Twincharging with Screw-Type SC instead of Roots

[jbond]

Twincharging an engine that features a screw-type supercharger (ie it already has internal compression) in my mind has the following advantages and disadvantages:

Disadvantages:
It would still feature parasitic loss when the supercharger is "bypassed" and the turbo is powering away at higher rpm

Advantage:
The efficiency being better than a roots blower, would mean lower temps and more power

Can one of the gurus post their thoughts? PatPrimmer and Warpspeed had a lot to say on previous TwinCharge topics.

In short: what is the suitability of screw-type SC for twincharging project?

PS: I really have the twincharge bug! help!

 

[BrianPetersen]

What's with the capital letter at the beginning of each word??

Anyway, the trouble with doing any sort of change-over is that it's potentially abrupt and can cause driveability problems.

The VW arrangement has the supercharger in front and the turbocharger second. The turbo "sucks" through the supercharger. When the turbo is "sucking" enough to draw the supercharger outlet below atmospheric, it bypasses the supercharger (and in that case de-clutches it, but a simple flapper that acts like a check valve will do the job as well, just won't reduce the mechanical losses). This arrangement gives a near-constant pressure ratio and even when the turbo isn't quite "on boost" yet, whatever boost it does create is unloading the supercharger to cut down power demand to drive it.

A parallel arrangement would have a significant risk of running the compressor in surge (too little flow through it) when the engine is in supercharger mode, and it would not make use of the turbo to unload the supercharger.

 

[patprimmer]

Sucking through a positive displacement pump does not work, other than to reduce effort to drive the pump.

To draw in extra air for the turbo you need a bypass. The bypass is an extra complication that you don't need if you run the turbo directly into thebelt driven blower.

Yes if you make more boost you need less compression or higher octane fuel, NO MATTER HOW YOU GET THE BOOST.

Have you read the previous treads on this as you seem to be missing some basic points that were covered previously.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BrianPetersen]

Before stating that it doesn't work, might want to take a closer look at how VW did it, because that's the way theirs is, and it's in production.

The VW twin-charged engine is not designed to make the absolute maximum possible amount of power. It is designed to have a flat torque curve and to be as efficient as a gasoline engine can be (by downsizing it). The supercharger is only used when the turbo is "off-design" (takeoff from a standstill). The turbo is designed to unload the supercharger as quickly as possible, as soon as the revs come up.

 

[patprimmer]

As I understand it VW has a bypass. I said it does not work to such through a positive displacement pump. With a bypass, it does not try to suck through the positive displacement pump.

I stand by my comment.

With the resources of an OEM you might be able to tune the bypass to switch over the boost source smoothly, however a a hot rodder that might be more complicated, which I why I advise the less complex method.

Al;so hot rodders typically are looking for more power, not more economy. I doubt that no matter how the OP does this that he will improve his fuel economy, but he will immprove power.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[HoofHarted]

I Have Read The Thread Again And Yes I Did Miss Plenty Good Points. Sorry About The Caps It's Habit More Than Anything.

I Must Admit Im Still A Junior To The Whole Supercharging Idea, I Know Plenty About The Turbocharger But Using Them Together Is New Grounds.

My Only Worry About Running The Chargers In Series Was The Extreme Increase Of Compression. And In That Lies The Reason Why I Opted For Parallel Charging.

I Didn't Say That Series Charging Was Impossible, I Just Didn't Read The Thread Properly.

The SuperCharger That I Have Is An Eaton Type And Not A Screw-Type As I Mentioned. Sorry

I Think That Having The Super Feeding The Turbo Is Rather Complicated In That A Bypass Valve Is Required, And Once Again Im Not Too Sure Of The Plumbing.
But The Turbo Feeding The Super Is A Better Idea, The Only Drawback Is The Two Intercoolers, I Would Rather Have Two Mainly To Prolong The Life Of The Super.

But Once Again Could Someone Advise If The Compression Would Change From One To The Charger To The Other. Thus Dramatically Reducing The Compression Ratio Of The Engine.

I Really Appreciate You Guys Bearing With Me And My Q's As I Come Up To Speed With Things.

 

[patprimmer]

Re he typos in my last post. Sorry I posted late at night and saw some serious misquotes of my previous statements and reacted to correct them when I should have been sleeping.

