Compound charging

[Jesus-Ninja]

So, here's what's been kicking around my head recently. A bit of a ramble and doubtless I will re-read and edit this many times....

There's the ongoing debate about twin or single turbo upgrades. The question has been asked a million times, and the purists on both sides have made their opinions clear, but to me the arguments are as follows:

Bigger Twins - maintains the characteristic of the car as it was intended. Shorter pipework to the tubs - less lag

Big Single - Scoobs use big singles fine with longer pipework. No custom manifolds or issues with fitment. Easy to upgrade again. Only one turbo to buy.

Given the choice, I'd personally go big single. However, I've been mulling over another option. Compound charging. A bit of background.....

Turbos move volume and compress by ratios. So, a turbo that makes 1 bar of boost is operating at 1:2 (atmostpheric being 1 bar) ie it's taking 1 bar of atmospheric pressure and giving 2 X atmostpheric pressure - and absolute pressure of 2 bar, which is 1 bar of boost. If that turbo had an inlet pressure of 2 bar absolute, then at the same spool would still give 1:2, but the boost it would produce would be 3 bar - ie it takes the inlet of 2 bar absolute, double that which is 4 bar absolute, or 3 bar boost.

This all sounds too much to be true, and in a way it is, for a numbr of reasons, the main one being the heat produced at each stage of charge also increases exponentially. It's generally the preserve of big ass tractors and sled pulling diesels where manifold pressures of 100+PSI are needed. However I don't know that it's entirely without merit on a petrol engine. It has been done with varying degrees of success, using one turbo to feed another as well as using combinations of superchargers and turbo chargers.

It's worth noting that this is not the same as sequential charging. That requires various combinationos of controlled valves etc on the exhaust and inlet systems to remove one (big) turbo from the system whilst the smaller one does the work, and then before the small one runs out of puff, the big one is allowed to spool and ultimately takes over. But these do not typically blow through each other.

A compound setup, in it's simplest form, has a small turbo mounted traditionally on a manifold, feeding it's charge into the plenum (we'll ignore intercoolers for now). The inlet to the small turbo is fed from the charge from a bigger turbo. The bigger turbo receives exhaust from the small turbo.

So, where am I going with this?

If you look at the boost curve of our twins, they make about 18 or 19 PSI and then boost steadily tails off over the rev range. What if we could keep that early spool and and hold the boost to the read line. Maybe even increase the boost and have a steady increase?

What you'd need is a charger before the twins that would raise the pressure that they "see" and it wouldn't have to be by much - the main thing is that whatever is producing the initial charge needs to flow well, even if it's not providing silly boost. For this reason using another TD03 probably isn't going to help, as although it doesn't need to provide bags of boost, it does need to let enough volume through. One interesting notion, as an aside, was to compound charge the existing twins with each other - if the curve boosts to 18 PSI and the trails off to 13PSI as twins, then compounding one into the other would see boost hitting 26PSI before trailing off to 16PSI, but as I say, I'm pretty sure that flow would be the issue.

So what would you put infront of the twins - well one idea is a centrigual supercharger. These seem to be only selected in the event that tubos or a positive displacement supercharger won't fit, but have some merit in that they produce boost which rises linearly with rpm - exactly what we want. At lower rpm the multiplication of boost will be slight, maintaining the cars chracteristic, but it would then give more lift to the stock twins at higher RPM. Given the same curve that boosts to 18 PSI and the trails off to 13PSI, overlaying a centrifugal supercharger that, on it's own, only makes 5PSI at 7000RPM, the resulting boost curve would hit 23PSI and hold a steady 22PSI to the red line. The problem is that superchargers incur significant parasitic power losses to just spin them, compared with a turbo which is "almost" free. That said, centrifugal superchargers are the least parasitic of superchargers, as far as I can tell. The benefit though is that provided it physically fits somewhere, a centrifugal supercharger is as easy to plumb into the inlet as a turbo, but without the need to manage it's boost and no exhaust plumbing.

