Maximum advance

[Bernd Felsche]

Dave Plummer tapped away at the keyboard with:

> More isn't better.  The more advance you have, the more the piston
> is working against rising cylinder pressure on the upstroke.  What
> you want is a peak cylinder pressure at a particular point in the
> downstroke that provides the best leverage on the crank.  No more,
> no less.  If you could somehow accomplish that with zero advance,
> you'd have the best of all worlds.

Zero advance would only be valid if the mixture is ignited either
instantaneously, or if the pressure developed in such a way as to
maximise the total torque produced during the whole power stroke.
The effectiveness of converting that pressure into torque depends on
the crank geometry; rod length and crank throw, as well as cylinder
offset if applicable.

At 6000 rpm, the engine completes the power stroke in 5
milliseconds. The ignition delay; to get the mixture to start
burning; is around 1 millisecond (dependent on engine design and
varies with AFR).  That's 36 degrees *before* any combustion
pressure develops.

At mid-crank, the piston speed (6000rpm) is about the same as that
of the flame front, so there'll be little increase in pressure
(approximately!) at mid-travel. After that point, the mechanical
efficiency of the crank will decrease (give or take a few degrees,
depending on precise geometry).  Most of the pressure build-up must
therefore be produced before that time; i.e. within 2.5 milliseconds
ATDC

The highest pressures are developed when the initial flame is
propagating into the minimum volume. Flame speed is constant for a
given mixture so the 50mm or so from the spark plug to the
most-extreme wall must be travelled as quickly as possible.
You really do want the mixture to start burning at TDC (ignoring
possible engine damage for now). The 2 milliseconds or so it takes
for the flame to travel to the cylinder wall is a considerable time.

-- 
 /"\  Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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