Once again. The compression is dependant on the maximum total boost and charge temperature. The combustion chamber has no idea how you get to that.

For instance, if the roots blower produces 15 psi at a certain rpm and charge temperature, it tolerates the same compression as a turbo producing the same.

If the roots blower produces 7.5 psi or a multiplication factor of 1.5 times atmosphere, and the turbo feeds it at 15 psi then you get 22.5 psi boost. You need the same compression as if you get 22.5 psi boost any other way



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[HoofHarted]

Thanx Pat For Answering My Q's

If I Were To Add The SuperCharger First Run With It And See Where The Power Comes In And Out (And After That Add The Turbo)

My Q Is Do I Need To Add A Dumpvalve To The SuperCharger Plumbing, I Don't Think So Cause The Supercharger Is Belt Driven.

Then If I Had Both The Super And Turbo I Would Still Need The Dumpvalve. I Know It's A Stupid Q But I'd Rather Make Sure.

Pat You Said

"The plumbing layout is easy.

Air into turbo via filter and bell mouth.

From turbo to intercooler if available.

From inter cooler to throttle body.

From throttle body to the belt driven supercharger inlet.

From the belt driven supercharger to the intercooler if available."

Does The Throttle Body Have To Be Before The SuperCharger And Not On The Inlet Plenum?

 

[patprimmer]

Yes

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[HoofHarted]

Why

 

[patprimmer]

Because, like I said in PREVIOUS THREADS, a Roots blower is positive displacement, and if fully throttled downstream, pressure builds up and bends or breaks things, like the throttle plates, then you have a damaged throttle that can't be closed, then you have runaway engine.

I am really fed up with doing your homework for you. READ THE PREVIOUS THREADS if you expect any more help from me.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Warpspeed]

Yes it does get extremely tedious Pat, I too am sick and tired of people asking the same very basic questions that have already been answered many times over and over, and then wanting to start an argument over it.

To put it simply, the best and simplest way to do this is to place the turbo compressor ahead of the positive displacement supercharger (in series), and use a load sensitive air bypass system directly around both the supercharger and turbo compressor, that opens at idle and small throttle openings. Leave the throttle body right where it is on the plenum, down stream of the supercharger for the fewest drivability problems.

To respond to the original poster, It depends on which is most important to you, full throttle performance, or part throttle fuel economy. A roots blower is well up to the job and has the lowest parasitic losses when fully bypassed. A screw blower will consume some power, even when fully bypassed, as the supercharger heat and noise will quickly tell you.

At the generally low supercharger boost pressures required for twin charging, the screw blower hardly seems worthwhile, unless you already have one on the engine all set up ready to go.

 

[PJGD]

Go to the Ant and study her ways. And while you are at it, check out those of Volvo as well; I'm sure they know a thing or two:

PJGD

 

[BrianPetersen]

That one most definitely is Roots blower and then turbo, with a flapper / check valve to allow the turbo to draw in air bypassing the Roots blower when it feels like drawing in more air than the Roots blower is supplying.

The VW arrangement is similar in concept.

These OEM applications are using the Roots blower to fill in the bottom end of the torque curve before the turbo "spools up", and use the turbo to unload the roots blower as it picks up speed.

 

[jbond]

Don't get confused with "what's best for OEM is what's best for you"

you may have a different set of priorities

it is common sense to understand and to acknowledge that OEM are in the business of making a profit ($)

when people buy a performance car or a performance motorbike, one of the first things they do is replace the exhaust system and the intake filters because of the recognised compromises that are made in the manufacture of them

OEM's calculate what is the cheapest implementation of something that will suffice and yet yield the greatest profits, and then that is the business case

forget VW, look how complicated their implementation is, there are guys on this forum that have implemented the turbo followed by roots and even posted videos of it on youtube and it rocks...

 

[Warpspeed]

It certainly looks that way, the pipework is extremely confusing follow just from those pictures.

But just remember, that is a marine diesel engine. There is no throttle butterfly to suddenly block off airflow. No wastegate either.

Accelerating up through a manual gearbox in a road car involves some fairly drastic throttle butterfly movement, and that places far more stringent requirements on throttle response than a marine or aircraft engine.

While that pipe layout obviously works just fine in a boat, It would create some HUGE drivability problems if copied in a gasoline powered a road car.

