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# Centrifugal Twincharge Idea

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## Centrifugal Twincharge Idea

(OP)
I have no industrial experience outside of my own garage. I consider myself humble about my current knowledge and am seeking advice on an idea I've had difficulty finding information about after extensive research.

What I'm exploring is the feasibility of a twin charged engine using a turbo feeding a clutched centrifugal supercharger that is oversized and designed for operational rpm before the turbo has "spun up." At this point the supercharger will disengage to prevent exceeding the max allowable impeller speed and a bypass around the supercharger will gradually open to remove the flow restriction. Pressure relief valves where necessary after closed throttle will be used. The thought process was to gain the benefits of a conventional twincharged (positive displacement) system with more flexibility in engine bay arrangement and the removal of any supercharger flow restriction during the turbocharger's maximum output.

As I said before I am a novice although I do have quite a bit of time under the hood and online for my short years on this planet. I'm looking for your guys' thoughts and predictions on this. There very well may be a major design flaw and I need help finding it or any advice for success. Thanks guys!

### RE: Centrifugal Twincharge Idea

On paper, at any given steady state operating point, your concept appears to feasible. The devil will be in the details, especially getting the transitions to happen as intended in both directions and under all conditions. If you can pull it off in a driveable, reliable and performance-competitive state I will be extremely impressed.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

### RE: Centrifugal Twincharge Idea

Hemi is right on.

Making it work at any fixed point in your speed/load plot won't be all that difficult; rubber meets road during transients, especially when you're switching from supercharged to turbocharged operation.

There's really two ways to skin that cat- you either make the transition as immediate as possible. Meaning that the turbo will be pressurizing the inlet of the supercharger (the interaction which is most complicated) for only a very short period, and the transition between intake air through the supercharger and intake air through the bypass is as fast as you can make it.

End result of this is a system which WILL have a little 'blip' in driveability during this transition; depending on your goals, this may or may not be OK.

The alternative is, of course, to optimize and make the transition as smooth as possible. This requires more time, better controls, and more mechanical ingenuity; the end result will be a car that is better for daily driving.

### RE: Centrifugal Twincharge Idea

Define feasibility. To an engineer that term relates to the usual fit/form/function. To the business folks that means ability to generate happy customers and maximum profit.

Your idea should be fairly simple to implement and calibrate since the supercharger and turbo aren't compounding, merely bypassing each other. In theory it should give the best of both worlds. In reality it will be expensive to install/maintain and fill already crowded engine bays to the limit. Whether or not its worth the cost and effort depends on who you ask. ;)

### RE: Centrifugal Twincharge Idea

I believe that something like this has been done electrically.
The scheme used an electrically driven supercharger. Drive an alternator with an exhaust driven turbine.
The motor may be speed controlled to give the level of boost that you require.
At low power levels the motor will be driven by the battery. At high power output the exhaust driven alternator powers the motor.
No need to worry about transitions from supercharger to turbocharger mode. The motor does not know or need to know whether the power is coming from the battery or from the exhaust driven alternator.
Has anyone seen this done? Does it work in the real world?

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: Centrifugal Twincharge Idea

It doesn't really make a whole lot of sense to use a centrifugal design for both - something biased towards making more boost as revs rise. If you want grunt right off the bottom then use a positive-displacement blower of some sort. The twin-charged engines that I know of have all been like that. The twin-charged engines that I know of that made it into production have all been superseded by newer designs that are just plain turbocharged.

What is the bottom line objective of what you are trying to accomplish?

At a certain point you just have to tell the driver (or, more commonly these days, the automatic transmission) to downshift instead of trying to make stupendous (detonation-inducing and main-bearing-shattering and clutch-vibration-damper-smashing) torque right down off idle.

I own a production vehicle with a mechanically driven centrifugal supercharger (Kawasaki H2), and it's driven using a plain ordinary fixed drive ratio from the crankshaft, and it's in combination with very mild bottom-end-torque-oriented cam timing on the engine itself. It will pass everything except a gas station. It has a bypass valve in the intake system, but it only operates under near-shut-throttle conditions so that the compressor is not operating in surge.

