Venting BOV to Exhaust...Thoughts?

[cdenton]

Kind of a backwards EGR.

My thought is that when the throttle plate is closed the gas flow stops and thus the turbo spools down. Why not send the recirc to the exhaust to keep the gas velocity up and to minimize spool down? (assuming you are not running a MAF or AFM)

 

The only thing I can think would be a big issue would be fuel metering with an O2 sensor might fluctuate. Seeing an artificially lean condition.

 

I suspect there is no real advantage here as I can not find any mention of such plumbing.

 

Please let me know what you think.

 

Thanks

Chris

[Tony D]

That's becase clean compressed air will spin a cold compressor wheel just like a pinwheel just as well, so the BOV recirculates to a tangential entry to the compressor hosusing to spin the compressor and keep the speed up.

 

And that doesn't screw with your O2 sensor reading, either.

And if you have a MAF or AFM, blowing into the inlet chamber also keeps your metering correct on drop-throttle as well. A Win-Win situation.

[Six_Shooter]

Sounds like you're talking about trying to vent the BOV into the exhaust pre-turbine, there's oine HUGE problem with that idea.

The exhaust pressure pre turbine is commonly twice that of the intake, sometimes higher. I.E. 10 PSIG boost pressure will have around 20 PSIG exhaust pressure pre turbine.

 

So the BOV would never open, due to the exhaust pressure holding it closed, and if it did some how manage to open, then exhaust gasses would then be introduced into the intake system.

 

Good to think outside the box though.

[cdenton]

Tony

I get the pinwheel concept, but it seems that what is really going to happen is that you will be just adding load to the compressor wheel by presenting more gas at the impeller or you will simply blow the air back out of the air filter. Given this thought it seems we are really just equalizing the pressure between two volumes of gas. I this why the high end cars choose to vent to atmosphere?

 

Six

I was thinking that when the throttle plate was closed the gas pressure in the exhaust would fall. I have no idea what kind of pressures would happen or how long it would take for the pressure to fall (too long I suppose). Excellent point about backflow.

 

Thanks for the responses.

Chris

[Tony D]

No, you will pinwheel the compressor. Trust me on this one! Industrial compressors will viod the warranty if you do not install a check valve on the discharge because as easily as they will spin with power to the gearbox, that same pressure will stop them dead and then spin that same 15,000 HP motor in reverse just as quick as you can say 'what's that noise?'! Same goes for automotive turbos, especially ball bearing units.

 

You will not be equalizing volumes and blowing out the air filter, you will be imparting pressurized air to the suction side of the wheel which is under several inches of water vacuum. This is exactly how Porsche does it. I guess they aren't high-end? In any case, with the turbine already spinning, blowing a pressurized air source at it in the correct orientation will pre-spin it briefly to a higher rpm. It will continue to coast down as normal if still off the throttle, but if you reapply throttle the turbine is closer to productive rpms in this case. Try aiming the nozzle wrong and see what happens. You will (if you have a turbo tachometer with a datalogger) see a marked decrease in speed when that air hits the wheel.

 

As the 'twice the pressure' in the exhaust, I know for a fact that JeffP had 20 psi in the exhaust when there was 20psi in the intake plenum, under load, at 7000rpms. Some of the 'book stuff' really is so out of date it isn't funny. Proper turbine A/R sizing and current technology will make for radically different exhaust pressures that even experienced as recently as the 1980's

[Six_Shooter]

As the 'twice the pressure' in the exhaust, I know for a fact that JeffP had 20 psi in the exhaust when there was 20psi in the intake plenum, under load, at 7000rpms. Some of the 'book stuff' really is so out of date it isn't funny. Proper turbine A/R sizing and current technology will make for radically different exhaust pressures that even experienced as recently as the 1980's

 

That wasn't based on what I have read in books, that's based on what (many) guys have datalogged in thier cars, and makes complete sense.

 

You need more pressure (force) on the drive side of the device, a turbo in this case, in order to not stall the device. A/R ratios and trims would change this a bit, but that's where the pressure at the nozzle tip would then be higher than before that point, and where a lower pre turbine pressure could be seen. In most turbo systems you will see a higher pressure in the exhaust pre-turbine than in the intake.

[cdenton]

Tony

I believe you. I have read enough posts to know that you know what you are talking about.

I have looked, and I can not seem to find any information on why wrc cars etc vent to atmosphere. Some enlightenment here would be nice.

