Supercharger vs. Turbocharger

[wmaxt]

Wouldn't manifold pressure go down if the turbocharger was damaged?

Depends if the slugs hit the turbo and did damage yes, if it hit ducting but within the pressure limits of the waste gate then no. If the waste gate was damaged open yes until the waste gate valve was overcome but if the wastegate was damaged closed it could go way up, to the capacity of the turbo anyway.

wmaxt

 

[DaveB.inVa]

The B-29 did in fact have two General Electric B-11 turbos in parallel feeding an engine mounted gear driven single speed supercharger. When used both were used simultaneously, there was no option to cut out one turbo as on the B-36. An engine driven supercharger was also present on the B-17 and B-24 in addition to the turbos. The supercharger is usually after the carburetor or injection system and serves as a way to mix the fuel/air. One of the big problems with early R-3350s was their magnesium supercharger case. This was the gear driven unit, not the turbos.

Look at this picture of the cutaway R-3350. The engine mounted supercharger impeller is clearly visible in the blue section. This was the way the R-3350 was and you couldnt get it any other way.
http://www.enginehistory.org/G&jJBrossett/RAFCosford/Wright Cyclone R-3350 cutaway view.JPG

Here is an R-1820 (from a B-17) cutaway. Note the internal supercharger.
http://www.enginehistory.org/G&jJBrossett/NMNA/Wright R1820 cutaway.JPG

Its a little harder to see but heres a cutaway from an R-1830 (B-24) Follow the induction path on the right and youll find the supercharger impeller. http://www.enginehistory.org/G&jJBrossett/NMNA/Wright R1830-64 slice view.JPG

In fact FIFI doesnt have turbos, it only runs on the internal mechanical supercharger.

http://rwebs.net/dispatch/output.asp?ArticleID=7

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.

 

[schwarzpanzer]

A point worth mentioning on turbos is that (basically) the compressor (inlet) side needs to be kept as cool as possible whilst the turbine (exhaust) side should be kept warm. Heat should be shielded/dissipated between the two.

Obviosly some parts can fail at extremly cold/hot/differing temperatures.

The materials used nowdays are the same, but the designs and possibly construction certainly aren't. They also require absolute precision, doubt they had that in WW2?

Another Question - I have also read that the P-38's turbosupercharger had problems in the frigid air over Europe, but never had heard of any problems with any supercharged engines. What gives....?

The location of the turbos on top of the boom really didnt protect the turbos and their components from the exceptionally wet European weather.

You've got to consider that B-17's and B-24's in Europe didn't have these problems and their turbos were under the wing, protecting them from the rain.

Wet weather doesn't harm turbos except for cooling on the exhaust side, a cold exhaust/turbo would severly hamper performance. If the water iced up it would be even worse.

However if this happened on the intake side, performance would drastically improve!

With the long, unprotected headers these engines seem to have?
I'd think the former to be at least 1 major factor.

air to turbo to intercooler to carb. NO mechanical supercharger was used.

The supercharger is usually after the carburetor or injection system and serves as a way to mix the fuel/air.

Well, there's a contradiction here, but apart from that:

The set-up wmaxt said is blow-through, Marshall's draw-through.

I expected the P38 draw through because of the its flame problem, blow-through is favoured for preventing that.

Wouldn't manifold pressure go down if the turbocharger was damaged?

If headers to the turbo were damaged, yes. aft of that it could maybe do that, but the wastegate problem seems more likely.

One of the big problems with early R-3350s was their magnesium supercharger case

I can believe it!

Cheers for the pics DaveB.inVa!

That crank-mounted R-1820 supercharger, what rpm would it spin at? It would have to be geared up from engine rpm surely? (like the one on the R-3350 pic appears to be?)

Unless it just served as a mixer?

 

[DaveB.inVa]

Im not particularly saying that the turbo on top of the boom was affected in flight from the elements. Im more concerned with the turbo thats being exposed while sitting on the ground. Having the turbos turned straight up while sitting in the rain and elements is my hypothesis for the P-38's problems in Europe.

Every other fighter or bomber with turbos had them underwing or under the belly (like the P-47). Here while on the ground they couldnt get as much exposure from the elements.

