Supercharger vs. Turbocharger

[Lunatic]

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.

=S=

Lunatic

 

[Gnomey]

Nice info Lunatic!

 

[Lunatic]

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.

=S=

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]

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?

=S=

Lunatic

 

[Lunatic]

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

I have limited time and only occassional internet access/freedom. I cannot research things as much as I'd like and my books are all over 1000 miles away. I pretty much reply to what I see in a thread as I see it - often someone else has made the same comment subsiquently. I'm trying to read a few posts ahead to avoid duplication of info.

I always enjoy reading your posts wmax and appreciate your diligence in researching/verifying info. I wish I had the time to do the same. My training is almost over (techincally it's over tommarow but until I get an assignment I'm sure it will continue).

=S=

Lunatic

 

[wmaxt]

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.

=S=

Lunatic

The sad fact too, was the cockpit cold was partialy due to the cruise operating procedures of high rpm, low manifold prs. and fine prop pitch. When used with slightly lower rpm and corser prop pitch the load was greater as well as the cockpit heat system (the heat was provided by hot exaust manifold ducting) and reduced oil cooling and wastegate cooling. It still wasn't enough but it was enough to help. I found this when comparing the ETO problems to the Aleution missions where they didn't have anywhere near the magnitude of cold issues even though it could be -40 on the Ground. They never implimented the heated flight suits either. The 8thAF did not support the P-38 adequately, it was also very new at the time.

At altitude the P-38 wastegates on pre J models did freeze, usualy in either hi or low boost positions and the intercoolers were not adjustable when it came to cooling the charge air. the J/K/L models with core intercoolers and adjustable flaps to control airflow cured those problems

wmaxt

 

[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.

wmaxt

I always enjoy reading your posts wmax and appreciate your diligence in researching/verifying info. I wish I had the time to do the same. My training is almost over (techincally it's over tommarow but until I get an assignment I'm sure it will continue).

=S=

Lunatic

Thanks,

Good luck in finding an assignment!

wmaxt

 

[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?

=S=

Lunatic

It was on the engine this the path after the turbo

The air through the intercooler then carb then entered the supercharger then engine intake. This part of the system was new to me because the supercharger is rarely mentioned, Ben, enlightend me.

wmaxt

 

[DaveB.inVa]

wmaxt

I think you're referring to the article and diagrams here
http://www.avweb.com/news/columns/182102-1.html

The cold (blue) air you see going into the PRT isnt being pressurized and sent to the intake. Its only there to cool the PRT.

 

[Lunatic]

Thats a Power Recovery Turbo, the Turbo-supercharger systems are shown below (with very little detail on the turbo section).

Thanks for the link

=S=

Lunatic

 

[schwarzpanzer]

wmaxt:

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.

Yes, on aircraft engines the rules change there, too hard for me to explain now, though someone else here might be able to?

Otherwise, running an engine hard is always going to shorten it's life.

The Tumansky turboprop for e.g.

The detramental issue might be overboost,

Unlikely, it's a switch usually so is deliberate.
They can grenade an engine though.

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),

Yes, though if the engine is hand-built this shouldn't be too much of a problem?

or overheating.

This could be because of a lean mixture at WEP and/or high revs?

From the sound of things the Allison shouldn't have this problem?

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.

Taking this into account, it is IMHO likely gonna be a NO2 problem causing a lean mixture, though:

1. Was NO2 even used?

2. The rules are different when in the air.

As for cam shaft timing for low rpm (>3,000rpm)emgines, tight overlaps are normaly used which would preclude cam timing as a problem.

Yes, they are nowdays, but what about then?

However with a supercharger to compensate, the cam timing becomes far more flexible, so, maybe.

A supercharger can cause/aggravate things like valve bounce or even force the valves open on poorly made/designed valve springs, this might be something?


Lunatic:

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.

I've heard that too, though I may have misread?

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.

Interesting, was that like a viscous coupling?

 

[FLYBOYJ]

Otherwise, running an engine hard is always going to shorten it's life.

The Tumansky turboprop for e.g.

Apples and Oranges - turboprops generally run 100% and are controled with propeller pitch. Recips are variable in RPM..

Actually running a recip at it's design RPM limit is actually good for it provided the proper air-fuel mixture is maintained.....

 

[Lunatic]

WEP usually means a higher boost level (manifold pressure), often accompanied by a small increase in RPM (usually about 10%).

WWII fighter engines were designed and tuned to run at one peak power RPM setting, usually around 2000-2500 rpm. Valve overlap was setup to be optimal at this RPM. Unlike a car engine, where increased RPM is how more air-fuel mixture is burned to gain more power, on an Aircraft engine usually forcing more air-fuel into the engine via the turbo/super-charger system was how it was done.

Increasing the boost means more power and this would, all other things being equal, result in an increase in RPM. However, WWII fighters had variable pitch props and the pitch angle was changed to deliver more thrust per rotation which requires more power and thus the engine RPM did not increase. This of course puts more stress on all the components making it more likely the engine will fail and also requiring inspection/rebuilding to ensure it will not fail on a subsiquent sortie. Any use of WEP on the P-51 required inspection.

