# Advanced Engineering Tech - auto ignition point fuel??

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camaro383
02-20-2011, 09:00 PM
hey I have found information on the kindling point for gasoline and e85 and their numericle values for comparisons
But i was also was trying to find information on how cylinder pressures would effect these numbers, maybe an equation or something
Ive been very determined on how to figure out what fuel I would be safe with, and how much I can push them

thanks

camaro383
02-21-2011, 12:41 AM
BTW I am already running e85 in my turbocharged 370, but i wanted to figure out the science end of it to make sure i am tuning my car safely and to really see what i am able to do to improve it, without doing it blindly like many people do since there are so many "i made this much horsepower with this setup" kinda threads

1 FMF
02-21-2011, 04:00 PM
Fuel Octane Calculator & Preignition Probability (http://horsepowercalculators.net/octane_calculator/fuel-octane-calculator-preignition-probability)

you would want to get the auto ignition point of the fuel your using. I see reports of gasoline being 536 F but I don't know what octane rating that is. Gasoline with a higher octane rating (93 vs 87) would have a higher autoignition temp and I don't know what effect the blend of gasoline has on it... whether it's 10% ethanol or MTBE or whatever else. The other important thing is the autoignite temp is usually based on 1 atmosphere pressure. When you compress the gasoline vapor in the cylinder, the autoignition temp decreases because of the increased pressure because oxygen molecules are closer together, and a higher a/f ratio than stoich will also decrease autoignition temp because there are more oxygen molecules present for a given amount of fuel. How you could calculate that I have no idea.

In just compressing air in the cylinder, you can follow the ideal gas law PV = nRT to figure out the relationship between pressure increase and temp increase. It won't be exact because when you mix in gasoline with the air I don't think it's an "ideal gas" anymore and you would also have other effects happening such as the cooling effect from a rich mixture and type of fuel that you are not accounting for.