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.
Loading...
Comments