high/low blower super charger gears

[Ardy123]

If an engine has 2 gear settings for the super charger, one for high alts and one for lower alts, and the engine is left on the high alt one when not at high alts, would it blow the manifold gaskets?

I ask, because HiTech stated he could see adding control for the super charger gears in engines. If he were to add this, and one was in combat and the fight fell below the alt floor which the high gear is for, how long would it take for the engine to fail?

[Charge]

That's a difficult question IMO. Some engines tolerate over boosting longer than others and it also depends on how close the FThs for different stages are to each other. If the first FTH is significantly lower than the second e.g. in a  full throttle dive below second FTH with high gear on it would cause quickly a severe over boost situation where the engine would break down rather quickly.

What would be the mechanism then? I don't  know but my guess would be that a blown gasket is the mildest result and I'd expect something a bit more radical when such a huge engine knocks severely. Maybe a destroyed piston or cylinder head i.e. what ever happens to be the weakest point, could also be a connecting rod or even crankshaft which could cause a bit more radical effect to the airframe, even an explosion.

Merlins also had an anti detonation screen to protect the engine detonating in direction of the supercharger but not all had this so I'd expect that overboost and too lean mixture could cause a detonation which could damage the supercharger. Not sure if this applies for this situation though...

-C+

[Mirage]

it wont happen every time but if it does, you would see a piston close to the size of your torso fly through the cowl, not changing the blower will also destroy the supercharger in some cases

[hitech]

I would have to do research, but I am fairly sure nothing would happen.

There is a waist gate after the Super Charger that controls max manifold pressure. You would just be dumping more air out this gate. So you would not over boost the engine. Now I am not sure if anything would happen to the Super Charger, but off hand I can not think of any.

2nd think about it from a design perspective, would you really design something that would blow an engine just by a pilot forgetting to change gears? I would be fairly sure that it has happened many times.

HiTech

[MiloMorai]

I would have to do research, but I am fairly sure nothing would happen.

There is a waist gate after the Super Charger that controls max manifold pressure. You would just be dumping more air out this gate. So you would not over boost the engine. Now I am not sure if anything would happen to the Super Charger, but off hand I can not think of any.

2nd think about it from a design perspective, would you really design something that would blow an engine just by a pilot forgetting to change gears? I would be fairly sure that it has happened many times.

HiTech

Turbo superchargers had a waste gate.

Merlins had a setup that restricted the opening of the butterfly. The throttle lever might be full forward but the butterfly would be only partially open. If 15lb boost was the max boost, then that is all the engine got.

[hitech]

Turbo superchargers had a waste gate.

Merlins had a setup that restricted the opening of the butterfly.

Any idea how this worked? But it seams to me that would tend to always over pressurize the inlet, and cause a large inefficiency in the charger do to the high out let pressure.

HiTech

[MiloMorai]

Any idea how this worked? But it seams to me that would tend to always over pressurize the inlet, and cause a large inefficiency in the charger do to the high out let pressure.

HiTech

from Allied Aircraft Piston Engines of WW2 by Graham White

To eliminate this chore (pilot monitoring manifold pressure) R-R and the RAE developed an automatic boost control that allowed the pilot to "firewall" the throttle without concern for over boost.This task was accomplished by a servo system that read manifold pressure and opened the throttle butterflies far enough to give the maximum rated boost and no more. As the a/c climbed, the automatic boost control allowed the throttle (butterflies) to open until the critical altitude was reached, that is, the point at which the the throttle (butterflies) would be wide open.

[SectorNine50]

Interesting that they couldn't make the supercharger change gears using this mechanism.

[morfiend]

from Allied Aircraft Piston Engines of WW2 by Graham White

To eliminate this chore (pilot monitoring manifold pressure) R-R and the RAE developed an automatic boost control that allowed the pilot to "firewall" the throttle without concern for over boost.This task was accomplished by a servo system that read manifold pressure and opened the throttle butterflies far enough to give the maximum rated boost and no more. As the a/c climbed, the automatic boost control allowed the throttle (butterflies) to open until the critical altitude was reached, that is, the point at which the the throttle (butterflies) would be wide open.

