[Vwdiesel] INCREASING AND DECREASING FUELING

[James Hansen]

I may indeed be wrong.  I get what was said regarding increasing fuel merely
increasing power up to the point where efficiency drops off.  That may be
true, but my imagination still has a little trouble with it.  I imagine that
boost
pressure is not just a function of fuel but also of the amount of air being
pumped in and out of the engine.
______________________

Well, not quite really.  This is not a gas engine- it's an unrestricted
engine, to say there is no throttle butterfly to restrict incoming air (on
the vast vast majority), it has a straight shot like a wide open throttle on
a double pumper.  The amount of air that passes through the engine is
regulated by the volumetric efficiency of the design at whatever rpm it is
running at.  The increase in gas volume from heat generated by combustion of
fuel is very very significant.   This is why a diesel is such an efficient
design- there is almost always O2 in abundance.  Your example of an engine
pumping air to spin the turbo would make no boost pressure whatsoever, not
even a negligible one.  If it did, you would have perpetual motion, but
entropy still calls the shots in these here parts.

Why a diesel smokes isn't so much that there is insufficient air, as
insufficient time for all the fuel to burn before an exhaust valve opens.
Grossly overfueled perhaps but not in an engine in tune, but the reason the
turbo turns red is because it has fire in it, not because all the air is
used up.  If there was no O2 left, it wouldn't be fire any more.  I've seen
the blue candles atop older truck smokestacks at night.  A 290 Cummins from
the 1970's will push between 6" to a foot of blue flame out the stack on a
hard uphill pull, visible at night. Looks pretty cool actually. :-)

You can actually increase power in a diesel until the valves, pistons, turbo
and head all melt.  Quite easily actually.  The Max fuel screw on a bosch
pump is really just a means of limiting how much fuel can be injected when
the internal pump governor sees less rpm on the pump shaft than the throttle
shaft input is calling for.  Fuel injected quantity is increased until the
shaft speeds up.  Just how much is increased is regulated by boost aneroid
to say that the aneroid can't add more than the max fuel screw allows, but
will temporarily reduce the max fuel if there is insufficient boost. Only
when there is full boost is full fueling allowed.

HTH
-James
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