Supercharger vs. Turbocharger

[DaveB.inVa]

Thats not a very good cutaway for what your looking for.

You have to look on the engine as the mechanical supercharger was mounted directly on the engine. In this case the engine is a Pratt and Whitney R-2800. Here is a cutaway of this engine. Note the blue section. This is the supercharger and right above it is the carburetor. The basic diagram on the google search shows right where the turbocharger feeds up to the carb.

http://www.enginehistory.org/G&jJBro...%20cutaway.JPG

[FLYBOYJ]

The CL-28 Argus used 3350's with turbo compounds..

[DaveB.inVa]

Here are some good cutaways of the R-3350 TC. Pretty inovative concept, Ive seen some big rig diesels that have experimented with this recently.

http://www.enginehistory.org/lsfm.htm

[wmaxt]

Originally Posted by schwarzpanzer

Hydro/Methanol (C-Stoff?) = no.

(NO2 Nitrous Oxide) = maybe.

- It may also mean overboost on a Supercharger.

[quote"wmaxt "]However fuel in the exaust system would throw a flame into the air on a rich start even with a blow through and is unrelated to type of carb or intercooler or even location of carb or intercooler in the system.

True, in that case it might have something to do with the camshaft(s?) duration also?

Nice info DaveB.inVa It's nice to see how ground technology applies in the air.[/quote]

WEP is often a throttle position that allows more power to be used. This position is presumed to be detrimental to engine life esp if it is used for more than 5 minuets. The detramental issue might be overboost, over revs (I've read that the extra force on con rods increased by 6 times with a rpm increase from 5,000rpm to 5,500rpm), or overheating. Depending on the peace/war situation a limit on total hours was placed on an engine before mandatory replacement/overhaul. Interestingly the manufactorures tested Allisons to 2,300+ hp @ 105+ boost and PW ran the 2800 to 3,000hp( from memory does anyone have the right numbers handy?) range for 250hrs without trouble the numbers the AAF ran are substantialy lower.

As for cam shaft timing for low rpm (>3,000rpm)emgines, tight overlaps are normaly used which would preclude cam timing as a problem. However with a supercharger to compensate, the cam timing becomes far more flexible, so, maybe.

I have a question how does trrbo-compounding work?

wmaxt

[DaveB.inVa]

Ive read of quite a few aircraft engines being "optimized" by the crew chief. Particularly R2800's, Ive read that a bunch were turned way up to run over 3000hp in WEP. Most Id read were in M model P-47's.

Basically turbo compounding is like a turbocharger except the exhaust turbine is connected back to the crankshaft instead of to a compressor.

If you look on the link in my post directly above youll see a R-3350 TC. Basically the TC R-3350 had 3 PRT (power recovery turbines) per engine. So thats 6 cylinders per PRT. These turned a fluid coupling for damping and this was transmitted directly to the crankshaft by bevel gears. These provided a way to reclaim energy otherwise wasted in heat. This gave the TC about 20% better effeciency than a likewise equipped R-3350.

[Marshall_Stack]

wmaxt,

Check out this other link that DaveB.inVa posted earlier in the thread. It explains turbo compounding and has adiagram of the construction.

http://www.avweb.com/news/columns/182102-1.html

[wmaxt]

Originally Posted by DaveB.inVa

Ive read of quite a few aircraft engines being "optimized" by the crew chief. Particularly R2800's, Ive read that a bunch were turned way up to run over 3000hp in WEP. Most Id read were in M model P-47's.

Basically turbo compounding is like a turbocharger except the exhaust turbine is connected back to the crankshaft instead of to a compressor.

If you look on the link in my post directly above youll see a R-3350 TC. Basically the TC R-3350 had 3 PRT (power recovery turbines) per engine. So thats 6 cylinders per PRT. These turned a fluid coupling for damping and this was transmitted directly to the crankshaft by bevel gears. These provided a way to reclaim energy otherwise wasted in heat. This gave the TC about 20% better effeciency than a likewise equipped R-3350.

