Twincharging SC>Turbo or Turbo>SC? 2

[Max965]

For those who are 'pro' twincharging I am interested in discussing the technicals of the combined Supercharger and turbo used in series.
The 3 states are 1) Supercharger by itself (low revs), 2) turbo by itself (high revs) and 3) the change over point.

Will there be any difference between an SC blowing into a Turbo vs the other way?

My view is that its more effcient to have SC>Turbo. Why? If a SC is 50% efficent and a Turbo is 70% (or just more than the SC) as the turbo begins to spoil to catch up with the SC, it needs less CFM than the SC needs for the SAME boost pressure, so the SC can provide the turbo with the air (as its trying to blow more) no problem, infact it will supply the turbo with more air than needed and so you will achieve a compound effect or boost pressure inbetween the SC and Turbo.

With Turbo>SC I would of thought the turbo cannot start boosting until it spins and sucks in more CFM than the SC needs so there will be no compound effect.

Clearly after these CFM points the turbo starts to need more air and the change over begins. But I am interesting in the point where the turbo starts to catch up.

What are the thoughts on this or I am way off?

Max

 

[Majik]

Are you using a Roots blower or a procharger style? Most roots blowers sit on the intake for that instant response. Or else they've still got to compress alot of air then. Most twincharged units have the turbo feed into the roots blower or feed into the engine and bypass the SC when enough boost is achieved. You can actually feed the compressed turbo air into the SC and the SC will compound it.

 

[MWPC]

It seems to me that the supercharger (roots type) will only gulp so much air. Once the turbo comes on it will be fighting for air. As Majik says if the turbo is first, the super will compound it. Ken

 

[Lou Scannon]

There was a thread about 1 month ago covering this topic. I can't find it now, so it may have been deleted.
In summary, the turbo needs to go upstream of the blower for a series compounding effect. If fuel economy is important you probably want to be able to declutch and bypass around the blower at part load and when the turbo is able to supply all the required boost.
If fuel economy is no big deal, a fixed configuration should work fine, assuming correct sizing and drive ratio.

 

[Max965]

Thanks for the responses. I am going to use a roots blower but will not be able to put it on the intake as my engine is already turbocharged with the intercooler ontop of the intake manifold, its a 3600cc 911 Turbo engine.

It seems there are two setups, the Lancia Delta T>SC and the new VW Golf SC>T. Both have bypasses, I am interested in the golf setup.

Just to respond... Having the SC close to the intake or not I would of thought it wouldnt make THAT much difference, the air should still be compressed as the SC CFM inlet is greater than what the engine needs.

Regards to the compounding, they should both be able to do it, if you feed compressed air into a SC or Turbo it will just multiply that by the pressure ratio.

I have ran through some numbers and will post them on here.

 

[patprimmer]

A Roots blower is a positive displacement pump.

A turbo charger is a centrifugal pump and is not positive displacement.

If you put the Roots blower first then blow through the turbo, The turbo will do nothing as it cannot draw in more air than the displacement of the blower.

If you put the turbo first, the Roots blower can suck in air through the turbo before the turbo comes onto boost, and when it comes onto boost, the turbo can pressurise the air going into the blower. The blower will then compress it further.

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[Max965]

Thanks, this is a long response because I have read many of these topics and they never come to anything too scientific, this is why i came on this forum. I know people are interested in it.

I realise SC and Turbos are different however these are my thoughts: a turbo can compound. Based on a turbo compressor map you have air inlet flow, the SC will just increase that flow and if the turbo is spinning fast enough it will compress that inlet air.
Now when the turbo is beginning to spin up in the crutial stage of switch over (i.e mid rpms) the airflow the SC gives it will 'choke it' and a turbo wont be able to compress it. But as it starts to spin up it will be sucking in that air and compressing it somewhat.

However the point of compounding is to get higher boost pressures, this is not needed for twincharging purposes- well not for me anyway because the turbo can boost enough on its own.
If you want the SC to be infront of the turbo and compound, thats fine but then when do you switch you off the SC for sake of having the benefits of twincharging?

I think everyone agrees with 1) the SC being used at low rpm for instant boost & help the turbo spoil up and 2) the turbo is on its own at high rpm with SC bypassed. But my thread is really about the change over between the two. If you had SC>T or T>SC points 1 & 2 should have the same HP outcomes.

Therefore the change over is when the turbo starts to need more air than the SC...

I have ran some (rough) numbers for my engine @ 3000RPM:

(SC on its own) CFM = 262, PSI=14, Density Ratio =1.72, with a HP loss of 24HP you get a net 157HP

With the Turbo on its own and with 228 CFM inlet, would be PSI = 8.8 , D/R =1.5 no HP loss (or little with some backpressue) so a net of 157HP.

