To appreciate the role these additives play, we need a basic understanding of hydrocarbons, the main ingredient of crude oil and gasoline. Hydrocarbons consist of hydrogen and carbon atoms only, but in a dizzying variety of permutations, which scientists name after the number of carbon atoms each molecule has. Crude oil contains lots of different forms, from methane, the simple one-carbon molecule, to 85-carbon chains with complicated branching patterns. Refiners break up the longer chains and eliminate some of the shorter chains to get a mixture that consists mostly of four- to 12-carbon molecules.
Refiners particularly prize octane, an eight-carbon molecule. Internal combustion engines work by compressing a mixture of air and gasoline in a closed cylinder, then igniting the mixture to create a controlled explosion. But if you compress the mixture too much, it can combust on its own before the spark comes along. This is called knocking, and it’s a quick way to destroy the engine. Octane is highly compressible, which helps avoid knocking.
That’s why higher octane ratings are better for high-performance cars, which need higher compression to generate more power. Eighty-seven octane, for example, means the gas is 87 percent octane or at least behaves like gasoline with that much octane.
That second clause is crucial, because, in reality, 87 octane gas — or 89, 91 and 93 octane gas, for that matter — never contains that precise percentage of octane. It’s too challenging and expensive for refiners to reach that composition consistently. Instead, they’re allowed to add other chemicals to the gasoline to get lower-quality fuel to behave like 87 octane.
One of the first gasoline additives used for this purpose was tetraethyl lead, which turned out to be an environmental disaster. Everywhere it went, this original octane enhancer left a nasty cloud of lead, which is really bad for our health and the environment. In the 1970s, the Environmental Protection Agency began forcing refiners and importers to reduce the amount of lead in their gasoline; by 1996, the EPA had banned lead from automobile gasoline entirely.
As the EPA began phasing out lead, refiners replaced it with a chemical called methyl tertiary-butyl ether. MTBE presents a fascinating environmental quandary. On the one hand, it helps gas burner cleaner. In 1989, with several U.S. cities gripped by smog, the EPA mandated that gasoline producers find a way to solve the problem, and MTBE proved to be the most effective chemical. In some areas, gasoline was 15 percent MTBE. Demand for the chemical nearly doubled between 1990 and 1994.
But while MTBE was great for urban air, it was bad for the water supply. When gasoline leaked from tanks, the MTBE mixed easily with water and soil and biodegraded much slower than gasoline’s other ingredients. The EPA found that MTBE had contaminated 5 to 10 percent of drinking water samples where the chemical was being used heavily.
Toxicologists aren’t sure how dangerous MTBE is at these concentrations. While some people can reportedly taste its turpentine-like flavor in drinking water at very low levels and claim that it causes nausea and dizziness, researchers haven’t been able to definitely link MTBE to any particular symptoms. Though the chemical causes cancer in laboratory animals and is classified as a “potential human carcinogen” by the EPA, it might take more MTBE than you’d get from drinking water to cause cancer in humans. Most states have banned or severely restricted MTBE use, and public water utilities have successfully sued oil companies to remove it from soil and groundwater.
Today’s most common alternative to MTBE is ethanol. Like MTBE, it increases the oxygen content of gasoline, which is supposed to lead to cleaner burning. But ethanol raises its own environmental hazards. While some claim that increasing the ethanol in gas could reduce our reliance on foreign oil and trim greenhouse gas emissions, it would also release higher levels of certain volatile organic compounds known to harm human health, such as acetaldehyde and benzene. A 2005 literature review suggested that E10, a gasoline mixture including 10 percent ethanol, produces more smog than ordinary gas.
Nitrogen is another key additive. A few years ago, some companies started pushing nitrogen-enriched gasoline, emphasizing its ability to clean a car’s engine. It’s not really a new idea: The EPA has required that a certain level of detergent, such as nitrogen, be added to a fuel for years. It’s also not clear whether the new gasoline makes a significant difference to an engine’s performance or longevity. While there’s little research on the subject, the added nitrogen probably doesn’t significantly alter gasoline’s environmental impact. Internal combustion engines do release nitrogen oxide, a smog-forming gas. But the small amounts of nitrogen being added to gasoline make a negligible contribution: Most of the nitrogen in that nitrogen oxide comes from ambient air, not the gasoline itself.
But let’s try to keep some perspective. Although gasoline additives are worrisome, fretting over nitrogen in your gas tank is a bit like cutting cookies from your diet because of the preservatives.
The Green Lantern is produced by the Web magazine Slate and can be read online at www.slate.com. Have an environmental question? Send it to .