Supercharger vs Turbocharger

[Shortround6]

I think it was better than that, the two stage superchargers would also be blasting their air into a low pressure ambient, which also improves thrust. Higher pressure should lead to both higher mass flow and higher exhaust gas velicity. The thrust is given by F =dm/dt x v. Jet thrust is also of greater value at higher speeds where mach effects reduce airscrew efficiency

Yes, the thrust ( and power) goes up with the reduction in outside air pressure. But the charge weight is decreasing once the plane is above critical altitude. As in the examples I gave above, the Merlin XX engine dropped from a charge weight of 144lbs at 9lbs boost (or close to it) at 20,000ft and an jet speed of 1695fps to 129.1 lbs at 25,000ft at just over 6lbs boost but with a jet speed of 1840fps. the difference in jet HP was 113 to 107. or look at the difference between 15,000ft and 30,000ft a difference of just 2.5hp in jut HP even though the charge weight went from 140.5lb/min to 107.2lb/min. the jet exhaust speed went from 1395fpm to 1901. the same book does give the details for 35,000ft. The charge flow is down to 84.5lb/min and the exhaust jet speed is actually dropping a bit to 1890fps ( or instrument error?) but in any case the jet hp is down to 65. Power to the propeller is 568hp.
power to the propeller for 15,000ft, 20,000ft (48.24in boost), 20,000ft (50.67in boost), 25,000ft, and 30,000ft are, respectively--- 1048, 1073, 1126, 960, and 778hp.

Please note that the combined power at 25,000ft is 1067.2hp which is quite a bit shy of what a turbo-supercharged engine would have capable of delivering. A Turboed MK XX would have been capable of 1280hp if not more at 25,000ft to the propeller. Wither the extra 20% in power is worth the extra 400-500lbs and the extra bulk and drag of the installation is the question.
The two stage mechanical drive engines will not only get more Jet thrust from the higher charge weights but because they are operating at higher cylinder pressures and exhaust pressures, but then they also need bigger, heavier superchargers/intercoolers and ducts and drag that go with them. They are also using more fuel to drive the supercharger. The Merlin XX was using anywhere from a minimum of 132hp at 35,000ft to drive the supercharger to a maximum of 236hp at 20,000ft (and the 50.67boost) for it's single stage supercharger in high gear. A two stage Merlin would probably need in excess of 400hp to run it's supercharger set up.

 

[wuzak]

A two stage supercharger should, in theory, require less hp to drive than a single stage supercharger with the same boost and mass flow. Of course in the case of the two stage Merlins they ran more boost, or a higher overal pressure ratio (that is, maintained the boost to a higher altitude).

 

[wuzak]

though turbo is conspicuously absent on the R-4360 of the B-36B

The B-36's R-4360s didn't have one turbo - they had two. They used two C-series turbochargers, similar to the ones used by the P-47.

The B-29 used 2 B-series (also used on P-38, B-17 and B-24).

The choice of turbocharger was based on engine size and/or power output.

 

[davparlr]

FW did try to develop the FW 190C "Kanguruh' with turbo charger: the result shows how difficult it could be to integrate a turbo-supercharger:

There is no doubt that the bulk of the turbo system is a major design issue, especially with a very trim design like the Ta-152. If it could have been made to work it would probably been impressive. The P-47 seems to have solved the mass problem with massive, and bulky, hp, kind of like using a sledge hammer to get a square peg into a round hole. It worked, though.

....

Great info on jet hp, thanks.

 

[Siegfried]

The B-36's R-4360s didn't have one turbo - they had two. They used two C-series turbochargers, similar to the ones used by the P-47.

The B-29 used 2 B-series (also used on P-38, B-17 and B-24).

The choice of turbocharger was based on engine size and/or power output.

Are you sure? The B-36A and B-36B seem to have had only mechanical superchargers: the R-4360-51VDT never entered service.

 

[fastmongrel]

And the wastegate which needs to be controlled and its own ducting and exhaust. Diesels don't need one.