It shows the same HP made but the turbo is boosting less as the temp is much lower and the HP drain is not there/less, even with the same HP drain its still better because its more efficent.

So whats my point? If 262CFM was coming in through the air inlet whats more effcient SC>T or T>SC? If you have T>SC then the SC will be 'choking' the turbo and the turbo wont boost as the SC needs all that air the turbo is recieving plus more. If its SC>T then the turbo will use the 228CFM to get to same HP, however the SC is sucking in more so this will be a) compressed between Turbo inlet and SC outlet or the turbo will take the extra CFM and boost it futher. Which will result in a higher HP with the same CFM!

So as the turbo begins to spool the pressure differential between the T and SC will be less and less... with this the HP drain of the SC will get less upto the point the turbo needs 262CFM, then the SC should be freewheeling.

But with T>SC the SC will be sucking in and compressing that inflow itself, the turbo is no good and does not do anything to help or reduce the HP drain etc until it gets above this 262CFM.

This point is when the change over/bypass should happen.

My other thought is with the SC>T setup when the turbo needs more air than the SC can produce, can the turbo suck the additional air through the SC? If so then you dont need anychange over point as the SC will automatically not be compressing and no HP drain...

If you think I am wrong with my logic please explain!

 

[patprimmer]

I already did.

Maybe there is a terminology difference. By first I mean on the open atmosphere side. By last I mean on the engine side.

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[Max965]

I didnt disagree with what you said or even thinks it wrong. its not.
But there is no point in using it like that unless you are trying to compound for more boost than the turbo can produce. Air is air, do you want it to be compressed by a turbo or an SC? if you need both then your turbo is too small and in that case diverts the point of twincharging because you use a larger turbo one anyway...

Its only phyics afterall. Unless people have actually done this or done the math then it doesnt carry much weight.

 

[patprimmer]

I will try again in more detail, starting at elementary basics.

Anything that increases the mass of free oxygen in the charge is supercharging.

For this discussion, we have two types of superchargers, a Roots blower and a turbocharger.

A Roots blower is a positive displacement pump. The volume (not mass) of air it displaces per single full turn is fixed no matter what the rpm, inlet air density, inlet air temperature or whatever. They are normally, but not necessarily belt driven. No air can be blown nor drawn through a stationary Roots blower. For air to pass through it, it must turn.

A turbocharger is actually two devices connected together. That is an exhaust gas driven turbine that is a source of power for the centrifugal blower. The centrifugal blower side is just like a belt driven centrifugal blower. As the turbocharger is turbine driven, You can suck or blow air through it while it is stationary. It will also freewheel if air is drawn through it as there is no mechanical connection to the crankshaft.

Say your engine is a 3 litre 4 stroke. At 100% VE it will pump 1.5 litres of ambient air per complete single turn.

Say the Roots blower is driven at twice engine speed, and say it displaces 1.5 litres per turn, so long as it is not throttled, it will supply 3 litres of ambient air into the manifold for every turn of the engine. As the engine only consumes 1.5 litres, the air is compressed to 1.5 litres at double ambient atmospheric density. The pressure required to maintain this density will vary with temperature change. This density change will be directly proportional to the VEs of the engine and the Roots blower. If well matched re size and speed, the VEs will be fairly similar across the normal usage range, hence the good bottom end response. A reasonably large proportion (like about 40 to 50%) of the extra power produced by a Roots blower is consumed to drive it, so compared to an NA engine, they waste fuel and put extra stress on the engine for a specific power output.

A turbo charger behaves very differently. It is not positive displacement, it is not tied to engine speed and its VE is not related to engine speed, and there is always a time delay between WOT and build up of boost. They do consume some parasitic power to drive them due to increased blow down pressure in the exhaust, but this is small compared to a Roots blower for a specific power output, but still larger than an NA engine for the same specific power output. They are very poor performance at low speed and they have a response delay at any speed until the wastegate opens.

To get high relative performance from a turbo, you need a big turbo, but a big turbo lags more at low speed.

You can get the best of both worlds by running a Roots blower and a turbo on the one engine.

These can be set up in parallel with a system of valves, but this is big, bulky and complex and requires a lot of calibration of the valve actuation system to get good progressive response and the turbo is completely wasted until it's boost is more than the Roots blower as it will be feeding against a closed valve so as to prevent boost loss back out through the turbo compressor. The Roots blower will also be inefficient when pumping against a high manifold pressure.

