<< Until this year, when the SCCA started making us use racing fuel, I used
 > 93
 > > > octane
 > > > pump gas with no alcohol, and added cheap 30 weight motor oil. 7 oz. to
 > 5
 > > > gallons of fuel, to kill off as much octane as I could. Daryl Drummond
 > says
 > > > we lose 8HP at the top just by using the high octane fuel.
 > > >
 > > > Lynn E Hanover
 > > > lehanover@aol.com

 norm doty wrote:
 >
 > that doesnt sound right.
 > the higher the octane the harder it is to ignite, so less chance of
 > detonating,
 > oil is harder than gas to ignite, you added oil, the gas should be harder
 > still to ignite
 > the gas is now even higher octane than it was.
 > right?
 >
 > Norm & Monda
 > Cozy MK IV #202
 > Ford V-6 Powered

 Norm. See Paul Yaws explanation later on.

 BTW Lynn, why motor oil and not two cycle oil?
  >>

The octane rating of a motor fuel is a way of quantifying it's resistance to
detonation over a large range of mixtures and temperatures. There are two
established systems
(methods) used to determine this number. Research octane system and the motor
octane system. Generally the two results are averaged to get the published
octane number. Some states require this formula posted on each gas pump.
Detonation is the spontaneous ignition of a fuel air charge after the spark
plug has fired but not associated with the flame front initiated by the spark
plug. After the plug fires, the pressure in the chamber begins to rise
quickly. While this would occur with no movement from the rotor or piston, it
occurs more quickly because the piston (or rotor) is still moving toward TDC
and making the combustion chamber volume smaller. Should the pressure rise
high enough, ignition will occur away from the spark plug location and two
flame fronts will travel towards one another in the chamber. The collision of
the flame fronts is what you hear when you full throttle the old pickup truck
in 4th gear. A metallic clicking sound. Or like BBs being dropped on the
hood. It goes away instantly when you lighten up on the throttle. That's
detonation.

This rapid burning produces more pressure than the system can handle, and
since it is pressure generated before (for the most part) TDC, it reduces
horse power. The additional heat generated overheats the combustion chamber
and makes additional detonation more likely. To eliminate detonation, we can
reduce the compression ratio, (reduce the heat of compression) improve the
cooling system (reduce combustion chamber heat), increase squish area, (
drive the unburned mixture toward the spark plug) retadard the timing (delay
heat build up in the chamber) or slow the speed of combustion. Since all of
this cure stuff tends to take away horse power, the piston people pour in
fuel that cures detonation most of the time. There are millions of things to
learn about this process but lets just say the high octane fuel is less
sensitive to heat and pressure than lower octane fuel. As a result it burns
at a lower speed (flame front velocity is lower) than low octane fuel.

In the rotary the combustion chamber is huge (the entire face of the rotor).
Lots of heat is removed from the chamber through the coolant and oil, (it has
low chamber temperature). The compression ratio is low (lower heat of
compression) The tips of the rotor, (the area near the apex seals don't run
close enough to the housing to act as a squish area, but they are close
enough together to quench the flame front (put out the fire). Notice how the
carbon build up on the rotor never quit gets to the apex seals. So the
unburned fuel in each cycle cools the mixture further, and adds to higher
exhaust gas temperature. Flame front travel is so poor that the Lemans
engines had three plugs per chamber. That's why a killer ignition system
perks things up so nicely. Anyway it would be difficult get a rotary to
detonate without some real work. I did it by using the stock igniters in a
racing application, and it destroyed the engine in one lap. So for high
performance applications we can use very low octane fuel. We can use less
timing advance than a piston engine, 27degrees for racing about 20 for
anything else. We can run over lean or over rich
with little effect other than slightly reduced power. Low octane fuel has
slightly more energy in it than high octane fuel. Exhaust temperature
readings give you a better idea of what is going on inside the engine and not
just what the temperature of a lot of high octane fuel burning in the headers
instead of the engine is. I used motor oil because it was cheap and
available. Over oiling the fuel reduces wear on the housings substantially.

Lynn E Hanover
lehanover@aol.com

 
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