[Vwdiesel] Fueling screw on VE pumps. --( Rabbits )

[Gavrik Peterson]

I just finished rereading all the posts in this thread.  While many
interesting ideas are introduced, the discussion seems to be centered
on the 'fueling screw' and its effect on how the pump does its job.

It appears to me that, with respect to the 'fueling screw' there are
three things that Hagar is asserting:

(1) That the 'fueling screw' affects the 'mixture'.  I am assuming
that for a fixed amount of air, Hagar's 'mixture' is equivalent to the
volume of fuel that the pump delivers per injection.

(2) That the 'fueling screw' setting effects 'mixture' over the full
range of engine speed.

(3) That the 'fueling screw' setting effects 'mixture' over the full
range of engine torque.

Hagar supports (2) and (3) by saying that adjusting the 'fueling
screw' changes the engine speed and therefore the mixture must have
changed.

I think that Hagar's assertion (1) and (2) must be correct.  But after
some thinking I have figure out a way to show that assertion (3) is
not true without knowing anything about the fuel pump.

I propose to show that (3) is untrue by examining a fundamental
property of the diesel engine -- the relationship between fuel volume
delivered by the pump and the behavior of the engine.  That each
combination of speed and torque, the engine uses a certain amount of
fuel.

Imagine a rectangular map where the horizontal axis represents engine
RPM and the vertical axis represents engine torque.

    torque
      ^
      |  ///////////////////////// <-- This boundary moves upward
      |  /////////////////////////     when the 'fueling screw' is
      |  /////////////////////////     adjusted inward
      |  /////////////////////////
      |  /////////////////////////
      |  /////////////////////////
      |  /////////////////////////
      |  /////////////////////////
      |  /////////////////////////
      |  /////////////////////////
    0  ----------------------------> RPM
      0  |                       |
        min                     max
   
The rectangular area shown with slash characters represents all the
possible combinations of engine speed and torque.  The left and right
sides of this area exist because of the limits placed on the travel of
the control lever by the two set screws.

It is my assertion that there is a one to one correspondence between
each point within this area and a volume of fuel the engine needs to
be operating at that point.  This relationship is a property of the
engine, independent of pump adjustments.

So if adjusting the 'fueling screw' changes something in the pump that
causes the engine speed to change, all that has happened is a change
in the relationship between control lever and engine speed.  The
engine will receive the same amount of fuel at a given point within
the operating area.

So one might ask if this is true, why Bosch bothered with the 'fueling
screw'.  The reason is that the 'fueling screw' changes the location
of the upper boundary of the operating area -- expanding or shrinking
it.  Therefore, one can move the boundary upward and have more torque
(and smoke) by allowing the pump to deliver more fuel.  This is why I
argue that the 'fueling screw' only matters at certain levels of
torque -- when the pump is delivering enough fuel to correspond to an
operation point near the top of the operating area.


  --  Gavrik