RE: Increased Compression?

["KOHLBRENNER,MICHAEL (A-hsgAndover,ex1)" <michael_kohlbrenner@xxxxxxxxxx>]

From: Mark Johnson <mjohnlit@domain.elided>

> The ratio is increased but the volume is less.  Displacement,
> i.e. bore x stroke is unchanged.
>
> Functionally, you are running a smaller engine with a higher
> compression RATIO.

The small reductions in combustion chamber volume that are
used to increase compression ratio have an insignificant
effect on overall engine "size".  More to the point, the
engine isn't really "smaller" in any sense that would effect
performance.  The amount of fuel/air pumped through the
engine is primarily a function of the swept volume or
displacement, not combustion chamber volume.  

Increasing the compression ratio of a given engine typically
improves performance and thermodynamic efficiency as long as
fuel with appropriate anti-knock qualities is used.

> My point on the detonation remains the same.  If the valve
> is open when detonation of the fuel/air mixture begins, the
> increase is well for nought.

Detonation is not going to occur while a valve is open -- or
at least not unless there is so much carbon on the piston that
it is on fire.

The danger with the carbon deposits is really a localized pre-
ignition thing where uncontrolled ignition of the fuel/air mix
occurs during the phase of rapidly increasing pressure just
after the spark has ignited the mix, but before the flame
front has reached the location where the hot carbon deposits
are lurking.

Most preignition actually occurs AFTER the spark event.  If
detonation occurs without a spark (dieseling), you've got
serious issues with the engine and there will likely be some
more dramatic symptoms than simple knocking or pinging.

Regards,
Mike Kohlbrenner

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