A Question on Supercharging

[[TWB]Sauerkraut-]

I don't have an intricate understanding of how every part of an engine works, but I know my basics. A while ago I was talking with my father (who was a damn good mechanic and knew quit a bit about high performance engines in cars and boats) about engines in WWII fighters. I mentioned how the Germans used this elegant solution to the problem of supercharger throttling, by using a fluid coupling so that the speed of the supercharger could be controlled independent of the engine.

 

Then he said to me, "Why didn't anyone think to use a simple dump valve to release excess pressure, instead of throttling the engine?"

 

I can't seem to find any sources on why this couldn't work. Could anyone with a more in depth understanding of aircraft engines in WWII explain why this wouldn't work? Could it be that no one thought of this as a solution back then?

[JtD]

This works on turbos and it was done this way (waste gates). On supercharger aircraft you have a mechanic coupling (or fluid in case of the DB60X), where you have no dump valves in the proper places.

 

Or I don't quite understand what your father means...

[Guest deleted@134347]

could it be that your farther confused 2 different technologies? In lament's terms the turbo wastegate is part of the system that is used to build up and release excessive pressure from the exhaust gasses, with the main focus being the reduction of the initial spin-up lag of the compressor (i.e. at low engine rpm's the turbo rpms are low as well without the additional pressure system). But in general the turbo spin depends on the input exhaust gas pressure.

 

The supercharger, however, operates using a different principle and requires a direct drive between the engine's rotational part (crankshaft) and a compressor. In cars it's a belt driven transfer system. The challenge with superchargers (as you can see on vvs fighters going above 3k) is that with a direct drive there's no way to adjust the speed of the super charger, hence the vvs introduced a second gear to their SC for the high altitude flight, so the SC spins much faster and compresses whatever air is left near it at high altitude.

 

The Mercedes-Benz introduced a new way to rotate the compressor, which is via this hydraulic coupling, and without any involvement of exhaust gases. The idea is that it's no longer driven by the 'crankshaft' but by oil pressure pumped in to it. Because you can regulate oil pressure using the oil pump rotation you can therefore infinitely regulate the supercharger speed via this 'hydraulic' energy transfer.

 

There's no gas pressure anywhere in this system, other than the output compressed air.

 

You can read it up in this doc:

 

https://www.google.com/url?q=https://www.flightglobal.com/FlightPDFArchive/1942/1942%20-%200810.PDF&sa=U&ved=0ahUKEwianpb03bPeAhWQr1kKHR3ADokQFggUMAA&usg=AOvVaw0Jel4MYeK0aW3MyguYptk_

Edited November 1, 2018 by moosya

[[TWB]Sauerkraut-]

1 hour ago, moosya said:

 

There's no gas pressure anywhere in this system, other than the output compressed air.

 

 

Right, that's what he was referring to. He was talking about a dump valve between the supercharger and the intake manifold. He was NOT talking about anything to do with a turbocharger.

 

--------------------------------------------------------------------------------------------------------

 

High pressure air                    ---------------->                  Air is now at a lower pressure

from supercharger                                                      (Safe for engine to use)

                                                 Dump valve       

-------------------------------------------------\    /  \----------------------------------------------

                                                         \        \        (Dump valve allows some air to leave, so the pressure on the right side of the valve will be lower than on the left side)

                                                           \         \

 

The problem with optimizing a supercharger at any given altitude is that below that altitude, you have to throttle the engine in order to prevent the supercharger from overboosting your engine.

The fluid coupling was Benz' solution to the problem, and it was a very innovative one, as the supercharger's RPM could be controlled independent of the engine's RPM to prevent overboosting.

 

As far as I can tell, the solution illustrated above would be a much simpler solution to the problem. You could still use a mechanical system with a fixed ratio, and you wouldn't have to worry about throttling the engine because  any excess pressure produced by your supercharger would be released at the pressure dump valve before reaching the intake manifold.

 

 

Edited November 1, 2018 by itsthatguy

[Guest deleted@83466]

I'm the least mechanically inclined guy in the world, and know next to nothing about engines, but hear me out

 

I don't see the advantage of using up power of the engine to run the supercharger at a given speed and then just dumping the excess boost potential overboard like that.   In the German solution, I gather that you aren't feeding the supercharger any more power than you need.  In your father's solution you are generating power that you don't need, or can't use because of engine limits, and then just throwing it away, so what is the advantage over just throttling down the engine, and not generating that excess boost potential in the first place?

 

But like I said, I know nothing of engines, and might very well not understand the concepts being discussed, so I probably should shut my trap...?

Edited November 1, 2018 by SeaSerpent

[Guest deleted@134347]

40 minutes ago, itsthatguy said:

 

Right, that's what he was referring to. He was talking about a dump valve between the supercharger and the intake manifold. He was NOT talking about anything to do with a turbocharger.

 

--------------------------------------------------------------------------------------------------------

 

High pressure air                    ---------------->                  Air is now at a lower pressure

from supercharger                                                      (Safe for engine to use)

                                                 Dump valve       

-------------------------------------------------\    /  \----------------------------------------------

                                                         \        \        (Dump valve allows some air to leave, so the pressure on the right side of the valve will be lower than on the left side)

                                                           \         \

 

The problem with optimizing a supercharger at any given altitude is that below that altitude, you have to throttle the engine in order to prevent the supercharger from overboosting your engine.

The fluid coupling was Benz' solution to the problem, and it was a very innovative one, as the supercharger's RPM could be controlled independent of the engine's RPM to prevent overboosting.

 

As far as I can tell, the solution illustrated above would be a much simpler solution to the problem. You could still use a mechanical system with a fixed ratio, and you wouldn't have to worry about throttling the engine because  any excess pressure produced by your supercharger would be released at the pressure dump valve before reaching the intake manifold.

 

 

 

it seems there was safe-fail 'secondary' throttle, which I believe was relying on a mechanical feedback of some pressure plate.  German Punk, man.. ?

 

[JtD]

9 hours ago, itsthatguy said:

High pressure air                    ---------------->                  Air is now at a lower pressure

from supercharger                                                      (Safe for engine to use)

                                                 Dump valve       

 

This solution would be less efficient than a throttle. Thermodynamically this would be the same as just feeding everything into the engine (full throttle all the way, with the effect of higher manifold pressure as altitude decreases). In the below picture I've used the supercharger power consumption curve from the BMW801 to illustrate. I've extended the second gear curves for a clearer picture, and as you can see, throttling back the engine is more efficient than dumping the compressed air would be. At sea level it would be 100hp lost, and the actual altitude used it's up to 50hp lost.

 

The extra power going into the supercharger would also increase supercharger losses accordingly, leading to higher temperatures in the supercharger and intake air, further reducing engine power output.

 

Now in theory you could use the dumped air for cooling of the remaining intake air, thereby increasing charge air density and thereby power output, but then I doubt it would give you a benefit equivalent to the extra power input at the supercharger. It would have to compensate not only the mechanical extra power, but also the increased charge air temperatures, and as there is no free lunch in physics, I doubt it could be done at more than 100% efficiency. Not to mention that any sort of intercooling requires weight and space and would not come for free, either.

 

A practical point of not dumping compressed intake air is that in many engines fuel was injected into the supercharger or prior to the supercharger to increase supercharger efficiency and reduce supercharger temperatures. With such designs, you'd not be dumping compressed air, but compressed air-fuel mix.

 

 

Anyway, it is an interesting idea...

 

[[TWB]Sauerkraut-]

JtD, thank you. That was exactly the type of answer I was looking for. I appreciate the time you took to explain with that level of depth.