twincharging estimated results 11

[bear1a]

ok, so, if you have a twincharge set up on a chevy small block 400 cu. in motor, turbo feeding roots blower... roots by itself at 4000 rpm produces 6lbs boost...turbo by itself at 4000 rpm produces 15 lbs boost..what happens when we put them in series...turbo feeding roots at 4000 rpm ..what approximate boost will be get?...i'll know soon enough i'm building an engine..but would like some theoretical input

 

[Warpspeed]

Water may be of doubtful value here, because detonation is definitely not going to be a problem.

Water will certainly seal the blower rotors and raise boost pressure, but as explained earlier, getting sufficient boost from the supercharger is never a problem when twincharging.

Where water really comes into its own is when a roots blower is run up to obscenely high pressure ratios without intercooling, or when a straight turbo engine is forced to operate at unreasonably high exhaust back pressure. Twincharging solves both these problems in a very neat way.

Twincharging is very benign. Combustion stability is easy to maintain up to very high boost pressures, because of the excellent positive scavenging possible during valve overlap. Fresh cool clean charge every cycle, with all those nasty hot exhaust residuals blown right out through the turbines.

Most people only think of mass flow and volumetric efficiency when planning a high power engine. But combustion stability and thermal management can go a very long way to gaining reliable horsepower that lasts.

 

[bear1a]

like i said this is fun, when you talk about positive scavenging and valve overlap, i presume you are referring to the exhaust stroke of the motor, where some of the intake is actually blown out the exhaust side, right?..i can see how that is affected by cam duration, but how is it affected by a different type of forced induction, i.e twincharging, vs. straight roots or turbos...

 

[Warpspeed]

It has to do with the relationship between boost pressure and the back pressure in the exhaust manifold that the engine has to fight against.

With a supercharger, boost is always going to be way higher than any exhaust back pressure.

With a turbo, the unavoidable back pressure created by the turbine can be greater than boost pressure, and very often is. That traps a lot of very hot exhaust in the combustion chamber that simply cannot escape. Brightly glowing red hot exhaust manifolds and turbine housings are not that uncommon. Realise that gas flowing through those glowing headers, will be hotter than the pipes! That "red hot" trapped gas dilutes the incoming charge, and undoes mostly all the useful cooling that the intercooler may have achieved. It also takes up space that could otherwise be filled with fuel and extra oxygen during cylinder filling.

This is all far worse in a low compression engine, because the unswept combustion chamber volume will be greater for a given engine capacity.

We all know how turbo engines make greater power (at a given boost level) whenever a larger turbine, or larger turbine housing is fitted. The really big disadvantage of doing this, is that the engine then becomes more peaky and laggy.

But adding a supercharger after the turbo compressor increases boost pressure, and lowers exhaust back pressure, because the turbo no longer has to create all of of the required boost pressure all by itself. So the exhaust turbine has to work a lot less hard.

Boost can be very easily kept considerably higher than exhaust back pressure over a very wide rpm range without the disadvantage of a high boost threshold or lag. The turbo is still there, providing the massive top end airflow that only a turbo can produce.

This is why earlier in this thread I suggested you monitor exhaust back pressure as well as boost pressure. If boost can be kept a few psi above exhaust manifold pressure, that is hugely beneficial to sweeping out all of the exhaust during valve overlap.

The engine will have a much higher specific power output, because every induction stroke gets a full charge of clean cool fresh air, not diluted by superheated trapped exhaust gas.

The temperature in the combustion chamber at the end of the compression stroke will be lower, and there is far less chance of combustion temperatures reaching the critical point of instability and detonation. The result is a safer and more powerful engine, as well as potentially a much wider power band in both directions.

Supercharging looks ideal for really high horsepower, but they all have the unfortunate disadvantage of falling volumetric efficiency as rpm rises. But a big turbo can really perk up the top end airflow.

To the turbo freaks that insist turbos are more powerful, that may be true, but at the expense of having an absolute minimal power band.

Where are all the four second turbo cars ???

Twincharging will give the flexibility, response and wide power band of a supercharger with the high specific top end power of a turbo. The advantages of each, without the disadvantages of either.

Where that boost pressure actually comes from, is extremely important to the final results. My own experience and success leaves no doubt that twincharging is superior to either supercharging or turbocharging.

