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KNOCK,
DETONATION OR PING
This is an intense
pressure wave within the cylinder created when a LEAN mix
of air and fuel ignites before the spark is delivered.The
fuel/air mixture goes off by itself a micro second before
the spark plug fires, at that time you have two intense
high pressure waves clapping together and that energy
wave hitting each other is the sound you hear. The sound
you hear is from the actual vibration of the cylinder
wall
Under normal circumstances, when the fuel-air mixture
in the engine burns, it does so in a controlled manner, releasing the
energy that produces the force that drives the pistons. Now, if you think
about it, it might seem that the correct time for the spark plug to ignite
the mix would be when the piston reaches the top of its stroke. Then,
as the mix burns and energy is released, the energy pushes the piston
down. The problem is that it takes a certain amount of time for the mix
to burn. If you wait until the top of the stroke to ignite it, the bulk
of the energy will be applied to the piston only during the latter part
of its downward movement.
In order to get the maximum use out of the combustion
energy, the mix is ignited while the piston is still moving up. This is
called "advance". As the engine speed increases, fuel has less
time to burn and advance is increased accordingly. Ignition advance is
controlled by the engine management computer. It varies in response to
a host of other conditions but the base advance curve is a function of
engine speed.
Now we have a new problem. As the mix is ignited earlier,
it is not compressed as much and has a tendency to burn at an uncontrolled
extremely high rate (it basically explodes). This is called "detonation".
When this happens, all the energy in the fuel is released almost instantly.
The resulting shock wave slams into the piston which is forced through
it by the inertia of the engine and the combustion in other cylinders
. The resulting stress on the engine is enormous.
Detonation can be caused
by several things not just a lean condition or bad choice
of fuel octane: Too much compression and too much timing
can also be to blame.
The most common way to describe the sound of detonation is: "like
shaking a can full of marbles", but the BEST way to verify the existence
of detonation is by reading the sparkplug's. If the plugs are very white,
show a very eroded electrode and signs of metal deposits, then you have
DETONATION.
The Cure: Less timing and/or higher octane gas and/or
colder plugs.
Higher octane gas is a good safety measure. Is the $.10 per gallon worth
that extra risk? Another myth about octane is that higher octane produces
more power, wrong. Higher octane fuel is less volatile, less explosive,
less prone to detonation.
A more 'volatile' fuel actually produces more energy per unit volume.You
just have to watch out for detonation more. That's why higher compression
engines use higher octane fuel, higher compression ratio (and higher altitude,
lower air pressure) increases the tendency for detonation. Run super unleaded,
but don't expect to gain power from the more expensive fuel. You CAN get
more power, but it's from running more compression and spark advance.
PRE-IGNITION
This is premature ignition before the spark plug fires,
usually caused by something glowing inside the cylinder
such as carbon deposits or too hot a sparkplug tip. Do
not confuse this with detonation. It is not the same.
Usually pre-ignition occurs when there are lots of "red hot"
carbon deposits and unburned fuel in the combustion chambers.
If you turn of the ignition and your stang wants to keep on running,
then you have pre-ignition.
The Cure: Eliminate
carbon deposits and/or colder plugs, check fuel system.
Glossary of Terms
OCTANE
Octane is a chemical molecule. When referred to as ISO-octane
in testing to find the "Octane" of a sample of
gasoline, it is the reference fuel with the value of 100.
In the testing of gasoline we use two reference fuels.
The other one is N-Heptane which has an Octane value of 0
or zero.
OCTANE
RATING
This is an anti-knock scale developed in the 1920's to
rate the quality of a gasoline's ability to resist
knocking or pinging. Samples of any gasoline are placed
in a laboratory Knock Engine (this is a small, one
cylinder engine with a variable combustion cylinder).
While the engine is running, the combustion ratio is
increased until the engine begins knocking. Now the
gasoline is replaced with N-Heptane with an octane of
zero and is mixed with the 100 octane ISO-Octane at
various ratios until the motor "knocks." If you
end up with 10% N-Heptane and 90% ISO-Octane ratio, your
test sample has an octane of 90.
STOCHIOMETRIC
AIR FUEL RATIO
The exact air fuel ratio required to completely combust a
fuel to water and carbon dioxide. You get all the energy
out of the fuel at this point. This air fuel ratio is
almost impossible to achieve. Racing gasoline ideally
burns at a 14:1 ratio. (14 parts air, 1 part gas).
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