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What’s the difference between premium-grade and regular gasoline?

Premium gas has a higher octane rating that may—or may not—make it a good choice for your engine…

By Peter Dunn

With slight variations—depending on the crude oil and the refining and blending processes used in production—all gasoline grades contain the same amount of chemical energy. When combusted, premium (high-octane) gasoline and the less-expensive (and less-glamorous) regular, and all grades in between, provide the same amount of thermal energy, or heat, which an engine uses to generate the mechanical power that moves a vehicle.

There is, however, another aspect to this question, notes Ahmed Ghoniem, the Ronald C. Crane Professor of Mechanical Engineering at MIT: How much of that raw heat energy can the engine actually convert into mechanical energy? “One can argue that using high-octane fuels in the right engine ultimately leads to more mechanical power from the same amount of fuel,” he says.

In other words, higher-octane fuel confers an advantage in some cars, but not others. It allows performance-oriented engines (specifically, those with higher compression ratios) to burn gasoline at higher pressures and higher temperatures. These conditions at the moment of combustion create better thermodynamic efficiency, so a greater percentage of the gasoline’s heat energy gets converted into motive power.

Octane rating is a measure of grace under pressure: how evenly a gasoline will burn under difficult conditions, like hard acceleration. Ideally, the vaporized gasoline inside an engine’s cylinder burns by the propagation of a wave of flame, ignited by the cylinder’s spark plug. This allows a smooth transfer of power to the engine’s crankshaft and the car’s wheels. But at higher pressures or temperatures, small pockets of gasoline vapor can prematurely explode, or self-ignite, creating a distinctive “knocking” sound, as well as potentially destructive shock waves.

Gasoline 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 electronic sensors and controls, are very good at preventing detonation of lower-octane gas (this is why drivers no longer hear much knocking). But high-octane fuel is still specified when designers want to achieve better acceleration and power output, and when they are willing to accept a slightly bulkier and heavier engine with higher operating costs.

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