Honestly, I've always thought the idea of belt drive / turbo compounding was sort of a waste of time, until now. I'd seen this on a little Japanese diesel years ago, but didn't put 2 + 2 together until I saw it again on a Honda on the Net. The idea is (obviously) to use the supercharger to build torque and get the (big) turbo spooled up sooner. The problem is, you have to either blow through the (now restrictive) supercharger, or come up with a complex system to bypass it when the turbo is pumping...or maybe not. The setup I saw (and I CAN'T find the pic again) , used a small factory Eaton supercharger like the one pictured, which employs an internal bypass valve.
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In this setup, the turbo blows into the supercharger inlet. Now here's the interesting part...the supercharger bypass is controlled by a vacuum diaphragm connected to the manifold. As long as manifold pressure (vacuum) is lower than ambient, the bypass is open and air goes past the supercharger, conserving power.



As soon as the throttle is opened enough; manifold pressure equals ambient and the valve closes, engaging the supercharger. Now, if we had a double port diaphragm setup (like a boost controllable wastegate pot),
where the "ambient" side were plumbed into the charge pipe from the turbo; we would have a means of internally bypassing the supercharger at low load and under turbo boost.

At low throttle cruise, manifold pressure is below ambient (in the charge pipe with no spool), and the engine is N/A. When the throttle is opened, manifold pressure equals ambient and the bypass closes, engaging the supercharger. When turbo boost exceeds supercharger boost, "ambient" pressure (charge pipe) again exceeds manifold pressure, opening the bypass and allowing the turbo to blow directly through into the engine. It should be totally seamless and automatic for any combination of throttle opening and boost...

Thinking about it, I wonder if a couple of big BOVs or wastegates could be used in the same way as external bypasses....

Any thoughts or ideas?

 

Realized wastegates or BOVs won't work. The bypass valve needs to be 3 way...inlet to supercharger or inlet to engine. The factory bypass in the Eaton supercharger is obviously too small for any sort of performance turbo application. Still, I can't see why the idea wouldn't work.

 

Your fundamental understanding of compond compressors is flawed. You need to read up on how they work. When you understand how the supercharger is not a restriction your starting to understand. Search this site as there have been good discussions.

 

Thanks Andy, I did read much of the discussion on compounding a supercharger with a turbo. My mistake in the original post was not pointing out that I do not see the supercharger adding to cylinder pressure once the turbo is spooled up. The roots supercharger would be strictly an "anti-lag" device in this system. I probably should have titled the thread differently... The idea of the vacuum/boost operated bypass is simply a more efficient way of bypassing the supercharger than pushing against spring loaded doors, or some of the other methods I have seen. I'm not eyeball deep in the forced induction stuff, so my apologies if the push/pull actuator idea isn't new. If it's already out there, I've never read anything about it... I'd envisioned a fairly small supercharger (for a given engine size) to get the engine up on the torque curve and quickly get some exhaust energy into the turbo. Obviously a small blower isn't capable of the air volume necessary for big HP numbers, so once we're done with it, it would be nice to have it "out of the way". I had in mind some sort of a lower manifold spacer with the (larger) bypass built into it and replacing the supercharger air horn.

I'm still sold on the idea of an efficient, seamless bypass... According to Kenne-Bell, a roots supercharger will add 15-18deg F of intake temp per lb of boost. If we go to the trouble of intercooling the turbo, why heat the air up again...?

 

A guy recently did this on performancetrucks.net in a newer model GM pickup. IIRC he had a TVS1900 blower and a rearmount (really midmount) S475. He also had the bypass valve begin bypassing boost once the turbo came up (they were both attempting to net 10 PSI, and as the blower got closer, it bled off some boost so it was still 10 psi total). He was definitely pleased.

 

There have been a few people who have done this on this site. If the setup is well designed i could see how it would be a good idea provided you were using the blower to eliminate other compromises in the system. Such compromises include high stall converters, big heads, big displacement, big RPM and huge cams to get a usable powerband with a specific sized turbocharger. In a non drag racing world I could see a lot of benefits from this as powerband becomes more useful but even with a manual transmission the powerband can be fairly narrow and it doesn't matter. If it's an automatic it can probably be even yet more narrow if the stall is high enough and not matter. The Eatons on the GM applications with bypasses use throttle position for the bypass but i doubt it would be hard to modify one to be "boost referenced." A stepper motor is still likely a better idea, perhaps some modified variant of a bypass valve that is boost referenced? If you actually wanted to put this into a vehicle a drive by wire throttle body would likely be easy enough to come by and work very well. Besides this on a smaller engine I will also point out that the Eaton M62s used in some Mercedes actually have a clutch on them so you can spool the turbo and turn the clutch off as well as bypassing the blower in theory. I have not seen anyone else really make a clutch for a blower bigger however.

 

In this setup when the bypass is open you should not think of it as the turbo's air going around the supercharger. Unless you spin it slower the supercharger will move a certain volume of air regardless. This means the supercharger's air is really what is going through the bypass not the turbo's so to speak. The air would be moving in the opposite direction than what you would visualize if you say the turbo's air is going around the supercharger. Since it moves a certain volume of air the only way to reduce the amount of boost it is contributing is to take air from its outlet and return it to its inlet.

