[A/S] Fuel and octane

["Maurice Riggins" <maurice.riggins@xxxxxxxxxxxxxxxx>]

The octane rating of the fuel is how well it will resist spontaneous 
combustion (knock) caused by the pressure/temperature rise in the 
cylinder and combustion chamber.  There are two types of knock.  

Preignition is when the air/fuel mixture ignites before (pre) the 
spark plug fires (ignition).  It can be caused by spark plugs that 
are too hot of a range, holding more heat in the tip, but is often 
caused by carbon buildup, where the cracks in the carbon glow like a 
glow plug.  It can also be a result of an overheated engine, and that 
overheat can come from the second kind of knock, detonation.

Detonation occurs after the spark plug fires and the flame front is 
expanding from the spark plug.  The mixture in the outer regions 
furthest from the plug are highly compressed, which causes 
temperature to rise, and this can exceed the octane rating of the 
fuel.  Poor distribution of air/fuel mixture can also cause fingers 
of flame to protrude from the advancing flame front and ignite the 
mixture in the outer regions.  Much design work goes into the heads, 
such as GM's Vortec, to create turbulence that ensures even 
distribution of the mixture.

Detonation is the most destructive.  The burning mixture does not 
touch the piston, chamber, or cylinder walls.  There is a boundary 
layer of mixture that is cooled by the thermal mass of the metal next 
to it, and this insulates the metal from the burning mixture under 
normal combustion pressures.  The very high pressure waves of 
detonation break through the boundary layer and allow the combustion 
to touch the metal.  Internal engine temperatures soar when 
detonation is occuring, and the overheated metal can then cause 
preignition.  

Listen for sounds of knock.  Preignition is a lighter, "tinkling" 
sound, while detonation is a harder sound.  Many people describe it 
as the sounds of valves rattling.  Valves don't rattle unless a valve 
spring is broken or a valve seat drops out of the head.  In both 
cases, the valve will probably hit the piston and be bent, or bend a 
pushrod or break a rocker arm.  Your engine won't continue to run 
normally.  Holes in a piston are a sure sign detonation was occuring 
and the driver didn't hear it or ignored it. 

Even cast pistons can withstand very high compression as long as 
detonation is held in check.  Forged pistons are stronger, but expand 
more with heat, so have to be installed with more clearance.  This 
sometimes results in audible noise when the engine is cold.  Pay 
particular attention to knock if you have cast pistons.  Even the 
modern hypereautetic cast pistons can't handle it.

Although the ignition fires before the piston is at top dead center, 
it takes time for the entire mixture to fully ignite and reach its 
maximum burn pressure (like the long whooosh of lighting charcoal 
soaked with lighter fluid).  This should occur when the piston has 
maximum mechanical advantage over its crankshaft throw.  There are 
many factors that influence the mixtures burn speed, and that's why 
ignition timing varies so dramatically.  Today it's based on rpm and 
intake manifold pressure, among other things such as intake air 
temperature, engine temperature, etc.  

The bottomline is that if the ignition fires too early, pressure will 
build before the piston has sufficient mechanical advantage over the 
crank, and pressure will rise faster than the piston can fall and 
increase the cylinder volume to hold pressure in check.  Today, 
engine manufacturers dial in a bit too much advance for all 
conditions and install a "knock sensor" to listen for detonation.  If 
it hears it, the computer retards the ignition timing, which results 
in slightly less performance (or much less, in the case of high-
compression engines designed for premium being used with low octane 
fuel).

High octane gasoline has no more or less energy than low octane, and 
burns no faster or slower.  If you get better performance (power and 
mileage) with higher octane gas, then your engine is having to retard 
the spark to handle the lower octane gas.  I recommend using only the 
highest octane that provides a _measurable_ performance gain in your 
vehicle.  With some, that will be 87.  Others may benefit from 89 or 
even 91.

For what its worth, the oil companies no longer use slugs between a 
shipment of high and low octanes (or gas and diesel for that matter), 
so there's a bit more mixing where they meet (there was some before 
anyway).  Each company puts in so much of certain type gas at the 
start of the pipeline, and draws so much out at the end.  In other 
words, you may be pumping gas put in by Shell at a Citgo station.  
Some companies DO suppliment the drawn gas with additives at their 
endpoint distribution points, but that's the only real difference 
between brands of gas.

Finally, air-cooled engines aren't as good as water-cooled when it 
comes to heat.  Hot spots can develop within the engine.  I invest in 
really good lawn equipment, and feel it's worth the extra price to 
feed them premium gas to help prevent knock under high load 
conditions.

Hope this helps,
--
Maurice
WBCCI 5446