  Interesting,sounds somewhat like a mass airflow sensor in modern fuel injected cars!I might be mistaken but isnt that how the idol works?

  end hijack.....



   

[hitech]

from Allied Aircraft Piston Engines of WW2 by Graham White

To eliminate this chore (pilot monitoring manifold pressure) R-R and the RAE developed an automatic boost control that allowed the pilot to "firewall" the throttle without concern for over boost.This task was accomplished by a servo system that read manifold pressure and opened the throttle butterflies far enough to give the maximum rated boost and no more. As the a/c climbed, the automatic boost control allowed the throttle (butterflies) to open until the critical altitude was reached, that is, the point at which the the throttle (butterflies) would be wide open.

Something I didn't know, but it still does not eliminate the possibility of a waist gate. Also was the butter fly before of after the super charger? If before it would make since to me for no need.

Ever seen the complete intake system diagram?

HiTech

[MiloMorai]

Something I didn't know, but it still does not eliminate the possibility of a waist gate. Also was the butter fly before of after the super charger? If before it would make since to me for no need.

Ever seen the complete intake system diagram?

HiTech

The book has a diagram. The butterflies (2 barrel carb) were before the supercharger thus restricting the amount of air being compressed.

The waste gate on the GE turbo supercharger was on the engine exhaust side, not the air intake side, of the turbocharger. The gate opened to restrict the compressor side pressure being produced.

SectorNine50, an aneroid switch was used to change between high blower and low blower. If the nominal change altitude was say 18,000ft, the change altitude would be say 16,000ft when going from high to low blower. If one was fighting around 18,000ft, one didn't want the blower to change every time one went below 18,000ft.

[Captain Virgil Hilts]

If you restrict the air that goes IN the supercharger, it makes less boost, less heat, and takes less HP to turn.

A waste gate will not work on a crank driven supercharger, the waste gate vents exhaust gas past the turbine of a turbocharger into the exhaust pipe, to keep the turbine from spinning the compressor faster so that it does not make as much boost. There is no exhaust side turbine on a crank driven supercharger.

As far as the original question, if you were to continue to over boost an engine, as you would if you left a two speed blower in high altitude gear at low altitude, you could easily destroy the engine. The excessive manifold pressure generated would make the engine detonate itself to pieces, probably knocking the rings off of the sides of the pistons, crushing the rod bearings, caving the pistons in, and seizing the pistons in the bores. Odds are, a Merlin rated at 80" of boost might make well over 100" of boost if the blower were left in high altitude gear. That kind of cylinder pressure could easily destroy the engine.

I thought I remembered reading that the two speed superchargers were commonly shifted by an aneroid bellows.

[hitech]

CVH , Obviously if you over boost an engine serious things will happen very soon, but the question we are talking about does leaving a supercharger in high alt gear over boost the engine. With an automatic butterfly valve or other device are you sure it would over boost?

HiTech

[jamdive]

I would have to do research, but I am fairly sure nothing would happen.

There is a waist gate after the Super Charger that controls max manifold pressure. You would just be dumping more air out this gate. So you would not over boost the engine. Now I am not sure if anything would happen to the Super Charger, but off hand I can not think of any.

2nd think about it from a design perspective, would you really design something that would blow an engine just by a pilot forgetting to change gears? I would be fairly sure that it has happened many times.

HiTech

I would guess it creates an adverse effect. At higher alts the air is very thin. Now at low alts you are trying to turn a fan with much more resistance. It would be my guess that also accounts for the reasons you see flat spots in engine performance at a certain alt. where the trade off in engine power vs. boost could go either way.

[SIK1]

Don't they use pop-off valves on Roots type blowers?