What of the Republic P-47 Turbo-Supercharger

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Here's a cutaway of a 2-stage Merlin.

https://i.pinimg.com/originals/5b/45/96/5b45968624361ffb447bb9d3393157c8.jpg

Note there are two supercharger wheels, both "half-compressors" since they have vanes on only one side of the wheel. The second stage sends the heated air through an intercooler to reduce in intake temperature.

Below is a drawing of a single-stage Merlin. Note only ONE S/C wheel.

https://www.bluebird-electric.net/b...e-Merlin-MkXX-Engineering-Cutaway-Diagram.jpg

These drawings are not as detailed as they could be if the format were bigger, but you can see one and two supercharger wheels in them at least.
 
GregP said:
Note there are two supercharger wheels, both "half-compressors" since they have vanes on only one side of the wheel.
Click to expand...

Most WW2 superchargers had single sided compressors.

There were some that had a double sided compressor, such as the Napier Sabre (later changed to a single side compressor). The Rolls-Royce R also had a double sided compressor.

Turbochargers of WW2 and modern turbochargers have single sided compressors.
 
Pat, Greg and Wuzak thank you for the info as it filed things in nicely! I was trying to envision how an automotive two lobe blower would be called a two stage. It's obviously not but now get it.

Cheers,
Biff
 
The P-47 was able to use such a system because it originally was based on a two seat airplane, so it had the room. And it was not the only single seat turbo we had; the P-43 setup was exactly the same, for the same reason.

The two rectangular slots on the side of the aft fuselage is where the air cooled intercooler air exhausts. The engine exhausts from the rear bottom of the fuselage, after passing through the turbosupercharger turbine.


 
Okay you hardcore engine guys, it is clear from the posts on this thread that in many cases the wastegate is a simple butterfly valve on the exhaust outlet side of the Turbo-Supercharger. I want to reduce the boost from the Turbo-Supercharger so I close the butterfly valve a bit, creating some back-pressure on the outlet side. This in turn slows the exhaust impeller, and since they are on a common shaft, the intake impeller which reduces the boost.

In the case of the P-47, if I open the wastegate entirely all the exhaust is dumped out of the system at the cowling and the Turbo-Supercharger is essentially un-powered. So how does fresh intake air make it to the Supercharger? Is there a bypass to allow fresh air into the system between the Turbo-Supercharger and the Supercharger? I would think that a Turbo-Supercharger that is not being powered by the exhaust stream would be pretty restrictive to intake airflow.

In the case of the P-38 and others with the wastegate located after the Turbo-Supercharger, as I close the butterfly valve I encounter the same problem on the intake side as the P-47. But it is compounded because I am starting to create back-pressure all the way back up the exhaust system to the exhaust valves. If I close the wastegate entirely I just choke the engine completely, something akin to Eddie Murphy and "The banana in the tailpipe" problem. So I'm guessing the wastegate cannot close entirely or there must be bypasses on both the intake and exhaust sides of the system between the engine and the Turbo-Supercharger.

What am I missing?
 

You would open the wastegate to reduces turbocharger rpm and reduce pressure at the outlet. Open the butterfly to have more exhaust bypass the turbine.

Close the butterfly to increase rpm and boost. Up to the critical altitude, of course.



The air would be drawn past the turbo in any case.

There may be a bypass inlet for the P-47 for when the turbo is not spinning, but I doubt all the exhaust goes out the wastegates when they are fully open, so the turbo wold always be spinning.



The P-38's wastegate is not after the turbo. It may look that way, but the wastegate is in the exhaust, before the turbine. It works the same as for the P-47, the wastegate causes the exhaust to bypass the turbine.

If the wastegate is fully closed, all exhaust is directed through the turbine.



From the thread Was the Republic P-47's turbo-supercharger system unique to only the P-47?

The wastegate is connected to the turbine housing, opposite the exhaust connection. You can see the exhaust turbine wheel at the top of the turbo, looks like a fan.

The exhaust has two options to exit the turbine housing - through the wastegate pipe or through the turbine. Adjusting the wastegate changes how much goes through each path.
 
Last edited:
wuzak,

There was a very similar thread to this one started at about the same time by the original poster, Was the Republic P-47's turbo-supercharger system unique to only the P-47?

I thought the wastegate was always located upstream of the Turbocharger in any application, but other posters in that thread indicated that was not the case in some USAAF planes during the war. Using the same picture you post of the P-38, the exhaust port and butterfly valve are ID'ed as the wastegate. Additionally in post #8 of that thread the pilots manuals for the B-17, B24 and B-29 all show the wastegate located after the Turbo-Supercharger. This was why my post started by stating that closing the wastegate would actually reduce boost.

