Compound Turbo Charging Theory
 

Ok, sorry if this all sounds nuts, just thinking out loud.

First of all, let's establish what compound turbo charging is. It's not twin turbo, or sequential turbo. I don't like to quote Wikipedia, but this isn't politics, so this should suffice.
Anyway, I've been told that on tractor pulling, they use this method to create high boost.I was told they turn out say 200 PSI from the turbos. They air is super hot. They intercool it to get temps down, then the boost goes through an orifice tube to drop boost, which causes air temps to fall. They often have charge temps that are well below zero degrees Fahrenheit.
So, could this be done to a miata? For example, have a t3/t4 blow into a really small turbo to get boost up to say 60PSI or something. Then, intercool it to get temps back down to 200*F or so. Then, run it through a fancy orifice tube that MS controls to modulate boost to whatever specified in the EBC settings. Seems like this allow us to remove a lot of heat from the charge. I'm not really wanting to do this, just thinking if this would work, and would it allow someone to make big power without detonation. Would removing all that heat really be beneficial?

Discuss.

I think I mentioned something like this in a previous thread, here or at m.net discussing intercooler thermodynamics and/or mass flow. Yes for sure this works. It is like a refrigeration cycle except the working fluid is air instead of a refrigerant (like R134 or ammonia). Same deal... compress the working fluid, raising the temperature. Run it through a heat exchanger which removes heat much more effectively after compression since there is a larger temp difference (delta-T in Engineering speak) between the working fluid and the surrounding air. After the heat exchanger, expand the air, and viola it is much colder than when it entered into the process.

This is also used on airplanes. I saw it on an AWACS, though I think it is common to all the airframes (707 or 727 I think). Anyhow, it is the same idea, except rather than expanding through an orifice, which is inefficient, the expansion happens through a turbine, which is used to drive the compressor upstream from the heat exchanger. This recaptures some of the energy that would otherwise be lost through the orifice only expansion. In this case it is not used to boost engine output (axial flow multistage turbine) but to provide a source of refrigerated air for the cabin.

A/C systems, at least the cheap ones, use an orifice for expansion because the energy lost is not worth the additional complexity the turbine/compressor adds to the assembly.

So I have wondered to myself about doing something similar in a car turbo. You could raise the boost to a higher pressure level, run it through an intercooler, then expand it through an orifice. The efficiency of this would be lower than just using a normal intercooler setup, though the outlet temperature could be a lot lower and/or the intercooler could be a lot smaller.

So a simple experiment could be to do this... Put an orifice at the outlet of the intercooler, before the TB. Hook the wastegate controller to the charge pipe between the orifice and the TB. This will raise the boost pressure upstream of the orifice since the wastegate will be driven by the expanded air after the orifice. I would start with large orifices and work down. No guarantees you are not going to blow your shit up trying out this idea. I would also keep an eye on the boost pressures at the turbo outlet. If all you want is cooler temps at the inlet and don't care about efficiency this would work. It would probably be pretty inefficient, but works for the tractor pull engines because they probably don't give a rat's ass about efficiency. 250 psi boost?! That is nuts! Awesome...

The orifice could be as simple as a plate with a hole drilled in it, like a washer. Better orifices are venturi shaped as this is more efficient. If you wanted an adjustable orifice, an old throttle body assembly could be used. Just adjust the throttle gap to vary the pressure drop. If you want to go nuts, hook up an actuator to the throttle and use it to maintain a set delta-p. OR better yet, set it up so it only closes down when under boost. That way during cruise there is no flow resistance which would be needlessly inefficient.

If you wanted to go REALLY nuts you could skip the orifice and add another turbo, with the turbine driven by the post-intercooler air (expansion) and the compressor driving the pre-cooler air (compression). This would be more efficient, but more complex, not to mention sizing the turbo properly. Maybe this would be a good use for a chinacharger off ebay.

This could all be calculated out using thermodynamics, or at least estimated. But I would have to scrape the rust off of my thermodynamic skills as I have not tried to solve a thermo problem this intense in years. It would be best done on a dyno since this could easily result in a net loss in power. Sure you get cold air at the inlet, but the turbine is going to be working harder (and hence the engine) to supply the energy that is lost by the expansion process. I'll bet someone in the F1 world back in the era of psychotic turbocharging figured out whether or not this works well.

