Getting facts straight! (2/3)

[Greg Hermann]

(continued from 1/3)


Another possible approach is to use staged injectors. Another, still more
fascinating, approach is to use air atomization of the fuel delivered by
the injectors--and thus get EVEN BETTER atomization of the fuel than is
given by GOOD carbs. This approach would also let one get still wider
dynamic range out of the injectors by allowing one to take the fuel rail
pressure MUCH lower (at light loads) than what gives even minimally
acceptable fuel atomization otherwise.
>
>BTW,  theory finally caught up with real life after I discovered a software
>glitch in the ignition timing that had my advance at 65 degrees at high RPM.
>(oops).  Now I (properly) limit advance on this engine to 34 degrees and
>maximum
>advance is in at 4000.  VTEC solenoid comes on at 3700RPm and we are achieving
>134HP at 6700RPM.   Suddenly my Volumetric Efficiency table matches theory and
>tuning the engine has become far more fun.

Sweet! How much stuff did you melt/break??

With regard to mixture requirements for max power/max efficiency:

With the superior fuel distribution that comes with port injection, it has
been learned that not nearly so much enrichment (beyond stoich) is
necessary to get to maximum power.13 or 13.5 , sometimes even 14 to 1
appears to be enough, at least with port injection.

There are TWO effects which lead to maximum power happening with the
mixture richer than stoich.

1.: The selective burning of the hydrogen component of the hydrocarbon fuel
before the carbon component. The hydrogen component has a higher heat of
combustion than the carbon, and therefore, for a limited amount of oxygen
available, the combustion process can actually generate a bit more heat
(and therefore power) by burning more  hydrogen and wasting some carbon.

2. A cooling effect on the engine cycle. SOME of this effect is brought
about by the adiabatic cooling of the inlet charge by the latent heat of
evaporation of the fuel--this effect makes for a denser charge, thus more
net oxygen and fuel to be burned passing through the engine and more power.
BUT--contrary to popular mythology, this is NOT the primary effect! The
charge can only be cooled SO much before it is saturated with respect to
fuel vapor, and that is IT! There is MORE than enough fuel present, even in
just a stoich mixture, to cool the inlet charge o a saturated condition!!
The heat energy necessary to evaporate all of the fuel (barring hot spots
in the manifold, or spraying fuel on the back side of closed, hot intake
valves (exactly as is done in MANY port injected engines) does not become
AVAILABLE (emphasis because this word is used in its context as the correct
thermodynamic term) until the intake valve closes and the compression
stroke starts! Absorbtion of the latent heat of vaporization of the fuel by
the charge during the compression stroke makes the compression part of the
cycle move from being an adiabatic process closer toward being an
isothermal process--which means several things: isothermal compression
requires less negative power, increasing the net output of the engine; the
peak temperature and pressure at the top of the compression stroke are
significantly lower, one can go to a higher effective compression ratio
because of the reduction in the tendency to detonate and/or pre-ignite
(with the lower T/P conditions), and, because the TP conditions at ignition
are lower, the peak cycle temperatures, cycle efficiency, and positive
power stroke output are ALL lower. BUT--the net effect on engine output is
about 2% positive, if the compression ratio remains constant. If the higher
possible compression is USED, then output goes up more, as usual.

Lots of folks don't realize it, but an alky (methanol) fueled engine, even
with a 13 to 1 compression ratio, does not have enough heat/work available
to vaporize ALL of the fuel, even by the time ignition occurs. The mixture
is still partially WET  at TDC on the compression stroke in an alky fueled
engine!! The first part of the combustion heat must therefore be wasted to
finish evaporating the fuel--which is precisely why alky engines can be
rather hard to light and like to foul their plugs, particularly when they
are cold. And is also why they are not particularly fuel efficient.

(cont'd.)

Greg