[roadie]

I am trying to understand the necessity of wastegates. I know they allow exhaust to bypass the turbo at certain boost levels (via a boost controller), but wouldn't an advancement in blow-of-valve technology be more effective ?? Shouldn't a wastegateless turbo maintain higher rpm and therefore have better throttle response ?? Or is there a point where you can have too much exhaust going thru a turbo and over spin it ?? Or would forcing all the exhaust thru a turbo restrict the engine in the high rpms ??
I am currently running a BOVless single turbo setup which has been argued to death already...

 

[tweenierob]

Lol, you have answered the question yourself already

is there a point where you can have too much exhaust going thru a turbo and over spin it ??

the wastegate controls exhaust gas flow to the turbine, the more gas flow the more boost.

Rob

 

[Gareth R34]

I am currently running a BOVless single turbo setup which has been argued to death already...

WTF :chairshot :chairshot why would you run it with out a BOV that is one way to kill the turbo :chairshot :chairshot when you lift of the throttle all the xs-boot will be going back throw the turbo and that's not good for the life of the turbo now is it

 

[roadie]

My apoligies for not being too clear....Firstly, there have been lots of instances where turbos in a BOV absent enviroment have worked perfectly for many years (ie. my T88 with 20,000+kms and no wear or damage). However. I have decided to add a blow off valve as a precautionary measure.
But, it doesn't seem right that boost is controlled by not putting as much "wind" thru the exhaust housing. Would it not be possible to vent the excess pressure (at say 1.5 bar) somewhere between the compressor housing and the intake ??

 

[MacGTR]

Would it not be possible to vent the excess pressure (at say 1.5 bar) somewhere between the compressor housing and the intake ??

Now, correct me if im wrong because i don't know too much about BOV's but aint that what a BOV is doing just at a different point?

 

[skyline69_uk]

I suggest just doing a google search or buying a book about turbochargers to understand them...

The easy stuff...

Turbocharger - Wikipedia, the free encyclopedia

Others...

The blow-off valve should not be confused with a waste-gate. The waste-gate is a
device that by-passes the turbine wheel of the turbocharger, limiting the
shaft-speed of the turbocharger. Therefore, limiting the boost
(pressure) that the compressor generates and keeping the turbocharger
from over-speeding.

A blow-off valve is mounted in the intake plumbing between the
turbocharger compressor and the throttle plate. The blow-off valve is a
second safety measure against the turbocharger over-boosting and damaging
the engine.

The blow-off valve is more commonly used to keep the turbocharger spinning
when the throttle plate is suddenly closed. When the turbocharger is
generating maximum boost pressure at full throttle and then the throttle
is suddenly closed, compressed air coming from the compressor slams
against the throttle, generating extremely high pressures that travel
backwards to the compressor stopping the compressor from spinning. When
the throttle plate is again opened, the engine must spool the turbocharger
shaft again. The effects of this high pressure can also be very damaging
to the turbocharger.

Brian Wright
Washington University in St. Louis Formula SAE Racing
=====================================================

 

[MacGTR]

Okay, now i completely inderstand how a turbo and a BOV works.

Well "the easy stuff"

 

[skyline69_uk]

Lol, those links were for Roadie but I'm glad they helped you also .

 

[MacGTR]

So, first question, i was wrong when i said its like a BOV because its more like a wastegate? Also you can have a BOV and a waste gate fitted? seeming as they are both fitted in different places....

 

[MacGTR]

So, first question, i was wrong when i said its like a BOV because its more like a wastegate? Also you can have a BOV and a waste gate fitted? seeming as they are both fitted in different places....

tell me if im wrong or right because i can't learn from my mistakes unless i know what they are

 

[kismetcapitan]

in a nutshell, a wastegate vents exhaust before the turbo. a BOV vents boosted air after the turbo but before the engine.

what I'd like to see are computer controlled servo-actuated wastegates and possibly BOVs. vacuum-actuated wastegates controlled by a solenoid seems crude and imprecise in this day and age - hell, BMW has heads that have infinitely variable valve duration as well as lift. If you shut the wastegate you'd spool damned fast, but then you'd have to open up again to keep from overboosting.

