Twincharged setup, Supercharger THEN turbo?

[Warmington]

Hi,

I have just been directed to and enjoyed reading the following post regarding twincharging setups, and correct SC/TC sizing:

thread71-174690

Some excellent words of wisdom from Warpspeed and Pat!

I have just designed and almost finished installing my own twincharged setup on a very small engine. It is 993cc, 3 cylinder, 12 valve that was originally TurboCharged from the factory. It has a IHI RHB5 turbo charger with a larger than standard compressor from a 1.6 litre RHB5 equipped car.

I have installed an eaton M24 (tiny!!) SuperCharger which is constantly driven without any clutch mechanism at 2 x engine speed. The SC feeds directly into the TC, providing around 7psi of boost at the TC inlet. When the engine manifold pressure (post Throttle Body) exceeds 10psi, a butterfly valve opens to allow the TC to draw air around the SC from a common filter. The SC is not blocked at input or output and continues to supply air.

After the TC there is an InterCooler, then the Throttle Body.

In addition I have used a recirculating dump valve from a large engined car which is connected to the pipework just after the TC, but before the IC. It's return is plumbed back into the SC intake port. The dump valve allows the boost from the SC to continually pass through the TC, keeping the compressor spinning, but does not allow the boost to affect the MAP sensor at idle or small throttle openings.

The engine has not been run properly yet, so it is purely theoretical, but soon to be tested!

I read with interest the theory of boost multiplication (or rather pressure ratios) when the TC provides the SC with pressurised air. I realise I will not benefit from this phenomenon in my setup due to the TC not being a positive displacement pump. Also I realise that my turbocharger should still be sized for max boost and flow as if the engine were TC'ed alone.

What I am hoping for is a lift in bottom end torque and quicker spooling of the turbo. The top end performance was previously about as far as I wanted to go with regards to traction issues with the TC alone, so outright power is not a huge concern (it was producing a reliable 135 Bhp at the wheels with 16psi in a very light car).

Would it really be worth considering rearranging the pipework so that I have the SC post TC, and choosing a larger turbo? Could I achieve similar power outputs whilst increasing engine responsiveness and efficiency by doing this? Is the decrease in exhaust back pressure from using a larger turbo going to give me a better 'feeling' engine with the SC?

Any advice or opinions would be gratefully recieved!

Greg

 

[patprimmer]

I would consider the losses in driving a Roots blower open to atmosphere at both sides as relatively small compared to when producing boost, but not negligible. We are starting to pick at straws now I think.

Roots blower installations typically have the throttle before the blower, as being positive displacement, you build up a very high boost pressure against the closed throttle is the throttle is downstream.

A pump with a vacuum on its inlet side does not pump much.

I have never seen a Roots blower with the throttle downstream nor with a blow off valve.

On thinking about throttle placement in your scenerio, it cant be before the Roots blower, as the blower bypass can open and the turbo will run away.

The throttle for speed control needs to be after the turbo with a very large blow off valve or it needs to be upstream of the Roots blower with the air duct to the bypass feeding from downstream of the speed control so the turbo does not run away.

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.

 

[Warpspeed]

Pat,

Take a look at the Toyota 4AG-ZE roots supercharged engine. This has the throttle mounted on the inlet plenum downstream of the roots supercharger. They can do this, because significant air is bypassed around the supercharger at anything less than full throttle.

http://www.geocities.com/MotorCity/7177/superchargerinfo.html

 

[bentheswift]

Warpspeed, if by downstream you mean air must go through the blower before it gets to the throttle, then you're mistaken.

The path the intake charge takes on the 4A-GZE is filter, throttle, blower, intercooler, manifold. There is literally no plenum on the 4A-GZE, it's just a very short set of runners.

Toyota kind of worked themselves into a corner when they decided to supercharge the high-revving 4A-GE. The only real space for the supercharger in the MR2 engine bay required them to make the intake path very complicated, which makes it easy to confuse the throttle's location in the intake path. The throttle is actually attached directly to the intake manifold, but it's screwed onto the end of it and so it looks like it feeds directly into it, but it does not.

My understanding has always been that if the throttle was located downstream of a positive-displacement pump, if the throttle was shut and a blow-off valve failed to open in time, that something would break, violently, due to the pressures that would result. Even in Toyota's case where they have an electromagnetic clutch to deactivate the blower, the inertia of the blower at 11,000 rpm would still want to pump air to the point of breaking something.

