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]

You will get more total boost than just adding the two pressures. The reason being that the pressure ratios multiply, not add.

So if your turbo has a pressure ratio of 2.0 (14.7 psi boost), and your supercharger has a pressure ratio of 1.4 (5.9 psi boost) you get a total combined pressure ratio of 2.8 which is 26.5 psi, not 14.7 + 5.9 = 20.6 psi.

So expect to see 14.7 out of the turbo, and 26.5psi out of the supercharger. The supercharger will work A LOT HARDER with much denser air being forced into into it.

The best way to handle this is to just run your wastegate sensing line off the total combined boost, and select a wastegate spring for whatever total boost you finally decide to run. Boost will quickly rise to that pressure and stay there over a very wide Rpm range.

If your exhaust housing originally produced full boost at 4,000 Rpm with just the turbo, expect full boost at more like 2,000 Rpm with twincharging. I am not kidding. You will definitely require a larger a/r exhaust housing, but try it first and see.

The whole thing will be far more responsive and have a much lower turbo boost threshold than you are probably expecting. But the top end power will still be there.

With boost pressure substantially higher than total exhaust back pressure, over a very wide Rpm range, it is going make some real horsepower, as well as having massive low end grunt.

 

[bear1a]

thanks for the input...very interesting..can't wait to build her...

 

[Lou Scannon]

I would give some thought to charge temperature rise and strongly recommend charge cooling. Ideally after each stage, but I understand packaging a cooler between the blower and the intake may be difficult. An intercooler between the turbocompressor and the blower will help a lot though.

 

[bear1a]

thanks..you are correct on heat..the current design has an intercooler for the turbo and alcohol injection after the blower....

 

[Fabrico]

It is likely that heat generation from this configuration should not be ignored. The blower will likely be making the most heat.

Diesels that run compounded use a fairly compact heat exchanger under the blower only.

 

[Warpspeed]

True, heat is always the number one enemy.

But this has a couple of things going for it. The supercharger pressure ratio is reasonably low at only 1.4, and if some fairly efficient intercooling is employed after the turbo, the temperature at the supercharger intake need not be very far above ambient.

The detonation problem that often plagues high boost turbo engines is usually more often due to intake charge heating by trapped exhaust residuals. The unavoidable high turbine back pressure, traps a lot of heat, especially where there is a low compression ratio.

With supercharging, or twincharging it is extremely easy to keep boost pressure well above total exhaust back pressure, and with sufficient valve overlap, the heat can escape due to much more efficient positive scavenging. With EFI, the injection point can be delayed until the exhaust valve has closed if a good fuel specific is required.

So the engine is going to have a far higher detonation threshold than can be had with a turbo.

High induction temperatures are never good, but at least with supercharging or twincharging, detonation should be far easier to avoid with some suitable (but not excessive) valve overlap.

 

[bear1a]

ok, how about sizing for the twincharge or in my case tricharge, is there a methodology to do this..for example,

500 cu.in motor, 1400hp at flywheel desired...size of twin turbos...size of roots blower...i'm assuming we want to underdrive the roots... this is what i was thinking.. two g35's and an 8-71, but i'm using seat of the pants...i think i want somewhere around 20 pounds of boost...

 

[Warpspeed]

I doubt if it is possible to design something like this entirely from first principles with just a calculator and a clean sheet of paper, and no previous twincharging experience.

But a fair start might be to size the turbo compressors to provide sufficient airflow at about half the total estimated final boost pressure. These days exhaust turbines are fairly well matched for speed and energy to the compressor, with typically three alternative sizes of exhaust a/r housing being offered.

The larger size of a/r would most likely end up being most appropriate. The compressors will be run at high flow but at a relatively modest pressure ratio. The turbines will also need to run at high flow and a similar low pressure ratio.

The required turbos will be very large by any standard. A 400 CID engine with a roots blower is going to look to the turbos more like a 700-800 Inch engine. The turbos don't "know" the engine is supercharged, they just see flow. So both compressors and turbines are going to be enormous.

GT35's will be far too small. Something like GT42's with 1.34 a/r may be more like it, even they would be a fairly conservative choice. I know of one three liter (183 CID) twin charged engine in a road car that runs this particular turbo,(GT4288) and he reaches full boost at 3,500 Rpm.

GT35's sound about right for a flexible 400 inch straight turbo engine. Many guys fit GT35 1.0 a/r to this same three liter engine, and it works well. But things change very significantly when a supercharger is added.

The 8-71 has plenty of displacement per revolution, certainly more than enough to provide a very good range of efficient boost pressure adjustment for a 400 CID engine. But a suitable drive ratio for it will need to be selected experimentally.

Probably something that experience tells would "normally" produce around 4-5psi may be a good starting point. That will increase to something approaching double that, with the turbos feeding into it. So yes, I would expect a rather conservative underdrive ratio may be required to begin with.

