Can a twin screw charger be adapted to be placed "before" the TB?

[bob8907]

In a custom application, we would prefer the instant performance that only a twin screw can offer. I have also not found much to support that a centrifugal type will really give any additional boost at peak rpm than the twin screw. (open to debate).

Problem is we really need something that can mount before the TB, not between the engine and TB as typically done. Is it possible to do this? Is there an air bypass or something that would be needed? it seems like it should work other than a possible runaway at idle due to trying to force open the TB?

 

[patprimmer]

It is possible. I believe it is to dangerous. Others disagree as I am sure they will declare.

The problem is a twin screw blower is positive displacement and can easily bend throttle plates leading to a run away engine if a bypass or blow off valve is not installed or fails to work when the throttle is closed.

You can always remove the throttle plates from the manifold or throttle body and install a throttle upstream. This does slow throttle response, but can't be bent by manifold pressure.

Regards
Pat
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[BrianPetersen]

Forced open throttles = runaway engine.

Think in your mind what happens when you close the throttles. You want to reduce airflow into the engine. The unthrottled positive displacement supercharger has no way of doing this, so it increases its output pressure to overcome the downstream restriction (the closed throttles). Sure, the additional output pressure will reduce the volumetric efficiency of the supercharger a little, but not by as much as the driver is requesting.

There absolutely must be a bypass valve around the supercharger. There should be a failsafe also (like a blow-out panel, or some such thing) in the event that the bypass mechanism fails. The strategy for controlling this bypass valve is not necessarily going to be simple, if you want normal-feeling throttle response.

A throttle upstream of the blower won't have any of these issues and doesn't need a bypass mechanism.

 

[patprimmer]

Yes

Forced open or bent both represent a runaway engine and the dangers that represents when jumping hard on the brakes approaching a sharp bend on a mountain pass for instance.

I believe that if you retain throttle plates downstream you need a doubled up fail safe system like a burst plate or a secondary throttle plate upstream that say activates with brake application or a bypass valve that activates under trailing throttle.

I think you need two of the above or similar to truly be safe.

Regards
Pat
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[70btdc]

The theory of bending throttle plates and forcing open throttles sounds plausible. But it just DOESNT HAPPEN. Manufacturers of some current supercharged pascar vehicles have the throttle after the supercharger, no burst plate, just a normal bypass valve that fails open. Thats it.... Nobody died yet (as far as I am aware). The benefit is throttle response like you say.

 

[bob8907]

Thanks a bunch guys for the info. Due to design issue, modding or replacing the throttle body will not be an option but designing a separate bypass system or building it into a supercharger IS a possibility. In theory, it sounds like we just need something that will mechanically open at a certain pressure to ensure an excess of pressure does not hit the throttle body.

This will be on a vehicle subject to on/off throttle frequently thus the reason for wanting to go this route (throttle response and no lag).

It would sure seem an internal bypass system could be installed in a twin screw to simply redirect output back to the input. Thoughts??

 

[bob8907]

I talked with Kenny Bell just for reference and they say it cannot be done. I still have to wonder if the right valving can make this work. It really is just a matter of pressure spikes when chopping throttle and bypassing flow at idle so it is not boosting.

 

[Five9dak]

How much more throttle response could you possibly need beyond what a normally configured twin screw provides? Has a conventional arrangement been tested and proved inadequate in this application?

 

[Warpspeed]

It depends on the total trapped air volume between throttle and engine. If you are planning on also having a large intercooler, with very large and long pipework, with the throttle up stream of the screw supercharger....

When you close the throttle, with all that pipework still full of boost, it can take an awful long time for the rpm to drop on closed throttle. If you run a manual gearbox, changing up a gear under hard acceleration will not be a pleasant experience.

With an auto box it will hardly make any difference, because you don't need to lift off to change up a gear.

The throttle response problem is not so much during throttle opening, as throttle closing.

Just because it has a blower, think long and hard about making the throttle response worse, especially with a very close ratio manual box.

 

[gruntguru]

So, if you have a 3 litre engine and 6 litres of volume (ie lots) between your throttle plate and the engine, after closing the throttle, each revolution of the engine removes 25% of whatever air is there, so the absolute pressure drops by approximately 25%, the bmep drops by approximately 25% and so on. After 2.5 revolutions the manifold pressure has more than halved. After 8 revolutions the pressure is 10% of the original value. At 6000 rpm, 2.5 revolutions takes 25 milliseconds, and 8 revolutions takes 80 ms. Don't get too excited about throttle response times unless you are increasing the volume after the throttle plate by a fairly large amount.

 

[Warpspeed]

All I can say is people that have tried fitting appreciable pipe (and intercooler) volumes between throttle and engine, always wish they hadn't done it after an initial road test.

Unless the blower is bolted direct onto the inlet manifold, a throttle placed before the blower is not going to improve drivability one bit.

But go ahead and try it for yourselves.
And if you think snapping the throttle shut is going to drop from redline rpm right back to idle rpm,in only two engine revolutions be prepared for a real surprise.

 

[waross]

Just because you can't remove the existing throttle body doesn't mean that you have to use it. Set the throttle wide open or remove the plate(s). Install a second, working throttle ahead of the twin screw.

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

 

[mattsooty]

gruntguru,

are you for real?

How many applications do you know where a closed throttle is indeed zero airflow through the engine?

As warpspeed suggests I think there is some serious misunderstandings of the physics involved here!