It seams to me that this would be more efficent as a way to drive the turbos at low power settings by getting power from the crank?

wmaxt

[DaveB.inVa]

Im not quite sure I follow what your saying. Are you saying it would be more efficient to drive the turbo from the crank at low speeds?

Even though the turbo compounded R-3350 didnt have turbochargers it did have an engine supercharger. I know for certain it was a single stage unit but I will have to check to see if it was a two speed unit.

When I was at a community college a few years ago a Super Constellation came an and you could tour it. I wasnt able to go and I regret that now. But I really didnt know much about turbocompounds then either, I really would've liked to have seen that.

[wmaxt]

Originally Posted by DaveB.inVa

Im not quite sure I follow what your saying. Are you saying it would be more efficient to drive the turbo from the crank at low speeds?

Even though the turbo compounded R-3350 didnt have turbochargers it did have an engine supercharger. I know for certain it was a single stage unit but I will have to check to see if it was a two speed unit.

When I was at a community college a few years ago a Super Constellation came an and you could tour it. I wasnt able to go and I regret that now. But I really didnt know much about turbocompounds then either, I really would've liked to have seen that.

The ones in your picture have both exaust and intake so they are real turbochargers. Turbos work best at high rpm/hi load. With the fluid coupling they would turn out a higher pressure lower from the crank (like a normal supercharger) then as the demand/rpm grew and the Turbo would be faster and working on its own, possibly even returning a little to the crank. Even if the turbo didn't return any power to the crank, I could see a 20% added efficency to the system.

I wish we had this conversation in '90 I used to work about a mile away and party right by the old Vought/PW factory where I could have gotten the info right from the exaust stack, so to speak.

As for the Connie I actually was a passenger on one in '60 at ~5 years old and one time at the AF museum. I also got a chance to take some pics from the foward compatment of Fi Fi. Pretty cool!

wmaxt

[Lunatic]

Originally Posted by wmaxt

Thats a good description of supercharging.

The difference between the two is that a Turbo is free wheeling depending on exaust gasses to spin the turbine and compress the air to the engine.

The mechanical supercharger turbine is geared to the crank/cam shaft to provide the power to compress the air to the engine.

Common terms, any device providing positive pressure to the engine is a supercharger. Mechanical supercharging came first and is assumed anytime the word supercharger is used by itself. A Turbocharger or Turbo-Supercharger is exaust driven and sometimes both, turbo into mechanical.

Common Supercharged engines/planes
Merlin - 2 stage - always
Allison early P-51, P-40 - single stage
PW-2,800 in fighters - F4U, F6F, P-61 - 2 stage

Common Turbo charged engines/planes
Allison - P-38
RW-2800 - P-47 - edit I oopsed and had it under Allison first.
PW-1,800 - B-17, B-24
CW-3350 - B-29

From memory and I'm not sure about the CW-1,600s in the B-25s. Interestingly, I knew a guy who built PW-1800s at a Crysler plant from '43 on.

wmaxt

Pretty good wmax. However I'd point out that some Merlins (early ones) only had one stage of supercdharging, and some R-2800's (such as in the Hellcat) only had one stage.

It should also be noted that (contrary to someone elses comment) that generally speaking if a 2 stage supercharger is being used the gearing not the staging is what changes with altitude. At the specified alt the gear is switched from a low gear to a higher ratio gear. Both stages are usually active at lower altitudes however for high-altitude designs there was usuallym also a neutral gear for. Usually the first stage is geared and the 2nd stage always runs at the given ratio (to RPM via the crank or cam).

The thing to consider is that the mechanical supercharger eats power (via the crank/cam). Typically on a P-51 for example it ate over 300 HP, which is over 15% (for both blower stages). At the critical altitudes (for low and high blower) more HP is generated than lost, but at lower or especially higher altitudes the HP drawn to drive the blower can easily exceed what is produced and in these cases it is better to switch to neutral.