### RE: Centrifugal Twincharge Idea

Variable guide vanes are used on turbos to get them to respond quickly from idle.

### RE: Centrifugal Twincharge Idea

Staged blowers works, if you always need both of them. But if all that you are trying to do is minimize lag then use an electrically driven blower, either centrifugal or PD. Done correctly you can get a very efficient system this way.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Centrifugal Twincharge Idea

You can feed a turbocharger directly into a positive displacement supercharger without any other special equipment.

### RE: Centrifugal Twincharge Idea

Information about the new Mercedes M256 inline-6 engine mentions it having both an electrically-powered compressor (supercharger) as well as a twin scroll turbo. I'm not sure about the arrangement

Regarding the transition control: "The trick is when to use either or both compressors, and that’s left to some complicated software developed by a staff of 10 in-house engineers, according to Vollrath. The result is a steady supply of forced air with virtually no gaps from idle to redline, generating forced-air power and torque with improved efficiency and no lag." Sounds easy, LOL.

Ref: http://autoweek.com/article/technology/why-mercede...

### RE: Centrifugal Twincharge Idea

(OP)
Thank you guys for all the input. I really appreciate it. To clarify the goal of this project is to explore the interactions of this design idea and how complex it would need o be. I am willing to spend the time to create a system that may start with a rough transition and refine the smoothness over time with increased control over the pressures and flow in various locations. One thing to add is I have full standalone control with decent resolution and eventually dyno access to combat transition roughness with spark or fuel changes where necessary. I'm committed to this project so I'm more or less looking for advice in implementing this centrifugal design as I'm currently designing and building my own centrifugal supercharger anyway regardless of whether or not it will be used for this application. Thanks again for all input.

### RE: Centrifugal Twincharge Idea

(OP)
I have another concept. What if I designed the performance of the supercharger to have similar performance to the turbo with respect to rpm and use it as a replacement for the turbo until it spools up and then is bypassed the same way. This way I can do away with a clutch and keep max boost at high rpm's instead low rpm. This way it essentially does the job of anti lag without the increased wear. The is with similar performance curves under wot.

### RE: Centrifugal Twincharge Idea

How do you propose to bypass the centrifugal blower?

If you divert its discharge to atmosphere it will consume a lot of power doing nothing - the power consumption of a centrifugal blower increases with increasing mass flow rate through it, and diverting discharge to atmosphere means the mass flow rate will be the maximum possible.

If you shut the flow off completely it will be operating in the surge regime.

If you clutch it, it will require time to speed up.

Study the production Kawasaki H2/H2R engine, please.

### RE: Centrifugal Twincharge Idea

Isn't your idea a bit similar to what BMW is currently doing with the 4 turbocharger B57 engine? I think they bypass the turbine side of the small turbocharger to control the speed. The work done by the smaller second stage compressor is likely just enough to overcome the flow losses through the small compressor.

Remember that the second stage compressor will see inlet conditions where the air is much denser than atmospheric, so you can pass more mass flow through the small compressor than you could if the inlet conditions were STP.

Corrected flow through a turbo compressor is m-dot x sqrt(T1/Tref) / (P1/Pref).

### RE: Centrifugal Twincharge Idea

Rather than needlessly complicate the system with two identical compressors you might be better off simply using either an electric supercharger or an electrically boosted turbo.

### RE: Centrifugal Twincharge Idea

Or a mechanically driven one.

### RE: Centrifugal Twincharge Idea

(OP)
To reiterate again I'm already building a Centri so I'm not going to use an electrical supercharger if I do twincharge. This is the only system I am considering because I have the resources already and I am committed to finding a solution with these components even if its not most ideal. I want to do something that hasn't been done before. Either I will reroute the air to the turbocharger inlet when bypass opens or have the setup transfer from a series to parallel state if operation. I'm ok with power consumed by the supercharger at max rpm. I have plenty of turbo for my desired power and fuel consumption is not a concern.