 

"When turbo boost exceeds supercharger boost, "ambient" pressure (charge pipe) again exceeds manifold pressure, opening the bypass and allowing the turbo to blow directly through into the engine."

A couple of comments.

1) A factory Eaton type bypass RECIRCULATES the compressed air back into the inlet of the supercharger during idle conditions. The by-pass circuit is downstream of the throttle valve.

2) If the throttle valve is almost closed then the engine is still pumping the same volumetric flow and the supercharger is still moving the same volumetric flow BUT the AIR MASS flow is very low therefore the power/ rpm is very low.

3) You open the throttle more, the the by-pass closes, and the engine "sees" more air mass as the recirculation process has stopped.

With your design, I am afraid that under most conditions you would be recirculating the
air mass and have less power vs just closing off the recirculation valve.

Most "Sequential" Boost Systems using a roots style supercharger or a smaller turbo (along with the larger TURBO), disengage the smaller boosting device completely from the system under higher mass flow conditions. They use a clutch on the drive belt of the supercharger or a wastegate on the smaller Turbo as well as the "Three Way" valve which sends the mass flow from the Large Turbo to the engine.

With your design, unless i misunderstood the concept, would be recirculating air at higher speeds vs blowing air into the intake through the by-pass hole (as the supercharger compressing the air would raise the pressure in the intake higher than at the inlet of the supercharger even with the turbo doing its thing).

Tom Vaught

 

You know what, Tom? I've installed a few of those as conversions on different engines (always with the bypass disabled) and I thought I had it figured out, but that was sort of a revelation...

I was always under the impression that the supercharger was bypassed when manifold pressure was below ambient (vacuum). In other words, the open valve was a "shortcut" for air to pass by the supercharger inlet and go directly into the engine.
That was where I developed the idea that as long as inlet pressure (in this case turbo boost) was higher than manifold pressure, the open valve would simply allow the turbo charge to blow right by the supercharger.

Guess that's why it's good to bounce an idea off of others before building one. The 2 way valve setup still intrigues me, as it would perform the bypass function rather than recirculation. Just need a non-recirc Eaton, I guess.... :hmm:

 

The TVS blowers actually bypass the rotors (supposedly will bypass up to 9 PSI at 6000 rpm).

Heres the link for those interested (in a homebuilt one that works)
http://performancetrucks.net/forums/showthread.php?t=464822&highlight=sequential

 

Wow, that's exactly what I had in mind...minus the whole rear mount turbo deal...

 

Had a idea some years ago about charging a turbo with a compressor.
The idea was that a compressor is using hp from the crank and the turbo get the power for free from the exhaust.
The compressor would charge the engine and the feed the turbo.
Once the turbo start to spool the pressure difference over the compressor would droop and the hp requirement from the crank would be reduced.
One of those things that newer got anywhere other than in my head.

 

You can call it a recirculation valve, bypass valve or what ever you want but when it is open the air is flowing from the oulet of the blower to the inlet of the blower.

 

Care to speculate on what would happen if you did simply close off the inlet to a typical Eaton, etc. type blower? Truly bypass all the air straight into the manifold when turbo boost exceeded blower boost...

It would take something along the lines of an exhaust cutout, where the flow was either one way or the other, and the opposite passage was blocked almost completely.

 

I can't get my mind around what would happen with the inlet blocked. I know if the blower were perfect it would still move the same volume of air regardless which means it would draw a perfect vaccume on the inlet side. There would then be some finite amount a volume flow at zero density which would equate to zero mass flow. I have no idea what would happen with a real blower at this extreme of a case.

I do know if you blocked the outlet (a perfect blower would theoretically consume infinite power) what would happen in the real world is the belt would slip.

 

I know of a company that developed an electric drive to do this. It was a downsized displacement engine with a relatively large turbo. It used an electric motor driven centrifugal blower to provide boost during transient conditions. The SR motor used for the project could accelerate the blower from 0 to 150,000rpm in about half of a second. The trick was two fold. It likely would be very expensive even in production volumes and it was a 42V system in anticipation of everything going to 42V.

 

Yeah, that was my line of reasoning as well. It seems like if you blocked the inlet, the blower would have to recirculate whatever air was in the case and basically "unload". Which should cut parasitic drag to just what it owuld take to spin the rotors??? :hmm:

I just can't see the blower killing itself running with the inlet blocked, and I doubt very much it would "vacuum lock" in a real world application...

 

I have a question-

Why would you bypass the supercharger? It does not turn into a restriction. If your turbo charger is feeding the supercharger with 1 bar of boost, and your SC has a 1 bar pulley your engine won't see 1 bar, it will see 2 Bar.

This is exactly what the diesel guys are doing with their version of "twins"

 

Contrary to my thread title , the idea wasn't to use both as sources of total boost pressure. My thinking was that a small blower would handle the air mass requirement at low rpm while cutting spool time on a big turbo. In that case, the air mass delivered by the turbo would be sufficient to kill parts; so why heat it up again running it through the blower when the engine doesn't need the extra boost?

 

X2, the turbo will make less heat than the blower (so more power from same amount of boost).