Quote:
Originally Posted by tjts1
Does adding diesel fuel to gasoline in any proportion increase or decrease the overall octane rating for the fuel?
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It decreases it. Dramatically.
Diesel has an octane rating around 20 or so IIRC. So, the more diesel fuel you mix in with your gas, the more the octane rating drops.
Even though a diesel engine and a gasoline engine are similar, there are significant differences between the engine cycles that allow them to work with their respective fuels.
A Diesel cycle only adds the fuel AFTER the compression stroke. At this point it ignites as it is injected.
An Otto cycle (gasoline engine) uses a mixed fuel/air charge that is ignited by the spark plugs. If your fuel is prone to autoignition, it begins burning early, possibly even during the compression stroke. However, since it's a well mixed charge, it detonates rather than burns evenly. This can be very damaging to your engine.
In small percentages, I can see diesel simply lowering the octane rating and the EFI pulling back the timing enough to compensate. Mix it in too high of a percentage and you will get detonation or the engine won't run at all. This is a very risky way to try to improve efficiency and I wouldn't recommend it to anyone.
Mike