If you set them in series, the turbo MUST be the first stage with the Roots as the second stage as you cannot suck extra air mass through the Roots blower, but you can easily suck extra air mass through a turbo when the turbo is not on boost, but the Roots is working effectively.

If the Roots is first, as the turbo spools up, it requires more air than the Roots supplies, so you in fact get a depression rather than boost in the manifold between the Roots and the turbo.

If the turbo is first, once the turbo comes onto boost and supplies more air volume than the Roots will pump, the air density in the manifold between the turbo and the Roots blower increases in density. The Roots blower can effectively double the air density again on top of the increased density at it's inlet port.



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[Fabrico]

Max 965, it's been done. Back to terminolgy, if it's not done for extra boost than it really should not be referred to as compounding.

The turbo should not be fed by the blower. The only thing this would compound is turbo lag. Or, if you don't believe that, then we will have to creat a new term to be known as "blower lag". The turbo should feed into the blower and/or directly into the engine.

A tried and true setup for one of my favorite engines... is to use the blower at low to moderate speeds, with the turbo going along for the ride upstream. At moderate speed and cruising, the blower bypass valve opens and the turbo supplies the air directly to the engine. On heavy power demand the blower bypass valve closes and the two then work together in great harmony. When the blower is by-passing it can continue rotating without wasting much power at all. It could also be decoupled. The after-cooler sits directly under the blower and cools all air entering the engine.

It is a very efficient setup for throttle response, cruise mileage, and high power output, without negatives in its operation. Any flowpath changeover is simply coupled to pressure production and can be electronically or pnuematically operated.

 

[Max965]

Thanks for detailed explantions.

One point was when ever the turbo needed more air than the SC, a bypass would open up for 1) letting the SC just freewheel and 2) the turbo can suck in all the air it needs. Sorry perhaps this wasnt clear.

One point Fabrico, you are right it shouldnt be called compounding but thats not what twincharging was meant to about. I am not clear on is why would there be lag if you blow into the turbo? The turbo can have air passing through it as stated above. So why couldnt the SC blow the air through when the turbo is spinning slowly and pressure the manifold as normal?

Just to point out I was just following the new 2006 VW setup, the SC blows into the turbo and then bypasses the SC when the turbo kicks in... this setup up apparently works i.e no lag, good emissions and high power with the big turbo. Can anyone comment on this?

 

[patprimmer]

That is because it has a separate feed that bypasses the blower. In reality in operation it is a parallel, not a series arrangement.

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[Fabrico]

Max965, I can see your argument if, and only if, you stray from conventional sizing and purpose. I would ass-u-me this system uses a smaller than "normal" blower and a larger than normal turbo. A larger turbo would allow less obstruction to the blower output by allowing air to more easily pass through. Even with a smaller blower, lag might be put aside. But a smaller blower and the configuration itself, rules the combination out for higher speeds or compounding. If the blower is by-passed at higher engine speeds, the turbo can again be larger as it is pretty much untethered to throttle response. It would presumable end up with good throttle response, and good power on top, from a smaller engine thus making a small engine feel bigger. Seems kind of expensive for a few MPG.

If that is the scenario, the "changeover" or blower by-pass should occur just about the time the inter-twincharging pressure drops to zero. This could be precisely controlled by air flow alone with a flap or swing gate valve.

 

[Max965]

Thanks Fabrico,
So what do you recommend for the bypass valve and how can it open up or gradually open when pressure = atomsphere? I thought of a BOV but these dont open until a decent vacuum is created.

 

[Fabrico]

I don't see why a simple self actuated swing device would'nt work perfectly. I doubt there would even be need to dampen it, although that would be easy to do.

But at the same time...I don't see how this system will improve a 3600cc 911 motor in a practical manner. If you use the original turbo the power will remain the same from mid to top RPM. It's been a while, but as I recall the 911 has good throttle response and low end torque. The blower will wake up the turbo, but at what price and complexity? Will that specific turbo respond as planned? Will it work better than a parallel configuration? If the combination works, it could lead to engine, driveline, or traction problems.

As far as an existing turbo charged motor, I don't see this conversion as doing much more than the electric air boost pumps made and sold for this same purpose. There was a very lengthy thread on this here a few months ago

http://www.eng-tips.com/viewthread.cfm?qid=157771.

 

[patprimmer]

A very short shot of nitrous oxide can be very effective at overcoming turbo lag.

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[NickE]

So is retarded timing, extra air and a hot manifold. (anti-lag or bang bang)

 

[patprimmer]

Maybe Max965 should look up this thread

http://www.eng-tips.com/viewthread.cfm?qid=157771&page=7

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[patprimmer]

And this one

http://www.eng-tips.com/viewthread.cfm?qid=101957&page=28

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