[Vwdiesel] Diesel octane or why can you run a diesel engine lean?

[slatersfb at aol.com]

Answer :  You can run a diesel lean  -- because it will not detonate like
a gasser. 

  I STILL contend you can't run a diesel "lean."    ;-)  That's the state 
it's 
supposed to and WANTS to run in.  You can ovefuel with poor timing, 
bad injectors, low compression and setting the pump so that it will 
allow more fuel than can be burned.  When you run more fuel than 
is required at a given load, the engine wants to accelerate.  Less 
fuel will not make it run more lean, it will make it slow down.
  The big point tha Hagar is making but just doesn't seem to click in my 
pointy head, is that you need to get ALL those other parameters correct 
in order to use the least fuel.  Turning the fuel down will simply make 
the engine run slower.  One tends to think of it in terms of lean but it's 
really achieving the best efficiency.  IMO.

 Loren

*********************
 
This is correct in my view. There is no throttle plate in intake tract on VW diesel. Speed is controled by metering fuel and pump timing. If we ignore pump timing for a moment for simplicity's sake, then it is seen that the injection of fuel begins at the same engine piston position always. So combustion always starts at the same point. The little slider ring on the pump piston determines when the spill port will be opened, thereby limiting injected quantity. So engine speed is controled by how long fuel is injected, and is limited by varying exposure of the spill port.  It is therefore possible by adjustment of the fuel screw, to inject fuel for too long a period, maybe even when the exhaust valve has opened. Engine piston position and connecting rod angularity near the bottom of the power stroke create less torque, so injection of fuel beyond a certain point is less efficient. I guess the idea is to get most efficient burn when engine piston position is optimum: pump timing, injector spray pattern & break pressure.
 
Bob in the entire state