Turbochargers & Bulk

[KJ_Lesnick]

How is it that turbochargers were generally more bulky than superchargers

[rickshaw]

'cause they were?

[Go4fun]

You have to remember that things like the metallurgy were not as advanced and thus turbochargers had to be bulky to keep from flying apart under the pressure. Plus turbos being driven by the exhaust were subject to heat which was trasnfered to the fuel/air mixture whih then needed coolers which were bulky.

[wuzak]

Because turbos needed the turbine and its volute.

Also, turbos were used in conjunction with intercoolers, where many superchargers were not.

[jcf]

Not really much bulkier than any other multi-stage induction system. It's all in how you arrange the bits.

http://www.enginehistory.org/installations.shtml

[PR19_Kit]

The Merlin and Griffon centrifugal superchargers were HUGE! Getting on for 3 ft in diameter and about 18" thick, with hefty ribs round their casings to conain the pressure. Lightweight items they were not.

[Hobbes]

These days, a turbocharger is generally smaller, but it needs more plumbing: you need to route both the intake and exhaust through the turbocharger, and those are normally on opposite sides of the engine so you get big tubes snaking everywhere.

Early turbochargers were hindered by metallurgy, as Go4fun says. A modern automotive turbocharger runs at more than 100,000 rpm, while a supercharger runs at 2-4x the crankshaft speed. Higher rpm means a smaller package to transfer the same amount of power, but also high demands on materials.

[eatthis]

These days, a turbocharger is generally smaller, but it needs more plumbing: you need to route both the intake and exhaust through the turbocharger, and those are normally on opposite sides of the engine so you get big tubes snaking everywhere.

Early turbochargers were hindered by metallurgy, as Go4fun says. A modern automotive turbocharger runs at more than 100,000 rpm, while a supercharger runs at 2-4x the crankshaft speed. Higher rpm means a smaller package to transfer the same amount of power, but also high demands on materials.

i think the rotrex (centrifugal) superchargers are basically tubo front ends with gearboxes on the back with similer rpm stats

[KJ_Lesnick]

You have to remember that things like the metallurgy were not as advanced and thus turbochargers had to be bulky to keep from flying apart under the pressure. Plus turbos being driven by the exhaust were subject to heat which was trasnfered to the fuel/air mixture whih then needed coolers which were bulky.

Wait... I thought the exhaust would blow through the turbine then go out the back?  I was under the impression that the impeller would compress the air and drive up it's temperature where it would need to be cooled prior to fuel and air being mixed in to avoid knocking?  I know the intercooler would need to have it's coolant routed through a radiator to get rid of the heat, but inline engines already have radiators so I don't understand how much bigger it would need to be.

Because turbos needed the turbine and its volute.

The volute was the "elbow" on centrifugal flow jets right?

Also, turbos were used in conjunction with intercoolers, where many superchargers were not.

That I know, but the RR Merlin had intercooling/aftercooling and cooling in the volute and it looked pretty compact.  Come to think about it, RR had done some work on a turbo-merlin and it wasn't too bad.

Not really much bulkier than any other multi-stage induction system. It's all in how you arrange the bits.

Then why were the bits on the YP-37 so obscenely large; the bits on the B-17 and P-38 fairly small; and the P-47 a bit clunky?

You'd figure they'd have all arranged it in a manner that stuffs everything in the smallest space, right?

[jcf]

The YP-37 used a GE B-2 turbo-supercharger, which is not bigger than
those used on the bombers. Mounting the radiator and intercooler
behind the engine is what created the look of the P-37. The P-38
also used a B-type, the XP-39 used an F-10. The B, in various dash numbers,
was the standard type during the period and hundreds of thousands were built.



Here is an excellent presentation on 'Supercharging the Allison' that might help you
with basic concepts:
http://www.enginehistory.org/Convention/2009/Presentations/SuperchargingAllison.pdf

[Hobbes]

You have to remember that things like the metallurgy were not as advanced and thus turbochargers had to be bulky to keep from flying apart under the pressure. Plus turbos being driven by the exhaust were subject to heat which was trasnfered to the fuel/air mixture whih then needed coolers which were bulky.

Wait... I thought the exhaust would blow through the turbine then go out the back? 