 

[waross]

Will scavenging lower the boost a little? With a lot of valve overlap, we will have some air going straight through. Pressure can't start to build until the exhaust valves close.
I understand the benefits of scavenging, but will replacing a cam with little or no overlap with one with a lot of scavenging overlap lower the boost a little? Is this one more of the complexities that make the difference between experts and beginners, or am I missing something?
respectfully

 

[Warpspeed]

You are quite correct.

But losing a little induction air is much better than having very hot trapped exhaust residuals displacing and diluting the fresh incoming charge.

In fact, if the whole thing is set up to perfection, no significant air volume need be lost. The exhaust valve is kept open just long enough, to see the last of the hot exhaust exit at low pressure, before it closes.

The degree of valve overlap is tied to the pressure differential between induction and exhaust. Exactly the same thing occurs with tuned headers and a high overlap cam, except it is the timed negative exhaust reflection that is the driving force. This only works over a limited rpm range, and the effect can be relatively weak.

With tewincharging it is the higher boost than exhaust pressure that is the driving force. And this is far more consistent in action over a much wider rpm range, and delightfully easy to control with the blower drive ratio.

So very much depends on the pressure differential, and valve overlap. But it is certainly far easier to set up than critical header and collector tuning, and vastly less valve overlap is required for it to be effective.

A blast of relatively cool air over the exhaust valves is beneficial too. The fuel injector can be timed to open only after the exhaust valve has fully closed, so it is only air that is lost, but the benefits of doing this are huge.

 

[patprimmer]

When optimising an engine to accept a Roots blower there are 4 important issues, reduce compression ratio to avoid detonation, reduce valve overlap so as to limit scavenging to the point where almost all exhaust gas is expelled, but little fresh charge is lost, increasing exhaust flow capacity and cooling the charge to avoid detonation.

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.

 

[bear1a]

as a follow up...i'm assuming a twin screw supercharger would work the same as a roots in the twincharging set up..yes?

 

[patprimmer]

Yes, but it will have a lower parasitic loss to drive it to the same boost.

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.

 

[bear1a]

that's a good thing i presume..

 

[patprimmer]

It certainly is.

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.

 

[bear1a]

as i'm getting close to pulling the trigger on parts..i'm trying to figure out the smallest size blower i can use for the system...whipple makes a 5 liter...a bit smaller than the 6-71 in physical size as well.and has a rear air intake which is nice...any thoughts as to whether the whipple will be enough

 

[Warpspeed]

My best guess would be a Whipple 3300AX would be about right.

This assumes a 400 CID engine, and an initial "guess" at a required (unaided by the turbo) supercharger pressure ratio of 1.5 That is a rather high, but let's be conservative here.

That is going to requires 300 inches of air to fill 200 inches of engine per revolution. Or going metric 4,916cc of air per rev.

The 3300AX has the rear entry option, and flows a theoretical 3,300cc of air per revolution. So the blower will need to be overdriven at 1.5 x crank speed. As this blower has a maximum continuous rated speed of 11,000 rpm, that allows a conservative 7,300 rpm engine redline.

I would start off with a blower overdrive ratio closer to 1.3, knowing you can go up comfortably to at least 1.5 if required. but the actual boost you will end up with in the car is impossible to predict with any accuracy. There are just too many difficult to pin down unknowns.

 

[Brettcelicacoupe]

warpspeed, you say it is unrealistic to calculate boost from a positive displacement blower.

i have done a few calcs and seem to have found a method which gives ballpark figures. i posted this on another forum so i will just C&P it here.

please prove/dis-prove this method.

note that a SC14 is a toyota positive displacement blower capable of displacing 1.4L of air/rev. the 3sgte engine is a 2litre 4 cylinder DOHC toyota engine.

