There are a number of contributing factors that can result in knock, including high cylinder temperatures, lean air-fuel mixtures, excessive timing advance, and especially the use of low octane fuels. While modern computer strategies can automatically retard timing and alter fuel injector pulse width as necessary to help maintain efficient combustion under differing engine loads, older vehicles rely on more hands-on tuning to find a compromise between economy and performance.
The relationship between heat and pressure also means that high-compression engines including supercharged and turbocharged engines can be more prone to engine knock, especially when running lower octane fuels.
While high-octane premium fuels to can help combat engine knock, some of our customers prefer using fuel additives to help increase the octane ratings of regular 87 octane pump gas. Fuel additives such as octane boosters and lead substitutes are popular DIY purchases, especially among classic and muscle car owners, but these two products have more in common than you might think.
Beginning in the s, tetraethyl lead was a common additive found in gasoline. While most of us associate lead with its ability to reduce valve seat wear, its original purpose was to increase the octane of early fuels. These two compounds outline the two extremes of the scale, with pure iso-octane having a rating of and n-heptane having a rating of zero. The higher the octane number, the more compression from the piston the gasoline can withstand before igniting in the engine.
The ability for gasoline to withstand being compressed is most important for vehicles that run on traditional gasoline. In contrast, diesel engines do not compress fuel but rather air and then inject fuel. These engines rely on different ratings, known as cetane numbers. The main difference between whether or not one uses high or low octane gasoline is determined by the level of "knocking" ones engine experiences.
Inside the engine, an air-fuel mixture is ignited by the spark plug. While this is occurring, the piston is also moving upwards, compressing the air-fuel mixture and simultaneously increasing its temperature since temperature increases as pressure increases. Share Flipboard Email. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels.
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Several EPA fuel regulations have concerned octane. Octane is a gasoline additive that is needed for the proper functioning of modern engines. Octane sources have taken many forms throughout the years, both renewable and petroleum-based. As adverse health and environmental consequences have been discovered for lead and petroleum-based octane providers, they have been removed from the fuel supply or decreased.
Today, there are two primary sources of octane used in the U. Knock is virtually unknown to modern drivers. This is primarily because fuels contain an oxygenate that prevents knock by adding oxygen to the fuel.
This oxygenate is commonly referred to as octane. At most retail gasoline stations, three octane grades are offered, 87 regular , 89 mid-grade , and premium. The higher the octane number, the more resistant the gasoline mixture is to knock.
Doing so would increase vehicle efficiency and lower greenhouse gases through decreased petroleum consumption. In the early 20th century, automotive manufacturers were searching for a chemical that would reduce engine knock.
In , automotive engineers working for General Motors discovered that tetraethyl lead better known as lead provided octane to gasoline, preventing engine knock. While aromatic hydrocarbons such as benzene and alcohols such as ethanol were also known octane providers at the time, lead was the preferred choice due to its lower production cost. Leaded gasoline was the predominant fuel type in the United States until the U. Environmental Protection Agency EPA began phasing it out in the mids because of proven serious health impacts.
Early in its use as a fuel additive, health concerns were raised regarding the use of lead in gasoline. In , 15 refinery workers in New Jersey and Ohio died of suspected lead poisoning. As a result, the Surgeon General temporarily suspended the production of leaded gasoline and convened a panel to investigate the potential dangers of lead use in gasoline. Despite these warnings, the Surgeon General set a voluntary standard of lead content, which the refining industry successfully met for decades.
It was not until the s, following extensive health research, that the devastating health impacts of low-level lead exposure were established. The health impacts of lead exposure in children include anemia, behavioral disorders, low IQ, reading and learning disabilities, and nerve damage.
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