I might have misunderstood you there but a turbo diesel definitely needs a wastegate. Without a wastegate a turbo is going to suffer damage wether its a petrol or diesel.

 

[Siegfried]

Please note that the combined power at 25,000ft is 1067.2hp which is quite a bit shy of what a turbo-supercharged engine would have capable of delivering. A Turboed MK XX would have been capable of 1280hp if not more at 25,000ft to the propeller. Wither the extra 20% in power is worth the extra 400-500lbs and the extra bulk and drag of the installation is the question.

We also need to absorb the extra high altitude power through a larger propeller and that propeller is only 85% efficient.

two stage Merlin would probably need in excess of 400hp to run it's supercharger set up.

Worth about 300lbs thrust (130kg or 1300N) this is worth at 440mph 200m/s (from P = F x v) 260kW at the prop but this would require around 315kW at the shaft, say 450hp?

 

[Shortround6]

Are you sure? The B-36A and B-36B seem to have had only mechanical superchargers: the R-4360-51VDT never entered service.

From Joe Baugher's web site.

B-36A
"Engines: Six Pratt Whitney R-4360-25 Wasp Major air cooled radial engines, each rated at 3250 hp for takeoff and 3000 hp at 40,000 feet. Performance: Maximum speed 345 mph at 31,600 feet."

B-36B "Six 3500 Pratt Whitney R-4360-41 Wasp Major air cooled radial engines. Performance: Maximum speed 381 mph at 34,500 feet"

There is no way that is done with mechanical superchargers.

 

[wuzak]

Are you sure? The B-36A and B-36B seem to have had only mechanical superchargers: the R-4360-51VDT never entered service.

Yes, I'm sure.

I was looking at a schematic of the B-36 engine installation, with its two turbos hanging beneath one end of the R-4360, when I typed that.

 

[wuzak]

The B-50's R-4360s also used two C-series turbos, the XB-44 used a mechanically supercharged R-4360.

 

[WJPearce]

It is my recollection (from things I have read) that the B-36 had two conventional turbochargers. I also recall that for high altitude cruise, all the exhaust would be routed through one turbo. Of course the 4360s still had their mechanical supercharger too.

The VDT 4360 did not enter service just as stated by Siegfried. I believe in its final form the VDT engine did not have a supercharger, and the throttle was basically controlled by restricting the output of the "turbo"; hence the name VDT - Variable Discharge Turbine.

 

[wuzak]

It is my recollection (from things I have read) that the B-36 had two conventional turbochargers. I also recall that for high altitude cruise, all the exhaust would be routed through one turbo. Of course the 4360s still had their mechanical supercharger too.

The VDT 4360 did not enter service just as stated by Siegfried. I believe in its final form the VDT engine did not have a supercharger, and the throttle was basically controlled by restricting the output of the "turbo"; hence the name VDT - Variable Discharge Turbine.

I think there were several variations on the VDT theme. Some certainly did not have an engine stage supercharger, but some may have. Some also had compounding - that is, the turbine in the supercharger also fed power back to the crank.

What killed teh VDT in the end was no suitable control system. The one that flew in a B-50 had to be constantly monitored and adjusted by the engineer. Imagine teh workload if teh engineer had to deal with 6 of them?

 

[Siegfried]

Yes, I'm sure.

I was looking at a schematic of the B-36 engine installation, with its two turbos hanging beneath one end of the R-4360, when I typed that.

Which version? B-36A, B or D? or latter?

 

[wuzak]

The diagram didn't specify which version.

But I believe they were all essentially the same.

 

[wuzak]

Seems I was wrong....they used B-series superchargers:

Each engine was also provided with two General Electric Model B-1 (or BH-1 on later models) exhaust driven turbosuperchargers arranged in parallel. The Primary purpose of the turbos was not to increase the power rating of the engine. Instead, they allowed the sea-level power rating to be maintained up to 35,000 feet, with a gradual degradation at altitudes above that.

