cutting off injection going downhill was [Vwdiesel] Why gasss cars mpg

[Val Christian]

To "Start" the engine, you need fuel, oxygen, and ignition.
Since the engine keeps "pumping air", it has oxygen, and it
also has ignition temperatures.  When the injection of fuel is
resumed, the engine "starts".  VAG-COM displays the total
injection volume.  Idle speed in controlled by the injection
volume.  Try turning on the AC and/or drawing a large electrical 
load (defroster, lights, etc.) and the injection volume goes up
to maintain the same RPM band.

Do you have VAG-COM so that you can watch the injection volume?

Val





> 
> The injection amount commanded by the ECU may drop to 0 but that  
> doesn't mean injection stops.  If the TDI actually cut off the fuel  
> to the engine as you went down hill, it would have to know when to  
> "restart" the engine.  How would it know when to do that?  I spent 6  
> hours in my car yesterday going up and down hills in East Texas and  
> Louisiana.  The engine never stops running even if I let off the  
> pedal.  It simply returns to idle status, meaning only enough fuel  
> being injected to keep the car idling.  The ECU injection quantity as  
> I see it is a demand ABOVE AND BEYOND what is needed to keep the  
> engine running, not the total amount of the fuel injected.  In other  
> words, there is some base amount - X that is injected all the time,  
> but when the ECU senses a signal from the pedal, injection quantity  
> goes up.  Let up on the pedal, injection quantity goes down.  There  
> is simply no practical way for these engines to literally shut off  
> when going down hill.  Just my two sense worth but as I said, I spent  
> 6 hours in my car yesterday thinking about this and trying to  
> logically justify the concept.  It simply cannot work.  Hayden
> 
> On May 15, 2005, at 6:22 AM, Val Christian wrote:
> 
> >>> A couple of things...first, the TDI DOES shut off all injection
> >>> in a coast operation.  I've observed that, and verified it as
> >>> correct behavior for the TDI.
> >>>
> >>>
> >>>
> >>
> >>   So when you top over a hill, (and keep going of course)  the TDI
> >> engine actually shuts off?  Seems like something DOT wouldn't
> >> approve of regardless of ramifications or not.  Seems like a
> >> freewheeling (ala E-mode) would save even more fuel.  If that's
> >> the intent.
> >>     Loren
> >>
> >
> > When you top a hill, and start down, coasting, the injection level
> > commanded by the ECU can drop to zero.  It's a function of power  
> > commanded.
> > Taking your foot off the pedal, it will drop to zero.  Depending upon
> > the hill, not moving your foot may cause some fuel to be commanded
> > to be injected.  But simply stated, yes, fuel can (and often does)
> > drop to zero.  It's not freewheeling, per say, because the entire
> > engine (altenator, compressor, power steering, pistons and valvetrain)
> > keep turning.  So when the available power exceeds those, you get a
> > free ride.
> >
> > Val
> >
> >
> >
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