Yea, I actually thought about using a TB as an orifice tube. I could be vacuum operated just like a wastegate, and let MS modulate it to maintain whatever PSI I wanna run. I would also have an MBC mechanism to control the wastegates on the turbos as that would be down right simple.

Yep a manual MBC on the turbo would be easiest. The key is connecting the boost signal line downstream of the orifice.

This could get really complex, like using a EBC to control the turbo wastegate, and another EBC like controller to vary the orifice, all based on some map in an ECU set up to maximize power, increase efficiency, make your BOV louder, whatever.

Well, I don't wanna make it more complicated then necessary. Hell, I could use MBC for both the orifice valve and the wastegates on the turbos for initial testing.

It just seems that this would make it possible to make big power on a stock motor. Heat and detonation are usually what kill motors. Seems like 5PSI with this setup would make more power than a regular setup at (10, 15, 20?) PSI because of all the extra oxygen in the air and total heat of compression.

......:eek5:

1 month I'm not around on the forums and we're up to 2 or more tc's...holy crap..lol

All kidding aside, pretty interesting stuff...I have absolutely no place in this discussion, but pretty interesting nonetheless

Heck give it a try! Just make a simple expansion orifice like a section of tube with a washer in the center that you splice into the charge piping. Keep the post-orifice boost low like you said. Move the AIT to the post-orifice section. Do a test at 5 psi before adding the orifice contraption to compare the results. I would also add a pressure gauge before the orifice to make sure the turbo is not boosting too much. If no dyno, then maybe a timed run from 3K-7K in third gear or something. Watch the AFRs, etc.

Of course to really see the benefit I think you would also have to advance timing right?

I'm just waiting for Corky to step in here with some story about how he already tried this 20 years ago :giggle: I am sure someone has tried this on a gasoline engine with a turbo.

Of course you could skip all of this complexity and just add a 5th injector :bowrofl:

Though I am not trying to be a killjoy here, there is another potential problem. Depending on how this is set up, the intercooler is going to be dumping a lot more heat, which for a FMIC could mean engine overheating due to the hotter air flowing through the radiator.

Haha, I'd love to hear from someone that's tried this. I've searched high and low and can not find anyone runing a setup like this. It makes me wonder. If it worked, you would think it would be seen in racing or in magazines.

It would be a chore to build the stuff. I would have to have two turbochargers and I'm not sure there is room for both. I could probably figure it out though, I'm sure I could.

I dunno, this seems too good to be true. I'm not sure about adding timing. I thought that at first, but then I wasn't sure. If you have denser air it's going to increase compression inside the cylinders.

Well I was thinking along the lines of a WI setup with the cooler air. I'll say up front I have not played with WI, at least not yet. But from what I have read the real gains come from being able to advance the timing more and/or leaning out the mixture.

The thing to remember here is that in thermodynamics, just like anything else in this universe, there is no free lunch. All of the power required to make the cold air is coming from somewhere, that being the engine. And, a lot of that power is being dumped back into the air by the intercooler as rejected heat.

Another way to look at this would be to think of how it would be to have a refrigerated intercooler. You could have another A/C compressor that drove an evaporator unit in an air-to-refrigerant heat exchanger just like in the cab A/C. The power to drive the compressor comes from the engine. Of course in this example the A/C intercooler would have to be comparatively huge to provide an appreciable temperature drop.

E-Cool the intercooler.:giggle: That could be dealt with. I could use a small IC that has it's own ducting for the heat to escape under the car and not interfere with the radiator. One way or another, I could deal with that part.

EDIT: Haha, the intercooler could go after the radiator. :) The air in the IC would be 500-700*F, so that nice cool 180*F air off the radiator would be like dunking it in an icebox anyway. Plus it will make the IC piping super easy.

Yeah that could work. The intercooler could be smaller and thus ducted away from the radiator more easily.

Someone(who understands all of this) should call up Corky and get to the bottom of this idea. It sounds interesting but like patsmx5 said, its absence in racing or any setup similar to the miata is worrying.

