Turbo Tim wrote:
As far as using the FCV valve to act as a brake, I believe that your on the wrong end of the horse. Every engine brake setup I have seen on a diesel has been on the exhaust side. Closing off the air into the engine would cause it to run in a vacuum. I don't see how that would help brake. On the exhaust side however, by closing off the escape route, then the compression would offer resistance and would slow you down.
By using the signals from the transmission it is very easy to have the FCV valve be switched back in before shutdown. This signal would make it active only when in park. Easy to do, but first, back to the circuit.......
Don't want to totally jack this thread,.. but the FCV is related so...
What I was thinking is that the vacuum would help it brake the same way it helps a gasser brake. When the vacuum is created with a partially closed FCV doesn't the engine have to pump against the vacuum the same way a gasser does (it is trying to pull air in the motor and that air is not there)? Is this not one of the reasons a diesel gets better mileage than a gasser-- it does not suffer from the vacuum pumping loss the gas engine suffers from constantly, except at WOT. I know this is one reason gas motors engine brake very well when downshifting, but most diesels don't slow as much when downshifting-- they just rev higher.
Maybe this vacuum loss is a reason why many have poor mileage-- the FCV is closing a bit even with the EGR off (like at idle) and causing the motor to pump against the little bit of vacuum.
A potential worry to using this to brake is out of spec heat generated by a different fuel/air mapping. Another is that you are lowering the high compression ratio of the diesel by putting less air in it and this may cause an ignition failue/poor ignition. Interestingly, could the latter also be a contributor to the .... "buck and shudder"? Especially when letting off the pedal after a hard acceleration as the software is busy trying to adjust from WOT back to cruise and closing the FCV valve (maybe more than it should), especailly with the shoot covered MAP sensor sending it data from the land of make-believe.