MS

 

[patprimmer]

Sorry for the long post, but I think it is about time we got a bit realistic here rather than using extremes to push agendas or support arguments.

I think it is beyond doubt that there are inherent real dangers with a throttle downstream of a positive displacement blower

I also think it is beyond doubt that a very large volume between the throttle plate and the the inlet valves seriously hurts the drivers ability to adequately control engine power in critical situations.

I think it should be easy to agree that where we most need instant response to a closing throttle, we can also least tolerate a stuck open or broken throttle.

I think it is also easy to agree that budget, existing components, class or government rules and application or intended use all play a major part.

If I where to set up a twin charger right now for my daily driver, I would be inclined to mount a fairly small supercharger directly on a shortish inlet manifold with the minimal plenum that does not significantly inhibit air flow.

I would have the throttle plate mounted directly to the supercharger inlet flange with water injection at that point.

I would have a fairly large turbo inter cooler etc upstream of that.

I would have the waste gate triggered by manifold pressure close to the inlet port on the cylinder that required most fuel to avoid a lean out.

If the budget was available I would replace the water injection with a small sandwich plate water to air inter cooler.

I have driven enough blown cars to know that a manifold mounted large blower with a manifold with large runners, medium sized plenum in the manifold and a medium sized plenum in the hat and VERY large throttle plates has no discernible delay on closing the throttle, certainly no more than a heavy flywheel or a dash pot or damper on the throttle plate. Dash pots or heavy flywheel have both been std practice in the past.

If for some reason I needed to have a large volume between the supercharger and the inlet valve, I would use a throttle as close as possible to the valve, but I would have a very simple reliable safety device incorporated, just like I always use dual return springs to different points if possible, as close to the throttle as possible.

While gruntgurus sums are very obviously flawed, a closed throttle does not flow much air as not much is required for idle. I can't be bothered trying to calculate the airflow past a typical throttle body at idle vs WOT. Matty, I suspect you have plenty of examples of fuel flow at stable warm idle vs WOT at high rpm. I expect only a few %age points.

It is pretty obvious that as the engine empties the manifold against a closed throttle a little air is leaking in for idle and the VE drops of quite a bit as the manifold pressure drops, so the closer it gets to idle MAP, the slower it approaches it. Same as you open the throttle, you do not get to full boost with one turn of the blower as the engine is consuming air as you over supply it and the usage rate increases as boost builds, so while it gets to 50% boost real quick and 80% boost pretty quick, it takes about 1 to 2 seconds to reach full boost.

On acceleration with a large roots blower, this delay to reach full boost is rarely a concern as you often actually have more boost than you want as the throttles open.

Regards
Pat
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[gruntguru]

"gruntguru,
are you for real?
How many applications do you know where a closed throttle is indeed zero airflow through the engine?
As warpspeed suggests I think there is some serious misunderstandings of the physics involved here!"

I am absolutely for real. For the purposes of the "back of the envelope" calculation I presented, zero airflow is a reasonable approximation. You should support your attack with facts and figures for example:

An idling engine at 33kPa abs manifold pressure has a volumetric efficiency of say 33% at best. ie 33/100 x 1.5 litres/rev x 800 rev/min = 396 l/min. The flow is choked so closed throttle at 6000 rpm and say 25 kPa the flow will be the same. WOT at 6000 rpm 100% VE (no supercharge) the flow is 1.5 x 6000 = 9000 l/min. Closed throttle = 396 l/min or 4.4% close enough to zero I would say! With 1 bar supercharge this drops to 2.2%

 

[gruntguru]

"And if you think snapping the throttle shut is going to drop from redline rpm right back to idle rpm,in only two engine revolutions be prepared for a real surprise. "

That would be a surprise indeed - probably a broken crankshaft for a start.

 

[1gibson]

Depending on the bypass setup, twin screws will have hotter temperatures at low pressure ratios than a roots blower (despite lower temps than roots at high pressure ratios) because of the actual compression taking place in the twin screw.

Something else to consider, I am not sure but I think temperatures would be higher with throttle after a twin screw.

 

[patprimmer]

At WOT it should make no difference, but at low load up until the point that the bypass valve opens it would be higher and would take more power to drive the blower.

Regards
Pat
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[gruntguru]

Depending on the bypass setup, twin screws will have hotter temperatures at low pressure ratios than a roots blower (despite lower temps than roots at high pressure ratios) because of the actual compression taking place in the twin screw.

Depends on a lot of things including throttle plate pressure drop and location. I think the effect would be quite rare and fairly insignificant (assuming the compressor is matched to the application).

 

[belgian1776]

I think maybe personal opinions with a lack of actual experience with this type of airflow question is showing up here. I am not an expert on supercharging, but can address the problem of airflow to throttle blades. I have a 1972 GMC pickup with a turbocharged & intercooled 454c.i. engine it. This is a completely homebuilt engine with all plumbing done by myself. The enigne has a carburator and the turbo is a blow-thru design, so the throttle would see boost pressure(15 psi plus) when closing. What I did to alleviate this problem was install a bypass that returns back into the fresh air box. The bypass works on a vacuum signal and is plumbed behind the throttle blades. When the throttle starts closing the bypass sees a vacuum signal and bypasses incoming air charge. The failsafe is that the bypass opens all the time...I have had this happen to me and the engine would not make boost. I have NEVER had the engine run away or had any difficulties with throttle blades. I run 2 return springs on the carb for safety in case one might break. This system is not fancy by any means but it just plain works everytime.