(OP)
Very interesting

### RE: Centrifugal Twincharge Idea

That's very cool- Pun intended.

### RE: Centrifugal Twincharge Idea

(OP)
Just a little update on the project. I have begun construction on the supercharger using a Borg Warner S475 compressor cover and wheel with a custom gear housing and shafts. Bear in mind the compressor in the picture is not Borg Warner it is only for demonstration. The final design will have a Precision 6769 turbocharger feeding the centrifugal supercharger with a mechanical vacuum operated bypass valve similar to those used on factory sequential turbo setups like the V6 TDI biturbo motor. Of all the possible configurations this was chosen due to simplicity. An electronic valve may later be implemented for further improvement. The bypass will open when delta p for the supercharger becomes close to zero and attempt to hold the supercharger in zero duty while the turbo is at maximum boost. Theoretically this will reduce supercharger power consumption to only friction losses and the pressure drop in the housing due to viscous forces. I will update when the supercharger has been completed and passed testing phase. I will be running simulations of all transients with ANSYS CFX to determine optimal bypass operation and if a mechanical valve is capable of meeting such requirements. More to follow.

### RE: Centrifugal Twincharge Idea

The idea of attaching an alternator/generator to the turbo to recover waste heat (and disabling the main alternator until loads exceed what the turbo generator covers) then using an electric blower as a supercharger is intriguing to me... an electric blower won't have lag and allows precise control. What are the pros and cons of this approach versus augmenting a turbo with a mechanical blower?

### RE: Centrifugal Twincharge Idea

(OP)
Cost. Obviously that system would be far superior. Perhaps a future concept.

(OP)

### RE: Centrifugal Twincharge Idea

"High" by what standard. "High" by vacuum cleaner standards is "peanuts" by forced-induction engine standards.

The mechanically driven centrifugal supercharger in my beast of a motorcycle spins at 120,000 rpm at engine redline and develops something like 1.2 bar of pressure. I don't know how much crankshaft power it takes in that condition but I'm sure it's several kW.

Using a leaf-blower as a supercharger is rather widely regarded as a joke ...

(OP)

### RE: Centrifugal Twincharge Idea

Nowhere in that ad does it say anything about the pressure that it makes.

It can somewhat be inferred from the "110 mph" claim. If it can blow at 110 mph then we can deduce the velocity pressure from the Bernoulli equation with the assumption that the nozzle is fully effective at translating static pressure into velocity pressure. It won't be dead-nuts accurate but it won't be a million miles away from the truth, either.

So what velocity pressure corresponds to "110 mph" flow velocity under the assumption of starting with standard atmospheric conditions ...

I have to do these calculations in units that make sense. 110 mph = 180 km/h = 50 metres per second.

Standard air density is 1.2 kg/m3.

So the velocity pressure is then 0.5 x density x velocitysquared = 0.5 x 1.2 x 50^2 = 1500 pascals.

Standard atmospheric pressure is 101325 pascals.

So this thing is capable of developing a "boost pressure" of about 1.5% of atmospheric pressure. If you prefer old English units, 1500 pascals is 0.2 psi.

Now, I doubt if the "nozzle" is converting ALL of the static pressure to velocity pressure. We don't really know the basis of the "110 mph" claim. (Is it at the tip of the nozzle, is it some standard distance away from the end of the nozzle, etc.) But it's not out by two orders of magnitude compared to what's needed!

### RE: Centrifugal Twincharge Idea

Good ole Wikipedia. Appraently it’s more common to assist the turbo with an electric motor. They also suggest an accumulator... do they mean an air tank pumped up during periods of no demand then discharged into the intake?
https://en.m.wikipedia.org/wiki/Electric_superchar...

### RE: Centrifugal Twincharge Idea

Compressibility of air at 1500 Pa "boost" pressure is insignificant and can be neglected.

A real supercharging blower needs to generate enough boost pressure that you can't neglect it any more.

