So you are at a petrol bunk and are trying to figure out which grade of fuel to top up with. You see all these different numbers, 87, 89, 91, 93 and now you are really confused on which grade to buy. To add to your conundrum, the cost ranges from Rs. 55 per liter to about Rs. 70! That’s a big difference. What do you do?

The numbers mentioned earlier are referred to as the Octane rating. Octane rating is basically how evenly the fuel burns under difficult conditions, like hard acceleration. Ideally, the vaporized gasoline inside an engine’s cylinder burns by the ignition by the cylinder’s spark plug. This allows a smooth transfer of power to the engine’s crankshaft which in turn powers the wheels. But at higher pressures or temperatures, gasoline vapor can prematurely explode or self-ignite, creating a distinctive “knocking” sound, as well as potentially destructive shock waves (detonation).
With slight variations, depending on the crude oil and the refining and blending processes used in production, all petrol grades contain the same amount of chemical energy. During combustion, premium (high-octane) grade fuel and the less-expensive (and less-glamorous) regular grade fuel, provide the same amount of thermal energy, or heat, which an engine uses to generate the mechanical power that moves a vehicle.
So what’s the difference? Efficiency. Higher octane fuel allows performance oriented engines (specifically, those with higher compression ratios) to burn at higher pressures and higher temperatures. These conditions at the moment of combustion create better thermodynamic efficiency, so a greater percentage of the petrol’s heat energy gets converted into mechanical power. Petrol with a higher octane rating does not self-ignite easily, and burns more evenly than lower octane fuel under harsh conditions, resisting detonation and knocking. Modern engines, with its electronic sensors and controls, are very good at preventing detonation of lower-octane gas (this is why you don’t hear much knocking). On your daily driver, chances are you won’t be faced with this issue. If you drive a high revving, high performance machine using regular fuel can reduce your engine’s performance as the knock sensors will retard the spark and give you less power and mileage.
For the regular Joe, the next time you are at the pump, consider saving the Rs. 15 per liter and sticking to regular fuel. It can add up to a significant savings at the end of the year.

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