History of leaded petrol
phenomenon is known as engine knocking, and may cause unwanted damage to the engine, decreasing its lifespan and causing nuisance due to the loud metallic pinging it produces.
Figure 2: The difference in shockwaves produced during normal combustion and premature combustion 2
Engine knocking can be prevented by using fuels better at withstanding compression, and the measure of the ability to do so without detonating is known as the fuel’s octane rating. To increase the octane rating of petrol, other substances or additives are mixed into the petrol, usually in the form of branched chain additives. Midgeley tested many potential additives to petrol, but he described most of them as ‘ no more effect than spitting in the Great Lakes ’ . 3 One additive that stood out was ethanol, which was already used as a fuel source and was able to effectively increase the octane rating and prevent premature combustion in high compression engines. The most direct route for converting energy from the sun into a material that is suitable for use in an internal combustion motor is through vegetation to alcohol. Alcohol is the only liquid from a direct vegetable source that combines relative cheapness with suitability. Alcohol will stand very high initial compressions without knocking, and at high compressions is smooth and highly satisfactory. 4 However, the use of ethanol as an additive to petrol was faced with displeasure from politicians as the 1920s coincided with prohibition of alcohol in the US, thus widespread storage and use of ethanol across the US was seen as a ‘ bargain with the devil ’ by some. Furthermore, the concentration of ethanol required to increase the octane rating is high, at around 10%, and thus even purchasing enough ethanol to produce alcohol-infused petrol would be more expensive than other 2 LibreTexts Chemistry, ‘Gasoline - A Deeper Look’, accessed 4 th August 2023, https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/03%3A_Org anic_Compounds-_Alkanes_and_Their_Stereochemistry/3.08%3A_Gasoline_-_A_Deeper_Look, 3 Derek Muller, ‘The Man Who Accidentally Killed The Most People In History’, YouTube, accessed 2 nd August 2023, https://youtu.be/IV3dnLzthDA. 4 Thomas Midgley Jr., ‘Our Liquid Fuel Reserves’, unpublished paper to the Indiana Section of the Society of Automotive Engineers, 12 th October 1921.
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