### RE: Centrifugal Twincharge Idea

This sounds like a bad idea to me. Brian summed it up in post #6 - the centrifugal blower is not the ideal machine for low rpm grunt. BTW to make it do that, it needs to be UNDERSIZED for the engine to keep it out of the surge region under low-rpm/high-boost conditions. On changeover to the turbo you will need to uncouple the drive to the supercharger to keep it from over-speeding (with the drive ratio needed to produce significant low rpm boost). An air con clutch will probably do.

The bypass will open when delta p for the supercharger becomes close to zero and attempt to hold the supercharger in zero duty while the turbo is at maximum boost.The delta p will increase not reduce as you raise the inlet pressure. The PR of the centrifugal supercharger remains the same.

Use a positive displacement blower or staged turbochargers if you must twincharge.

Easiest route is not to twincharge at all - just use a positive displacement supercharger. If you must have the outright power of a turbo there are ways to fill in the bottom end - careful exhaust design with divided housing for blowdown, advance exhaust valve timing, larger exhaust valves, a shot of nitrous brings on boost nicely.

je suis charlie

### RE: Centrifugal Twincharge Idea

(OP)
I guess I didn't clarify this will not be for low end grunt. Merely throttle response as the car will live in high rpm's on the track. If it was a street car you would be absolutely right about the positive displacement being better suited. The car will stay above 5000 rpm's on track with my gearing and rev limit. The purpose is to get instant throttle response out of the corner without antilag or nitrous. A pd blower will be more parasitic and difficult to fit in my engine bay.

### RE: Centrifugal Twincharge Idea

Adding complexity to your system makes fast throttle response LESS likely, not more.

I either missed it, or that last post was the first time you mentioned the application for this design; i.e. this isn't a street car. As you move from a street driving profile toward a race car (i.e. much more time @ WOT, much higher mean RPM levels, etc) the advantages of a supercharger over a turbocharger fade rapidly.

This sounds like a heavier, more expensive, less reliable version of a well tuned turbocharged race engine.

If this engine really will spend all of its time in anger above 5000 RPM, and unless we're talking about a motorcycle or a very expensive/advanced car engine, you're trying to provide peak pressure and maximize throttle response over a range of maybe 4000 RPM.

With current variable vane turbo technology and even remotely competent tuning, achieving both of these goals simultaneously is not hard.

If this is for a trackday toy and your goal is just to have fun and do something crazy, by all means soldier on. If your goal is to build the fastest car possible for X amount of money- this is not it, pretty much regardless of what X is.

There are a lot of problems to solve which you've glazed over- namely what you're going to do with the high volume of high pressure air coming out of the supercharger discharge when you shut it off. Clutching the supercharger doesn't solve this problem, because the supercharger has a moment of inertia greater than zero; when you declutch, it will not stop, potentially for several seconds. Using giant gears to drive the compressor makes this problem worse, not better.

You also have to do all this in a way that doesn't cause the compressor to operate in a state across the surge line- unless you want to destroy a lot of compressor wheels and rebuild a lot of engines.

### RE: Centrifugal Twincharge Idea

Once again, I implore study of the Kawasaki H2 and its mechanically-driven centrifugal supercharger, and the bypass valve that is built into the design to address the near-shut-throttle situation.

### RE: Centrifugal Twincharge Idea

Thanks for clarifying. The main cause of poor turbo engine throttle response is the downstream throttle plate. Move it upstream. Trailing throttle through corners maintains reasonable turbo rpm and throttle response is vastly improved. The formula goes something like this:
- turbo operated well below its boost limit (to avoid overspeeding)
- modified compressor seal arrangement (two piston ring seals with atm' venting between)
- ball bearing or modified thrust bearing to handle greater thrust loads
- water-air intercooler (or no intercooler) to minimise intake volume

je suis charlie

### RE: Centrifugal Twincharge Idea

Has this setup been mentioned yet?
A super charger discharging into and through the turbo.
You size the supercharger to give the boost that you want.
Blow through the turbo.
As the turbo spools up, it will be be sucking air away from the supercharger. At the point where the turbo mass delivery equals the mass delivery of the supercharger, the pressure between them will equal atmospheric pressure. As the turbo increases the mass delivery further a check valve will open and allow more air to enter the turbo inlet.
At this point the pressure drop across the supercharger will be close to zero and it will be doing very little work. It can be left spinning without wasting a lot of energy and be ready for a smooth transition when the turbo contribution falls off.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: Centrifugal Twincharge Idea

Waross - seems like a practical idea.

