Kento

Because the SC is driven off the engine, creating a lot of drag as the rpms rise. It would be foolish to continue losing power to that drag, so bypassing it once its role has played out is the logical solution. Also, "accounting for pressure differences" would be a lot tougher than you think as pressures rise; it would be much too easy for the SC to start backfeeding the turbo, since it is driven directly off the engine.

Skich

iTrader: (4)

Here is a link to that new VW that uses the supercharger and turbocharger. Its pretty sweet and 48mpg is nice...
http://www.greencarcongress.com/2005..._vws_new_.html

WaLieN

I don't know what method they are using, but one could use a solenoid that allows an actuator to close off gates to isolate the super charger from the turbo charger.

Prôdigy2nd

iTrader: (3)

even with the solenoid closeing off the flow from the SC you still have it being powered by the engine which is the main concern with the whole double induction, with super cahrging and turbo charging. if you can figure out a way to use on of the old Mercedes superchager clutch, then you would be half way in the green, but thats a riddle for another day......





Prôdigy

Roen

iTrader: (2)

The thing is, you're using the turbo to feed the supercharger, so there will always be enough flow into the supercharger, as it only moves a fixed volume of air per revolution. Also, once you disengage the supercharger, you'll see an instant loss of power due to the fact that you lose pressure gains from using two compressors in series with each other. It's a simple mathematical relation really, A X B > A + B as long as A and B are > 1. Using any form of compression with a compressor ratio less than 1 would just be dumb, it wouldn't be compression anyway.

The drag isn't as bad as you think, especially if the power gained from using two compressors in series out weighs the power lost due to extra friction from the supercharger pulley at high rpm. The role of the supercharge is not to spool the turbo, rather, the role of the supercharger is to add power on top of what the turbo can add on its own.

In the Mini example I gave, the only "accounting for pressure differences" that they do is installing BOV's for both the turbo and the supercharger. For the supercharger, they replace the bypass valve on the roots-type supercharger with a BOV. It proves an efficient form of boost clearing and there doesn't seem to be any problem with backfeeding.

Finally, from what I see, the popular consensus for using compound induction is to use the supercharger to spool up the turbo faster, thereby reducing lag. With that setup, it would consist of a positive-displacement turbo, rather large turbo, and a clutch to disengage the supercharger at a pre-defined rpm. My method of using compound induction is to use the "instant-on" properties of the power from a supercharger and augment that power to the power given by a turbo, without increasing lag. Flowing the compressed air of a small turbo through a positive-dsplacement supercharger and making the same power as a bigger turbo, with a lag similar to the original smaller turbo.

dradon03

iTrader: (2)

The invention of Supercharge/Turbo cars can be summed up to equal the idea of twin turbo, small turbo quick response followed by a big turbo for high end power.


Alex

Kento

Uh, what? The SC is running off the engine, so it doesn't depend on anything for its flow. The turbo feeds off the supercharger, since the SC doesn't need exhaust energy to get spooled up.
You're not really making sense here...anyway, the two compressors aren't really running in series, it's actually more of a parallel configuration. The SC initially boosts the engine, which spools up the turbo; it's not feeding into the turbo directly.
Um, have you ever worked with superchargers? It's not the pulley friction that's contributing to the drag, it's the internal vanes inside the blower, and the pressure produced (the fact that they're "moving a fixed volume of air per revolution"). As the rpm goes up, the drag increases significantly, even at lower rpms. This is reason for the internal bypass valve, which helps ease some of that drag at cruising rpms so that you can get instant response when you need it without killing fuel efficiency trying to overcome that drag when you don't need it.
The role of the SC is to spool the turbo. Otherwise, why increase the intake temps unnecessarily by compressing the intake charge "twice", when you can just add power without the higher intake temps using a single large turbo?
They probably replace the bypass valve with a BOV because the recirculation isn't needed, since once the turbo is spooled up, there's really no need for the SC.
"Compounding the induction" will lead to unnecessarily high intake temps because you're working two compressors on the same intake charge. Ever seen how a modern jet engine works?

Roen

iTrader: (2)

Regarding the twin turbo, not that it has any relation to my idea, but if you look at the stock FD twin turbo setup, you'll notice that it's not a small turbo/big turbo setup.

