Supercharger vs. Turbocharger

[Marshall_Stack]

Which form of supercharging an aircraft engine for altitude was better - the variable stage supercharger or the turbocharger?

[wmaxt]

Originally Posted by Marshall_Stack

Which form of supercharging an aircraft engine for altitude was better - the variable stage supercharger or the turbocharger?

The Turbocharger was best at altitude for several reasons:

1, Turbochargers with a wastegate are variable. At low workloads a turbo puts out little boost, as the demand increases (not RPM only) the boost goes up with the wastegate preventing overboost.
2, A properly sized turbo will still boost higher because of the waste gates ability to allow a larger boost capability.

Superchargers (mechanical) are geared so there speed is in direct control of the engine speed. The fixed relationship limits the quantity of air pulled in. The gearing on the Merlin for instance allowed the first stage to work from about 5,000ft to 12,500/15,000ft (depending on pressure altitude) when the second stage kicks in, there is a speed graph on the Planes and Pilots web page http://home.att.net/~C.C.Jordan/index.html that shows both the kick in of the second stage on a Mustang and the super vrs turbo envelopes. Look in the 3rd installment of the 'Der Gableschwanz Teuful article. At about 25,000ft the fixed speed of the supercharger starts falling behind.

wmaxt

[Sal Monella]

How did the "turbo-supercharger" on the P-47 work?

[Lightning Guy]

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.

[syscom3]

Plus the exhaust gas's if they are of sufficient velocity and quantity, can give you a small amount of thrust for propulsion.

One drawback for a turbocharger is a sometimes complicated ducting system. The P38 was a plumbing nightmare that was hard on the groundcrews and added a layer complexity to manufacture.

[wmaxt]

Originally Posted by Lightning Guy

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

[DaveB.inVa]

The Merlin wasn't always 2 stage. A version of the Merlin found in some P-40's (F and L models) was only a single stage version but it did have 2 speeds. Most of what your referring to were 2 stage 2 speed superchargers.

Also the Allisons on the P-38, the PW R-2800 on the P-47, the PW R-1830 on the B-24 and the CW R-1820 on the B-17 had the turbos feeding a single stage engine mounted supercharger. The CW R-3350 on the B-29 had two turbos feeding a single engine mounted supercharger.

Sal the turbo on the P-47 had quite a lot of ducting to it. The actual turbo unit was mounted near the tail, right in front of the tail wheel. Yet the wastegates were all the way up near the engine!

The engines on the B-25 were CW R-2600's and had a single stage two speed supercharger. The B-26 had PW R-2800 with a single stage two speed unit as well.

The B-36 is of some interest here too because it had two turbos feeding a single engined mounted supercharger on each of its 6 R-4360's. At very high altitude however, exhaust volume would fall to a point that 2 turbos couldnt be driven efficiently. This resulted in power pulsations. To solve this the B-36 could switch over and run on a single turbo. The exhaust volume for that turbo would rise and performance would improve.

[Sal Monella]

I am hearing here that the P-47 had a turbocharger but have always read that it had a turbosupercharger.

[FLYBOYJ]

The P-38, during starting, would sometimes have fuel vapors build up in the turbo charger ducting. While cranking the engine a flame would emit from the turbo port on top of the engine booms. I was told by a former P-38 flight line inspector that sometimes these flames would rise 50 feet from the aircraft. If you discontinued cranking the engine, you run the risk of igniting the vapors and possibly blowing up the aircraft. If the engines didn't start, you had to shut off fuel and continue cranking until the flame dissipated. This "Ole Timer" said this looked real cool at night!

[DaveB.inVa]

turbocharger=turbosupercharger

The P-47 had a turbo that fed an engine mounted supercharger though.

[schwarzpanzer]

Turbo +

The turbo basically uses otherwise wasted energy below 300 mph.

It is quieter and more economical than a supercharger.

Turbo -

Take off and restarting would be harder.

They are extremely heavy nowdays, back then due to inferior materials/design they were even heavier!

Temperamental.

Can be dangerous.


The supercharger saps more power at higher revs, but at 2000rpm this would be negligible, also in WW2 most could be de-activated.

