Maximum advance

[Bernd Felsche]

944Technologist tapped away at the keyboard with:

> What is the maximum advance that is safe for a 4 cylinder engine?
> Does anyone have a Timing Advance versus RPM graph for a high
> performance 4 cylinder?

That's primarily determined by the engine geometry (crank throw and
conrod length - and cylinder offset if applicable) and ignition
speeds; how long it takes to ignite. There's a point of advance
beyond which the pressure buildup due to combustion is too early,
reducing the effectiveness of the crank. Mixture has some effect on
the time it takes to ignite and as to the flame speed and hence
pressure buildup.

Typical ignition times are around the millisecond, flame speeds
around 60 m/s. These in themselves are independent of engine speed.

You can therefore determine the required advance for engine crank
torque from the integral of instantaneous torque over the power
stroke. Well, at least in theory...

As a major limiting factor; there's the need to avoid detonation.
Detonation will scour the protective boundary layers off the
cylinder walls; and in particular the piston top, exposing the
surfaces to temperature extremes.

The optimum advance for maximum torque should be the target; use of
water injection, charge-air conditioning by humidification, higher
octane fuel and lower cylinder temperatures are just some means of
reducing propensity to detonate. One can also use a richer mixture.
The use of (perhaps cooled, or even internal) EGR, like those
measure mentioned, also slows the burn.

Airflow effects such as swirl are speed-dependent; the swirl speeds
are typically of the same order of magnitude as flame speeds.  Swirl
exposes a larger flame front to the end gas. That reduces the
propensity to knock, possibly for a number of reasons; larger
flame-front area reduces the concentration of energy; the volume of
end-gas subjected to secondary compression is reduced more rapidly
and hence less likely to detonate; etc.

These are just a few variables. And the variation in some is usually
out of your control; water vapour content for example can alter the
octane number requirement by 10 points or more. [Rule of thumb is
that 4g of water per kg of air is worth one octane point.]

The starting point is to work out the optimum advance based on
engine geometry, nominal breathing and nominal fuelling
characteristics.  Then watch for detonation and if detected, retard,
while trying to manipulate other factors so that you can get back to
optimum for a particular mixture.

If you want a "safe" advance curve, then you need to retard ignition
until the piston speed is such that the advancing flame front
doesn't cause excessive secondary compression. This will be
determined by the dimensions of the engine. You won't nett very much
power from the engine. There's still the possibility of knock with
lean mixtures so you need to maintain adequate fuelling.

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