Wastegate for BOV?

[mdb4879]

OK, this is going to sound really wierd, but it's just a theory I came up with a couple of days ago. Now I don't really have any experience with turbo'd cars, so someone who does could probably tell me what's wrong with this idea, but I do like to think I know a fair bit of theory, so here it goes.

The purpose of a blow-off valve is to release excessive pressure from the intake plumbing when the throttle plate is closed to prevent compressor surge, which would slow the turbo down and create harmful pressure on it's bearings. Well when this pressure is released it takes a moment for it to build again when you hit the gas, producing a slight lag in power delivery.

When cornering, letting off the gas or hitting the brakes will shift pressure off the rear tires and onto the front, possibly causing the rear tires to break loose. One may "feather" the throttle (not sure if that's the terminoligy to use) to control the car while cornering. If a BOV is used, then when you let of the gas in a turn the weight transfer may be very abrupt because of the drastic loss in power to the wheels because all the manifold pressure is suddenly lost, causing the rear end to unsettle.

So my theory is to place a wastegate (as would be used for exhaust gases) on the intake plumbing as well, instead of using a BOV (a wastegate would still be used to divert exhaust flow to control turbo speed). By doing this, I would believe that when the throttle is closed and the MAP builds, the wastegate would open to release this pressure, preventing undesired compressor surge. But, as soon as the desired boost level is reached, the wastegate would close retaining the boost for when you hit the gas again. The pressure in the intake will still slow the turbo down, but not near as fast as if the intake were sealed, and the boost will still diminish, just more slowly (for when you let off the throttle completely to deccelerate). I believe having this set-up would make the car more controllable and responsive, feeling like a larger NA motor once the turbo is spooled.

So what do you guys think?

[float_6969]

What? draw a picture.

[1200ute]

I have heard of Waste gates being used on the intercooler piping but it was to help control the amount of boost the engine would see rather than acking as a BOV. It was basically a fail safe so that boost spikes wouldn't affect the engine on a pretty high powered drag car.

With control ability of the car, abrupt lose of power and unsettling the rear end, they are all to do with how the car is driven - so you are correct in saying about feathering the throttle. It doesn't matter if you do or don't have a BOV or you make up some other system to control the turbo pressure when the throttle is released - when you take your foot off the accelorator you are removing power from the driven wheels. All the BOV is going to do is help keep the turbo spinning for a bit longer.

You do have the basic understanding of the BOV and turbo, The turbine is driven by exhaust gases, the more fuel you burn the more drive for the turbo, and this inturn drives the compressor via a shaft. When you take your foot off the accelorator you reduce the drive to the turbo and without a BOV the pressure in the intercoolor and piping build up and cause pressure on the compressor. The two of these actions cause the turbo shaft to slow down. With the BOV the pressure is released and it basically means the turbo can 'free wheel' for a bit longer until you accelerate again which should help with the response.

GoFast Bits has developed a BOV which is meant to reduce lag and improve throttle response - http://www.gfb.com.au/images/stories/PD ... ase_NA.pdf

[mdb4879]

I had actually just thought of using this set up to prevent overboost as well just after I made the post. And I've never had any turbo car until the project I have right now, so I don't know how much you have to let off the gas for the BOV to open. I know that when you completely close the throttle all power will be taken away from the wheels, and even some drag due to the motor wanting to unwind. But if the BOV opens when you partially close the throttle then almost all the power is taken when only a percentage was wanted to be taken away. But then again I also don't know how much the throttle must close to actuate the BOV, so that could be irrelevant.

My thinking was that with a BOV all the pressure is lost, and it takes time for it to build back up again. Sure, the turbo is able to spin freely and build boost much quicker once the throttle is reopened, but it still takes time to receive full power however little time it may take. The idea with the wastegate is to retain the boost for immediate response.

I can try to draw some pictures in the morning to illustrate the idea I'm portraying if you would still like.