"im trying to calculate the effective boost of running a SC14 on a 3SGTZE engine

im looking at running a belt ratio of 1.2:1, overdriven

so one rev of the crank forces the SC to displace 1.7litres of air (1.2*1.4)

since the engine consumes a fixed volume, i thought (suggested by someone else) it must be possible to use the formula P1V1 = P2V2,

the pressures are in absolute so;
P1 = 14.7 psi (atmo pressure)
P2 = unknown
V1 = 1.7 litres (amount consumed by SC)
V2 = 1.0 litre (volume consumed by 2.0L engine @100VE)

P2 = (P1*V1)/V2
= 14.7*1.7/1
= 25psi (abs)
= 10.3 psi boost

this figure sounds ballpark but i havent looked at any system losses. if the VE for the SC and engine is the same (probably not) , then they cancel as a fraction because the V1 would multiply by a number eg 0.9 and V2 would also multiply by a number ie 0.9..... therefore, 0.9 would be on numerator and denom so would cancel.


its seems so simple but ive read that calculating PD SC boost is complicated and not accurate at all?


i would like anyone with a SC engine (prefer SC14 or SC12) to provide me with engine size, SC type and SC pulley ratio so i can validate this formula


just something ive dug up about another forum member's setup

he has a 1g-gte (2 litre I6 dohc), ~1.7pulley ratio and claims 16psi boost

putting the formula to the test;

P2 = (14.7* (1.4*1.7))/1.0
=34.986psi (abs)
= 20psi boost

close!

from that formula, the only way that value can be lowered is if the denominator is bigger (meaning the engine consumes more than 1.0L air/rev - should be opposite) - so prob not that case.

or, the numerator is smaller. this makes sense because its highly likely the efficiency of the SC deteriorates with RPM. since the atmosphere pressure shouldnt budge from 14.7psi and the pulley ratio cant change..... the calculated displacement of the SC is;

~31psi (abs ) = (14.7* (actual SC disp. * 1.7))/1

actual SC disp = 31/(14.7*1.7)
= 1.24L/rev

this equated to 88% VE

seems ballpark also

so now to redo this with engine VE of 95% (realistic?)

~31psi (abs ) = (14.7* (actual SC disp. * 1.7))/(1*0.95)

actual SC disp = 29.45/(14.7*1.7)
= 1.178L/rev

equals 84% efficiency

now i shall use the same forum member's efficiency figure for my calculated boost

P2 = (P1*V1*SC VE)/V2
= 14.7*1.7*0.85/1
= 21psi (abs)
= 6.5 psi boost

maybe the SC VE is better when less boost is run?



sounds about right to me but seems too simple to be accurate. anyone like to check my math?

any ideas how thos SC VE's equate in real life?


ideas and thoughts welcome


cheers

brett
"

 

[patprimmer]

Brett

Your method P1V1=P2V2 is what I use, but there are several sources of error.

1) The air is heated during compression so that increases the pressure at the compressed volume.

2) The blower has some clearance between case and rotors, so there is some leakage.

3) Restriction in the cylinder head will resist flow, but as it is pretty much positive displacement, the mass of air must flow through the engine, therefore the pressure will continue to build until the mass flow into the engine equals the mass of air at atmospheric and displaced by the blower.

4) Valve overlap allows boost to escape across the chamber and out the exhaust, thereby reducing boost.

5) Fuel added to the manifold will both displace air and cool the charge.

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 is spot on. Plus probably the biggest source of potential unknown error of all... exhaust back pressure.

Think of it this way.

The supercharger pumps a "fixed" volume of air into two restrictions connected in series downstream of the supercharger. The first restriction to flow is the engine itself, the second and sometimes much greater flow restriction is the exhaust system. The pressure drops across both these restrictions are directly additive. The actual boost pressure at the supercharger outlet is not created directly by the supercharger, but is a direct result of the ultimate total restriction to flow downstream from the supercharger. And that will be directly dependent on exhaust back pressure.

Nobody talks about this, and I have never seen it in any of the reference texts on supercharging either. but there is almost a 1:1 correlation between a change in exhaust back pressure and a directly resulting change in boost pressure.

Any blower drive ratio formula that only considers supercharger and engine displacement can be way off, if the exhaust system is restrictive. Realise also that exhaust back pressure rises almost square law with any flow increase. Double the horsepower will raise total exhaust back pressure FOUR times. This simply cannot be ignored, and often makes a complete nonsense out of any preliminary boost pressure guestimate.

Pat mentioned thermal effects, and an intercooler can cause a dramatic drop of boost pressure. The air literally shrinks in size as it cools. The drop in boost pressure occurs right at the supercharger discharge, it is not measured as a pressure drop between intercooler inlet and outlet. Although obviously there will always be some finite pressure drop across the intercooler.

