Ignition theory

[Greg Hermann]

>I'm sure everyone on this list is familiar with the idea that
>ignition advance increases with engine speed.  However, a number
>of the newer ignition systems take load into account to alter
>the timing.  I would suppose that one retards as load increases
>(wild guess there).  Any general relationships?  Most of the
>systems I've seen use throttle angle to indirectly measure load.
>Is it likely they take a rate of change into account as well?
>If so, what would be the general behavior for a given rate of
>change?
>
>Just trying to get a discussion going...
>
>james montebello

It sure does. Used to be called "vacuum advance" in the old, analog days,
and most engines had it. Basically, there are no quantitative rules on
timing. The qualitative ones are that you want the peak cylinder pressure
(after ignition) to occur at a particular point relative to crank angle.
And this point varies somewhat with connecting rod length to stroke ratio.
>From there, the relationship between spark timing and crank angle depends
upon the speed of flame travel in the combustion chamber (which can
sometimes even be faster than it is on this list! :-) !) Factors which
affect flame travel speed and rate of pressure rise in the chamber include:
type of fuel, mixture strength, presence or absence of diluents to the
mixture, density of the mixture, temperature of the mixture,  both
organized and turbulent (random) movement of the mixture in the chamber,
spark plug location, and chamber geometry.

Engine speed and load (which relates directly to manifold abs. pressure)
can be used to give a pretty good matrix of any given engine's advance
requirements, since most of the other factors are pretty constant in any
given engine. Hence the use of centrifugal advance weights in combination
with a vacuum pot in the old days. But way too many variables exist to
allow coming up with a really meaningful quantitative general relationship.

Some things you will observe, though--generally, advance requirement will
not increase as fast as one might expect with engine speed--because chamber
turbulence usually also increases with engine speed, and can make up the
difference. Higher compression and boost will both decrease advance
requirements, because both increase density and temp at the time of
ignition. Higher compression with proper design can also reduce advance
requirement by increasing mixture velocity and turbulence at the time of
ignition (proper use of squish). Light load will increase advance
requirement because of reduction in charge density. EGR and H2O injection
will both increase advance requirement because of dilution of charge.
Leaner mixtures will reduce advance requirement.

Regards, Greg