolling, takeoff and initial climb. In contrast, a car engine would go through it's gears, adapting its speed over and over again to match the cars road speed. Otherwise the tires would slip. This adaption is somehow made by soft air for the airplane case.

Today all those engines are computer controlled of course to keep them in safe operating conditions and give better economy.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .