It seems to me that this is way beyond what a food science group can advise
you about. I can tell you that used frying oil will contain peroxides and
polymerised oil, but what effect that would have on an engine I can't say. My
gut feeling is that you can't just put your oil straight into your car and
expect it to work perfectly. I think you need to talk to a few automotive
engineers and diesel mechanics.
I also seriously doubt that you can pick up frying oil for the same price as
diesel, unless you happen to know someone who runs a fish and chip shop and is
willing to give you their used frying oil. That, I suspect, is the real reason
biodiesel hasn't taken off yet.
Lincoln University, P.O. Box 84, Canterbury, New Zealand.
Socrates was a famous Greek Teacher who went around giving
people advice. They killed him. (school history howler)