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I thought I would
take this opportunity to expose one of the least understood areas of the
automotive engine. Why? Because I like a challenge and Catholicism is too
easy a target. If you step back from an engine you can view it as a chemical
reaction harnessed by a mechanical system to produce work. To produce peak
power the mechanical requirement is for peak cylinder pressure to occur
at the same crank angle under all operating conditions. Through testing
that angle turns out to be between 15 and 20° after top dead centre
(TDC). Any earlier than that and there’s either too much power lost to
rising cylinder pressure before TDC, or a risk of knock. Any later than that
and the pressure front chases the piston down the bore rather than forcing
it down.
The mechanical system, having a fixed
geometry, works the same regardless of operating conditions. The same cannot
be said for the chemical reaction it harnesses. Combustion speed of an
air/fuel mixture is dependent upon numerous factors, some set during engine
design time, a lot that vary during operation and some that vary even under
steady state conditions. Indeed, just like snowflakes, no two combustion
cycles are the same. Fortunately, the cycle-to-cycle variations can be
safely ignored, as can some other factors, but most cannot. It is important
to identify the physical controlling factors when changes are made to a
stock engine so that an initial advance curve can be determined. The other
factors, let’s call them environmental factors, have to be accommodated
in operational controls or at least safeguards provided to ensure the engine
does not operate outside safe ignition limits.
What this means is that to ensure the
peak cylinder pressure occurs when it can do the most work, the flame front
has to be initiated at the correct time. Unfortunately, this time varies,
and because we are talking about a rotating machine, so does the angle.
Because a cylinder charge takes a particular amount of time to burn and
the engine speed varies, so then must the point at which combustion is
initiated in advance.
The major factors affecting ignition
timing requirement are volumetric efficiency, engine speed and burn rate.
Volumetric efficiency, or VE, is an expression of how much of a lung full
a cylinder gets for each breath. As it turns out, a full cylinder burns
faster than one that is only partially full so any improvements to VE also
affect timing. Improvements to cylinder head ports, inlet manifold or camshaft
improve VE at certain rev ranges so the ignition timing must also be changed
to suit.
Lets look at a bunch of factors that
affect timing requirements and discuss a few:
|
| Factor
|
Cylinder
filling |
Flame
speed |
Burn
time |
Timing
effect |
| bore/stroke
ratio |
-
|
You
wouldn't have thought so, but this has an effect on rate of compression
and piston dwell time at TDC |
-
|
a
long stroke/small bore requires as much as 10° more advance than does
an equal displacement engine with short stroke/big bore
|
| Camshaft
with more duration |
improved
at higher speed, worse at low speed |
-
|
-
|
less advance at high
speed, more at low speed |
| combustion
chamber shape |
-
|
-
|
compact
designs take less time to burn to farthest reaches
|
Minimized
for compact designs, maximized for in-piston chambers
|
| fuel
atomization |
-
|
liquid
fuel does not burn |
fuel
globules have to be atomized by combustion in other areas before they will
burn |
Minimized
when atomization is complete - atomization also improves with speed
|
| improved
exhaust efficiency or lowered backpressure
|
-
|
the
presence of residual exhaust gas in the cylinder retards the flame front
|
-
|
less
advance required when exhaust extraction effect is working, ie, higher
up in the rev range |
| improved
induction efficiency |
The
engine has an easier time getting a 'lung' full
|
faster
with improved VE |
-
|
less
advance as VE improves |
| increased
bore size |
may
increase valve shrouding and lower VE |
-
|
more
distance from plug to far side of cylinder
|
more advance required
|
| increased
compression ratio |
-
|
a
higher CR results in faster burning |
-
|
less
advance required for increased CR |
| mixture
swirl in combustion chamber |
-
|
-
|
a
well mixed cylinder charge will burn uniformly
|
minimized
with good swirl. Westlake type heads have heart shaped chambers to improve
swirl |
| piston
shape |
-
|
-
|
pistons
which force the charge into a confined space limit the distance the flame
front has to travel |
minimized
for squish pistons, maximum for dished pistons
|
| spark
plug position in head |
-
|
-
|
varies
with distance from plug to farthest point in the cylinder, ideally centered
in cylinder |
minimum when centered,
more advance as it moves to the farthest corner of the cylinder
|
| air/fuel
ratio |
-
|
anything weaker or richer
than the ideal 14.7:1 will burn slower |
-
|
maximized at
stoicheometric |
| coolant temperature
|
-
|
increased engine
temperature affects final charge temperature
|
-
|
less advance required
as engine temperature increases |
| fuel octane rating
|
-
|
octane slows burn rate
|
-
|
more advance with
increased octane, or, more importantly, less risk of over advancing without
changing advance |
| heat transfer rate
|
-
|
the higher rate at which
the cylinder head can get rid of the combustion heat, the lower the final
charge temperature will be |
-
|
less advance needed for
poor cooling iron heads, slightly more for efficient turbo heads, even more for
aluminum |
| induction air temperature
|
Colder charge means
denser air and higher VE, warmer charge, lower VE
|
higher temperature charges
burn faster |
-
|
less timing with increased
temperature |
| engine speed
|
VE increases with speed
|
-
|
-
|
as VE increases, advance
decreases, but because flame speed is finite, it must be initiated much
earlier with increasing speed |
| Ambient moisture
conditions |
-
|
moisture in the cylinder
charge will slow the flame speed |
-
|
more advance required
under humid conditions |
|
As you can see, there are a lot of
factors that decide what a particular engine’s advance curve looks like.
If you look at advance curves for a dozen different engines you will have
a dozen different advance curves, but, they all have the same general pattern.
This has a lot to do with the physics of an internal combustion engine.
The fine tuning of exactly what the advance curve looks like for a particular
engine is not so much designed in as pulled out of a running engine. The
advance curve for a similar engine can safely be used as a starting point
and in fact, the engine may run just fine. This is good news for the home
tuner, however, until the optimum timing point is found there is horsepower
going unharnessed.
If you are planning an engine modification,
or have an already modified engine the above table will not help pinpoint
where to push and pull the advance curve, or by how much. Some combination
of factors cancel each other out, some amplify and some are so subtle they
can be ignored. But how each of these factors affect an engine can only
be determined by testing. When an engine has been changed in many areas,
typically cylinder head, camshaft, inlet manifold and compression ratio
all at once, any advance curve that will allow the engine to run should
be considered nothing more than a starting point.
There are a few suppliers that offer 'performance'
distributors, notably Aldon, Piper and Accel. Although they have sexy names
like "Red", "Rally" and "Dominator" they are in fact no different from your
original distributor except for the advance curve. As far as the engine is
concerned it couldn't care less if it were a distributor made by Lada or a
gold plated one with a sexy name as long as the spark hits at the right
instant. Considering the long list of factors affecting ignition timing, how
is it possible for a 3rd party to supply an advance curve that
optimizes an engine they've never seen? They can't so to avoid
pre-ignition problems they supply a generic conservative advance curve that
may be just as conservative as your original. The actual difference between
the 'performance' distributor and your own is 2 tiny springs.
That's it. And the chances of those springs delivering the optimum
advance curve your engine needs is very low indeed. At the price of a new
distributor, that's an awful high price to pay for a shot in the
dark.
Save your money. Don't trust your ignition
advance curve to anybody, but take your car to a rolling road dynamometer
with an operator that knows what he's doing. It will be cheaper in the
end and you'll maximize the return on all the mechanical modifications.
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