Re: Word/Re-circulating valve fuction - Saab Performance Bulletin Board - Saabnet.com
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Re: Word/Re-circulating valve fuction
Posted by craigharroff [Email] (more from craigharroff) on Fri, 27 Feb 2015 09:23:19
In Reply to: Word, ELaw [Profile/Gallery]
, Fri, 27 Feb 2015 04:18:53
As Jlaw has noted, the holding pressure is higher at WOT. That was another great design feature about the Bosch that I had not considered. Thanks for pointing that out.
From what I have read and from personal experience, my understanding of what the blow off/ re-circulation valve tries to accomplish is as follows: At partial throttle or WOT the pressure in the intake (plus spring) holds the valve closed. When you let off the gas the throttle closes. Pressure in the PLENUM drops. Without the blow off valve/re-circulation valve present you are left with pressure (above atmosphere) in the intake between the turbocharger which is still compressing (for a very short time) and the throttle body which is now in a closed/partially closed position. This is a substantial volume of air:it includes the intercooler and all the piping volume before the throttle body. At this point several things are happening. Because you are off throttle (or partial throttle) exhaust gas heat/ pressure drop (worst case is a closed throttle). Less energy causes the turbo to slow down. In a closed throttle position, it slows down a lot depending on the length of time/bearing loss/wheel design/etc. At the same time the elevated pressure in the intake is looking to "escape" to the lower pressure of atmosphere which is now (WITHOUT a blow off/re-circulation valve present) BEHIND the TURBO.Because the turbo is no longer compressing due to the exhaust energy being partially or greatly reduced, the barrier to atmosphere is no longer present. As I understand it, this is the single most important event that slows down the rotating turbine as the air pressure "vents" backwards through the now slowing compressor wheel. This back flow will either slow or stall a compressor wheel. This happens every time you lift off the throttle. Given enough volume and pressure (larger aftermarket intercooler and intake pipe plus higher than stock boost), I suspect the compressor wheel would begin to rotate in a non-compressing direction AKA backwards. I took a Bosch valve and turned it into a blow off valve. The noise was so loud that I constructed a highly vented shroud to reduce the noise. It screwed together. My first design totally blew apart when I first tested it.The volume of air is significant and the rapid slowing of the compressor wheel can be a violent event given a high pressure differential. The valve allows the air to vent to atmosphere AROUND the turbocharger thus avoiding most of the shock and, hopefully, maintaining turbine speed so that when exhaust energy is again applied to the turbine wheel the initial starting point of rotation is higher than it would have been without a venting blow off valve.
Not all boosting events are WOT followed by a closed throttle so the effectiveness of the valve depends upon quite a few things such as time between getting on and off and then back on throttle. Are you shifting or braking? There are trade offs here. It would be logical that some optimal compressor speed be maintained while all these things are happening and, as Jlaw has posted, this function is now coming under computer control. That is very interesting!
posted by 192.230.18...
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