Higher octane for better mileage?

[Charon]

Gasoline (petrol) is compounded to resist ignition under conditions of high temperature and pressure such as those at the end of the compression stroke. It is supposed to ignite only when the spark ignites it, and is then supposed to burn smoothly.

Diesel and jet fuels are compounded to ignite easily under high temperature and pressure conditions. This works because the fuel is not present in the engine while the air is being compressed.

Every year a few piston aircraft are lost from misfuelling. The pilot orders fuel. The fuel truck driver sees some variation of "Turbo" on the airplane and thinks it needs turbine (jet) fuel. He fills the tanks with jet fuel. The pilot checks but doesn't notice the error. Engine start is normal because the fuel lines are still filled with gasoline. Engine run-up is normal even if the mixture of gasoline and jet fuel has reached the engine, because the gasoline will vaporize normally and because the pressures in run-up are relatively low. The takeoff seems normal, even though the low octane rating of the jet fuel is causing severe detonation in the engine. The pilot doesn't hear the detonation because of other noise, such as propeller noise. About the time the airplane reaches 500 feet above ground one or more pistons fail from the detonation, the engine loses power, and the pilot gets to try to make an unplanned landing.

[trollbait]

Octane rating is a measure of how well a fuel resists igniting from heat and pressure.

Cetane is the measure of how well it ignites from a heat and pressure. A higher value could give better performance and efficiency by burning more completely than a lower cetane fuel.

[Matt715]

Originally Posted by Draigflag

Diesel engines are stronger, Fifth gear tested the "wrong fuel" scenario a few years ago, the petrol car with diesel didn't drive very far, but the diesel car with petrol in it drove ok, and kept going and going, and then they drained it, refilled it with diesel and it still kept going. They were pretty old cars though, modern engines are far more sensitive.

In the owner's manual of my old 1986 VW Golf diesel, it said that in extreme cold temperatures you could mix a small amount of gasoline in with the diesel to prevent gelling. Unfortunately, I don't recall how much they specified and I never tried it as our diesel is blended for the severe cold here.

[Charon]

Decades ago Mercedes recommended mixing up to 25% gasoline with diesel fuel in really cold weather to reduce fuel gelling problems. They no longer make this recommendation. I believe it is because if the weather warms up the diesel fuel system is not designed to control the gasoline vapors, and there is the risk of fire.