About hybrid turbos. --->

[VonK :: Retired]

As we know, production turbines and compressors are carefully matched on an engine dyno with their specific engine so as to keep both turbine and compressor operating near their maps' max efficiency island.

If, then, only the compressor wheel is "enlarged", it will demand more power from the turbine to actually flow more air at the same boost. That, in turn, requires more turbine throughput (less waste gate flow), driving the turbine into lower efficiency with increased back pressure and lag. In an extreme case, sonic speed is eventually reached at the turbine throat, defeating any further manifold pressure increase, and melting the turbine wheel.

To do it right, shouldn't wheel changes in a turbo involve both sides? I know, It's harder to do for the turbine.

What say you? Please, no techno-babble.

[VerrÜckt]

[PSmalls::DoggieStyle]

[Funkweasel]

compressor doesn't flow all THAT much more air, perhaps 15-20%. But what it does do is flow that air quite a bit more efficiently. As far as the pressure choking, bollocks' car ran pretty hard all the way up to 6800rpm(on race gas, so high pressures were used) even with slightly increased displacement. Shouldn't be a major problem, especially on pump gas cars. Whether it would be of large benfit to use another wheel or play some tricks to improve the gas flow through the turbine/turbinehousing....we'll see.

[Imolavirus@BuehnEngineering]

While from the factory, turbochargers are matched to their engines, the factory is not always looking for what we are with the hybrids. They (in my opinion) match a turbo to the engine to provide the necessary HP, while maintaining reliability, drivability, fuel efficiency, ect... which in not what we usually look for in a turbocharger setup. We (the enthusiast) want a turbo which will flow enough air/boost at an efficient level, while maintaining a reasonable level of drivability.

So, specifically regarding the K04/RS6 as an example. This combination of compressor wheel/turbine is used on the 225TT. I don't see us maxing out the turbine wheel in terms of flow when we are only pushing what, 1.34 liters/side?

"If, then, only the compressor wheel is "enlarged", it will demand more power from the turbine to actually flow more air at the same boost." This is the point I'm trying to make. With the addition of the larger compressor wheel, you are able to flow that more air at same boost level with less rpm's of the turbine wheel. Once the desired boost/air flow level is reached, the wastegates will dump the excess air. This is why proper wastegate flow out of the turbine inlet area is crucial, as to not put the turbine wheel into an overspin situation. I want to say something else, but you may think its techno-babble, so I'll leave it out so no accusing of that.

And my last thought.....have you ever tried to melt a turbine wheel? Inconel aint no puss.

And in the end, I'm not trying to sound like I know all about this subject matter in the slightest, we are all constantly learning. Hybirds however are a great way to achieve increased boost/air flow, while maintaining reasonable lag, to a certain extent. Don't go trying to put a T72 compressor wheel on a K04 shaft : )

[PSmalls::DoggieStyle]

[VonK :: Retired]

There's just one point:

A compressor's power consumption is proportional to the boost multiplied by the volume flow rate. Therefore, increasing the flow rate at the same boost increases the power consumption. The turbine must deliver that added power through increased shaft torque at the same speed, or even greater torque if the shaft speed is reduced. This means that the turbine will be forced to operate way beyond its design pressure ratio (back pressure).

Which brings up the question whether anybody has ever instrumented a hybrid turbo on an S4 to measure the actual shaft speed/boost/airflow relationships. It would be interesting to see whether, say, the K04/RS6 combo can actually maintain boost while delivering more air flow at lower shaft speed. I don't think so.

BTW, I've destroyed a number of SS turbine wheels. But I've seen deformed incanel wheels. I assume, the K03 and K04 wheels are incanel.

Havin' fun.

Harald

[Imolavirus@BuehnEngineering]

Not trying to be a dick, but i've seen my fair share of destroyed turbine wheels and the melting of inconel....well, you'd have to have some SERIOUS problems. the melting point of inconel is something like 1410 degrees C, that's hot!! before the head melts, other things are going to happen to destroy the turbo, the bearings are going to fry, shaft can break, ect...but this isnt what we are really discussing

Specifically talking about the K04/RS6: I dont understand how you can say that the turbine wheel is going to be working out of isnt "designed efficiency range." Like i said, the 225TT uses this combination from the factory. If the K04 turbine wheel wasnt operating in its efficiency, they would have stepped up the turbine wheel..dont ya think?

Now, if we have two compressor wheels, say a K03 and K04 wheel, both on a K04 turbine wheel. (hypothetical numbers) Lets say for the K03 wheel to produce 1.2bar of boost, it has to spin at 175K rpm. For the K04 wheel to produce the same amount of boost, it is going to be spinning at 135K rpm, while also pushing more air. The effect of the turbine wheel have to work "harder" to spin the K04 wheel is negated by the benefits the compressor wheel is producing.

BTW...in 1995 when porsche put out the GT1...what turbos did they use? They took the K16 and K24 and made Hybrids

[VonK :: Retired]

Re your paragraph 1: Turbine wheels can be destroyed by manifold after-burning. But, as you say, that's not what we're discussing.

Re your paragraph 2: I agree, though what is a perfect match for the 225TT is not necessarily perfect for the S4 engine.

Re your paragraph 3: The example should really be a K03 turbine driving either a K03 or K04 compressor wheel at 175k rpm or 135k rpm respectively. Producing the higher output of the K04 compressor wheel would require the tiny K03 turbine to produce higher torque at lower speed than what it was designed for. I can't see what "benefits" the larger compressor wheel would produce to negate that.

BTW, I have nothing against hybrids if they match the engine (GT1). But I wonder whether a hybrid K04/RS6 is really a better match for the S4 engine than a K04/K04, how much lower the shaft speed really would be, and how much higher the turbine backpressure would have to be. To answer that would require instrumented and controlled comparative testing. With just VAG logs, I suppose we're reduced to educated guessing.

H

[Imolavirus@BuehnEngineering]