The turbo-supercharger (where the turbo feeds into a supercharger) has the advantage of using exhaust gas pressure to drive the pump. While this does eat a tiny amount of power in terms of the loss of engine power due to increased backpressure, but this amounts to only a percent or so of engine output as opposed to ~7-10% for a mechanical supercharger stage. It has the disadvantage of requiring more plumbing and more intercooling than a supercharger stage.

Another option is the variable-speed mechanical supercharger, which uses a fluid coupling to vary the speed of the 1st stage of supercharging as was done on most Bf109 variants. This has the advantage of allowing higher power over a wider range of altitudes, and minimizes power lost off the crankshaft. However it has the disadvantage of wasting power at all but the unit ratio (1:1 on both sides of the coupling) and as the fluid in the coupling heats (especially as it slips) the unit becomes less and less efficient.

Finally, about the "exhaust stacks providing thrust". Yes it was found that properly designed exhaust stacks could provide a small amount of thrust. However, this typically only added about 2.5% or so to the maximum speed of the plane, and provided no benefit for climbing or acceleration. It was done because it was basically free, but lets not make this out to be more significant than it was.

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Lunatic

[Lunatic]

Originally Posted by syscom3

Didnt the P38's in the ETO have wastegates that froze up from the cold? Something about the turbochargers would be stuck on full boost and it would ruin the engines (if it didnt fail right out).

Most of the problems with the P-38's in the ETO with cold at altitude had to do with the fact that there was no heating system for the pilot. The pilot would freeze his ass off up above 20-25K. The guns would also tend to freeze unless the pilot was dillagent about periodically running his gun heaters.

Once the engines were running, freezing while in flight was not a huge problem. However, some early models did have problems with the intercoolers being too effective causing problems at altitude in all theaters. This was rectified in the E model IIRC.

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Lunatic

[Gnomey]

Nice info Lunatic!

[Lunatic]

Originally Posted by DaveB.inVa

Also about the P-38. The Allisons on it were much the same and did indeed have an engine mounted supercharger in addition to the turbos. I had this discussion with someone else another time. They insisted the P-38 had no internal supercharger. I have been to see Glacier Girl a few times in Middlesboro Ky and met a bunch of nice people. I emailed Bob Cardin, part of the Glacier Girl crew. Here is my email and his reply.

Dave:

I am curious are the Allisons on Glacier Girl equipped with a single stage supercharger along with the turbocharger... or does it have a two stage supercharger in addition to the turbo? Ive been under the impression that all P-38's had a single stage supercharger plus the turbo. Thanks for any information you can provide.


Bob:

David, the Allison has a single stage engine driven supercharger. Only the P-38 has a turbo to go along with it (the supercharger).
Bob

There you go, short and sweet.

I think the issue is that the Allison on the P-38 had a supercharger stage, but it was not "internal" in the way it was on the Merlin and some of the other engines we're discussing. On most radials, the supercharger stage is really external, but so tightly packed onto the rear of the engine that for all intents and purposes it's internal. On V engines where to put two external supercharger stages was always a bit of an issue. The Merlin solved this nicley by integrating it on top of the engine partially within the cylinder V.

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Lunatic

[wmaxt]

Lunatic,

Your not the only one who pointed out the, Merlin - 2- stage fax pas, and your all right, whats worse I knew that.

One of the things that keep me coming back th this site is there is always more to learn, and even the Die-hards Like me and the P-38, are willing to learn more. Most threads are black/white and still haven't done the research.

wmaxt

[Lunatic]

Originally Posted by wmaxt

The air path on the P-38 was exhaust to turbo and out, Intake air was outside (small airhorns under wing) air to turbo to intercooler to carb. NO mechanical supercharger was used. I have also seen the engine setup for the P-38 and it did not include a mech supercharger.

Are you sure about this. I thought there was a small mechanical supercharger mounted on the back of the engine?

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Lunatic