### RE: Centrifugal Twincharge Idea

I assume you mean a positive displacement supercharger. If it is sized to provide a useful amount of boost, I'm failing to see what is the probable net gain of adding a turbocharger to the setup. I suggest you do a back of the envelope calculation of the component inlet and outlet conditions and component sizing at the crossover point, and then another one at the design point of the turbocharger with the same component sizing.
In the case of the PD blower, by component sizing I mean the volumetric displacement per 2 crank revolutions (assuming 4-stroke engine), as a ratio of the engine displacement.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

### RE: Centrifugal Twincharge Idea

A supercharger normally gets its energy from the engine that it is supporting. It provides a constant boost ratio throughout the load range.
A turbocharger gets its energy from the heat of the exhaust gases, energy that otherwise would be wasted.
Thus the turbocharger is more efficient than a supercharger.
However, the heat of the exhaust increases with the loading on the engine and so the boost from the turbocharger varies from little or none at low loading to more than enough at high loading.
The low boost at low loading produces throttle lag until the turbo spools up.
The plan is for a smooth and simple transition from constant boost at low loading to avoid throttle lag and then a transition to turbo boost, with the supercharger basically moving air against zero back pressure, at which point the power demand of the supercharger will be almost nil.
The best of both worlds with a check valve allowing the turbo to overtake and unload the supercharger when the turbo comes into its working range.

Note: A two cycle GM diesel does not have a supercharger, it has a blower. The purpose of the blower is to scaveng the exhaust gases out of the cylinder during the time when both the intake valves and the exhaust ports are open.
Yes, the GM blower does make an effective supercharger when fitted to a high performance engine, but it does not increase the charge volume.
Add a turbo ahead of the blower and it works against the back pressure of the exhaust impeller of the turbo to produce boost.
There is an interesting range of GM blower/supercharger sizes.
A 2-71 blower from 2 cylinder engines.
A 3-71 blower from 3 cylinder engines.
A 4-71 blower from 4 cylinder engines.
A 6-71 blower from 6 cylinder engines and 12 cylinder engines.
An 8-71 blower from 8 cylinder engines and 16 cylinder engines.
Then there is the 10-71 supercharger. GM blowers/superchargers became available from independent manufacturers when they were either too expensive or no longer available fro GM. I response to requests for a larger supercharger than the 8-71 the 10-71 supercharger was developed for the high performance after market, but there never was a 10-71 engine.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: Centrifugal Twincharge Idea

The net gain would be either:
- higher top end power thanks to maintaining or rising boost. Also exhaust energy takes over the role of driving the compressor from a (probably inefficient) positive displacement blower.
- or (in the case where waross's check valve/bypass is eliminated) better efficiency by using exhaust energy to take over the compression role AND return some work to the crankshaft by "driving" the blower with a PR less than unity.

je suis charlie

### RE: Centrifugal Twincharge Idea

(OP)
This is essentially my design only with the turbo feeding the supercharger. I'm starting to realize nobody shares my vision and I'm getting little advice that is benefitting my progress. This project is mostly for the fun of it however I have spent a lot of time researching and am reaching a point where free information is less available And more trade secret. I appreciate all the input guys. I'll let you know how it goes. If I fail at it feel free to laugh. I won't have any regrets.

### RE: Centrifugal Twincharge Idea

No one is going to laugh at you- this is a community of engineers. This isn't bimmerforums.

### RE: Centrifugal Twincharge Idea

With the turbo feeding the supercharger you are going to end up with a lot of boost. If you include a means for de-clutching the supercharger you will experience a sudden drop in boost and a pause waiting for the turbo to catch up.

je suis charlie

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