Chemistry Question #473

shaheena, a 38 year old female from Karachi/Pakistan asks on December 15, 2001,

The name for gas written on CNG filling station is octane. There are other members in the alkane series also, so why is only octane used and not other members of this series?

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The answer

Barry Shell answered on December 15, 2001

You are most likely confusing the fuel's Octane Rating with the 8 carbon chain hydrocarbon "octane". There is probably little or no octane in CNG (Compressed Natural Gas) fuel, however the octane rating is based on historical developments in the automotive fuel industry.

According to my university organic chemistry textbook, Morrison & Boyd (1970), the mechanism of the explosive reaction during the combustion of fuel and air under compression within an automotive engine cylinder is not well understood. In some cases, depending on various factors, a pinging or knocking sound can come from engines when the hot compressed gases spontaneously explode before the spark and flame gets to them. One wants an orderly explosion to get smooth power from the engine. Early in the 20th century experimenters tried all sorts of pure alkanes in internal combustion engines and rated them on a scale for knocking. The worst was n-heptane which knocks very badly and was given an arbitrary value of zero. The best was 2,2,4-trimethylpentane (also known as "iso-octane" because it has a total of 8 carbons) which was given a value of 100. Scientists discovered that highly branched alkanes had higher octane ratings, hence modern gasoline is a blend that includes some of these to bring up the octane rating and avoid knocking. Also, additives, such as tetraethyllead (C2H5)4Pb, which is now illegal, were found to increase the octane number of fuels. These have since been replaced by other additives such as Methyl Tertiary Butyl Ether (MTBE) which is also controversial.

CNG is usually around 70-90% methane with 10-20% ethane, 2-8% propanes, and decreasing quantities of the higher hydrocarbons up to pentane.

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