The other option of course is a third turbo running off the exhaust gasses from the twins. Boost would probably have to be managed independently, monitoring it across the big turbo. There are plumbing questions around whether to use just the wastegate gasses from the small turbos or all of it. I would suspect you'd want the lot, even though some of it has been "spent", as you'd potentially get into difficult situations where you may fall into a valley of death where the twins trail off and fall below their preset boost level (wastegates close) and so no gasses get to the big turbo which doesn't then spool.

Finally, there's an overall issue of how would you control boost on the twins? The whole system relies on the multiplying effect of twin charging, but because it's compounded the control of the waste gates on the twins isn't really based on the total boost above atmospheric, it needs to be based on the pressure on the outlet of the turbos compared with their inlet.

It might be possible to do it arse about face, and have the twins feed thair air into the single charger - their boost would be easy to control then, but they'd be expected to flow more as their outlet is being compressed by the next charger, and I suspect they'd be too restrictive.

This is all conjecture as well, of course. And there are very good reasons why people don't do this, but if you wanted to maintain the characteristics of the car but add top end power, then this might be of interest, and would be more bolt on (certainly the Centrifugal SC would be) than either big single or big twins.

I've been mulling it over so I thought I'd present it to the wide CLubVR4 to pick holes in it and shoot down my balloon before I go too high in it!

compound boost.jpg

[foxdie]

Using a combo of twin turbochargers and single supercharger has been done before on a 3000GT, including comments on how the setup performed compared to other solutions:



Although notably, they're using turbos to charge the supercharger, which charges the engine.

[Jesus-Ninja]

Interesting that they are using a roots blower, which are quite costly to run - I think they sap something like around 100HP to run, and so need to return a lot of power. EG A 300BHP engine which then gets a SC that draws 100HP to run, needs to make an extra 100HP just to break even, and then of course you want a decent power hike as well, so you're looking for an extra 200HP from the system to make a net gain of 100HP, hence lots of injectors! As you say, interesting that they're boosting the SC with the turbos, and not the other way round.

[Jesus-Ninja]

It's a good video though. Basically saying what I was trying to, although they explain it better! And saying "Don't bother", or certainly not with a roots blower anyway. The parasitic losses high up are the issue.

Still, maybe a third turbo? But then the plumbing isn't much different to just going single turbo.

[wintertidenz]

I saw a forum thread about this being done on an Eclipse - they used a big ass turbo to compress the air into the smaller one.
What they did was feed the gasses out of the small turbo into the larger one, so as the smaller turbo closed the wastegate and was at fully spooled, the larger turbo would start to spool up.

I've thought about this for a while on our cars - we could use something a bit larger, like a TD05, to feed the smaller turbos and still have some extra boost at the top end.
You could regulate this turbo off an EBC or even just wastegate spring pressure - remember once the large turbo is fully spooled, it will be putting around 10 PSI into the small turbos, which then compress that again to 20 PSI from each turbo (based on 10 PSI set on them).

The piping will be a pain in the ass, and even more so for auto owners as there is a big transmission in the way, but I think it would be worth it. Aside from the extra power you would gain, it is also unusual and quite unique - who expects to see three turbos under the bonnet?!

Here's the link: http://www.dsmtuners.com/forums/cust...bo-set-up.html

[scott.mohekey]

This is something I've been thinking about also. I've seen it done quite successfully on I6 engines which obviously have a bit more space in the engine bay to achieve it. The biggest problem would be the piping, but I think if you turn the stock turbos around, the exhaust outlet can then very easily be routed to just above the transmission. It's then a matter of routing the output from the larger turbo into the two smaller ones.

[Kenneth]

I have been thinking about compound turbo charging for a couple of years and plan to eventually implement it using 3 turbos. 1 off each bank and then one big one above the gearbox.

That video looks to me to be dubious. It appears they have the wrong supercharger for what they are trying to do and so are over working it which is causing issues. Indications are the air temp problems they had (at Idle) and the fact they had lots of belt trouble.

[Jesus-Ninja]

If you used the stock turbos in the system you'd certainly need them to be closest to the engine in terms of charge. They would struggle to flow enough volume if they were charging another turbo, but a big lazy turbo that makes a bar of boost could double the boosr that the twins can pump.