I suppose you could assume that the original engine was equipped with an exhaust system sufficient to handle the original amount of exhaust flow. But some standard factory systems can be fairly restrictive at flat out maximum power.

My own experience is that I initially guess a suitable supercharger drive ratio in the way you suggest, based on relative supercharger and engine capacity. Then just road test it with a boost pressure gauge and an exhaust back pressure gauge. Armed with some figures, it will then become obvious what needs to be done to get it where you want.

It is a most curious thing to modify the exhaust system, and then have a very significant reduction of boost pressure, yet have the car accelerate much faster as a result.

But getting back to twincharging. The exhaust turbine and wastegate will have a major impact on total exhaust back pressure. And that directly effects the supercharger. The whole thing is just far too complex to try to design on paper. Give it your best shot, but be prepared to change both the supercharger drive ratio and turbine cover a/r to get optimum results.

 

[Brettcelicacoupe]

pat & warpspeed, your replys very much appreciated.


i am now seeking some advice on a suitable turbo


the engine i will be using is a toyota 3SGTE, 2 litre I4 DOHC. the supercharger (which i already have) is a SC14. it displaces 1.4Litres of air per rev.

it would be good if you could recommend something from the garret range (type, trim, a/r). im new to reading turbo maps so im unsure how to go adding in a SC to the map and its consequences :S.


im aiming for a peak output of about 400hp at the fly. the standard injectors are 540cc so 540*4/5 = 430hp.



im assuming with wastegates, bigger is better. somthing around the 40mm diameter would suffice?


regards,

brett

 

[Warpspeed]

First, I doubt if the injectors will be up to the job. The engine will require around 6cc of fuel per minute per horsepower. Flat out at 100% duty cycle that is 550 x 4 = 2200cc/minute. Divided by 6 is 367 Hp. Run at something more sensible like 80% duty cycle that would be only 293 Hp worth of fuel, a long way short of 400 Hp.

I have no idea what combination of boost pressure and maximum rpm you plan to push the engine to achieve 400Hp. The decision you make there, is going to have a very great influence on success, and also reliability. It is the very basis of everything else, including selecting a turbo.

The SC14 with its two lobe plastic rotors was designed to only run intermittently for very short periods on the original engine. Do not expect it to last very long if it is overdriven, or used continuously. A three lobe Eaton M90 is about the same size (1474cc/rev), and is a far better and more robust design in many ways.

 

[stimpee]

Just wanted to chime in and say that this is a very interesting, and valuable thread!

I have been developing supercharger systems based on Lysholm type compressors for several years now. The twincharging idea is intriguing, but I will probably not get into that for a while yet.

What are your thoughts on a twincharged setup, but with the throttles located AFTER the positive displacement blower/compressor? I know that the sizing, and operation, of an appropriately controlled bypass valve (or valves) will be absolutely critical in this type of application.

Are there any other considerations that might come into play on top of what would already be necessary to make a blow-thru the throttle positive displacement setup work by itself?

Thanks,
Steve

 

[Warpspeed]

Steve,

Any supercharger setup will always benefit from improved throttle response by having the throttle(s) as close as possible to the intake valves. That is the simplest and most obvious way to do it with a centrifugal supercharger (or a turbo). But it requires a little more thought when a positive displacement supercharger is involved. The benefits of improved throttle response are quite significant, and well worth the trouble. Moving the throttle ahead of the supercharger is always a retrograde step. Sometimes it is unavoidable, but doing so is never an improvement.

The basic problem is fairly obvious. With a positive displacement supercharger, and a suddenly closed down stream throttle, there would simply be nowhere for the supercharger discharge air to go.

Some type of air bypass system around the supercharger is required to recirculate any air not consumed by the engine. This system can also relieve the supercharger of any back pressure (boost) upstream of the throttle at small throttle openings, if it is set up to do that. This can completely unload the supercharger and reduce the pumping and supercharger drive power at light throttle when no boost is required. The benefit is less supercharger heat, noise, and vibration, and significantly improved small throttle fuel economy (expect ~10%). All supercharged factory production cars have such an air bypass system fitted, but few if any of the supercharger kits do.

The idea is simple, but putting it into effective practice usually baffles and defeats the do it yourself hot rod fraternity. It is much easier for the Auto manufacturers because they can use the existing electronic engine management system to control the air bypass. They can build in some nifty extra features, such as not allowing boost in reverse gear, or reducing boost if engine water temperature is too hot or too cold.