I'm probably missing something, but a wastegate that can open, close, and be anywhere in between within milliseconds seems a much better solution than VNTs or electric-assist turbos. Maybe servos aren't that quick yet?

 

[MacGTR]

oh okay so a BOV stops the air from going back through the turbo?

 

[SteveN]

WTF :chairshot :chairshot why would you run it with out a BOV that is one way to kill the turbo :chairshot :chairshot when you lift of the throttle all the xs-boot will be going back throw the turbo and that's not good for the life of the turbo now is it

You ever TESTED that?

I have. Trust me when i say theory makes it sound millions of times worse than reality...

I'm probably missing something, but a wastegate that can open, close, and be anywhere in between within milliseconds seems a much better solution than VNTs or electric-assist turbos. Maybe servos aren't that quick yet?

Lol, yeah, you missing a lot if you think a more accurate wastegate is gonna do anything to help spool. Why would it? Hopefully your not running boost control directly off the wastegate or via a bleed valve, so nothing else will improve spool with regards to the wastegate.

 

[roadie]

Cheers...Thanks for the info.
I understand that wastegates control the boost levels for the engine. I also understand that bov's vent excess presure during lift-off etc. and accuate during episodes of vaccum.
Some factory bov's are designed to leak air during episodes of boost spike. They vent pressure thru the pressurized area of the engine. My question still remains, why do we not regulate the pressure this way ??
Look at it like a water hose. Open the tap all the way. Take your thumb and slowly push down on the end. You increase the pressure, right ??. Current turbo mentality sugests that when you reach a certain pressure, close the tap partially until you reach equilibrium. Why not instead use your thumb to regulate the pressure and therefore have all the flow still available ??

Sorry if this sounds dumb. I know I deserve one of these :lamer:

 

[SteveN]

Cause it would overspeed the turbo, killing it at worst, and masivley dropping the efficiency, creating higher charge temps and therefore less power and more chance of det, at best.

It IS done like this now and again tho, but very rarely, for the above reasons...

 

[roadie]

That's the answer I was looking for.....

 

[skyline69_uk]

You ever TESTED that?

I have. Trust me when i say theory makes it sound millions of times worse than reality...

It's a reliability/weight/cost etc issue. If you can protect from surge you can build the components lighter leading to less lag etc.

Things don't just break but stretch, compress and bend (ALL materials act like rubber to differing degrees) and over time they fatigue. One of the things that Auto Engineers note all the time about turbos is that on each surge the rotating shaft is pressed slightly off centre and is stressed as it is levered against the bearing (and the Assembly heats up as you have lots of energy with nothing that wants to turn). Over time this results in play in the shaft that although is not visible will one day (during a surge probably) break the oil film and cause scoring. Over time this scoring kills the turbo unless the turbo has been built with a large safety factor in mind.

That's the point some turbos deal with it as they are over engineered while others built to reduce lag/cost will not - unless you know which your turbo is and by how much, removing the BOV will reduce it's life expectancy then you are safer leaving it alone. These things cost money to fit and can bugger up other things so engineers would love to remove them to save money etc but the downsides (reliability, having to make others things stronger/heavier) are too great in the majority of cases so they remain.

Having a few dozen turbos appear OK after 30k or so miles is just a gain of sand on a very large beach compared to all the other research that has been done on them over the last 102 years. The reason they didn't break is that some guys built them better than they needed to as they wanted them to last 250k miles if properly maintained.

 

[Gareth R34]

You ever TESTED that?

I have. Trust me when i say theory makes it sound millions of times worse than reality...

QUOTE]


Yes i did on my first turbo car and that was a MG Metro Turbo