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.
Octane is an index number that measures a gasoline's ability to resist engine knocking. American refineries produce gasoline with octane levels ranging from 85 to 94. The lower the octane level in gasoline, the less expensive it is for refiners to produce.
Colorado and several other Rocky Mountain states have minimum octane levels of 85 for regular gas, while most states with lower elevations have a minimum level of 87. Research several years ago from the American Petroleum Institute showed that lower air pressure at higher altitudes allows vehicles to perform as well with 85 octane as they would with 87 at lower altitudes.
But a 2001 study by the Colorado Legislative Council, the state legislature's research arm, concluded that the altitude difference may apply only to older cars. Most cars which use high octane are able to accept regular gas because they are equipped with knock sensors which will automatically adjust the engine cycle timing, thus reducing engine knock or detonation.
"Research findings indicate that newer vehicles manufactured in and after 1984 are equipped with sophisticated electronic control systems that minimize this altitude effect and may perform better using higher-octane gasolines," the report said.
So why does Colorado still have 85 octane in regular gas, even if most cars are better suited to 87?
The wheels turn slowly in the world of octane.
Colorado's 85 octane standard was set decades ago based on a recommendation by ASTM International, a body that establishes standards for everything from detergents to paving materials to gasoline octanes.
Petroleum industry officials and state regulators say that for years they've gotten periodic complaints or inquiries from the public about octane.
But it was only last week that ASTM International officials determined that the newer automotive technologies that apparently have rendered 85 octane less useful warranted a discussion about possible changes in Colorado.
Many car manufacturers recommend that motorists use at least 87 octane.
"They used to put in fine print that 85 octane was acceptable at higher altitudes, but mostly the manufacturers no longer agree with that," said Dick Piper, director of Colorado's Division of Oil and Public Safety.
Yet Colorado motorists continue to show an overwhelming preference for regular- grade gas, even at 85 octane.
In the first half of 2006, 80 percent of Colorado's 35 million gallons in gas sales was regular, according to the U.S. Energy Information Administration. Midgrade at 87 octane accounted for 9.5 percent of sales; premium at 91 octane represented 10.5 percent of sales.
"Sales of midgrade and premium have declined over the past few years as consumers have traded down octane levels in reaction to escalating gasoline prices," said Jeff Lenard of the National Association of Convenience Stores.
Denver motorist Jason Sullivan said that although his 2004 Toyota Corolla specifies 87 octane, he'll follow many experts' recommendations to use the lowest octane available as long as the level doesn't cause engine knocking or poor performance.
"It's tough enough to fill this thing up even at regular (grade) prices," he said last week while pumping 85 octane. "Until my car starts complaining, I'm going to stick with the cheaper stuff."