And while typing this post I think I just realized that vacuum opens the BOV, so it won't release any pressure until the throttle is completely, if not very close to being closed. Is this correct? (I've never looked into how a BOV works, I just know what it's used for) And if so then I guess this set up really wouldn't have any advantage over a BOV other than overboost protection, huh?

And going on a slight tangent the overboost protection wouldn't do much good anyways. It may prevent harm to your motor if the vacuum line came off the exhaust wastegate actuator and spooling the turbo by releasing the intake pressure, but the turbo would still overspool causing damage to it. I like the idea I read on ka-t.org i think it was where you use a pressure activated switch wired into your ignition so if an undesired boost level is reached it will turn off the ignition until the pressure resettles, allowing you to still drive home and figure out the problem without harming the engine or overspooling the turbo.

Man that's a lot of typing for 3:20 in the morning after getting of work, lol.

[TheMAN]

what you are describing is a bypass valve.. it does pretty much what you describe
instead of venting excess pressure out into the atmosphere to get that mad tyte whistle, it shoots it back into the suction pipe for the air to be circulated back into the turbo... throttle response is much better than plain BOV.... turbo lag can't be eliminated without an "anti-lag" system, which tremendously reduces turbo life

if what you want worked in the first place, engineers would've put this in place in many modern stock turbo cars because the emphasis these days is responsiveness and reduced emissions

[D_Stirls]

From what I think you are saying your are describing a BOV. It's just that you are using a external gate instead of an actual BOV. I think you are saying that you want to vary the pressure that the BOV actuates and you are going to (I assume) change the springs in the gate to achieve this. If you do a search though you will find that a lot of BOVs have an adjustment screw on then and this is what the screw does, varies the preload pressure on the spring that load the diaphragm.

There are some cars that use your idea about using a BOV to control boost pressure, a lot of these are race cars that have a restriction put on boost pressure for the class they are running, if you look for poppet valves you should find some. In street cars however if you are running an air flow meter then this idea does work as you are releasing metered air and there for it will cause the mixtures to be come rich when ever the poppet releases, which is find on gear change but will cause the engine to stumble under power. To get around this you would have to return the released intake charge to the intake pipe between the turbo and the AFM.

[mdb4879]

I hope this helps portray the idea in my head:



I wasn't sure how to draw the valves other than with butterfly valves, so I do know that probably isn't the kind of valve wastegates use. The intake wastegate is set to open at a slightly higher pressure than the exhaust wastegate to keep them from interfering with each other when the throttle is open and you are in boost. I used 14psi of boost for the example and guessed that it might take around 0.5 seconds to reach full boost again after the throttle is reopened using a conventional BOV set up. the double squiglies means "approximately" for those who don't know, and the < symbol with a squigly under it means "approximately less than or equal to." If a more in depth explanation is needed than what I made in the flow chart just ask and I'll do my best to type one up. I'm even having a hard time to put it into words. It's more like one of those things I can just visualize how it works in my head.

I wasn't thinking of reverting the excess pressure back to the beginning of the the intake. But since there would still be pressure in the intake it would reduce turbo life, just not near as bad as without this set up or a BOV (just a sealed intake). And there wouldn't be any compressor surge until the turbo was wound down enough for it to be negligible I would think.

I figure there must be something wrong with this idea be cause like you said, if it did work it someone would have already thought of it and it would have been implemented many years ago. But hey, maybe I just hit the 1 in a 5,386,204,491 chance that I had a truly original idea that will forever change automotive turbo technology, putting me in the history books... Yeah, right lol. But yeah, so I guess that really I'm trying to find where I'm wrong rather than argue to prove my point. So if I sound like I'm not listening to you guys, it's really that I want to figure this this thing out and I probably need all the angles on it I can get.

I'm not talking about changing the pressure at which the valve opens, but rather when it closes. With a BOV no matter what pressure opens it will release all the intake pressure. I want it to close once pressure normalizes to retain boost.