BTW: 5 minutes was the limit for the Merlin, though it was hardly a fixed figure. Many P-51 pilots ran at WEP for more than 5 minutes. On the R-2800(C) the limit was 10 minutes (coincidently the limit of water supply for the water injection). On some of the German engines it was even more.

German planes also utilized SEP power (Special Emergency Power), usually in the form of Nitros-Oxide injection (some Mossie's also used NOS injection). However it should be noted that this was, for the most part, not a combat power setting. It was used mostly to climb quickly prior to combat, the exception being at very high altitudes. The nature of NOS is that it gives a sudden power increase and there was little if any variability in how much power was going to be added. The pilot put the plane in a climb and when at full power and slowing down engaged the NOS. The prop pitch adjusted to ensure sufficient load on the engine to prevent over-revving. In level flight at less than maximum speed, or even worse in a dive, engaging the NOS would cause almost instananeous over-revving and subsiquent engine failure. This is not to say it could not be used in combat, but such use could very easily result in disaster. It would probably be used to escape combat not to engage in it. SEP would kind of be like having someone else floor the gas peddel on your car while your driving. Works okay on a strait-away or if your pointed up a steep hill, but for just about anything else...

=S=

Lunatic

 

[Gnomey]

Nice info Lunatic.

 

[wmaxt]

Swarz,

to keep this short I will try to answer this way
WEP
1. is is just a gate in the throttle to make sure the pilot is aware of implementation of the throttle position.
2. Is like, Lunatic said, normaly an overboost (your into boost already).
3. Overboost is normaly any positon beyond whitch the Manufacturer/Owner feels will cause unreasonable wear/damage to the engine. The throttle will be adjusted below the range of total destruction of the engine if used judiciously.
4. When someone is on your ass, trying to kill you, time limits don't apply. The atempt is made to provide a "safe zone" with throttle settings to prevent Grenading and inspection/overhaul rules are ment to ensure that.

Over heating
1. Part of the deffinition of WEP is often system limitations. The extra boost creates heat that, all planes in WWII had while in WEP at some altitudes/situations coud not expel heat quickly enough causing an overheat problem.
2, I don't belive high rpm alone caused over heating, but a lean condition could (did in early P-38s when the mixture wasn't advanced soon enough.

Overboosting
1. The Hi power test reports do not indicate any liquid boosters were added.
2. American planes, to the best of my knowledge, never used No2 but the Germans did, as Lunatic pointed out. We did use water injection esp in air cooled radials. Some late P-38s were reported to have it too.
3. Water was only used at max throttle to cool the mixture and prevent detnotation but it also added a little power.

Cam timing
I don't know if they used larger overlaps or large lifts. Overlap is much less critical on supercharged engines but the high idle these engines have is indicitive of high overlap. Large lifts become liabilities at high operation gut again not likely under 3,000rpm.

Overboost and valve problems
Unlikely esp after the hi boost tests I mentioned above. If they won't lift at 105in/hg+ the I'm not going to worry much at 64in/hg the P-38L-o5s ran.

wmaxt

 

[Marshall_Stack]

All,

I am amazed at how much you guys know about aircraft engines. I'm not a mechanic of any sorts but just an enthusiast. Is there any books or websites that anyone can recommend that explains the internal workings of aircraft engines (both in-line and radial)? Some thing along the lines of "aircraft engines for dummies".

I have learned a lot so far from your entries. I just want something that puts it all together for the apprentice.

 

[syscom3]

The next time I go to Chino museum, I will take some pics of the engine they have there on display. I cant remember whether its a R3350 or a R4360.

One thing I remember is the main fuel supply pipe. It was so big it looked like you could stick your fist in it.

 

[wmaxt]

All,

I am amazed at how much you guys know about aircraft engines. I'm not a mechanic of any sorts but just an enthusiast. Is there any books or websites that anyone can recommend that explains the internal workings of aircraft engines (both in-line and radial)? Some thing along the lines of "aircraft engines for dummies".

I have learned a lot so far from your entries. I just want something that puts it all together for the apprentice.

Marshal,

Go to to your local hot rod shop for basic engine, blueprinting, fuel injection, and turbo/supercharging how-to books. These apply to auto engines but the basics are the same though some things are signifigantly different. Then go on the internet and check out the pages from the engine manufactures pages Pratt Whitney, Curtis Wright, Allison and Packard for those made in the US Rolls Royce in Brittain and Dalmer Benz in Germany. This off the top of my head there are more and spelling may be off but these will get you started.

I've been around this stuff 30+ years and I'm still learning esp about the aircraft related stuff so keep asking questions esp the dumb ones, you'd be suprised at how many others don't know it either.

wmaxt

 

[FLYBOYJ]

All,

I am amazed at how much you guys know about aircraft engines. I'm not a mechanic of any sorts but just an enthusiast. Is there any books or websites that anyone can recommend that explains the internal workings of aircraft engines (both in-line and radial)? Some thing along the lines of "aircraft engines for dummies".

I have learned a lot so far from your entries. I just want something that puts it all together for the apprentice.

This site is geard for someone who is learning to work in the aircraft industry....

http://www.aerolearn.com/

There's tons of sites on line and if you live close to a local general aviation airport, you could usually find aircraft meachnic's text books there. There is a book written by a guy named "Treager" that has technical infomation about every type of engine you could imagine, including rocket motors...