Quote:
Originally Posted by Old Henry
But, all of my research indicates otherwise. Octane rating (aka AKI or Anti-Knock Index) is nothing more than the fuel's volatility, i.e. how much pressure it can take before exploding.
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Another way of putting it is octane rating is that quality of gasoline that is resistance to ignition. The higher the octane, the more resistance to ignition. In other words you want the gasoline to ignite by spark and not by pressure.
Higher octane gasoline components tend to be slightly denser aromatics so you may be getting slightly more pounds per gallon.
On the other hand cetane rating for diesel engines is that quality of diesel that is how easily it ignites. With a diesel engine the fuel should ignite easily by pressure.
Back in the 30s gasoline was much lower octane than today. Gasoline consisted of the gasoline and naphtha cuts from crude which run around 70 octane. Some low octane thermally cracked gasoline was blended as well.
The really high octanes came when alkylation was invented in WW2 for aviation gasoline. Alkylation produces iso-octane. BTW 100 octane is defined as the resistance to ignition of 100% iso-octane. 0 octane is defined as the resistance to ignition of n-heptane.
After WW2 catalytic reformers was invented also know as platforming using platinum catalysts. Reforming produces benzene, toluene, and xylenes which are high octane from low octane naphthas. Remember the old Chevron TV ads that said "our gasoline has Platformate?" All gasoline has some platformate now!
One reason Fords had such low compression ratios was the low octane gasoline that was available. The higher the compression ratio, the higher the octane requirement.
I retired as a chemical engineer in an ExxonMobil refinery that made around 10 million gallons a day of gasoline. I've never bought anything other than 87 regular unleaded for any car I've owned. My 63 Corvette 327 runs fine on the stuff. So few cars actually require Super. Super is marketed to make the oil companies extra money and is not actually needed by most people. It used to be that the cost of manufacture for super was only 6 cents above unleaded but sold for 20+ cents more.
It used to be that regular unleaded had some thermally unstable components(color yellow or green) in it but not anymore. Virtually all gasoline now is hydrotreated to reduce sulfur to very low levels. The hydrotreating also fixes the unstable components. All gasoline now is "white" like Amoco used to advertise.
Remember in the 60s you used to here about 100 octane gas? That was research octane. Today octanes are reported as road octane. 100 octane of the 60s is equivalent today to Super which is about 98 research octane or 93 road octane.