The intake wastegate wouldn't necessarily be used to control boost. If I relied on that then even though I could control my boost I couldn't control the turbo speed. So both the intake and exhaust wastegates would have to be used. I'm not saying you didn't know that, just the way it was worded may have looked like that to other readers.

And I see what you're saying about the AFR's going screwy if it were vented into the atmosphere. Although I know it would have to go in between the MAF and the turbo, would some of the air backtrack through the MAF causing some sort of false reading? I know turbo would try to suck it back in, but would the fact that it's pressurized be too much for the turbo to pull it all back in? I guess not if it is forcing the air to that point in the first place.

And once again, while typing I came to another realization. The use of a recirculating BOV would keep all the mass of the air from the intake in the intake, preventing boost lag completely. Or do you still start back with no boost when a recirculating BOV closes and it just aids in keeping the turbo spooled?

[blownhemi]

Some problems with your event flow in "X is IW":

TB closes -> MGP rises to 14.5 psi

Manifold pressure does not rise when the TB closes, manifold pressure *DROPS* to vacuum, when TB closes. Maybe you meant "IC cold side pipe pressure" (pre-TB) under MGP?

IW opens -> Turbo spools down to until up to,

You have no control over how much the turbo will spool down. When the TB closes, fuel is not injected, the engine is just pumping air, no high pressure exhaust gases are produced to drive the turbo. (This is where an ALS systems come in.)

Really not wanting to sound rude, but I think you have some misconceptions about the elements of a turbo system, so I'll give you a quick rundown.

First of all, BOV and wastegate are mechanically the same thing: an air valve, the opening and closing of which is controlled by a separate air "circuit". Only, a WG is made of stronger materials, since it has to withstand high exhaust gas temperatures, and the occasional small particles, whereas a BOV only has to be able to withstand the flow of warm, clean, pressurized air. Thus, it doesn't matter, which one you put on the intake piping, you are achieving exactly the same effect.

I can see you understand, what a WG does, so I''l just explain what a BOV does. Yes, mainly it serves to protect to compressor from backflow (surge). There are two ways a BOV can be installed in:
- dump BOV: release the extra stuff into the atmosphere
- recirculated BOV: routes the extra stuff back before the compressor. Why this is much better, than a dump valve: the pressurized air that the turbo already produced is blown back into the turbo compressor. This puts almost the same pressure pre-turbo, than post-turbo, and this takes the "load" off the turbo, as the compressor is not slowed down by the vacuum before it. The turbo is allowed to freewheel this way, and it spools down much slower. But as I saw, you started to recognize it there towards the end. As for disturbing the MAF, my opinion is, you have to mate the BOV recirc piping into the intake at a sloped angle, so that it merges nicely into the air flow coming from the MAF. Having the MAF further away from said merge point probably helps, too.

And coming back to your original goal, reduced lag, and reduced instability of the rear-end when slowing into a corner(in case of RWD):
- lag: you can't really do any more about it, than correct turbo and manifold choice, and recirculated BOV (or an ALS, if you're very rich). With divided entry turbos and matching manifolds people seem to experience much better throttle on/off behavour, and part throttle response. I'm aming for a divided entry setup in the long run. Or with a little luck, it may be ready this year.
- rear-end break-out: has nothing to do with the turbo, really, since you're supposed to be off the accelerator (TB closed, so nothing before the TB matters!), and stomping the brakes. Brake setup, brake balance, diff. settings, driver skill (rev-matched downshifting) have much more role in this, than what you do with your intake air. As for how much the feathering of the throttle mid-corner upsets the rear-end, that's really up to the driver more than anything else.

[themadscientist]

Quality blowoffs have adjustable spring pressure. You want it tight enough to slam shut immediately when vacuum is released. I have seen a wastegate used as a blowoff, but just for s*** and giggles.

