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Everything You Wanted to Know About Ignition
Timing, but Were Afraid to Ask |
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This is an article on ignition timing that
is long and technical. It came about after an online conversation
with Tom Brooks last year. The article will not have universal appeal
but it may be enlightening to some. Last year Tom Brooks went to a Miata event
in California and while there he had the opportunity to have Ms.
Merlot put on a dynamometer. The high speed dyno test introduced
some pinging and the report indicated this might result in a loss of power.
This lead to a discussion with me about ignition timing, pre ignition,
and other things that affect combustion, such as heat and related matters.
I wrote the following treatise for Tom which he suggested would make interesting
reading for some Miata club members. Let me start with some basics. If the
burning of gas/air in the engine's combustion chamber took place instantly,
it would be appropriate to ignite it, with the spark plug, at the very
end of the compression stroke when the piston reaches the top of the cylinder
- Top Dead Center (TDC). That would provide instantaneous pressure
on the top of the piston and push it down and turn the crankshaft on the
power stroke. However, in relation to the rapid movement of the
piston, the burning actually takes a measurable amount of time.
Consequently the ignition point is started early by firing the spark plug
while the piston is still on its way up to TDC on the compression stroke.
This allows the time needed for the gases to reach their maximum burning
point, and therefore, greatest pressure, just when the piston is going
down on the power stroke. So the plug fires before - in advance
of - reaching TDC or Before Top Dead Center (BTDC). Timing could be measured by the linear distance
that the piston is BTDC, when the plug fires, but it's not.
The measurement used is the amount of rotation of the crankshaft, in degrees,
before the piston reaches TDC. In the case of the Miata, the stock
setting is 10 degrees BTDC. This means that as the crankshaft is
turning, it has 10 degrees more to turn before the piston reaches TDC
when the plug fires. Fourteen (14) degrees BTDC means that the spark
is firing the mixture earlier - even more in advance of the piston reaching
TDC. As the engine's RPM increases, the timing must advance
- occur even earlier in the cycle - because the piston is now moving faster
but the TIME required to reach maximum burning and pressure is essentially
the same. So we start the burning earlier in the cycle to allow
the same amount of TIME to reach maximum combustion pressure just as the
piston is ready to go down on the power stroke. In addition to RPM,
several other factors determine how much BTDC the plug must fire
in the cycle while the engine is running. These factors affect the
burning rate of the fuel - how long it takes to reach maximum pressure. 1. Compression Ratio - This term refers
to the mathematical relationship between the volume of the cylinder plus
the combustion chamber, compared to the volume of the combustion chamber
alone. On the intake stroke, the piston moves to the bottom of the
cylinder and the entire cylinder and combustion chamber (space in the
cylinder above the piston) fill with air and gas. When the piston
is at the top of its stroke on compression, all the gas and air are squeezed
into the combustion chamber. The ratio of the volume of uncompressed
air/gas to compressed air/gas is the compression ratio. The higher
the compression ratio the greater the internal engine pressure as the
piston reaches the top of its compression stroke. Higher compression
creates greater heat of the air fuel mixture and therefore it burns faster.
So, under these circumstances timing must be retarded - the spark must
ignite the gas closer to TDC or the maximum pressure - power - will occur
too soon. It should be understood that the compression ratio is
a function of engine design and, therefore, is not variable. 2. Fuel Octane - Oddly as it
may seem, low octane gas burns faster than high octane. Since it
reaches maximum combustion pressure faster, the burning of the fuel can
be started (spark plug ignition) closer to TDC. The timing should
be retarded - delayed until the piston is higher in the cylinder.
Conversely higher octane fuel, because it burns slower, requires more
advance. Also, if the engine timing is set with more advance, as
many Miata owners do, it may require high octane fuel to permit the slower
burning to take place. 3. Altitude - Due to thinner air at
high elevations the combustion pressures are lower because the engine
is drawing in less air on each intake stroke. If the pressure is
lower there is less heat. Less heat equals slower burning.
Slower burning requires greater advance. More time to let the gases
get to max pressure. This is why at high elevations
low octane gas is 85 or 86-octane. Travel to the coast or below
four or five thousand feet and low octane gas is rated 87-octane.
The lower octane, used at high elevations burns a little faster and compensates
for the lower pressures and lower heat and slower burning that takes place
at higher altitudes. Some vehicles have altitude compensation which
automatically adjusts the fuel mixture and spark timing. More on
this later. 4. Ambient Air Temperature - Higher
ambient air temperature causes the air fuel mixture to be hotter even
before it gets compressed. The result is higher heat leading to
faster burning. This requires that the burning be started closer
to TDC - retarded - because it takes less time to reach max pressure.
However, to complicate matters, hotter air is less dense, consequently
it burns slower. So we have two factors here that affect what the
optimal timing should be. 5. The weight of your accelerator foot
- As you press down on the go-pedal the throttle opens and more air and
gas are admitted into the cylinder and combustion chamber. This
increases pressure and consequently increases heat and faster burning.
So the timing must retard, fire the plug closer to TDC. Any factor that causes the gas to burn faster
must be compensated for by retarding the timing - delaying the spark plug
firing. That is, starting the burning later in the cycle - closer
to TDC. The reason is that MAXIMUM burning is now occurring sooner
because it's faster. If the timing is too advanced, burning
too early, it would try to force the crankshaft and piston back the opposite
way they were going. As you can imagine, this puts tremendous pressure
on the piston head, rods and crankshaft. The pinging or knocking
that you hear is this pressure taking place in the cylinder, trying to
turn the crankshaft backwards, and it can cause serious damage as well
as a loss of power. Of course the reverse is true also.
Any factor that causes the gas to burn slower must me compensated by for
by starting the burning earlier to give it time to reach max pressure.
This means advanced timing. If the timing is too far retarded, that
is taking place too late in the cycle, max pressure takes place as the
piston is part way down the cylinder on the power stroke causing a loss
of power. In older cars the ignition timing was set
at idle and there were only one or two automatic timing compensations
for varying needs. Timing was changed relative to RPM and how hard
you were trying to accelerate. Higher RPM required advanced timing
and heavier acceleration required retarded timing because of higher cylinder
pressures and faster burning. Modern cars use fuel injection and on board
computers to monitor the factors that affect ignition timing and cause
it to change. There are numerous sensors on the car measuring air
temperature, throttle position, RPM, engine temperature, altitude and
even if the engine is pinging. If the computer senses that there
is a ping caused by the timing being advanced too much, or because the
gas you are using is of too low an octane, it causes the timing to automatically
retard. Older Miatas did not have this knock or ping sensor.
So if you set the timing too far advanced or use too low octane fuel your
ear will tell you, if you were sensitive to these sounds. Newer Miatas have more sophisticated computers
and if you choose too low an octane fuel or set the timing too far advanced
the engine will automatically compensate by retarding the timing and you'll
never know. This protects your engine from the possible damage of
pinging but it also robs you of some power. Most true blue (or red)
Miata aficionados, who are technically inclined, prefer the older cars,
in this respect, because their ears tell them what is going on and they
can set the timing the way they like. This is probably more than you ever wanted to know about ignition timing. But, if you got this far, I hope you have a better understanding of what is happening and what some of the factors are that influence the need for timing variations. |