 

[patprimmer]

Not only does it break the throttle shaft or whatever, but it disables the throttle causing a runaway engine which will quite likely break the car and quite possibly break the driver.

Depending on a blow off valve to prevent this would scare the s**t out of me.

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.

 

[Warmington]

Hi pat,

So we agree then that the power required to 'freewheel' the SC is small. I think also the ehat added to the intake charge in this configuration will also be small.

When you suggest that the main flow control valve is upstream of the TC with a big Blow off valve, it is in my application. I may also consider having another BOV located very close to the throttle body/manifold as a safety measure and more importantly, to eject any boost that the main recirculating valve could not flow.

I concur with the concerns that if the BOV bypass fails shut, then the pressure may break something aganist a closed throttle, but i have never had a failed BOV yet. They tend to get 'farty' and fail to close completely when they need to be re-built and I cannot envisage the spring becoming jammed shut, i.e. the only way the valve would fail in the closed position. If the spring breaks, the valve just flutters open.

I really do appreciate the time you guys have given to considering this application, and depsite the general concencus seeming to be "It wont work" I have yet to be convinced of any real insurmountable fluid problem.

I will not get a chance to test the system until between Christmas and new year, but I will record MAP sensor outputs to show boost characteristics with a data recording multimeter conected to my laptop. This should show the curves of boost at the manifold, for different conditions e.g. WOT, changing gear, part throttle etc.

Thanks
Greg

 

[Warmington]

One last question!

Do you think it would be better to have the TC sucking from the same air filter as the SC, or just route the TC to its own filer?

this is the second option:

http://i21.tinypic.com/rwi7oi.jpg

The only differences would be that the flow into the TC would not have to cope with the turbulence of the y-piece before the SC inlet for the bypass. On the other hand, the SC would not be spinning in a vaccum at its inlet any more, only at its outlet. Also the second option gets rid of two bends in the intake piping.

I am sure as long as the first option has the overall pipe size large enough to flow the maximum air in front of the SC , then it will be fine, although maybe a straighter path into the turbo would be beneficial for flow loss.

Any opinion?

Greg

 

[patprimmer]

Did you read my comments on throttle position.

It will be a lot easier to use one air cleaner and one throttle body, then split into two pathways.

If you are chasing efficiency, go to the turbo feeding the Roots blower method.

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.

 

[Warmington]

Pat,

Did you mean this comment?:

"The throttle for speed control needs to be after the turbo with a very large blow off valve or it needs to be upstream of the Roots blower with the air duct to the bypass feeding from downstream of the speed control so the turbo does not run away."

My Throttle for speed control is attached to the inlet manifold. Thinhs are in this order then:

Air filer - SC/TC - intercooler - throttle for speed control.

The boost making system is all downstream of the throttle.

Do you mean that the SC boost provided cannot be metred with the throttle in its current position? That the SC boost will always be pushing past the throttle?

Greg

 

[patprimmer]

Warmington

My comments on power consumption with supercharger running but producing no boost was relatively small when compared to producing boost. Roots blowers do consume a considerable amount of power under boost, so comparatively small is not negligible.

The post I made on throttle position was not clear and I have made a typo that is confusing.

On a straight turbo application it is OK to put the throttle between the turbo and the head, but with a positive displacement blower in my opinion, it is not.

Valves can jam closed, and even though unlikely, it is possible and can have a disasterous result.

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.

 

[jcd06]

Hi,

I'm very interested to follow the discussion about this because soon or later I might give it a try too.
This seems to be a "SC then TC" setup that seems to work

http://www.geocities.com/motorcity/lane/1231/20-valve.html

Regards,

Jean

 

[stimpee]

I have been thinking about this myself as well.

I have already done a twin-screw system (with integrated air-water heat exchanger in the manifold) on a BMW, and am interested in doing something a bit different.

The engine I am working on has individual throttle bodies mounted directly on the cylinder head, and high compression.

How much (or how little) manifold pressure could you run as a minimum on a compounded setup? I would like to do a low pressure setup (maybe 6-7psi) and develop and test. If things work out, then perhaps drop engine compression and go for higher boost levels. At only 6-7psi I could just go with the twin-screw only and be done with it, but it would not have the strong top end of the turbo or compounded setup, and the engine I am working with breathes VERY well at high rpm.

