Superchargers- how do they work?!?

[kool kitty89]

Red Admiral, what do you know of the use of a "waste gate" on mechanical superchargers. (not the exhaust gate on a turbo)

I don't see how it would work with the arrangement used on the carburetor fed aircraft engines as the fuel/air muix is inside the supercharger. (I can see it possibly being used on fuel injected engines or a supercharger stage preceding the carburetor)




On the altitude performance SFC issue, ive seen this too (on some V-1710 models as well), and it also seems to get worse above the critical altitude. Perhaps it has to do with the mixture control in these conditions or the decreased compression ratio.

 

[red admiral]

I'm afraid that I don't have anything that goes into that much detail but I'll have a look at one a sectioned Merlin over Christmas and see what I can find.

The decreasing boost at altitude would also account for increased fuel consumption as the efficiency reduces, although that would be balanced to some degree by reduced back pressure.

 

[<simon>]

Thank you all very much for putting such time and effort into answering my relatively short question!

I certain never expected such a response and i have copied and pasted a fair bit of it the use as references in the future.
Theres a huge amount of info collaborated here and its well worth the time it takes to read throught it all!

Thankyou all again.

Regards,
Simon

 

[Burmese Bandit]

A quick question with, I suspect, a looooong answer. I know that central supercharging, which means charging one or two engines from a supercharger driven by a third engine, was tried by the Germans. Apparently this was a dead end. What are the mechanical and other difficulties of this method of supercharging?

 

[Burmese Bandit]

Ah...any central supercharging experts here? I know there must be some!

 

[net_sailor]

I have read recently about cental supercharging sytem ACN-2 developed in Soviet Union for TB-7/Pe 8 bomber. There were engine unification foreseen (one AM-34FRN providing compressed air for four AM-34FRNW engines) but due to lack of space a M-103 additional engine was mounted. The system was developed to operational stage. Finally only 4 serial a/c with ACN-2 were equiped. The main reason was.... lack of manufacturer! This system had been constructed by engine research institute and only four prototype installation were produced. There was no space for large scale production on their small workshop. In USRR production couldn't be moved to another plant without an orders but orders never cames.
Later project was totally abandoned on behalf of turbocharger development (not lucky also).
The central supercharging system takes space in fuselage and adds some weight. Additional fuel and cooler is required (even larger than normal engine installation - there is no extra cooling effect from rotating proppeller). In case of failure the power loss on all engines occures.

 

[Burmese Bandit]

Yes...and this SEEMS to confirm my suspicions, that there is nothing inherently mechanically wrong with central supercharging...which now brings me to an idea I've long had:

with Mechanical supercharging, you have to 'lose some to win more'. In other words, you have to spend about 150 hp to run your supercharger from the engine, in order to get back 250-300 hp. Extra Hp, 100-150.

What if you ran the supercharger from a SEPARATE, smaller, auxiliary engine? You would get the full 300 hp benefit, but suffer the weight penalty for an additional auxiliary engine. This may not improve power to weight ratio - in fact it might well worsen it - but there are situations in aircraft design where you need 'power at any cost' rather than 'power at the best power to weight ratio'.

One possible example to think about. If central supercharging had been used for the B-29, may it not have been possible to put four narrow merlins with 2,000 hp each inside the wings of the B-29, like the design of the B-36? IMHO the weight penalty would have been more than offset by the tremendous drag reduction.

 

[tommayer]

Red Admiral,
I have long suspected what you say about altitude, fuel consumption, and superchargers--the more boost, the hotter, so P&W and CW dumped extra fuel in at altitude to forestall detonation and preignition.
I'm too old and busy to do it, but I'd love to put a modern turbocharger and good intercooler onto a 985. I think what would happen is the backpressure from the turbo would fool the engine into thinking it was running at efficient altitudes, and you'd get truly spectacularly good fuel burns without hurting anything, plus you could raise the critical altitude a good deal. Result: an old airplane with a very efficient old engine going very fast.
The BSFC numbers for 985s and 1340s, as long as MP does not exceed ambient, are pretty spectacular, .4 and even under .4 at 60%. Those guys knew what they were doing.
Tom Mayer

 

[Aurum]

I have read recently about cental supercharging sytem ACN-2 developed in Soviet Union for TB-7/Pe 8 bomber. There were engine unification foreseen (one AM-34FRN providing compressed air for four AM-34FRNW engines) but due to lack of space a M-103 additional engine was mounted.

That was the only first aircraft ANT-42 "bear".

Pe-8 (serial, without fifth engine) flew across the Third Reich for several times in 1942. And it transfered mr. Molotov (min. FA of SU) delegation to Washington at once in may 1942. They landed in Scotland and Reykjavik and then flew over Atlantic to Washington. They successfully returned to SU aprox. the same way.

If Ki.77

had had 3-rd engine to drive supercharger, it could have much more chances to reach Berlin from Tokyo and had not be lost somewhere after Singapore. Axis states could have quite regular stable communication by such planes if they had even small number of them.

In the case of Ki.77 even small inline aircooled Argus type engeen of 400 h.p. seems to be enought.