I think you might be overanalyzing the situation and making a needlessly complicated system. There is arguments over ideal placement for a blowoff. I try to mount it as close to the throttle as possible and my reson is as follows.

By placing the blowoof close to the manifold the vacuum hose is as short as possible. The lag is the pipetract filling up and beginning to move again. By releasing the pressure near the throttle the pressure continues to flow through the majority of the pipe tract. This means not only will it take less time to achieve positive pressure when you get back on the gas, it will already be moving so it will just have to speed up again. Also, the air will be flowing through the intercooler to keep it cooler.

[mdb4879]

I realized before I posted that once the TB is closed the MGP turns to vacuum regardless of the plumbing pressure. I changed the drawing before I uploaded it to have the boost signal for the wastegate from the plumbing and not the manifold, I just forgot to clarify that it should be plumbing pressure, not manifold pressure in the post. I had a lot of things to remember to type that I didn't write down, and this was one of the things I forgot.

I know there isn't any control over how the turbo spools once the throttle is closed, but if there isn't any drive then it will slow down and eventually the turbo won't be spinning fast enough to flow enough air to hold 14.5psi, at which point the wastegate will close (it may cycle opening and closing as it releases pressure and it builds again, idk. I just know it would remain closed once the pressure drops below the set point and the turbo can't force enough air into the system to make it rise above that point.

A wastegate and a BOV are both valves. But a wastegate opens once a peak pressure is reached and closes once pressure has dropped to or below that point. A BOV is pulled open by vacuum, giving no control as to what pressure it will close again other than by opening the throttle and getting rid of the vacuum (at least that how I think they operate, based off of vacuum. But I've never actually looked it up). So in regards to the intake plumbing, using a wastegate would only relieve the pressure down to a set point rather than relieving all of it. Or so I would believe.

After the first post I realized that if a BOV opens by vacuum, then all boost would be retained until the throttle was completely closed, or very close to it. So mid-corner stability wouldn't be affected. When I made the first post I was under the assumption that a BOV opens under a higher boost than what is desired, and I thought that even slightly letting off the throttle (feathering) could open it. Like say your at 75% throttle and drop it to 60%, I initially thought that this could open the BOV, making it like going from 75% to 0% throttle when it wasn't desired. Now I know it doesn't work like that and mid-corner stability is irrelevant. I must not have clarified I came to this understanding earlier.

I think your right, I probably am overanalyzing it now and am just blowing smoke. I might try this out some day for s***'s and giggles, but it obviously isn't very practicle for the relative benefits (if there would be any). This has been a very educational experience guys, thanks.

I'm still open to discussion of this topic if you would like, though.

[Buddyworm]

By placing the blowoof close to the manifold the vacuum hose is as short as possible. The lag is the pipetract filling up and beginning to move again. By releasing the pressure near the throttle the pressure continues to flow through the majority of the pipe tract. This means not only will it take less time to achieve positive pressure when you get back on the gas, it will already be moving so it will just have to speed up again.

I don't agree with this. BOV's help accelerate boost transition after shifts because they prevent compressor surge which slows down the compressor's RPMs. The BOV can't release pressure in one point of the intake tract and maintain pressure in the other sections without some sort of bulkhead or flow restriction in the piping*, it's just the way fluids/gasses under pressure work. As pressure inside a gas-containing vessel increases (and assuming the increased pressure is from more molecules entering the vessel and not increased kinetic energy from heating) the distance between the molecules in the chamber decreases uniformely and vice-versa. Since the charge piping is all connected, under boost it is, for all intents and purposes, a gas-containing vessel. So when the BOV releases it reduces the pressure in the piping uniformly throughout the whole tract as compressed gas escapes and allows the distance between the remaining molecules to expand and fill up the volume of the pipe. Think of it like a balloon full of water. If it springs a leak, regardless of where, the whole balloon begins to shrink and collapse at the same time and at the same speed.



*The IC core itself can be one such restriction, but it's effect is negligible because they are not that big of a restriction in the first place.