The point is, why not run two intercoolers and run the compressors in series? Not all twincharged setups are in a parallel fashion. Series is exactly how the twincharged mini is running and it hasn't blown up now has it? I don't really see these "unnecessarily high intake temps" that you speak up blowing up the engine. And it's only running one intercooler. Plus it makes close to 300 whp from a Cooper S engine, with 15 psi @ 3500 rpms, and full boost at 6000 rpms. Not bad for an engine that's supposed to blow up due to "hotter intake temps"?

Series configuration has one advantage that's rarely talked about. Response to power. Running a turbo that gives 8 psi and a supercharger that gives 8 psi will provide 20 psi of boost, with only the lag of the 8 psi turbo. And the math isn't fuzzy. True, you can make the power with a single large turbo, but have fun dealing with the lag in the middle of a corner. There are ways to get around that lag, but that's for another argument.

Hypothetically speaking, (and I know this is a false assumption, but for this comparison, I'll need to make it), if compressors were all 100% thermally and adiabatically efficient, what would heat up the intake charge more, a compressor that makes 1 bar of boost (2:1 compressor ratio) or two compressors (1 turbo and 1 supercharger) that make .414 bar of boost (1.414:1 compressor ratio) connected in series?

After that elementary exercise in PV = nRT, I realize that no compressor is 100% thermally and adiabatically efficient. However, some ball-bearing turbos that I've read about have about a 78% thermal efficiency and twin-screw superchargers have about an 80% thermal efficiency. What all this means, at least how I see it, I could be wrong, is that intake temperatures will be similar to using a single compressor with a 78-80% thermal efficiency that makes about the same amount of power.

If the car was built for drag, then sure, I'm an advocate of big single turbo since it's simpler and easier to pull off. If a car is built to actually turn and take corners, I'm more an advocate of using twin-turbo or twin-charged systems to lower the lag while making similar power. Using a series configuration with a supercharger and a turbocharger isn't to raise power as much as possible, it's to make a certain power with as little lag as practically possible.

I'm aware that superchargers create drag, but if the goal is not to make as big power as possible and the only problem that you brought up is fuel efficiency, when you're at WOT trying to spool up a turbo from a low rpm, fuel efficiency is thrown out the window, isn't it?

Ooh....and I apologize for that A X B > A + B thing, I just reread it and yeah, I didn't make much sense there.

Kento

But they're not running in series and compounding the same intake charge, are they?
For just that reason: Why run two intercoolers at all? Why unnecessarily complicate things?
Don't be so defensive . I never said the engine was going to "blow up" due to these high intake temps, did I? I'm saying that compounding the compression of the intake charge results in higher intake temps than necessary. Why not just run them parallel, where you won't need to cool the intake charge so much?
You completely missed my point, and judging by your responses, it appears you don't have any real experience with superchargers. The reason for combining a supercharger with a turbo is that there basically is no lag with a supercharger. You're making all these claims about "full boost at 6000 rpms" with the twincharged Mini Cooper-- hell, I'll bet that many FDs running huge T70 and larger turbos achieve full boost quicker than that...
Please see my previous response on the relationship between lag and a supercharger...
Sigh...you didn't read very carefully. Fuel efficiency is not the "only problem I brought up"; a blower's drag increases as the rpms increase. It's part of the reason why there are different size pulleys available for them, so that you can balance the drag/boost ratio at lower rpms. Obviously, once you get past a certain point, the drag no longer becomes a concern, but it's also one of the reasons why you don't often see them used on smaller engines.
In your haste to use my point on the SC bypass valve function out of context, you overlooked this fact: why are turbos used more often than blowers in production cars? Because they're more fuel efficient at lower rpms when you're at partial throttle. It's the reason MB uses the clutch mechanism on their blowers.

Look, if you're so hot on this twin-charge in series idea, then have at it. Go and build one for your FD, or convince an aftermarket manufacturer to build one. I'm just pointing out the reasons why it's not something you see every day.

Roen

iTrader: (2)

I would run two intercoolers out of concern for safety. I could just use one intercooler, hell, the twincharged mini uses just one intercooler and a roots-type supercharger. It's not a claim, read SCC's June 2005 issue or search online.