Originally Posted by wmaxt

At low workloads a turbo puts out little boost, as the demand increases (not RPM only) the boost goes

Not exactly, a disadvantage of a turbo is it can be temperamental.

Originally Posted by syscom3

Plus the exhaust gas's if they are of sufficient velocity and quantity, can give you a small amount of thrust for propulsion.

Very, very, very hard to implement on a turbo.

One drawback for a turbocharger is a sometimes complicated ducting system.

It doesn't have to be that way.

Originally Posted by wmaxt

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.

Bang on wmaxt, very well put.

Originally Posted by DaveB.inVa

The B-36 is of some interest here too because it had two turbos feeding a single engined mounted supercharger on each of its 6 R-4360's.

The twin-turbo!

A trick now can be to have 1 big turbo and one little having one or both on, was this done in WW2?

In WW2 an intercooler was really needed for each charger, so the B36 would have 3?

Also don't forget the turbo-compounder!

Originally Posted by Sal Monella

I am hearing here that the P-47 had a turbocharger but have always read that it had a turbosupercharger.

Like DaveB.inVa said, they're the same thing.

Originally Posted by FLYBOYJ

The P-38, during starting, would sometimes have fuel vapors build up in the turbo charger ducting. While cranking the engine a flame would emit from the turbo port on top of the engine booms. I was told by a former P-38 flight line inspector that sometimes these flames would rise 50 feet from the aircraft. If you discontinued cranking the engine, you run the risk of igniting the vapors and possibly blowing up the aircraft. If the engines didn't start, you had to shut off fuel and continue cranking until the flame dissipated. This "Ole Timer" said this looked real cool at night!

Yeah, you've got compressed fuel/air right next to a hot (1000 degrees) exhaust.

[DaveB.inVa]

The B-36 turbo setup was pretty much like the B-29 setup in that two separate turbos fed into a single engine mounted single speed supercharger. They were in parallel. Both had an intercooler between the turbos and the supercharger (for that matter I cant think of one right off that doesnt).

Also I dont know of any aircraft right off the bat that didnt use an engine mounted supercharger.. even when a turbo was used. Im sure there probably were some, but nothing you'd ever expect to perform.

Both turbos were the same size.



Sequential turbos are a pretty neat trick and can provide plenty of boost and CFM. Basically youve got the compressor of a big turbo feeding the compressor of a smaller turbo. With the exhaust first you feed the small turbo then on to the big one. With well matched turbos doing this can keep all turbos well within their efficiency range and allow them to produce tons of boost and flow. If losses are ignored, for a given flow if the big turbo were to make x psi and the small turbo were to make y psi then the output is the product x times y! This can get big quick. You see it a lot on tractor pullers and some diesel fellers who have a lot of fuel run them to get enough air.

Turbo compounding is an awesome thing but isnt really in the realm of a turbo feeding the engine. Basically recovery turbines in the exhaust are coupled through gearing and a fluid coupling to the crankshaft. When things get to going the PRT (power recovery turbines) send otherwise wasted energy back to the crank. The R-3350 was the only engine I know that ever got into this. These R-3350s are not to be confused at all with the R-3350's on a B-29. They are the same engine but the whole affair with the PRTs makes them a different animal. These R-3350s didnt have turbos used to compress air to be sent to the intake. They did use a single stage two speed supercharger however.

[Marshall_Stack]

All,

Thanks for the education! I have read many, many books on aviation but the only difference that I could find is that one was gear driven (the supercharger) and the other exhaust driven (the turbosupercharger). I could never find any info on the advantages and disadvantages of either system.

Another question - I have read that the P-61B Black Widow had a hard time with some of the Betty bombers that could fly higher than them. Why is that? Also, the P-61C addressed this issue plus gave more speed. What did they do?

[Marshall_Stack]

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....?

[evangilder]

I remember reading that the early P-38 ducting went along the leading edge of the wing and that backfires sometimes caused the leading edge to deform.

One thing with the P-47 ducting. It ran under the fuselage toward the rear of the airplane. The benefit of that was when the P-47 had to land on it's belly, it provided a "crumple zone" that saved the pilots legs from injury.