Also, another thing to consider, tractor pulls don't last all that long, maybe there is some kind of sustainability issue associated with this idea like heat exchangers not putting up with the abuse for long periods of time, just a thought, but I really don't have much experience in this subject.

Indeed, the power is definitely coming from the engine, specifically, the exhaust. From what I understand, about 1/3 of the total energy goes out the exhaust. Usually this is simply wasted. In these scenarios, we would be using this energy to ultimately increase the density of the air going in the engine. I guess a better way to say it is the ultimate goal is to get more oxygen in the engine with less heat.

Dude, you are right! If the compressed air in the intercooler is hot enough then it could be post-radiator. Easier plumbing indeed. Hmm....

I want to know why people don't do this now. With this setup, we could put denser air in the engine. That's HP. I swear, I think too much. I want to put 2 SC's on my car, a big turbo, and now a compound turbo setup. So, how could I calculate how much oxygen is in air that's 0*F or -100*F compared to air that's 100*F?

I agree totally. I think the difference here though is that the turbo is going to be making more boost, but the engine is not seeing as much of it (due to the presure drop) during the intake stroke like it normally would. So to achieve the same boost level, the engine will have to make more exhaust energy than it normally would to drive the turbine.

In other words, I think it has to create more waste heat to drive the turbine, and not just tap into more of the existing exhaust energy.

See, I think that's wrong. Granted, I can't prove it, I'm no engineer (yet, but I signed up for thermodynamics I today!) I've always heard that 1/3 the energy goes into the exhaust, 1/3 into the cooling, 1/3 pushing the pistons. No matter what you do, these ratios hold true. If we could change them, we would.

EDIT: Whoops, I read your post wrong. Indeed, it's gonna use more exhaust energy to drive the setup than it normally would. That's for sure. I think the word "boost" is relative. Let's put that aside. The motor will see all the air the turbo compresses, just with less heat. To make the same power with my setup than a typical turbo setup, mine will use more external compression (the compound turbo setup) and less internal compression (the engine). This would allow us to make more HP as we can pull the heat out of the charge but not when the engine compresses it.

Nah its cool (pun not intended) I'm thinking about it right along with you, it is an interesting discussion.

The oxygen content is the same. I think what you mean is the higher air density with the cooler air. Boyle's gas law will give you an idea. PV=nRT. Solve for n. Assume V is constant (not really the case but it keeps it simple). Vary P and T to your heart's content.

Yes both good points.

In such instances these things have been tried (at least on paper if not for real) and did not work in the past, which is why you do not see them used.

But then again, just because something does not exist does not necessarily mean that it is a bad idea. That is where innovation comes from after all.

There can also be specific applications, like the tractor pull engines, where it works well and thus it is used. Another example... There is no doubt that ice-cooled water-to-air heat exchangers work well for boosted cars. Problem is carrying around all that ice in a daily driver. But for a drag racer that is no problem. He/she only needs that ice for a few seconds, and hence it is used there.

Oh yeah good luck with your Thermo class, really :) It is tough, but really interesting. Plus when you are done, your knowledge will be fresh (much fresher than mine) so maybe you can work the equations for all this discussion. It would be an interesting class project for sure.

could you just route the exhaust gases out of one turbo into the other?

Yea, that's how it's setup. The order is manifold, small turbo, big turbo, downpipe.

then what have a 2-1 intercooler?

What? Explain your thoughts a little bit better. What's a "2-1" intercooler? The charge isn't intercooled until it comes out of both turbos usually.

ok nevermind i went brain dead for a minute. What are the odds that you will get the temps down that low though? also your car is gonna have some major heat cycles

Down how low? I dunno, I think they could be pretty low, 0*F or lower. On paper it seems they could be much lower. It's weird. What do you mean by heat cycles? Feel free to explain yourself. I'm still wondering why nobody does this.

If this was beneficial and efficient enough someone would have this on their production cars...I don't get it, why would you want to raise boost higher than what's normal, add to the complexity, add to the cost of a turbo system just to drop it later down stream to get lower IAT's? There are much simpler, cheaper, easier, better ways to do it.

But none of these other ways result in charge temperatures UNDER say...40 degrees Fahrenheit and this talks about possibly less than 0 degrees. Think about it, he's gonna have frost on his intake manifold.