The right-hand turbosupercharger on each engine also provided air for cabin pressurization. The flight engineer had teh ability to select dual or single opertaion of the turbosuperchargers for each engine. When single mode was selected, all exhausts gases were passed through the right hand turbo - there was no option to select using the left turbo only.

From Dennis R. Jenkins, Magnesium Overcast, The Story of the Convair B-36

Again, no particular model was specified - perhaps because all production models used the same engine setup.

 

[WJPearce]

I think there were several variations on the VDT theme. Some certainly did not have an engine stage supercharger, but some may have. Some also had compounding - that is, the turbine in the supercharger also fed power back to the crank.

You are 100% correct. A number of different configurations were mocked up but I do not know how many were actually run. Some VDTs did have the mechanical super and some were turbo compound. The one I mentioned I think was intended for the B-36. I just wanted to separate out the standard turbos used on the B-36 vs the VDT engine which was not used.

The Jenkins book has a better schematic for the turbos but this one will do. I'm not sure of the specific model but you can see that number 16 is the turbocharger and it says there are two (2). It says the engine is a R-4360-41, which I think were used on the B-36B and some Ds, and Es.
http://www.air-and-space.com/ficon/handbook%20GRB-36D-III%2010-11%20l.jpg

 

[wuzak]

Nice picture. Illustrates the turbos well.

 

[Gixxerman]

My limited understanding of the supercharger v turbocharger in aircraft was that whilst the supercharger provided sterling service and was stretched (thanks to multi-stage/speed inter-cooling) to meet just about everything asked of it in allied service, the turbocharger was the superior unit for larger aircraft and very high altitudes.

Germany in fact I think illustrates the limitations of the supercharger better than most, whilst they had a beautifully elegant design (the infinitely variable fluid coupled supecharger) - according to a Robbie Coltrane TV show on the DB engines - they simply didn't have large supplies of the fuels required to max out the supercharger, unlike the allies, in general service.
Hence the permanent high altitude issues almost all German planes had.

Of course jets rendered this moot it is quite obvious Germany was prepared to sacrifice 'otto' engine R&D resources for the jets.....and had things not collapsed so quickly who knows maybe their choice could have paid off, tactically at least even if the strategic picture would change little.

Even turbo's only had a short period in the sun as turbo-prop engines rendered the piston engine obsolete and offered much better outputs altitude.

 

[wuzak]

My limited understanding of the supercharger v turbocharger in aircraft was that whilst the supercharger provided sterling service and was stretched (thanks to multi-stage/speed inter-cooling) to meet just about everything asked of it in allied service, the turbocharger was the superior unit for larger aircraft and very high altitudes.

Germany in fact I think illustrates the limitations of the supercharger better than most, whilst they had a beautifully elegant design (the infinitely variable fluid coupled supecharger) - according to a Robbie Coltrane TV show on the DB engines - they simply didn't have large supplies of the fuels required to max out the supercharger, unlike the allies, in general service.
Hence the permanent high altitude issues almost all German planes had.

Of course jets rendered this moot it is quite obvious Germany was prepared to sacrifice 'otto' engine R&D resources for the jets.....and had things not collapsed so quickly who knows maybe their choice could have paid off, tactically at least even if the strategic picture would change little.

Even turbo's only had a short period in the sun as turbo-prop engines rendered the piston engine obsolete and offered much better outputs altitude.

The fuel limited the amount of boost that could be used, but was not the source of the high altitude issues.

All of the German engines that saw widespread service had single stage superchargers. With a single stage supercharger the pressure ratio that can be achieved is limited. At high altitudes their superchargers couldn't give enough boost to maintain power. There were several projects for 2 stage superchargers and turbocharged engines, but I don'tthink many, if any, saw service.

 

[riacrato]

Jumo 213 E had two stage, 3 gears, supercharger and saw limited service near the end of the war in Fw 190 D-13, Ta 152 H, Ju 388 and I think some Ju 88 nightfighter versions.