So I just assumed that being very new technology, this must have been the best they could muster back then. As I thought about it more, placing the wastegate after the Turbocharger just doesn't make any sense, hence my last post. Have a look at that thread and let me know what you think.

Kim
 
You could turn the turbo 180° and the wastegate would be ahead of the turbo.

The wastegate in that picture is integral with the turbine housing, but it releases the exhaust from before the turbine.

There was an attempt to control the turbo at its outlet, called the Variable Discharge Turbine (VDT). VDT was developed for, or by, Pratt & Whitney and tested on the R-4360 and R-2800. The control systems were not sophisticated enough in the day to control the turbo accurately, test flights with the R-4360 VDT requiring the engineer to constantly monitor and control the turbo.

Some VDT designs Pratt & Whitney came up with also did away with the engine supercharger.
 
wuzak,

So the pilot's manuals previously mentioned contain simplified diagrams which do not represent the actual operation of the wastegate and it placement within the exhaust system?
I had a feeling that was the case, hence my request for more detailed documents on the installation of the Turbo-Supercharger.

With regards to the VDT, wasn't that an attempt to provide thrust from the exhaust rather than functioning as a wastegate? Although to some degree it would. Did the VDT engine(s) have a normal wastegate upstream from the Turbocharger, or was the VDT the only device to control boost?

Kim
 
Just recently in another thread (I hope), there was a comparison between the P-47 and the Hellcat. Both were somewhat similar weight-wise and had the same engine. Yet the P-47 was considerably more expensive, due to the turbosupercharger by chance?
 
MIflyer said:
B-17 Turbo

View attachment 589181
Click to expand...


I would note in the photos posted by MIflyer that the captions are a bit off. The turbo in the lower photo has been flipped over from the from one in the upper photo.
as used in the bombers the turbine was on the bottom and the compressor was inside the nacelle.

All the wastegate has to do is provide an easier escape route for the exhaust gas than going through the turbine. In the lower picture the exhaust gas comes from the front (bottom of picture) to the turbine housing, If the gate is open most of the gas will flow through the casing and out through the pipe and open butterfly valve. As the valve is closed more of the gas will take the route out through the turbine blades as that becomes the path of least resistance.

I have no idea why the caption talks about the exhaust contaminating air circulating through the engine. the intake air is in separate ducts from the exhaust and the air intakes are in the front of the wing on B-17s and in the front of the nacelle on B-24s. Well away from any possible contamination from the exhaust outlet.
 

Which pilot's manuals? Canyou point me to a post?



Yes, VDT was aimed at utilising exhaust thrust, but the variable nozzle also served to control the turbine mass flow, therefore the turbo's speed and boost.
 
I took the pictures but the people at Airpower Magazine made up the captions rather than using the ones I provided. Where they got that part about the exhaust feeding back into the intake I do not know.

By the way, the drawings I provided of the R-2800 supercharging are not of the same system. The one on the top, in color, is of the P-47 with the turbo. The one on bottom is of the R-2800 with two stage mechanical supercharging as used on the F4U, F6F and P-61A/B.
 
Dash119 said:
So the pilot's manuals previously mentioned contain simplified diagrams which do not represent the actual operation of the wastegate and it placement within the exhaust system?
Click to expand...

I think they are accurate, but maybe lack a little explanation.

Here's the B-17 turbo installation diagram from the post you referenced.



The wastegate is to the right of the turbo, and looks like it is after the turbo. It must be realised that the wastegate is connected to the turbine housing and is, in effect, at the end of the exhaust duct. Some more arrows to indicated exhaust flow would be helpful - the exhaust either flows through the wastegate duct, or vertically down through the turbine wheel in that schematic.
 
wuzak,

Okay, so on these installations the wastegate is integral to the Turgo-Supercharger rather than being a standalone device. That clears up the confusion on my part.

So the real difference between the P-47 and the other Turbo-Supercharged aircraft in the USAAF during WWII is the standalone wastegate. I would also assume that is the difference between the 'B' series and the 'C' Turbo-Superchargers.

Thank you for your patience with me.

Kim
 
First became aware the the P-47 turbocharger configuration when building a model many years ago.
Have seen WW2 film of P-47's taking off, with that exhaust exiting at the rear of the plane. Reminded me of a car exhaust in wintertime.
 
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