I suspect Porsche vents to the intake for many reasons. Noise, emissions regulations are two that I can think of.

FYI, I plan to recirc my BOV to minimize noise. I happen to be fond of quiet when I can get it.

 

As far as exhaust pressure goes it seems that will depend on many things. My intuition is to think that it would be higher than the intake since everything that is coming out came in etc and we can not forget that the fuel that was previously a liquid is now more gas. Now, given the right size exhaust the difference may not be worth worrying about.

 

Chris

[Tony D]

. In most turbo systems you will see a higher pressure in the exhaust pre-turbine than in the intake.

 

I can agree with that statement, just not that it will be twice the inlet plenum pressure. In the old F1 days, the turbo sizing was enormous for the size of the engine, but for the HP they produced on mass flow, it wasn't so big. And they had very low preturbine pressures compared to inlet pressures. They weren't at a crossover point where reversion was an issue during valve overlap at least.

 

Yes, the pressure would be higher, but in one that is sized adequately it should be well less than 2X inlet.

[Tony D]

I have looked, and I can not seem to find any information on why wrc cars etc vent to atmosphere. Some enlightenment here would be nice.

 

Well, being they employ 'flatshift' technology that retards the spark, and gives full fuel to turn the turbo into a gas turbine on full boost, the control scenario would be to blow off the pressure to atmosphere to keep minimum flows stabilized on the compressor side. This is exactly the same as a comercial compressor. These turbos during shifting and drop-throttle are not operating as conventional turbochargers but rather gas turbine compressors. They will make boost with no throttle opening, or WOT staging at the line on a rev limiter.

 

In that instant, to keep turbine speed up when running your EGT's skyhigh, you need to dump the pressure overboard as the engine simply can't process it. It's a totally different dynamic than drop-throttle in a street-driven car.

 

In that instance, you have no motive force on the turbine side---and you begin coastdown. By dumping pressure you minimize the coastdown, and if you give that brief pressurized 'boof!' of air into the compressor side when there is no pressure load on it, you spin it up to speed thereby minimizing coastdown effects.

 

Also, keep in mind the big flames from the WRC cars...when a team mechanical engineer in charge of fuel systems was asked by a design engineer for a Rally Rag 'why do they do that' sheepishly the engineer for the WRC team admitted they indeed could make the cars run without the big flames, but 'the fans love it, and it makes for startling sponsorship photos, so we go full rich (instead of fuel cut) on drop-throttle for the effect!' So there IS showmanship in all forms of motorsport...

 

Most OEM's blowdown within their intake tract for metering concerns. But Porsche is one of them that uses tangential entry of that blowdown energy to actually do something positive in their turbosystems. Waste not, want-not!

[m4xwellmurd3r]

well not just that but the big flame is also a production of the anti lag system that's in there that causes the spark to ignite on the exhaust stroke. which is how they keep that 1-2bar of pressure at idle. DEFINITELY not somethin you want on a street car if you want to keep your exhaust system and turbo haha. I see why they would vent to atmosphere though. if it's making 1+ bar at idle, if you were to vent back into the intake the pressure would cause quite a lot of issues wouldn't it.

[cdenton]

Very, Very informative.

Now, how far from the compressor and at what angle should I make the recirc re-enter the inlet? Is there a particular model and year of Porsche that I can research to see how they do it?

Thanks again

Chris

[Tony D]

On my car, I used a 17 mm piece of tubing at a slight angle in the turbo's inlet piping, about 10cm from the turbine wheel. This 17mm tube is smaller than the BOV dump at around 25mm. I use a 25mm tube from the BOV to a tapered fitting that necks it down to 17mm. This gives a more directed velocity shot, and the hasn't seemed to affect how quickly the plenum blows down.

By blowing some compressed air at the compressor wheel you can best figure out what 'angle' seems the best for getting the wheel spinning. Keeping it 10cm away (er, 5" maybe) keeps that larger tube from screwing up the flow too much from sticking in to the intake ducting.

 

Same thing for your PCV, btw, similar setup like the OEM lines---if you play it right, you can use the stock upper PCV hose to hook to your fabricated intake tubing.

 

Argh...it's off the car and in the shed right now...I don't know exactly where...this is an example of where one photo would make what I'm trying to say much clearer!

[datman]

So is this what Mitsubishi were trying to do with the CBV on this turbo (TD04)