You're right, I am being defensive, and I do not have as much supercharger experience as you do. I'm looking at this from a conceptual point of view. What's the difference in the low end with the power a supercharger connected in parallel contributes and a supercharger connected in series? Probably nothing, maybe a small contribution from the turbo in series. What's the difference in the high-end? Well,in parallel, you have the turbo providing all the power, with the supercharger disengaged. In series, you have the turbo compressing the air into the supercharger, which makes for a comparable power boost. I obviously have no proof that this is true, but I suspect that the added compression over what each compressor can individually give due to the series configuration makes up (or at least partially) for the drag that the supercharger produces as the rpms increase. Like I said, I have no proof, you know more than I do on superchargers, what do you think?

After reading some aftermarket supercharger companies' websites (Whipple, Kenne Bel), I was under the impression that superchargers are only on at WOT, or at least there's were. I thought that was the function of the bypass valve. Once again, I'm not sure.

The Mini achieves full boost at 6000, but it makes most of its boost at the low end. I'm hoping to do even better than that, making full boost at 3000-3500 rpms. I just want to make sure the power is there throughout the whole rpm range, so when I'm cornering, I don't accidentally boost and spin the wheels in the middle of a corner.

From what I can see, even a supercharger and turbocharger in parallel will have lag. The supercharger has bascially no lag (well, 0-2000), and you're just using the supercharger to fill the range where you have less boost coming from the turbo than the supercharger. My idea is similar, where you have boost coming from the supercharger which goes up as the turbo spools. I'd like to think that series makes more power under the curve, while parallel has a steeper increase in boost, with the potential to make more power. I just don't like using a clutch setup since i think sequential or parallel is complicated in it of itself.

KevinK2

Ever hear a whipple at full rpm? a bit like a radial arm saw in action.

I like the VW concept, with blower feeding turbo at x psi, then as turbo builds boost, blower bypass starts opening then it's all turbo at x psi. blower can be small size with bigish pulley, as it's only used at low rpm, then clutched out and bypassed. Minimal thermally inefficient compounding, or using a small turbo that must work at low pressure ratio and high mass flow (= low efficency).

Of course just a mental exercise, and not worth cramming in an FD.

Speaking of compunding, interesting BMW staged turbo diesel set-up for 3L sedans:

http://www.germancarfans.com/news.cf...eng/bmw/1.html

Kento

Sigh ...You're not really grasping what I'm trying to say because, as you admit, you don't know very much about superchargers. Before you try to look at things "from a conceptual point of view", it would be best to have a firm understanding of how everything works.
I could care less about whether the "claim" is true; my point is that reaching "full boost by 6000 rpm" is a bit late in the rpm scale.
I'm not really sure why you're so stuck on compounding the intake charge into the supercharger; there's basically no need, because the SC's intake cfm progressively rises directly with the rpm (depending on pulley size, of course). If this configuration has such supposed potential, then why isn't it used more often? Why do TF/FunnyCar racers just have a hood that covers the blower intakes? Why not make a configuration like you speak of, or even some sort of serious ram-air setup? Because it's not necessary with a blower.
The parallel configuration's premise is simple: The SC covers the disadvantages of the turbo (lag), while the turbo covers the disadvantages of the SC (drag power loss). This allows you to run a larger turbo that would normally have a lot of low-end lag; the larger turbo has higher efficiency, so it can produce more boost with less intake temp increase. Use the SC to increase low-end power and generate enough exhaust energy to spool up the turbo, then shut off the SC to stop the increasing drag that comes with it. No matter what configuration you run in your series setup, you're always unnecessarily losing power to the blower drag.
It'd be kind of stupid for car manufacturers like MercBenz to use something that adds weight and complication to an engine just so that it will only work at wide open throttle. Once again, blowers are always producing boost, because they're directly connected to the engine. I explained the role of the bypass valve in a previous post.
Basing your knowledge on company websites (or even just the internet) not only puts you at the mercy of their ad sales staff, it also is a very myopic view of an incredibly more expansive world of information.
The reason is most likely due to the fact that the SC unit they use is comparatively small and runs at a relatively low boost configuration, so as to decrease its effect on intake temps and power loss due to drag.
As you said, you don't have any experience or much knowledge of SCs, so you're not seeing very much...
Not really sure where you're coming from there. You can make a SC produce major boost at 1000 rpm if you wanted to.
The difference is that with a parallel configuration, you're not constrained by drag-induced power losses at higher rpm. You can make the parallel configuration make as much "power under the curve" as you want; just crank up the blower boost.