Also, as far as the potential failure modes of a blow-thru setup, why not just use a properly fitted bypass valve (or wastegate as a bypass) and then use a second blow-off or pop-off valve as a redundant setup in case the main bypass failed. At least that would keep you from breaking stuff.

Interested in opinions...

 

[Warmington]

Hi Stimpee,

If you have SC into TC, then the main overboost protection is still maintained by the existing turbo wastegate. The maximum fail pressure will then only be the SC pressure, if you have two valves (blow-off) to dump SC boost off throttle, the chances that both will fail is very slim. You could of course use an external wastegate to replace one or both of the valves giving better adjustability and the option to use them for boost control at part-throttle. In SC into TC max boost is still of course taken care of by the TC wastegate.

In my idea, I was just using a blow-off valve for re-routing SC boost when the throttle is closed as my SC setup has neither a clutch or a bypass valve fitted as standard. Second BV is precautionary and is still in the normal BOV location. If you have an SC with a bypass fitted as standard, then you dont need to worry about the BOV so much. The MR2 setup detailed above has the clutch and bypass fitted.

If you are going to use the TC into SC setup, then I would suggest using Warpspeed's method for boost control/protection outlined in the thread:

thread71-174690

I would say that using your screw supercharger to provide low boost is pretty good as it is. The TC will add efficiency higher up in the rev range at the cost of complexity. If you still want to keep low pressure ratio (like 1.5) then there isnt much call for the turbo over the SC, if the SC is already sized properly. You will of course have the benefit with the TC of being able to turn the pressure up though should you be running out of efficiency with the SC alone. You could also bring in higher pressure ratios earlier on with the compound setup by spinning the SC faster and then changing to turbo. If you were running say 15psi, you could double the SC speed and have 15psi (Pr = 2) accross the rev range with a suitably sized turbo.

With regards to your project, did you make a custom manifold for your application? Was it a Jaguar heat exchanger by any chance? I dont suppose your car was the one featured in PPC a few months back was it?

Greg

 

[stimpee]

If I do it, I was planning on turbo into SC. In that case, I would run a smaller screw compressor (1.6L) at a moderate pressure ratio, to keep the speed of the compressor down and get excellent boost response on the low end in the sweet spot of the VE range. Then I would use a decent sized turbo (have not really thought about sizing yet).

The thought of being able to set up the SC for low end boost response, with the turbo adding to it in the midrange, and then being able to use the turbo WG to ultimately control the manifold pressure on the upper end is kind of intriguing. However to take advantage of the setup, I would have to build the engine to get over PR of 1.5(ish)

If I forego the turbo, I would use a larger compressor (2.3L), which will make for a slightly "soggier" lower end, but have 600whp of potential on the upper end and be a lot simpler, but certainly not as "cool"!

I do not think the PPC article was about me, my car, or my kit. I have had it featured in Roundel (BMW club magazine) twice on an E36 and E46 3 series, Bimmer magazine on an E36M3, and Eurotuner on a 330ZHP.

My kit included a cast aluminum intake manifold using 3 integrated Laminova cores and the compressor mounted directly to the manifold. It is sold by Active Autowerke in Miami, FL.

http://www.activeautowerke.com/supercharger/tse36m3/main.php

I developed it since I was bored with my engineer-turned-manager job, and did most of the development myself out of my garage. Now I am looking for my next challenge. Doing the blow-thru twin screw alone should be enough, but the idea of a twin-charged setup sounds kind of fun (but expensive!) and certainly unique...

 

[jomor]

Apologies if I am restating anyone but I did not see it mentioned.
Detroit Diesel 2-stroke engines, generally 6V92 and 8V92 had the setup you are discussing.

The blower is in the original design anyway due to Positive Pressure Intake air needed for a Two stroke. Normally the turbocharger will dump into the top of the blower. These blowers usually have a bypass that opens when the pressure on top of the blower is greater than the pressure below it, and the rootes type rotors are just freewheeling.

Also, they were putting an extra turbo on a Dodge Cummins Engine this weekend on Two Guys Garage. Small stock went to large second to give good bottom and top end.

Hope this helps.

 

[Lou Scannon]

Also, they were putting an extra turbo on a Dodge Cummins Engine this weekend on Two Guys Garage. Small stock went to large second to give good bottom and top end.
Can you clarify, were the turbos strictly in series, no bypass?, and which turbo was drawing ambient air, the small one or the large one?