I think it's a very interesting idea, you'd end up with some rediculous extremes in your engine bay. Exhaust is gonna be so hot with 2 turbos keeping heat around.

Seriously...someone should talk to someone who knows something about this. In the meantime I leave you with another great example of tractor pulling tech.

http://lh5.google.de/GreenMonsterTea...jpg?imgmax=912

Yeah, that's 3 V 16's strapped together.

Ok, seriously....tractor motors are rediculous, I mean I know none of these are for any real purpose tractors. but common, these things are awesome.

http://lh4.google.de/tractorpulling....jpg?imgmax=912

Totally stole that rotary motor out of a plane.

and this guy also stole his shit from planes.

http://lh3.google.de/tractorpulling....jpg?imgmax=912

Given how nuts most of the motors are, it really just makes me curious as all hell what this one is.

http://lh3.google.de/tractorpulling....jpg?imgmax=912

END Thread Jack.

For the same reason you run an intercooler or WI or lower your compression. Your trying to stop detonation and make HP. You would use a turbocharger to bring the air above ambient pressure, and then you would cool with an intercooler it to remove some of the heat so that your charge going into the engine has less heat, but more oxygen. That makes HP. More oxygen and more fuel with less heat seems to be the winning combination. We all know that the less heat and more oxygen there is, the more reliable HP you can make. This is basically just a larger scale version of what's normally done.

Are there really easier cheaper ways to do this? If so, I wanna hear them. On paper, it seems that you could have -100*F intake temps pretty easily.

EDIT: Oh yea, and the idea of compound turbocharging is not just limited to tractor pulling. There are kits available to compound big diesel trucks like the Fords, Dodges, and Chevrolets. They don't try to expand the air back after compressing it though.

If the intake temps get too low there could be some problems trying to get the fuel to atomize and combust. That might be your lower limit of practicality with the inlet temperatures.

A direct injection engine, like a diesel, or some of the newer gas engines (like the Mazdaspeed 3) could overcome this since the air is reheated during the compression stroke before the fuel is injected into the charge.

Yes you could figure it out cause there is room for 2 turbos in there in series. I had a nice seq. setup modeled at one time and I know you could figure it out too. It was actually easier to package than the parallel twins.

Well, it would still be compressed in the engine, so wouldn't it reheat and atomize then? I understand it's not atomize as well because of it being cooler, but would this really cause any noticeable effects? I guess it's really vaporize right? The injectors spray a fine mist of fuel to better atomize t so it can vaporize.

From just thinking about it, this system probably wouldn't even make "boost" after the orifice, because the air would loose a lot of it's volume when passes through the orifice.

I had an old ford ranger 2.3 pickup in highschool that I swapped an 88 T-Bird turbocoupe engine/trans in. Sold that truck to my brother whos been rebuilding the engine and obsessing about the idea of adding a second turbocharger and secondary A/C system to the truck -- He has the space.. The second A/C evap would be a secondary intercooler after a retard sized 4" thick TRUST FMIC to cool the charge down even further.

If I remember correctly, he figured out the charge temps would be worst case right around 35 degrees F at the throttle body and parasitic drag on the motor right around 14% total.

Hes also been planning on a compound boost retrofit for another project of his.. Like couple a 3.2 liter whipplecharger with a T76 turbo on an 8:1 355 chevy motor.. I think hes crazy (not like, tarded crazy.. but has too much time / money / plans, etc) crazy.. *shrug* It sounds like a good idea, and its definatly going to be more efficent in the long run than standard water to air intercooling.

I don't understand how this expansion orifice will lower MAP. Is it in the thermodynamics of uncompressing then re-compressing a gas?

Aren't the differences in MAP recorded on a single car fitted with various turbos flowing 300lb/min a function of compressor efficiency and turbine resistance?

What ever happened to a simple sequential setup? Seems like it'd offer more!

I'm still interested in hearing how this expansion chamber lowers map (since, of course, the air has to be put back into the intake manifold).