Now how you wish to set all this up depends on the application and your preferences. The wastegates will limit boost to the 20psi set point, but that 20 psi will be made up of contributions from both supercharger and turbos.

Depending upon supercharger drive ratio, and turbine a/r the engine can behave quite differently with regard to boost profile, torque curve, and exhaust (turbine) back pressure.

Both supercharger and turbos could both be driven fairly hard to reach full boost below 1,500 Rpm, but that may not be what you want, as top end power would be compromised.

This is where it becomes tricky, it is fairly easy to change either the supercharger drive ratio, or the turbo exhaust housing a/r, or the wastegate boost set point. Not so easy to predict beforehand exactly what is going to happen with a particular untried combination.

There are three pressures that need to be monitored. Turbo compressor boost, combined total boost, and exhaust (turbine inlet) pressure. They will give an excellent idea of what is actually going on, and what needs to be changed to get to where you wish to go. It is all extremely flexible, and the engine can be readily matched to the application.

One handy instrument I often use is a dual boost gauge from a twin engined aircraft. It has two pointers that sweep the same scale. I have painted one pointer red, the other blue.

It is easy to monitor two pressures, and see the exact difference between two pressures as the vehicle is driven. The only disadvantage is that these aircraft gauges are fairly heavily damped, and the pointer movement perhaps not as rapid as it could be. But it is excellent for watching for example, where boost and exhaust pressures cross over on an ordinary turbo engine. Or for measuring turbo and total boost simultaneously in a twincharge setup.

 

[SMOKEY44211]

I respecfully disagree with an earlier post suggesting that a roots type air pump will work harder once the turbo gets up to a speed where its providing positive pressure. In an application where an engine is equipped with a belt driven roots pump and the drive belt is removed the engine will continue to run and the blower will simply "wind mill" as a result of the air flow going into the engine. If you add a exhaust driven supercharger into the mix it will simply spin faster once the turbo gets up to a pressure greater then atmospheric. My conclusion is that roots blower will behave like a restriction if the turbo is capable and allowed to achieve high boost levels.-----Phil

 

[Warpspeed]

Yes it will certainly windmill freely without a drive belt, but there will also have to be a net pressure drop across the supercharger rotors required to do that.

A positive displacement supercharger when properly driven by the crank just gulps a fixed volume per revolution of whatever air density is there, and passes it to the output port. The boost pressure rises because the supercharger pumps a greater volume of air than the engine could otherwise inhale by itself. The supercharger can never be restrictive while a positive boost pressure is being produced.

If the incoming air density to the supercharger doubles, the supercharger mass flow will also double, and so will the torque required to drive the supercharger. Interestingly the supercharger pressure ratio stays about the same because the supercharger displacement and engine displacement (per revolution) maintain the same set drive belt ratio relationship. The flow through both supercharger and engine increases due to the turbocharger increasing the incoming air density to the supercharger.

There is a fairly widespread and seemingly immortal urban myth, that once the turbo winds up, the supercharger will become restrictive. That can never be the case if the boost pressure at the outlet of the supercharger is higher than at the supercharger inlet pressure. It can never be restricting the flow if that is the case.

As explained in an earlier post, the measured pressure increase across the supercharger actually increases fairly dramatically when the turbo begins producing more dense incoming air for the supercharger to work from. There is no question of there ever being a flow restriction by the supercharger.

If a supercharger operates at one atmosphere, or 14 psi absolute intake pressure, and has a pressure ratio of 1.5, the output pressure will one and a half atmospheres, or 21 psi absolute (creating 7psi boost)

But if the supercharger intake is operating forced up to two atmospheres, or 28 psi absolute, the pressure ratio of 1.5 still holds. That is the supercharger will have increased the incoming two atmospheres pressure up to three atmospheres or 42 psi absolute, or create a 14 psi boost increase, not the original extra 7 psi increase.

As the turbo winds up, the additional supercharger added boost pressure also increases as well. That can hardly be called restrictive.

 

[patprimmer]

This has been done several times before, but I will do it again.

A Roots blower is a positive displacement pump. For each turn of the blower it displaces a given volume of air irrespective of pressure.

Say it is attached to a 4 litre 4 stroke and is driven at 1:1 and displaces 4 litres per turn.

Putting aside variations in VE at different speed and the effects of cam timing, the 4 litre 4 stroke will displace 2 litres per turn of the engine.

Say atmospheric pressure is 15 psi, if the blower sucks in 4 litres, and pushes it into an engine that displaces 2 litres, the 4 litres of air will compress to 2 litres, but at twice the density, and putting aside temperature increase, it will also be twice the pressure.

This means the manifold pressure will increase from 15 psi to 30 psi (if there was no temperature increase) due to the compression from the blower.

