The Chemistry of Leaded Petrol, Unleaded Petrol & Diesel

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Tomorrow (18th May) marks the date of birth of Thomas Midgley, who made significant contributions to something many of us make use of on a regular basis: petrol. Midgley was the research assistant to Charles Kettering, and the duo were responsible for the addition of the compound tetraethyl lead to petrol, an innovation that would have a lasting legacy – although perhaps not in the manner that they may have originally envisioned.

Some general background information on petrol (gasoline for our US readers) is probably necessary before we discuss the finer points of Kettering and Midgley’s contributions. Petrol is obtained from crude oil, as is diesel. The two do differ slightly in their composition and properties however. They are obtained from crude oil by fractional distillation, where the oil is heated until it boils and vaporises, then fractions at different boiling point ranges are distilled off. Petrol is formed from fractions that have a boiling point between 35 to 200 degrees celsius, whereas the fractions that form diesel have a boiling point between 250 to 300 degrees celsius.

Both petrol and diesel are composed of mixes of hydrocarbons – compounds, unsurprisingly, containing carbon and hydrogen only. Petrol contains hydrocarbons with chains between five to twelve carbon atoms long, with diesel’s chains being slightly longer at ten to fifteen atoms. Diesel also contains more energy than petrol per litre, making it a more efficient fuel, albeit a more expensive one.

Petrol and diesel engines also work in slightly different ways. In petrol engines, the engine takes in both fuel and air, which a piston then compresses, before the engine’s spark plug ignites the fuel. The combustion reaction that ensues produces energy, and the engine then expels the waste gases produced by this reaction. In diesel engines, only air is taken in at the start of the process, and it is only after this air has been compressed that the fuel is injected. Diesel engines don’t use spark plugs to trigger the combustion reaction – instead, the fuel auto-ignites due to the heat generated by the greater compression used in diesel engines.

In petrol engines, premature combustion can be a problem. Since the fuel is injected at the start of the process, burning of the fuel can sometimes be triggered during the compression process, before the spark plug ignites the fuel at a precise time. This is known as pre-ignition, and can lead to another phenomenon called engine knocking. Knocking occurs when the peak of the combustion reaction doesn’t match up with the stroke of the engine’s piston. This leads to an actual knocking or pinging sound, and can cause damage to the engine – so it’s something we want to avoid.

In order to prevent engine knocking, scientists have added a range of compounds to petrol over the years. You’ve probably come across the octane rating of fuels before – this is essentially a measure of how well the fuel avoids the knocking problem. It’s referenced to two compounds, isooctane and n-heptane. Isooctane is given a standardised octane rating of 100, whereas n-heptane is given a rating of 0. The higher the rating, the better the fuel is at preventing knocking. The numbers between 0 and 100 refer to comparison to mixtures of isooctane and n-heptane; for example, a fuel with an octane rating of 95 would have the same knocking ‘resistance’ as a mixture containing 95% isooctane and 5% n-heptane.

Note that this isn’t the same as the fuel actually consisting of only isooctane and n-heptane, as the scale is just a comparison between the fuel and this mixture. It’s also possible to get octane ratings above 100, as there are other compounds that are even better at avoiding knocking than isooctane. An example is benzene, which has an octane rating of 101.

Knocking is a problem that automobile manufacturers have been trying to solve for decades. As car engines became more powerful in the 1920s, there was a greater necessity to find petrol additives that could reduce knocking. Kettering and Midgley appeared to hit on the perfect solution; a compound called tetraethyl lead appeared to be very successful at minimising knocking, and had the added bonus that it could be patented. It could be added to petrol along with 1,2-dibromoethane, which would react with the lead and prevent it from being deposited in the engine.

Somewhat startlingly, Kettering, Midgley and their colleagues had done next to nothing in the way of research on the potential health effects of tetraethyl lead before its roll out began. Today, this would be unthinkable, but it’s all the more remarkable because the effects of lead poisoning were already comparatively well known at the time, even if it was not fully appreciated that low exposures could still be a cause for concern. Several countries had already banned lead-based white paints in the early 1900s due to concerns regarding lead toxicity – although notably the United States did not do so until 1978.

Kettering and Midgley must have been aware of the potential negative associations at the very least, because their additive was marketed as ‘Ethyl’ by General Motors, pointedly avoiding any mention of its lead component. Midgley himself had to take a break from his work at one point due to developing mild lead poisoning, but still seemingly had complete confidence in the safety of the compound.

It’s worth pointing out that there wasn’t a lack of initial backlash to tetraethyl lead’s inclusion in petrol. Workers at the plant producing the compound started experiencing serious symptoms – collapsing, convulsing, gibbering nonsense, and requiring hospitalisation. Several of the workers died as a consequence, and it wasn’t long before tetraethyl lead was fingered as the culprit. Subsequently a number of cities banned the sale of petrol containing tetraethyl lead, and its production was suspended pending federal investigation.

You might think that that would have been that, but General Motors had had difficulty finding such an effective anti-knock compound, and were loath to discard it after the money they had ploughed into its development. They claimed that no suitable alternatives were available, though later uncovered correspondence shows that Kettering, at least, was fully aware of some of the additives being explored by other competing companies.

The federal investigation found, from rushed and limited experiments with flawed conclusions, that the addition of tetraethyl lead to petrol was not likely to be harmful to the health of the general public, and that its production and sale could be resumed. However, they did note in their summarising comments that their conclusions were subject to criticism, and that future increased motor use could still pose health issues. They concluded by stating that continued investigation into the effects was necessary, and specifically stated, “the committee feels this investigation must not be allowed to lapse.”

Sadly, lapse is pretty much exactly what it did. It wasn’t until the mid-1980s that, as realisation of the health issues that even low levels of lead in the body could cause dawned, countries started to enforce bans on leaded petrol. Its use was slowly scaled down, with most countries completing this phaseout by 2000; however, in a select few countries, leaded petrol is still sold and used. It’s clear the effects of the lead spewed out by engines running on leaded petrol were far more serious than probably even Midgley and Kettering suspected – increased lead blood levels have even been linked with increased rates of violent crime, though this is a link that’s still to be indisputably confirmed.

Today, unleaded petrol still contains anti-knock agents, but a range of different compounds that don’t contain lead are used. Ethanol is one such compound, as well as methyl tertiary-butyl ether (another compound which has attracted some controversy), benzene, and toluene, amongst others. Tetraethyl lead’s legacy still remains though – levels of lead in the soil near roads are still much higher than those in areas further from traffic.

Back to Midgley, and his tale doesn’t end with tetraethyl lead. He was also involved in the discovery of Freon, the widely-used refrigerant gas that was later found to be contributing to the destruction of the ozone layer. He didn’t live to fully comprehend the huge negative environmental impacts of both of these discoveries, however; he contracted polio at the age of 51, leaving him seriously disabled, and died four years later in 1944 when he became entangled in a contraption that had been designed to allow him to be lifted from his bed.

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References & Further Reading