Ken, I agree with you on their use of SC. A roots or twinscrew draws too much power, hence their losses. A centri or tub might still have mileage though. I'm struggling to find any clues as to how much more efficient centris are than PD, although tubs are the most efficient.

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[Jesus-Ninja]

Scott - the piping should be no harder than a single big tub. The only minor niggle is where to draw oil lines from.

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[Jesus-Ninja]

Wintertidenz - it doesn't quite work that way. If the first and second chargers can both make 10psi, it doesn't equate to a compoumd charge of 20psi. Both are operating at 1:1.66, so you get 1:2.75 which is 26psi. That's the beauty of it!

I've seen that eclipse before though, and he seemed to get some good results.

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[foxdie]

I suppose the next question is, does someone have a centrifugal supercharger lying about they can temporarily bolt onto a 6A13TT and dyno it?

[scott.mohekey]

I have been thinking about compound turbo charging for a couple of years and plan to eventually implement it using 3 turbos. 1 off each bank and then one big one above the gearbox.

That's pretty much exactly what I was thinking. Do you have any concerns about the balance of the compressor outlet from the large turbo to the compressor inlets of the two smaller turbos?

[scott.mohekey]

I suppose the next question is, does someone have a centrifugal supercharger lying about they can temporarily bolt onto a 6A13TT and dyno it?

Funny you should say that. I think my flatmate has one from a toyota of some sort that he was going to put on his honda.

[Kenneth]

No. Is pretty much the same concept as our current turbos except that the inlet to the 2 smaller turbos will be above atmospheric pressure when the large turbo is producing boost.

That's pretty much exactly what I was thinking. Do you have any concerns about the balance of the compressor outlet from the large turbo to the compressor inlets of the two smaller turbos?

[scott.mohekey]

Ah of course. Were you going to use the stock turbos off each bank?

[Kenneth]

No, though they should be adequate really.

[scott.mohekey]

Ok, I've decided to do this. I'm going to build up a forged block, and use the ported heads with the uprated cams from adam's group buy. I'll be using the stock turbos flipped so their exhaust outlets point towards the gearbox with a larger turbo (undecided what yet) sitting above the gearbox with its exhaust outlet pointing towards the rear of the engine bay. The outlet from the larger turbo's compressor will be split above the gearbox and route infront of and behind the engine to the inlets of the stock turbos. The stock turbo's compressor outputs will point down and be joined below and to the drivers side of the front one, feeding a large FMIC. The outlet of the FMIC will then come up over the gearbox close to the passenger's side rail, before entering the throttle body.

The manifolds will be custom made from mild steel, with external waste gates bypassing the stock turbos to the larger turbo so as not to push the smaller turbos too far. The larger turbo will also have a wastegate to manage overall boost.

One thing I'm not sure about just yet, is how to manage the waste gates. I've seen other people have the waste gates on the primary(s) set to an arbitrary total boost level, and the waste gate for the secondary (larger turbo) controlled with manual/electronic boost control. But I'm wondering if it might be possible to get more finely tuned results by having both sets of waste gates electronically controlled independently.

[Kenneth]

The challenge with wastegating is going to be keeping a pressure ratio rather than absolute boost on the small turbos.

As such, you really need to be able to pressure each side of the wastegate diaphragm (as you can with an external wastegate) so that inlet pressure forces the wastegate shut against the manifold pressure trying to open it.

Electronic wastegates would be ideal, but you need to have another MAP sensor in the inlet for the smaller turbos so that you can calculate pressure ratio.

You are also going to want to think about how much exhaust gas you need to bypass on the small turbos. The internal wastegate might not be sufficiently big enough to stop boost creep.

[scott.mohekey]

I was thinking of welding the internal waste gates on the stock turbos, and using larger external ones for them. If I use mechanical waste gates and take my pressure measure from after each of the turbos, would that suffice? (I have it in my head that the pressure after the primary would control the waste gate for the primary, but I'm not sure if using the pressure after the secondary to control the waste gate of the secondary is right or not).

[SEAN-NZ]

in regards to pressure regulation, how about a something similar to what we use, but before the small turbos, to control the big turbo, then just another at the manifold as we have it now for the little turbos, to me seems fairly easy in theory, although would likely prove problematic