Im with you, a little moisture ingestation will surely up the power! This would be getting into ADI or water/meth injection.. but thats a whole nuther thread.

I don't know the supercharger ratios right off. I have seen them before but cant recall. I seem to remember values around 5 or 6 times crankshaft speed though.

The WPAFB site gives the R-2600 (on B-25's) having a low blower ratio of 7.06:1 and a high blower ratio of 10.06:1.

http://www.wpafb.af.mil/museum/engines/eng42a.htm

Here is an excellent page for some basic descriptions of the supercharger, turbo and PRT and combinations of them. I was looking for this last night but it got too late and I needed to study!

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

 

[Marshall_Stack]

DaveB.inVa,

Great article on superchargers and turbos-chargers!

Two more questions (I ask a lot of questions) -

1. The P-39 is always maligned of its lack of high altitude performance. The P-40, although not a high flier, doesn't get criticized as much even though they have the same engine. Is the P-39 worse at altitude (compared to the P-40) because of its smaller wings?

2. War Emergency Power. I thought at one time I read that if this feature is engaged the engine needs to be overhauled upon return to base. Is this true? It seems like many pilots often used this for more than defensive purposes.

 

[wmaxt]

Interesting info guys. I've never run across that info and I have seen several engine set-ups and cut aways without the single stage supercharger, oh well.

The cut away shows of the 38J/L shows the carb after the intercooler so from the turbo its blow through, ALL P-38 intake air was near the turbo from the air horn below the wing. I don't remember flames in the intercooler ducts but the heat was very hi at times and aluminum will buckle at low temps. Look at ths side wrinkles on a B-52 and those are just from heat generated at 600mph. 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.

The turbo wastegates froze at altitude on P-38s, by then the water from the ground should have been evaporated/blown out by the end of the climb. Thats not to say your theory is wrong either, the issues may have been related.

I will be doing some research too, like I said I'm willing to be educated.

Marshal, WEP power got some added limitation after the war in my P-51 TO-1 (1952) it lists several perameters for inspecting/overhaul:
1. overspeed 3,300rpm to 3,600 rpm inspection prior to further flight.
2. Over 3,600rpm overhaul period.
3. WEP is limited to 5 min to preserve engine life
I didn't find the WEP limit but one is in there. It also must be remembered that in the war life span for the whole plane was `~50 missions so such limitation were not always used.


wmaxt

 

[FLYBOYJ]

The P-39 is always maligned of its lack of high altitude performance. The P-40, although not a high flier, doesn't get criticized as much even though they have the same engine. Is the P-39 worse at altitude (compared to the P-40) because of its smaller wings?

Consider the whole package - Airframe, engine, and very important but sometimes neglected - propeller!!!

 

[schwarzpanzer]

2. War Emergency Power. I thought at one time I read that if this feature is engaged the engine needs to be overhauled upon return to base. Is this true? It seems like many pilots often used this for more than defensive purposes.

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. [/quote]

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.

 

[schwarzpanzer]

On 1st glance at that article, it doesn't mention pulse-tuning or RAM-air.

These increase Manifold absolute pressure to way beyond ambient pressure without forced-induction and are deliberately used in F1 cars, though can exist naturally.

I think the latter was deliberately used in a WW2 engine?

 

[BillytheKid]

Hello: Go to Google (imagines) type in P47 Thunderbolt, click,
second row on the left, is a picture of a cutaway P 47, click, then the
turbo setup.
I don't see any mechanical supercharger in there.
anybody see one?
Billy the Kid AGF

 

[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&jJBrossett/Pima/P&W R-2800 cutaway.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]

2. War Emergency Power. I thought at one time I read that if this feature is engaged the engine needs to be overhauled upon return to base. Is this true? It seems like many pilots often used this for more than defensive purposes.

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]

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]

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]

Well, for the most basic explanation. A turbocharger (such as that used on a P-47, P-38, B-17 etc.) used a turbine to compress air before feeding it into the engine's carb. This allowed a greater oxygen concentration at altitude and thus greater engine power.

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.

=S=

Lunatic