Impeller (supercharger) and turbocharger interaction?

[SmirkingGerbil]

Already some great resources here regarding how the fixed gear impeller (supercharger) is driven directly by the engine, and is also fed by the turbo-supercharger (turbocharger) just forward of the tail.

 

No need to rehash airflow etc. However the one thing I am not clear on, is the interaction of the turbocharger and supercharger (engine coupled impeller) as controlled by the Boost lever. Or more specifically is there any interaction, or control of the engine driven impeller (supercharger) by the boost lever?

 

Does the boost lever only control the turbocharger (turbo)?

 

Is the throttle lever essentially the supercharger (impeller) control?

 

I did find a couple interesting diagrams that show a schematic of the P-47 supercharger and turbo-supercharger function. Attaching for reference, but the diagram only shows the boost lever affecting the turbo-supercharger (turbo). The attachments show the complete schematic of the P-47 induction system and a hydraulic regulator detail, which is shown in the induction system schematic as controller of the waste-gates, and thus the turbo-supercharger.

 

The diagrams are relative to the following paragraph I copied from a site describing this turbo -

 

Fig. 17 diagram of a typical hydraulic regulator. There is a tube leading from the nozzle box to the top bellows. The bottom bellows is evacuated and serves to prevent the top bellows from acting in response to atmospheric changes in pressure. Inside the top bellows is mounted a spring, one end of which is connected to the junction between the two bellows, and the other end to the range-shifting control lever. This lever is connected by linkages to the cockpit boost control. The purpose of the spring and control-lever assembly is to allow the pilot to vary the pressure on the spring for different nozzle-box pressure, corresponding to different engine powers. This spring tension just balances the pressure in the top bellows to the point where the servo-valve ports to the servo piston are closed.

As pressure changes occur in the top bellows, they act on the servo valve and shift its position, thereby opening ports which direct oil under pressure into one side of the piston. The piston then moves under the unbalanced oil pressure, and moves the waste gate with it. The servo piston continues to move the waste gate until the pressure in the nozzle box has been corrected. When corrected, the pressure in the bellows restores the servo valve to its closed-port position, and stops the piston motion.

The regulating process, described above, occurs in a very short space of time, a few seconds at most. Therefore, in actual operation, as soon as the exhaust pressure starts to change, the hydraulic regulator starts to move the waste gatem a direction to counteract this change. That is, for a given setting of the cockpit boost lever, the regulator always acts to maintain a constant exhaust pressure, and the actual exhaust pressure varies from this constant value only temporarily during those few seconds required by the hydraulic regulator to move the waste gate in a direction which restores the pressure.

 

My understanding then, would be that the Boost lever, controls the waste-gates, and the regulator in turn adjusts waste-gate operation due to changes (increasing or lowering) ambient pressure.

 

 

So my logical conclusion (yikes! Correct if Wrong!) is that the throttle essentially controls the supercharger (engine coupled impeller), and the Boost lever controls the waste-gates (via regulator) which in turn affect turbo-supercharger operation (along with the regulator measuring nozzle box back pressure)?

 

 

Thoughts, knowledge, corrections??

 

 

 

 

Edited July 8, 2020 by SmirkingGerbil
clarification

[randomTOTEN]

I don't have the P-47 in DCS, but I've used Twincharged systems in other simulators (FSX).

 

How about letting the USAAF explain it themselves?

 

The throttle is not the only control of the supercharger! Try this test in DCS (I know it works with other modules).

 

1.) Leave the boost lever at 0 (= idle supercharger = waste gate open).

2.) Climb at your chosen climb power setting until you reach "full throttle height." (Full throttle required to maintain desired MAP)

3.) Change the propeller RPM in different directions by different amounts...leave the throttle full.

4.) Observe MAP and RPM relationship.

Edited July 8, 2020 by randomTOTEN

[Tinkickef]

The carb throttle plate controls air pressure in the manifold. The turbo increases the air pressure upstream of the throttle plate as required via the wastegate system. The boost lever controls the wastegate setting. The more boost the turbo produces the less throttle is required to maintain manifold pressure which is the relationship between throttle and boost must be balanced, or screwy things will happen like hoses popping, turbine /impellor disc failure and maybe even the throttle plate jamming as well as the mixture going to pot.

