QUESTION - So what is compression,
timing and octane rating?
ANSWER - (By Chris Weinbeck)
Compression is exactly what it sounds like, the reduction of the
volume of the intake mixture by a certain mechanical ratio. The
ratio is dependent on the construction of the engine incorporating
the bore, stroke etc. and final volume of the combustion chamber.
A 9:1 engine reduces 9 volumes to 1 volume. This compression creates
heat -called the heat of compression.
The fuel-air mixture is intended to withstand this increase in
heat without self igniting or detonating in a gas engine; instead
undergoing controlled ignition by the timed introduction of a spark.
A diesel engine depends upon the intake mixture igniting solely
due to the heat of compression. The higher the compression ratio
the greater the utilization of the energy released by combustion.
Octane rating is intended to quantify a comparison of the particular
gasoline to pure octane -the liquid form of straight chain eight
carbon saturated hydrocarbons. This liquid was given the rating
of 100 (100% octane) early on in the research of internal combustion
engines and fuels because of it's excellent ability to resist detonation
due to the heat of compression. A given gasoline motor fuel is given
a rating (as compared to a mixture octane) based on the average
of two "measures" of it's ability to resist pre-ignition. The research
method -what all the equations say should be it's "octane rating"-
and the motor method -an actual test of detonation due to mechanical
compression of the mixture- are averaged to create the octane number
posted on the pump labeled R+M/2. Ether,used as starting fluid,
self ignites all the way down to 3:1 compression -the practical
lower limit in construction of an internal combustion engine.
Timing refers to the adjustment of the points during the compression
stroke at which a spark is introduced to ignite the intake mixture.
If the fuel-air mixture could ignite instantaneously and fuels with
precise octane ratings were available to run in our perfectly constructed
engines timing would be permanently set to top dead center "TDC",
where the piston is at the exact top of it's stroke and the charge
is fully compressed for maximum power and efficiency. But in our
less than perfect world it takes an imprecisely known amount of
time for the flame front to propagate throughout the mixture of
less than perfectly rated fuel and air compressed at something close
to a 8:1 ratio. Therefore we are forced to set our timing for something
in advance of TDC in most cases, and then use various measures (vacuum,
centrifugal force) to determine engine speed and farther increase
the "advance" and higher RPM.
Uncontrolled or improperly timed ignition reduces efficiency and
can destroy an engine. Detonation/pre ignition can occur at odd
locations inside the combustion chamber creating stresses for which
the engine wasn't designed. Pre-ignition or improper timing also
can cause the force of combustion to peak while the engine is still
trying to compress the mixture. Excessively late ignition lowers
engine power and causes too much heat to be transferred to the cylinder
Summing it up;
The greater the compression the greater the heat of compression
and need for fuels resistant to detonation or pre-ignition.
The greater the speed of the engine the farther in advance of TDC
the spark must be introduced to have ignition occur at the correct
point in the piston's motion for maximum efficiency.
The higher the octane rating of a fuel the slower it ignites.
The lower the quality (or reliability of the quality rating) of
the fuel the farther from peak efficiency timing should be set for
controlled ignition. Reply © 1999 Christopher Martin, Weinbeck
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