[Diy_efi] injector advance

[Greg Hermann]

At 2:43 PM 8/7/04, Adam Wade wrote:

>
>This is a good textbook answer, but my research
>indicates that such is not the case in practice, at
>least not with port-injection systems.

But it IS there.
>
>> and cooling the intake valve reduces efficiency.
>
>It reduces thermal efficiency slightly, but if the
>result is increased homogeneity of the intake charge,
>then overall you are getting "more bang for your
>buck", quite literally.
>
>> Evaporating (well atomized) fuel inside the cylinder
>> once the valves are closed provides internal cooling
>> during the compression stroke,
>
>Which...  reduces thermal efficiency, like you said it
>did for the intake valve, only more globally and less
>locally.  How is this "better" from the standpoint of
>thermal efficiency?

Yes, it reduces thermal efficiency, because the charge does not reach quite
such a high peak temp. However, the loss is SLIGHTLY more than made up by
the reduction in negative power required for the compression process. The
elephant stomping around in the closet is that a lower peak temp during
combustion significantly reduces heat loss to the piston crown and chamber
walls--particularly the radiative component of this loss --since, of
course, radiative heat transfer (including transfer from a hot gas to a
solid) varies with the fourth power of the (absolute) temperatures of the
bodies involved. THIS savings in heat loss makes the gain from internal
cooling rather significant.
>
>> thus reduces bearing loading !!
>
>Bearings are only really "loaded" during the power
>stroke, either peaking with the PPP or with a
>detonation event.  Compression losses are negligible.

In an (Otto cycle) engine with no internal coolant in the working fluid,
the negative power needed for the compression stroke amounts to about 25 %
of the NET power output at the flywheel (before accessories). Do the math.
The power to provide the compression must come from the power stroke. If
you make a significant reduction in the negative power for compression, you
are also reducing the power needed during expansion equally---in other
words, the bearing loadings are reduced because the engine does not need to
pass as much power around internally between the cylinders. A 33% reduction
in negative power needed for compression using alky and/or water injection
is NOT AT ALL beyond reach. Just as reduction of mixture temp  just prior
to ignition from about 900=B0F (depending on compression ratio) to more like
250=B0 F with alky and/or H20 is WELL within reason !! This CAN and DOES
translate into significantly lower bearing loadings for equal NET power
OUTPUT !!!
>
>> The fact that properly TUNED IR carbys (Webers, et
>> al) will yield lower full power bsfc than
>> conventional port injectors  at WOT pretty  much
>> proves the point.
>
>I don't understand how you are drawing that conclusion.

They DO. The point is that good IR carbys deliver very thoroughly atomized
fuel and deliver it ONLY during the time of high gas flow velocity in the
inlet ports. Air assisted injectors can do the same thing.

Greg


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