Turbosupercharger question?

[Jafferson]

I understand the Turbosupercharger is a exhaust gas driven turbine, its mentioned that the engine loses 300 HP when engaged at sea level or below 7000 feet. I want to understand what is causing the HP lost, maybe the pressure in the exhaust tube? Or is the info wrong?

More MP shold give more HP??? and how is this modeld in DCS?

Would be nice if someone can explain..

[grafspee]

1 hour ago, Jafferson said:

I understand the Turbosupercharger is a exhaust gas driven turbine, its mentioned that the engine loses 300 HP when engaged at sea level or below 7000 feet. I want to understand what is causing the HP lost, maybe the pressure in the exhaust tube? Or is the info wrong?

More MP shold give more HP??? and how is this modeld in DCS?

Would be nice if someone can explain..

Lost of power occur when throttle and boost lever is interconnected. Way to avoid it is to open throttle first to 100% then add boost.

Power loss comes from excessive inlet air temperature and increased back pressure.

 

[Schwarzfeld]

The power loss results from the turbocharger and supercharger being engaged/pressurized: the engine has an internal supercharger (Americans call it a "Blower" at the time, and still today in car culture often) as well as a turbocharger that is controlled by a wastegate moved by a motor. A diagram of the system is below.

A turbocharger is driven by the pressure of engine exhaust; therefore it does not "pull" very much power, if any, from an engine at all when it is metered (via wastegate closure) to increase boost. I say "not very much" because in some cases, when a turbo's gate closes up to generate boost, this increases backpressure in the engine exhaust runners, which, in some engine designs can lead to reduced performance.

A supercharger is a linear-drive air compressor that is driven directly by the engine's crankshaft rotation - in cars, this is typically done with a belt. In WWII era radial engines, the supercharger was (as shown below) driven directly by the crankshaft, or a direct gear drive from the crank. The supercharger "screw" or "turbine wheel" depending on where you're from "pushes back" against the engine a little bit; the engine must expend energy to spin both itself, it's load (the propeller) and also the supercharger screw. On a graph, there is a point at which you can point out where the "parasitic loss" of the supercharger is overcome by the boosted air its feeding the engine generating, and the engine is generating more power than it is losing by a profitable factor. 

Because air is thick at low altitudes, allowing the turbosupercharger to run full tilt would literally blow the motor up in pretty short order; the turbosupercharger exists for performance at altitude, solely. The Severskys were very smart guys, they designed the T/S system of the jug (and the rest of the plane) to protect the pilot from himself in many ways, so if you move the boost lever forward at low altitude, you'll close the wastegate some, but it won't allow you to close it enough to make (at that altitude, dangerous) boost; I assume this is achieved by a simple pressure switch (responds to air pressure). The supercharger is always "engaged" because it is physically attached to the crank. 

Presumably, looking at the layout below, closing up the wastegate at all at an altitude where the air is thick and the engine is outside of its volumetric efficiency envelope, you're just beating the crap out of the motor trying to force feed it far more CFM (volume of air) than it can physically pull in, combust, and exhaust, due to the air density at that altitude, so as a result, the engine will perform poorly and also get beat up. As the manual states, never move the boost lever ahead of the throttle; reason is, if the boost lever is ahead of the throttle in position, you are asking the engine to take in more air volume than you are allowing it to exhaust... you are asking it to bite off more than it can chew.



 

Edited January 27, 2021 by Schwarzfeld

[grafspee]

Engine power depends on how much oxygen can be delivered to combustion chambers.

Both supercharger or blower and turbocharger are increasing inlet air temp, which is reducing air density= less density less oxygen. 

You can open throttle to 50" in to ways.

1st open throttle to 50" w/o using turbo in that case inlet air temp will be increased only by supercharger and ram air let say our inlet air temp = 50C

2nd interconnect throttle and boost and you get 50" at 85% throttle position in that case supercharger ram air and turbocharger are increasing air inlet temp which will be 100C 

2nd way increase inlet air 50C above 1st way which directly reducing amount of oxygen in combustion chamber. Even if your engine running 50" 2700rpm the second way gives less power.

also back pressure is higher and engine loses additional power in 2nd way.

Edited January 27, 2021 by grafspee

[Jafferson]

Thank you guys for taking time to explain it, so its the exhaust backpressure and the air temperature. Very nice picture of the system 

[SmirkingGerbil]

On 1/27/2021 at 3:31 PM, Schwarzfeld said:

The power loss results from the turbocharger and supercharger being engaged/pressurized: the engine has an internal supercharger (Americans call it a "Blower" at the time, and still today in car culture often) as well as a turbocharger that is controlled by a wastegate moved by a motor. A diagram of the system is below.

 . . . .

 

Schwarzfeld,

 

Thank you so much for this. The diagram you sourced is the best one I have seen to date, and I have researched this "exhaustively", pun intended!

 

Your explanation is one of the better ones I have seen, and parallels what I have learned from other sources.

On 1/27/2021 at 3:31 PM, Schwarzfeld said:

I assume this is achieved by a simple pressure switch (responds to air pressure).

Hopefully I can return the favor, here is the pressure switch (regulator) that achieves this function. A combination of hydraulic, boost lever, and evacuated bellows working together.

 

One question if I may. A few sources indicate the Supercharger (Blower) is two stage, yet I can find no detail on gearing changes, or other mechanisms to change the ratio of the blower drive. Do you have any insight as to whether it is only a geared coupling with no ratio (gearing) changes and is only single stage? If it is two stage, do you know how the gearing ratio's were changed?

Thanks for your write up, very informative!
 

[grafspee]

P-47's engine has only single stage single speed supercharger.

R-2800 used by navy planes had 2 stage 2 speed superchargers, for example F6F hellcat used 2stage 2 speed supercharger in r-2800 engine.

Mentioned power loss  happens only when turbo is engaged with throttle not open to 100%, if throttle is open to 100%before boost is added by turbo, there is no power lose in that case. 

Explanation that power loss results from supercharger and turbocharger being engaged, isn't 100% true. Everything depends on throttle position, if opened to max there is no power loss only gain then.

Manual states that engaging turbocharger below 7k ft will result in power loss, probably because supercharger + ram air is sufficient to provide 52" of MP so using turbocharger is pointless there.

But if you want use high power settings like 64" 2700rpm you can use turbo at SL i guarantee, you will be faster:)

Turbocharger is much better then multi stage supercharger, a lot less power losses, but system size makes application in fighters planes extremely difficult. 

B-17 used turbo supercharging and in year when it come out, was faster then any US fighter at high altitude

 

Edited January 30, 2021 by grafspee

[SmirkingGerbil]

6 hours ago, grafspee said:

P-47's engine has only single stage single speed supercharger.

. . .

 

 

 

 

Thank you Graf!

You have shared a lot of knowledge about this warbird, and I appreciate all your input on various topics.