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Old 04-03-2008, 08:48 PM   #1
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Default 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.
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Staged turbocharging, often called compound turbocharging, is a technique used to achieve extremely high pressure ratios by having one turbocharger pressurize the air coming into the inlet of another. It is common in racing with diesel engines (For example tractor pulling) due to their combustion properties that take well to high boost pressures and are not limited by fuel stability like spark ignition engines. Boost pressures of around 100 psi are common and as high as 250psi in some cases.
A normal turbocharger has a maximum pressure ratio of around three but there are turbochargers in existence specially designed for high boost which have maximum pressure ratios of typically 4-5.
In this configuration one turbocharger is used to pressurize the air coming into the inlet of the other, resulting in a multiplication of the pressure ratios. Same goes for exhaust plumbing. For example if both turbochargers are running at pressure ratios of 3.0 and the atmospheric pressure is one bar the resulting pressures will be three bar absolute pressure at the inlet of the second turbocharger and nine bar absolute pressure (eight bar gauge) at the inlet manifold of the engine. The pressure ratio in this example becomes nine.
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.
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Old 04-03-2008, 09:48 PM   #2
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Originally Posted by patsmx5 View Post
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 **** 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 *** 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.

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Old 04-03-2008, 10:08 PM   #3
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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.
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Old 04-03-2008, 10:14 PM   #4
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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.
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Old 04-03-2008, 10:21 PM   #5
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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.
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Old 04-03-2008, 10:31 PM   #6
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......

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
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Old 04-03-2008, 11:00 PM   #7
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Quote:
Originally Posted by patsmx5 View Post
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.
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 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
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Old 04-03-2008, 11:06 PM   #8
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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.
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Old 04-03-2008, 11:09 PM   #9
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Quote:
Originally Posted by ZX-Tex View Post
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
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.
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Old 04-03-2008, 11:13 PM   #10
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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.
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Old 04-03-2008, 11:18 PM   #11
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Quote:
Originally Posted by ZX-Tex View Post
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.
E-Cool the intercooler. 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.
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Old 04-03-2008, 11:21 PM   #12
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Originally Posted by patsmx5 View Post
E-Cool the intercooler. 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.
Yeah that could work. The intercooler could be smaller and thus ducted away from the radiator more easily.
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Old 04-03-2008, 11:28 PM   #13
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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.
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Old 04-03-2008, 11:29 PM   #14
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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.
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Old 04-03-2008, 11:35 PM   #15
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Originally Posted by patsmx5 View Post
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.
Dude, you are right! If the compressed air in the intercooler is hot enough then it could be post-radiator. Easier plumbing indeed. Hmm....
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Old 04-03-2008, 11:39 PM   #16
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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?
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Old 04-03-2008, 11:40 PM   #17
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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.
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.
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Old 04-03-2008, 11:44 PM   #18
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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.
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Old 04-03-2008, 11:45 PM   #19
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I swear, I think too much.....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?
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.
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Old 04-03-2008, 11:51 PM   #20
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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.
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.
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