An effective bypass needs to be very positive and progressive in opening and closing, and it needs to be sensitive to engine load. Crude blowoff valves simply slam open and shut and are extremely unpleasant to drive. They usually also usually allow full boost upstream of the throttle when plenum vacuum is negative, for example during constant speed highway running. The extra wear and tear, engine harshness, and fuel consumption is never good.

I have experimented with all this for years, and have developed a very simple solution that anyone can copy quite easily. More on this in my next post.

 

[Warpspeed]

The ideal thing to use for a positive displacement supercharger bypass is an external turbocharger wastegate. This should be mounted so it bypasses the flow directly around the supercharger when open. It does not need to be very large. Try to imagine how much boost pressure you would lose if you drilled a one inch hole in your supercharger pipework!

It absolutely must be mounted so it flows in the correct direction. That is, increasing boost pressure would try to force the poppet valve off its seat.

Now most if not all external wastegates have two pressure inlet ports, one above and one below the control diaphragm. And the internal wastegate spring always holds the wastegate shut.

The trick here is to connect two control air lines, between the wastegate to directly across the throttle body. The idea here, is that the wastegate diaphragm senses the pressure drop across the throttle body, and increasing plenum vacuum opens the wastegate against the internal spring.

It should be obvious that at wide open throttle, the wastegate diaphragm sees zero differential pressure, no matter how high the boost pressure rises. That is the secret.... The air bypass is load sensitive and depends more on the combination of airflow, load, and throttle position, and not so much on just raw manifold air pressure.

At engine idle there will be sufficient vacuum to snap the wastegate wide open. The supercharger bypass will completely unload the supercharger at engine idle.

At light throttle constant vehicle speed there should still be enough vacuum to hold the bypass open far enough so there is zero measurable boost upstream of the throttle.

A little bit more throttle, and the wastegate closes slightly further and boost may just rise slightly. More throttle still and the wastegate fully closes, giving full available boost pressure. it is all wonderfully smooth and progressive in action.

Same when you lift off, the wastegate opens. For a more violent type of throttle movement, the wasteagate slams shut instantly at full throttle, and springs instantly wide open on throttle opening, as in gear changing.

Any boost spikes up stream of the throttle force the wastegate off its seat, so it acts as an over pressure relief valve.

Getting it working properly depends entirely on fitting a suitable spring to the wastegate, and that requires a great deal of thought and attention to get right.

The first requirement is that the spring be stiff enough to hold the wastegate closed against full boost pressure without leakage. But it also has to be weak enough to open the wastegate against plenum vacuum at part throttle.

The problem is the ratio between poppet valve area and control diaphragm area. A ratio of at least 2 x diameter (4 x area) would be a minimum requirement, and many cheaper wastegates do not have sufficient difference in areas to work properly in this application.

But you can work all this out for yourselves. You may wish to be able to drive around with the supercharger unloaded with plenum vacuum higher than perhaps 5 inches Hg. That is 2.5 psi (roughly) But it must also seal against say? 10 psi of boost. You can see that if the two area have only a four to one ratio, it might be very difficult trying to juggle a spring that can achieve both these requirements.

So try very hard to find yourself a wastegate with a huge diaphragm diameter, and a reasonaby small poppet valve flow diameter. The Chinese gates with the crap diecast bodies and nasty low temperature rubber diaphragms work fine in this application, provided the area ratios are sufficient. And that can take some searching. The more expensive "brand name" Japanese gates are mostly excellent, but far from cheap, not all meet the area ratio requirement so be careful.

It may be possible to build a "Frankenstein" hybrid wastegate by mixing parts, to get a higher than standard area ratio. The larger the ratio, the less critical spring selection will be.

While setting all this up, drive around with the wastegate on the passenger seat or floor, and watch what it does. It will respond instantly and progressively to throttle opening, just like the needle on a vacuum gauge does. Get the spring sorted, then install it onto the engine. I think you will be extremely pleased with the results.

This system works equally well on supercharged or twincharged engines. I have been fitting this to street vehicles for well over eleven years now, with great success, and others that have copied it are also very pleased with the results.

Sorry for the barrage of words. It is really a very simple idea, just difficult to express in very few words.