If the Roots blower takes in air from a manifold fed by a turbocharger at say 30 psi, for each turn of the system, it will still take 4 litres into the blower and compress it to 2 litres as it passes through the engine, so 4 litres at 30 psi equals 2 litres at 60 psi.

The Roots blower does not know what pressure the air is, it just pumps 4 litres per turn irrespective, and the engine pumps 2 litres of air per turn irrespective of pressure. Due to this 2:1 volume overfeed, pressure must double.

In reality it will more than double as it will also be heated due to the compression.

Regards

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[rpmag]

so what is the downside to twin charging?

 

[patprimmer]

Cost, weight complexity, space.

Regards

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

 

[thundair]

Could you increase tunability, with a Waste gate before and after the Roots Blower?

That would also be adjustable so that the pressure against the roots could be controlled to keep the turbo from spinning heat at extreme pressures.

Also adjusting Fuel Pressure to compensate.

Cheers

I don't know anything but the people that do.

 

[patprimmer]

A wastegate will be essential. In would think that between the turbo and the Roots blower should be the pressure pick up point as you control the turbo directly by it's output.

Another thought is that by taking wastegate operating pressure from the manifold between the Roots blower and the head, you can allow the turbo to correct for lost efficiency of the roots blower.

If you need to change the second stage boost multiplication at the Roots blower, it is best changed by changing drive ratio, normally by pulley size change.

Regards

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

 

[bear1a]

ok..on a 454 cu. in. motor..if i run an 8-71(568 cu.in)..at 1 to 1...i get about 10 psi...that's about 1.67 - s/c to engine..568 into 227 .. so if i add 10psi of turbo boost to the intake of the s/c...then on paper i should get 10 * 1.67 = 16.7 + 10 or 26.7 psi of boost...right?....

 

[patprimmer]

My figures for an 8:71 are 436CI per turn or about 500 CI per turn if it's high helix retro Teflon stripped.

At 1:1 it will act as a restriction. You are going to need about 70% OD to get a 1.67 boost multiplier factor.

You really need at least a 10, or even a 14:71 if you want to run small OD ratio.

The data for GM based Roots blowers are:-

6:71 small diameter
Rotor dia=5.505", length=14.975", displacement per full turn of rotor=339CI.

6:71 big diameter
Rotor dia=5.778", length=14.975", displacement per full turn of rotor=411CI.

8:71
Rotor dia=5.778", length=15.905", displacement per full turn of rotor=436CI.

10:71
Rotor dia=5.778", length=17.000", displacement per full turn of rotor=466CI.

14:71
Rotor dia=5.778", length=19.000", displacement per full turn of rotor=521CI

This is theoretical displacement.

Retro or high helix will change this considerably.

A worn blower will reduce this a little.

A very good tight new blower and a Teflon stripped blower should be about the same.

Regards

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

 

[Warpspeed]

In every twincharged installation that I have been involved with over the last ten years, the pressure reference for the turbocharger wastegate has always been taken from the combined final boost pressure going into the engine.

That pretty well fixes the maximum boost that the engine ever sees, and holds it constant over a very wide power band. That is one important tuning parameter.

Another tuning parameter is the supercharger drive belt ratio. That alters the amount of boost produced by the supercharger which can influence boost threshold, and turbo response. But much more importantly it can raise boost pressure with respect to total turbine exhaust back pressure, and that is extremely beneficial to engine volumetric efficiency, combustion stability, thermal stress, and the detonation threshold.

The third tuning parameter is exhaust turbine a/r, which also effects the turbo boost threshold and response, as well as final turbine back pressure.

Despite the obvious cost, complexity and packaging disadvantages, it has huge performance advantages, especially for a very tractable high horsepower road car. It is especially good with small capacity engines.

The biggest advantages are the extreme tractability and low end response that the supercharger provides, as well as the massive top end airflow that only a suitably large sized turbocharger can give. you get the advantages of both, without the disadvantages of either.

Superchargers by themselves all suffer from a rapidly falling volumetric efficiency at high engine Rpm, that is inescapable.

Turbos, (within a fairly limited flow range) can offer increasing flow and pressure, with rising rpm. Wonderful for top end airflow, but excessive turbine back pressure, a high boost threshold, and lag, can be less than wonderful.

But combine the two in the correct proportions and it is pure magic. Getting the proportions right is just a case of experimenting with the blower drive ratio and the exhaust turbine a/r. This is not difficult, but the results are always extremely rewarding.

 

[SMOKEY44211]

Plumb the exhaust up to the turbine section of turbocharger. Don't hook up anything to the compressor. Install pressure guage on discharge side of compressor.( no restriction) Guage reads zero. Now introduce a restiction. More restriction higher guage reading. Now lets assemble it all together in the scenario described. Install guage in conduit between turbocharger dischage and roots blower inlet. If guage reading is above atmospheric then the roots blower has to be providing the restriction. Where is the flaw in this logic?-------Phil