Octane ratings and biofuels

In a petrol engine, a piston squeezes a mix of petrol vapour and air, and a spark sets it alight at just the right moment. The flame spreads smoothly through the mix and pushes the piston back down.

Squeezing a gas heats it. When some of the fuel catches fire by itself from that heat, before the flame reaches it, it burns in a sudden burst that makes a metallic rattle called knocking. Knocking wastes energy and can damage the engine.

How a molecule is built matters here. Heptane, a straight chain of 7 carbons, catches fire easily when squeezed. Isooctane, the branched isomer of octane, holds out until the spark. The alkane naming rules give it its full name, 2,2,4-⁠trimethylpentane: a chain of 5 carbons with three methyl branches, two on carbon 2 and one on carbon 4.

These two molecules set the scale. A petrol rated 95 resists knocking as well as a mix of 95% isooctane and 5% heptane, tested in a special engine. Refiners raise the rating of petrol by blending in branched chains and rings. Regular petrol in the UK and Europe is rated 95. Pumps in the US and Canada show the average of two different tests, so regular petrol there reads 87.

Fuels from plants

Biofuels are made from plants. Ethanol is one: yeast makes it by fermenting the sugar in crops such as sugar cane and maize. Ethanol resists knocking well, with an octane rating of about 108. Standard petrol in the UK, sold as E10, holds up to 10% ethanol, and in Brazil many cars run on ethanol from sugar cane.

Biodiesel is made from vegetable oils and used cooking oil. As you have seen, an oil molecule is glycerol joined to three fatty acids by ester links. Warmed with methanol and a base as a catalyst, each fatty acid lets go of the glycerol and joins methanol as a new ester. One oil molecule gives three of these smaller esters, which make biodiesel, and one glycerol. Diesel at UK pumps holds up to 7% biodiesel.

Every fuel with carbon in it gives carbon dioxide when it burns, biofuels included. Plants take carbon dioxide from the air as they grow, so the carbon in a biofuel was in the air a short time before. The carbon in petrol and diesel spent millions of years underground, and burning it adds new carbon dioxide to the air. Growing the crops and making the fuel use energy too, so how much a biofuel helps depends on how it is made. Hydrogen burns to water alone, and how much it helps depends on how it is made too: most hydrogen today is made from natural gas.