If you compress a gas, it gets hot and expands. Roughly speaking, temperature rises 5*F per 1 PSI of increase in pressure. If use anything to compress a gas, it's temperature will rise. The turbo isn't a fixed displacement, so it will maintain a given pressure ratio, even if the intercooler pulls heat away from the charge, causing it to drop in pressure and increase in density. That's what it does. Anyway, air coming out of the IC is cooler and denser then it was, then you drop the pressure. Now, for every 1PSI you drop pressure, air temp drops roughly 5*F.

Right, but that doesn't explain how you are going to drop the pressure with the orifice. Whatever you do after the intercooler you are going to have to reverse it as you pass the intake charge through the TB and manifold, then into the ports.

To change manifold gauge pressure, you have to impose or eliminate a restriction. Will putting a 5" section in the middle of your 2.5" charge piping do that? I'm sorry, but I'm still terribly confused.

I just don't see how the expansion orifice will allow the manifold gauge pressure to change.

I'm sorry you don't understand it. It's physics. Do some research on it or read a book on A/C systems or something. I understand it, but I'm no teacher. I suck at explaining things.

Ok, this is the same Opek that was posting in the thread about exhaust flow earlier? If you understood all that, I don't see you having a problem with this.

no it's not.

An AC system works like the small turbo and intercooler. To extend the AC analogy to this the entire motor would need to a have it's cross sectional surface area increased. The AC unit cools the cabin air while the refrigerant is expanded (post compression and radiation). In this case you are expanding the intake charge, then recompressing it again before you put it into the motor.

In this case we are going to take air and compress it to say 100PSI. That should put the air temp around 700*F give or take. Agreed? Ok, now what happens when we move 700*F air through an air to air intercooler? A shitload of heat will be dumped. The air will become denser. Now, out of the IC we have air that's at a lower temperature then it was, but still under the same pressure. Agreed?

Ok stop. Assume the engine is not getting boost. Let's just dump this air out into the engine bay instead. What's gonna happen when that air is dumped to atmosphere? It's gonna get really really cold.

In this case, we are compressing the charge, radiating a lot of heat from it, then expanding it back. Same as an A/C system except there is no phase change involved as there would be with a refrigerant.

We are together up until the bolded part. This extra cooling offered by an expansion tube makes no sense to me as the intake charge will have to be recompressed by going through a reduction orifice to get into the intake manifold.

Well, I don't understand what your saying either. :) Uh, I don't think you understand exactly what I want to do. It's literally JUST LIKE an A/C system. Fluid goes into compressor to raise boost to increase the delta T so that heat can be removed more efficiently. Then pump the hot fluid through a heat exchanger and dump some heat. Now, expand it back and it's colder, right? Because we removed heat.

So, what part don't you understand? Why would it have to be recompressed by going through a whatever? I never said I was doing that.

This theory sounds as if it might actually work. I just wonder why it hasn't been done before if it could work so well. Were talking LOW intake temps, which translates into ALOT more power per lb of boost.

Not saying it cant work, you might be on to something.. subscribed to this thread, maybe corky will chime in ;)

Yea, it's basically just exaggerating what everyone already does with the typical turbocharger setup. Seems advantageous to me. I'm so tempted to try it. Probem is I love my power steering and A/C, and don't want to remove them to make room for the two turbos.

How are you going to expand the air? It comes out of the 2.5" intercooler, goes through the 2.5" charge piping, then through the 2.5" (?) TB. Where are you going to insert this 'expansion orifice' and how will it operate.

I imagine it would be inside or right before the throttle body, maybe an extention of the throttle body itself? A malfunction of this device would surely result in engine failure unless it has some kind of fail safe. Overboosting 100+psi doesnt sounds good haha.

Maybe just have an orfice that the compressed air will come out of- but make it small enough that manifold pressure will only be 12 psi max or whatever boost you want to try to run. Have different sized orfices for different levels of boost?

Im not sure how linear it would be or if that would work at all really.

Sorry, read the first page. I already answered all that crap like 4 times in this thread in detail.

I understand, now. :noob:

How did you account for the orifice when calculating the pressure ratio?

Any kind of update on this? Are you going to try to do it?

I concede that this may work at WOT. Note that the tractor motors run unthrottled. The orfice tube would reach zero pressure drop, and thus zero charge air cooling, as the air dammed up against the back of the throttle plate.

you better concede digging up a 9 month old thread is lame. bad newb!