Yes, that's correct. The exhaust will heat up the turbo: turbos run at several hundred degrees. Some of this heat gets transferred to the intake air, but as you say, the air compression is the main source of heat for the intake air.

I was under the impression that the impeller would compress the air and drive up it's temperature where it would need to be cooled prior to fuel and air being mixed in to avoid knocking? 

That's one reason, yes. The object of air compression is to get as much air (by weight) into the cylinder as possible. Engine power is a function of how much fuel you burn. The more air, the more fuel you can burn.

Air expands as it heats up, so the weight increase is less than the pressure increase. The intercooler recovers that by cooling down the air, so you end up with more power. 

I know the intercooler would need to have it's coolant routed through a radiator to get rid of the heat, but inline engines already have radiators so I don't understand how much bigger it would need to be.

I'm more familiar with automotive engines, they mostly use air-air intercoolers that are set in front of the radiator and are almost the same size as the radiator. You want the lowest possible temperature for the intake air, much lower than the 80ºC or so of the engine coolant.
Their drawback is you need massive air hoses from the turbo to the intercooler, and from the intercooler to the inlet manifold. This adds turbo lag and complicates installation.

An air-water intercooler is more compact and easier to install: you can put it on a direct route form the turbo to the inlet manifold. Its drawback is you need a separate water radiator, so you get two heat conversions and probably a lower efficiency.

[wuzak]

I think all US turbo installs used air to air intercoolers.

Rolls-Royce Merlin and Griffon 2 stage engines liquid cooled intercoolers and aftercoolers (the supercharger housing was cooled, and the big aftercooler in the intak was there for all to see).

I think 2 stage P&Ws used air to air intercooling.

Some Allison 2 stage engines used liquid:air, but many went without, using ADI instead.

[KJ_Lesnick]

joncarrfarrelly

The YP-37 used a GE B-2 turbo-supercharger, which is not bigger than those used on the bombers.

Okay, so the issue was clearly installation.

Mounting the radiator and intercooler behind the engine is what created the look of the P-37.

Why didn't they just put the radiator under the plane (would have taken up way less space) and the intercooler between downstream of the turbo but upstream of the engine?  It looks like it would have been the layout that made the most sense...

The P-38 also used a B-type, the XP-39 used an F-10.

What characteristics differed the B-types and F-types if it wouldn't take too long to explain?

Here is an excellent presentation on 'Supercharging the Allison' that might help you
with basic concepts:
http://www.enginehistory.org/Convention/2009/Presentations/SuperchargingAllison.pdf

I really like this.  It is interesting to note that in order to expedite the development of the Allison V-1710 the US Navy co-sponsored it; I'm wondering why the USAAC didn't just re-engine all their B-10's at the earliest possible date with V-1710's to drive up the demand for the engines (you'd have 306 engines on the spot plus spares and replacements)?


Hobbes

I didn't know to what extent they understood the effect of cooling the air to increase density.  Considering they didn't use them on superchargers and gas-turbines I assumed they'd did not fully grasp this.


[buwzak[/b]

I think all US turbo installs used air to air intercoolers... Some Allison 2 stage engines used liquid:air, but many went without, using ADI instead.

ADI is basically water methanol right?

[wuzak]

What characteristics differed the B-types and F-types if it wouldn't take too long to explain?

The GE Form 13 turbocharger was redesignated B-1 at soem point in the late 1930s. This was then developed into subsequent B-series turbos.

The Form 10 turbosupercharger (and the Form 8 turbo on which the F-10 was based) had a reversed compressor impeller. That is, instead of the air being drawn in through the very end of the turbo, as in B-series and modern car turbos, the air was drawn in between the compressor and turbine housings.

ADI is basically water methanol right?

Yes

[Hobbes]

I didn't know to what extent they understood the effect of cooling the air to increase density.  Considering they didn't use them on superchargers and gas-turbines I assumed they'd did not fully grasp this.

This is basic physics, and intercoolers have been used for this purpose on air compressors since around 1900. It's possible that aviation engine designers were unaware of the work done on air compressors, but IMO not that likely.

The reason intercoolers aren't used on gas turbines is they move huge amounts of air. An intercooler large enough to cool all that air is very large and would add a lot of drag, not to mention all the piping creates lots of air resistance for the compressed air.