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

[Roger Brown]

This is usually tied in with the engine's RPM, so that if the throttle is closed and RPM above some threshold or brakes applied, 
fuel injectors are not opened, effectively cutting off fuel.  When the engine RPM drops below that threshold, fuel injectors are 
turned back on.  I've not driven a TDI but I do know for certain that the fuel injected engine in my Toyota does exactly this. 
Fuel cutoff RPM is about 1200 and you can see the cutoff on the air/fuel meter that I run and you can feel it as well.

dieseltdi at verizon.net wrote:
> Yeah I have a VAG-Com, I just cannot mechanically justify the concept  
> of 0 fuel to the engine.  I guess the way to test this is simple, go  to 
> the top of a hill, put the car in first gear and let it roll down  the 
> hill without using the accelerator pedal.  If the fuel is really  turned 
> off then the car should reach the bottom of the hill and stop  because 
> it is getting no fuel.  However, I suspect you will find that  as the 
> car slows it will begin to buck and sputter because of the  slow running 
> engine trying to move the heavy car at idle speed.  I  understand what 
> you are saying about the VAG-Com showing 0 injection  quantity, however 
> I don't think that is what is really happening.  I  still think that 
> there is some minimal quantity of fuel always  reaching the injectors.  
> In the old IDI engines, towards the end of  their run, they added a 
> device to the top of the injector pump that  looked like a turbo 
> enrichment unit.  In fact it was but the pressure  was applied in an 
> opposite manner.  When the car would decelerate  or  the engine would 
> overdrive, as they say, a vaccum was applied to the  diaphram which 
> would pull up on a needle valve allowing most of the  fuel to by pass 
> the distributor for the injectors. (In a turbo pump,  pressure is 
> applied on the opposite side of the diaphram and pushes  the needle in 
> the opposite direction directing more fuel into the  distributor part of 
> the pump.)   In this way they decreased, but did  not eliminate the fuel 
> to the injectors.  A small amount still made  it to the injectors and 
> the ignition process in each cylinder  continued but at a much decreased 
> rate.  In this way they lowered the  smoke and pollution produced.  I 
> suspect that the process is similar  on the TDI.  On deceleration, the 
> total amount of fuel is decreased  but is never really 0.  Injection 
> simply takes place at some default  rate, just enough to continue 
> combustion but not really enough to run  the engine.  If the engine shut 
> off, you would be able to tell, the  clattering of the diesel combustion 
> process would cease and it does  not.  Next time you free wheel your 
> engine listen closely, although  it is quieter when doing this, the 
> unmistakable sound of diesel  combustion is there.  hayden



-- 
    Roger