Octane number is a standard measure of the performance of a motor or aviation fuel. The octane rating was developed by chemist Russell Marker at the Ethyl Corporation in 1926.
The octane number of a fuel is a measure of its ability to prevent engine knock. In another word it’s ability to resist self-igniting.
Self-ignition leads to inefficiencies if it occurs during compression prior to the desired position of the piston in the cylinder as appropriate for value and ignition timing. The problem of premature ignition is referred as Engine Knock, which is a sound that is made when the fuel ignites too early in the compression stroke.
Engine knock gets its name because of the sound it produces: a knocking or pinging sound coming from the engine.
Severe knock causes severe engine damage, such as broken connecting rods, melted pistons, melted or broken valves and other components. The octane rating is a measure of how likely a gasoline or liquid petroleum fuel is to self ignite. The higher the octane number, the less likely an engine is to pre-ignite and suffer damage.
But the most typically used engine management systems found in automobiles today monitor the level of knock that is being produced by the fuel being used. In modern computer controlled engines, the timing of the ignition will be automatically altered by the fuel management system to reduce the pre-ignition to an acceptable level. There is no point using fuel with a higher octane rating than the engine was designed for. So long as the fuel doesn't cause engine knock, it's good enough.
Some cars are optimized for higher octane fuel and we must use relevant octane rating fuel for such vehicles.
A common misconception is that power output or fuel efficiency can be improved by burning fuel of higher octane than that specified by the engine manufacturer. The power output of an engine depends in part on the energy density of the fuel being burnt. Fuels of different octane ratings may have similar densities, but because switching to a higher octane fuel does not add more hydrocarbon content or oxygen, the engine cannot develop more power.
Similarly, octane rating do not guide us to purity level of the fuel. It is depend on the external factors such as quality of storage tank, pumping methods.. etc.
We should use fuel with the octane rating recommended by the car manufacturer. For the Toyota Prius it says “Use unleaded gasoline (Octane Rating 87 [Research Octane Number 91] or higher)”
In simple terms, if we used octane 95 instead of octane 92 (which is cheaper) we won’t get any performance advantage or long life for the Prius engine. Instead it would drain out our pocket money little by little for nothing.
