Ignition theory (longish)

[William Woodruff]

-----Original Message-----
From: Chris Morriss [mailto:crsm at oroboros.demon.co.uk]

In message <003901be2b84$966918a0$602005cf at tegu.talarian.com>, James
Montebello <jamesm at talarian.com> writes
>
>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 
>

>I've often wondered why advance is given in degrees instead of in time
>units (milliseconds).  It's a lot easier to set the correct advance if

I think I can add a little insight here.  James, Ignition systems have been
taking load into account for quite some time, in fact older mechanical
distributors did this by means of the vacuum advance.  Chris, I agree with
you that it sure would be a lot simpler if ignition advance was referenced
to time instead of crankshaft degrees.  I think the reason is tradition.
Points based ignition system operation is fundamentally tied to crankshaft
degrees and I think everybody has gotten used to thinking in those terms.

As I'm sure you are aware, ignition systems are complicated and there are
many factors which influence their design and operation.  However, I think
you can understand about 95% of what the system must do by looking at four
fundamental properties of fuel/air combustion (here I am referring
specifically to air and gasoline).  First, the fuel air reaction time is the
same order of magnitude as the piston cycle time.  Second, the fuel/air
reaction time is not constant. Third, if the fuel/air mixture is brought to
a high enough temperature and pressure it can autoignite.  Fourth, the
products of combustion can be changed by altering the timing of the
combustion relative to the piston timing.

The reaction time influence on the ignition system is relatively easy to
understand.  In an "ideal" engine, the fuel/air mixture would be ignited at
the point of maximum piston compression.  This way, all the combustion would
take place at the highest temperature and pressure which would yield the
greatest useful work.  This is not possible in practice because the reaction
is too slow.  Therefore, combustion is started prior to the point of maximum
compression so that combustion will take place at a highest average
temperature and pressure.  There is a competing factor here; that is, if you
start the reaction before the piston reaches top dead center, the combustion
expansion will resist the motion of the piston.  The best compromise between
these two factors is called the maximum brake torque (MBT) ignition timing.
This forms the fundamental relationship between engine speed and ignition
timing.

The fuel air reaction time changes quite a bit over the range of typical
engine operating conditions and the ignition system must also account for
this.  When you say the ignition system accounts for "engine load" what it
really does is account for the changes in reaction time due to changes in
cylinder pressure.  The old vacuum advance did this by reacting to manifold
pressure.  There is a direct relationship between manifold pressure and
cylinder pressure.  When the throttle is closed, the manifold pressure is
low and the cylinder can't draw in as much fuel/air mixture.  Consequently
the cylinder pressure is relatively low.  When the throttle is open, the
opposite is true.  The fuel/air reaction slows down as cylinder pressure
decreases.  Therefore, the ignition system must advance the ignition timing
at part throttle conditions to achieve MBT timing.

The biggest gotcha in all this is a phenomenon known as autoignition.
Sometimes people mistakenly refer to this at detonation.  Since this is
potentially a touchy subject, I will refer you to Heywood's Combustion
Fundamentals book where he says that most researchers do not believe engines
experience detonations, they experience autoigntion.  The point of this is
that if a fuel/air mixture can autoignite if it is brought to a high enough
temperature and pressure.  When it autoignites, the entire fuel/air mixture
in the cylinder reacts at once instead of the normal process of igniting
through an expanding deflagration.  Because of autoigntion, the ignition
timing cannot simply be set at MBT timing.  It usually must be retarded,
sometimes significantly, to reduce the maximum cylinder temperature and
pressure.  One of the greatest advances of electronic engine control is
active knock detection.  This allows the engine to be operated very close to
the autoignition limit.

The final factor is that the combustion products can be influenced by
changing the timing.  The engine designer will use this in concert with the
other pollution control devices to meet emission requirements.

I hope this helps and it wasn't too basic for this list...

Bill W.