The century-old test that determines octane numbers | Fuel Facts | Articles

How are those octane numbers at the pump actually determined? Is it some kind of chemical test involving test tubes, beakers and a Bunsen burner? Would you believe resistance to knock is determined using an octane test engine setup with roots going back some hundred years?

“As much as technology has advanced, this scale for measuring octane has remained consistent,” says Zachary J. Santner, manager of Product Engineering and Business Optimization at Sunoco Performance Products. CFR Engines Inc., one of the dominant suppliers of these testing units, released its first one back in 1929.

Then and now, the basics remain the same: A sample of fuel is tested at different compression ratios until it knocks. “We actually force the fuel to detonate so we know how much resistance it has,” Santner explains.

The test engine features a single, electrically powered cylinder. It almost resembles an air compressor hooked to various coolers and controls plus a computer console.

The unit’s compression ratio is infinitely variable via a screw mechanism that raises and lowers the cylinder head. A multi-bowl carburetor allows the operator to compare fuels against a reference sample. Temperature, barometric pressure and other parameters can be tightly controlled in accordance with the government standards. The entire rig, Santner notes, allows both very precise adjustments and measurements.

These rigs don’t come cheap, though, as each one represents easily a six-figure purchase–plus the reference fuels at about $100 per liter. Santner has access to several of these setups, as Sunoco tests each batch of its race fuel.

And the fuel sold at the corner station? As per government standards, the fuels passing through the distribution network are tested throughout the day, so should a deviation pop up, it can be quickly addressed. This process also ensures that octane ratings remain constant from one pump to the next no matter the brand, location or time of year.