Kokua Line June Watanabe

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Octane rating takes ethanol into account

Question: The ethanol Web site says that 10 percent ethanol gasoline has an octane rating three points higher than straight gasoline. Shouldn't the gas pumps be revised to show this? It would seem that most cars would not need the higher-octane gasoline due to the cheaper regular being boosted to 90 from 87.

Answer: The explanation can get complicated but the short answer is that octane ratings shown on gas pumps do reflect the addition of ethanol, according to William Pierpont, manager for the state Department of Agriculture's Weights and Measurements Branch.

Yours is "a good question," but the answer is "very complicated," involving not only what the octane stands for, but also what you're getting when you buy gasoline, he said.

Hawaii's two refineries, Chevron and Tesoro, have reformulated their gasoline into an ethanol blend called Hawaii blend stock.

That blend meets the "ASTM" (American Standard Testing Method) standard, which specifies how fuel should perform, Pierpont said, "but also doesn't boost the ethanol any higher than what is advertised on the pumps."

He explained the effects of evaporation on the lighter components in gasoline (ethanol is one of those that evaporates relatively quickly at "room temperature" or in a hot parking lot) and what the octane number actually describes.

The octane number describes how the fuel will burn across a cylinder.

The bottom line, in answer to your question, is that, "If you took gasoline that meets the octane minimum number and you add ethanol to it, yes, it would boost the octane to a higher number," Pierpont said.

But because Hawaii refiners have reformulated the base stock of gasoline to compensate for the addition of 10 percent ethanol, "it compensates for that (additional) three points, so that the gasoline is still meeting the minimum octane rating, but it's not three points higher."

In other words, you can't make the assumption that if you usually buy 92 octane gas that you can now buy 89 and be at 92, Pierpont said.

"That's not the case," he said. "The number that you see on the pump is the minimum amount the octane will be in the fuel."

Pierpont's inspectors randomly sample and field-test fuel -- "coming from different refiners, distributors, marketers"-- at gas stations every two to three months or so to make sure there's no false advertising.

In the five years he's been branch manager, "We did not ever find octane that's below what's advertised on a pump. If we did, we would take action to remedy that."