[Vwdiesel] Brakes --- engine brakes -- ( The physics of it ).

[Paul]

On Thu, 08 Jan 2004 12:38:31 -0500 (EST)
Val Christian <val at swamps.roc.ny.us> wrote:

> If just one person asks, I'll bore everyone with why a TDI seems to brake 
> less than a NA VW diesel.  There are two primary factors, and several
> secondary ones...oops, I'll wait.

Well val, I guess I brought it up.. So I might as well be the one to ask..

I personally think it is how they have the feerback loop setup in the ecu.  It seems as though they set up the feedback loop to be gentel on the decel from high rpms.  This is to avoid stalling out under minimal or negative engine load. Thus preventing stalling when going downhill and the fuel is cut too much.  I imagine they have a dynamic minimum fueling floor, with a hard offset constant if a certain mimum rpm is violated.

Observations that lead me to believe this.  Sit in the driveway idling.  Peg the throttle(about 1/4 to 1/2) and let off real quick(dont let the engine rev very high 2200 max).. The trick is timing and a quick foot(all people driving rabbits should have quick feet, right?).  It seems as though there isn't a low pass filter in the user input, so if you pull off of the pedal at just the right time it bypasses the fueling floor limit.  If you do it just right you hear the engine almost stall out.  Either it hits the minimum rpm where a hard fueling offset is added or the feedback system gets the next sample and realizes it overshot the idle speed by a mile and overcompensates bringing the engin back to life.

Intrested in you'r thoughts too,

Paul.

p.s.  where you at from in rochester val?  I'm a native from gates.. Nando is the only other diesel guy around that I know of..  might be nice for all of us to have a gtg.. I've been thinking about giving Martin from u of r a shout also..