 

I suppose once a wide open throttle plate cannot provide enough manifold pressure on its own due to altitude, it could be seen that one would then use the boost lever as the manifold pressure control, much like the throttle is used in denser air. I always clip them together and use them as one.

Edited July 8, 2020 by Tinkickef

[SmirkingGerbil]

. . .

 

The throttle is not the only control of the supercharger! Try this test in DCS (I know it works with other modules).

 

1.) Leave the boost lever at 0 (= idle supercharger = waste gate open).

2.) Climb at your chosen climb power setting until you reach "full throttle height." (Full throttle required to maintain desired MAP)

3.) Change the propeller RPM in different directions by different amounts...leave the throttle full.

4.) Observe MAP and RPM relationship.

 

randomTOTEN - sir, I owe you one! That video was awesome.

 

Just watching the disassembly and the connection of the exhaust pressure tube into the regulator ( cleaning) made a lot of things fall in place!

 

Of course, I forgot about the Propeller (pitch) control on the quadrant, and propeller pitch will control engine speed and thus the supercharger (engine coupled/driven impeller).

 

So I have my answers, and some great references! Much appreciated. :thumbup:

[SmirkingGerbil]

The carb throttle plate controls air pressure in the manifold. The turbo increases the air pressure upstream of the throttle plate as required via the wastegate system.

 

. . .

 

 

Tinkickef - you bring up great points. After the throttle plate (depending on how far opened or closed) manifold pressure will adjust accordingly. Being a "car guy" I have witnessed this for years, but it is a simple thing that is often overlooked. Thanks for the reminder. :thumbup:

[grafspee]

If Turbo regulator is maintaining back pressure level then in case of closed throttle(closed too much).

If the turbo outlet will be closed by throttle

then Air flow will be greatly restricted, then back pressure will jump up but since turbo regulator is maintaining back pressure it will open waste gate and send just enough exhaust gases to spin turbo to get proper back pressure.

So amount of gases send to turbo will be reduced greatly, so any over heating problems should not appear, same with too large outlet pressure.

Unless there is the limitation for this turbo regulator.

[SmirkingGerbil]

If Turbo regulator is maintaining back pressure level then in case of closed throttle(closed too much).

If the turbo outlet will be closed by throttle

then Air flow will be greatly restricted, then back pressure will jump up but since turbo regulator is maintaining back pressure it will open waste gate and send just enough exhaust gases to spin turbo to get proper back pressure.

So amount of gases send to turbo will be reduced greatly, so any over heating problems should not appear, same with too large outlet pressure.

Unless there is the limitation for this turbo regulator.

 

 

I think I am following your reasoning, that there is no relationship (direct connection) between throttle and turbo behavior. Turbo is controlled by boost lever, and regulator function. Sorry, I had to read your post a couple times, and I hope I have understood it correctly. Thanks for the illustration.

[grafspee]

I think I am following your reasoning, that there is no relationship (direct connection) between throttle and turbo behavior. Turbo is controlled by boost lever, and regulator function. Sorry, I had to read your post a couple times, and I hope I have understood it correctly. Thanks for the illustration.

 

Opposite.

For example we are flying p-47 with throttle and boost set to max.

Then we start closing throttle only.

Turbo outlet is getting restricted more and more. So pressure between turbo and throttle plate starts to increase -> turbo rpm starts to drop -> back pressure increases(from what i know gas generation level is so high that reduced power will not affect turbo behavior much) -> turbine is getting hotter.

But P-47's turbo has regulator which will maintain back pressure level. This should act like turbo protection system which will not allow for harmful conditions for turbine.

If turbo regulator was governing MAP then closing throttle would be much much more dangerous.

Throttle it self will affect turbo behavior.

Closing throttle with advanced boost lever still will be dangerous for turbo, back pressure will remain constant but cold air flow which is main cooler for turbo will be restricted, so turbo may have encounter overheating problems. But i think gap between throttle and boost lever must be quite big, if we put boost level a little ahead nothing extraordinary will happen.