For instance, where would you insert the throttle butterfly? And those total pipe volumes look enormous. All perfectly fine for the application, but completely unsuitable for an air throttled gasoline road application.

 

[BrianPetersen]

VW's gasoline (TSI) implementation has the throttle body before the intake manifold (after the turbo and intercooler etc), same as every other turbo gasoline engine.

Regarding throttle closure, I am rather sure that VW's system will use the same style of internal bypass valve around the turbo compressor that they do on their other production turbo engines (2.0T, 1.8T). On that engine, the supercharger is clutched, and the computer is programmed for the conditions on when to engage or disengage this clutch (and the flapper that bypasses the supercharger).

No question that aftermarket may have different priorities than OEM. No question that cost is an issue for an OEM (but what's the difference in cost whether the turbo or super comes first? I can't see it). No question that there are multiple ways to accomplish this. The fact that this thread has so many responses and discussion is evidence of that on its own.

It's equally important to understand why the OEM's do it the way they do it, and what the advantages and disadvantages of each arrangement are, and what all the issues are that need some sort of work-around or some extra control devices, and how those controls should work.

So far, we've discussed two OEM implementations, one gasoline and one diesel, both of which are supercharger first (on the low pressure side), turbo second, with some means (flapper valve and/or clutch) of bypassing the supercharger when it is not needed.

 

[Warpspeed]

The OEMs have one big advantage over the do it yourself Hot Rodder, in that they can use the existing electronic engine management system to control any air bypass system or clutch.

So for them, the control system costs almost nothing (once fully developed) because most of it is already there.

Any air bypass around the supercharger absolutely must be smooth, linear, and extremely progressive in action. The infamous noisy and abrupt turbo blowoff valves are totally unsuitable for this application.

As you slowly open the throttle from light load, to something slightly more, you certainly don't want valves or flaps suddenly flying open or slamming shut, and massive sudden changes in engine induction air pressure. Clutching and de-clutching the blower smoothly is not as simple as it looks either.

If you suddenly slam your foot on the throttle, how do you start up a completely stationary blower without creating a massive mechanical and pressure shock load ?

Instead of a clutch, use a turbo external wastegate (fitted with a light spring) as a supercharger air bypass, actuated by the pressure differential across the throttle body. Use this to completely unload, and SMOOTHLY load up the blower, but keep the blower rotors turning always.

My advice to anyone planning to build a twincharge system with EFI would be to leave the throttle body in the original factory position if at all possible.

Where you fit the turbo into this is entirely up to you. But placing it ahead of the supercharger requires no additional controls on the induction side.

If you place it after the supercharger, some type of air diverting flap will be required. That will almost certainly introduce a whole series of extra problems you could well do without for gasoline road application.

One of the less obvious advantage of placing the turbo compressor ahead of the supercharger, is you will never have any problems with compressor surge, or compressor stall. The supercharger is always drawing significant air volume through the turbo compressor, even with a fully closed throttle. It has been proven to work extremely well countless times, and is simple. Why complicate things unnecessarily ?

 

[PJGD]

Warpspeed: Just remember that Volvo-Penta marine engines have air-to-water and water-to-water heat exchangers, not the air-to-air and water-to-air exchangers that this engine has.

PJGD

 

[BrianPetersen]

The latter point is only true if the air being drawn through the supercharger is being spilled / vented to atmosphere. Normally on supercharged applications, the throttle is in front of the supercharger, which means that latter point won't be any more true than with a naturally aspirated engine. (Closed throttle = next to no airflow, supercharged or not. Same situation for the turbo.) "Unmeasured air" on an EFI system that uses a MAF (mass airflow) sensor will cause trouble.

This stuff ain't simple, no matter which way it's done.

 

[Warpspeed]

That is the whole point Brian.

With a positive displacement supercharger, and the throttle placed down stream, air MUST be recirculated to unload the supercharger at very small throttle openings. And to prevent destructive boost spikes on sudden high rpm throttle closure.

The closer the throttle(s) are to the intake valves, the crisper the throttle response will be. The really serious normally aspirated guys are well aware of this, but the forced induction people often choose to ignore this simple truth.

If you plan to run a seriously large intercooler after the supercharger, (and you should), placing the throttle over the supercharger intake is a very backward step. How bad it ends up, depends on what you have. But it will definitely feel distinctly worse to drive.