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Centrifugal Twincharge Idea

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.

RE: Centrifugal Twincharge Idea

(OP)
Very interesting

RE: Centrifugal Twincharge Idea

That's very cool- Pun intended.

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