I am learning a lot about hydrocarbon fuels from Morgan Downey’s Oil 101. For instance, that the common understanding of the phrase ‘high octane’ is somewhat misleading. In the context of gasoline-powered internal combustion engines, such as those in cars, the octane rating of a fuel refers to how much it can be compressed along with air before it will spontaneously ignite. In these engines, fuel and air are mixed together and compressed in a cylinder. They are then ignited at a precisely controlled time by a spark plug. Cases where the mixture explodes before then are called ‘engine knock’ and are damaging. As such, engines are designed to use fuel above a certain octane number, in order to be confident that knocking will not occur.
When it comes to choosing fuel to buy, this means it is appropriate to use a grade with an octane rating as high as cited in the operating manual of a vehicle. Going higher, however, may be a waste of money for two independent reasons. Firstly, higher octane fuels are more expensive because they cost more for refineries to produce. Unless your engine is tuned to take advantage of the extra opportunity for compression, no additional power will be generated. Secondly, higher octane fuels often contain less energy per litre, because the hydrocarbons that comprise them have less energy in their chemical bonds. As such, a litre of more-expensive high octane fuel likely will not take a vehicle as far as a cheaper litre of adequate-octane fuel.
Octane numbers are assigned based on how a fuel compares to two specific hydrocarbons: isooctane (which is hard to ignite by compression) and n-heptane (which is easy to ignite that way). 90 octane fuel is thus as resistant to pressure-induced ignition as a mixture of 90% isooctane and 10% n-heptane. Some fuels are even better at resisting pressure-induced ignition than isooctane, and can therefore have octane numbers over 100.
In diesel engines, this is reversed. They do not have spark plugs and rely upon the ability of fuel to ignite spontaneously in the presence of pressurized air. In diesel, the cetane number refers to the propensity of fuel to autoignite on compression. Here, a higher number is more desirable.
One other thing I didn’t know about liquid transport fuels is that the fuel used by piston-driven aircraft, such as small propeller planes, still uses tetra ethyl lead to increase its octane rating. This practice has been discontinued in cars both because it interferes with catalytic converters and because it was massively increasing human exposure to lead – a known cause of brain damage. In aviation gasoline, tetra ethyl lead is used instead of alcohols to boost octane. This is because alcohol-blended fuels are less energy dense, more prone to vapour lock, liable to separate at low temperatures, as vulnerable to corrosion. Such aircraft are a relatively tiny share of the total market for hydrocarbon fuels; still, it isn’t particularly comforting to know that they continuously disperse lead on whatever is below them.