Edited July 9, 2020 by grafspee

[SmirkingGerbil]

Opposite.

For example we are flying p-47 with throttle and boost set to max.

Then we start closing throttle only.

Turbo outlet is getting restricted more and more. So pressure between turbo and throttle plate starts to increase -> turbo rpm starts to drop -> back pressure increases(from what i know gas generation level is so high that reduced power will not affect turbo behavior much) -> turbine is getting hotter.

But P-47's turbo has regulator which will maintain back pressure level. This should act like turbo protection system which will not allow for harmful conditions for turbine.

If turbo regulator was governing MAP then closing throttle would be much much more dangerous.

Throttle it self will affect turbo behavior.

Closing throttle with advanced boost lever still will be dangerous for turbo, back pressure will remain constant but cold air flow which is main cooler for turbo will be restricted, so turbo may have encounter overheating problems. But i think gap between throttle and boost lever must be quite big, if we put boost level a little ahead nothing extraordinary will happen.

 

Thank you. This makes sense, as under normal conditions we wouldn't put the boost lever ahead of the throttle. The conditions you describe are what other documentation mentions.

 

I would think that the Boost Regulator would indeed open the waste-gates to reduce back pressure, but as you mention, throttle pulled much further back from boost lever would still create adverse conditions for Turbo.

 

I think I followed it correctly this time.

[grafspee]

Thank you. This makes sense, as under normal conditions we wouldn't put the boost lever ahead of the throttle. The conditions you describe are what other documentation mentions.

 

I would think that the Boost Regulator would indeed open the waste-gates to reduce back pressure, but as you mention, throttle pulled much further back from boost lever would still create adverse conditions for Turbo.

 

I think I followed it correctly this time.

 

Yes. But take look at another case. What if pilot would retard throttle to 0 as fast as possible, then gas generation level would be so small the boost level location don't really matter here.

[SmirkingGerbil]

Yes. But take look at another case. What if pilot would retard throttle to 0 as fast as possible, then gas generation level would be so small the boost level location don't really matter here.

 

I am not a mechanical engineer, so my thoughts on this are probably flawed.

 

However, my assumption would be, yanking the throttle to zero power, would still yield some combustion gases albeit greatly reduced. So immediate effect would be some back pressure however slight (due to throttle closing off charge feed to engine), until the boost regulator via the exhaust pressure feed detected this, and compensates by modulating or opening the waste-gates a bit.

 

By looking at the regulator diagram, it would appear a small increase in exhaust gas pressure would drive the pressure bellows down, and moving the metering pin down, allowing pressurized oil to enter the lower part of the servo, driving up the piston, emptying the upper servo chamber due to current pin position, and open the waste-gate. I would imagine this would happen in short order???

 

Just a guess, based on my limited understanding.

Edited July 10, 2020 by SmirkingGerbil
clarity added

[oreste]

when the plane reaches a certain altitude the turbo starts to work, you can see it from the turbo revs indicator. Even if you keep the turbo lever fully pulled back reaching 15/16 000 ft it will start working automatically. in the P 51 where we can see a sudden increase in pressure which forces us to make an adjustment of the throttle knob.In the P 47 instead the pressure will rise gradually in fact you can only notice it by looking at the turbine rpm indicator. we can still manually add pressure by acting on the turbo lever but we must always keep an eye on the rpm / turbine indicator so that we do not go to exceed the maximum safety speed.In addition, the temperature of the air arriving in the carburetors, as the turbine revs and as the altitude rises, this temperature varies at times unexpectedly and it is very important that it remains within the safety margins. I return the intercooler or heat it by closing it, I do not know if the ice formation is simulated .. that would be another problem again :)

[SmirkingGerbil]

when the plane reaches a certain altitude the turbo starts to work, you can see it from the turbo revs indicator.

 

. . .

 

I return the intercooler or heat it by closing it, I do not know if the ice formation is simulated .. that would be another problem again :)

 

Good points. I wasn't aware of how the P-51 worked, as I have not flown it much. Thanks for the clarification.