As a motorist refuelling your vehicle, you are familiar with the question, “unleaded or lead-replacement petrol?”.
A few years ago, the question was, “leaded or unleaded?”, which prompted a further question, “What on earth is lead doing in my petrol in the first place?”
For the answer, we need to go back to aircraft falling out of the sky during World War I, an era when people were unaware of octane. Most of Europe’s petrol was extracted from Arabian light crude oil, which would have had a modestly high octane rating (on account of the aromatic content).
Obviously, the European war planes were designed for maximum performance when using this petrol. When the US joined the war, it helpfully sent its allies some shipments of American petrol, which had a much lower octane.
Since the planes had no exhaust silencers, the characteristic knocking combustion sound associated with too low octane was inaudible. The result was a spate of engine failures and flight catastrophes.
A small team of boffins was assembled to find solutions.
They knew by then that fuel composition was at the heart of the problem, so they started adding variations to the petrol in the hope that this would either improve or exacerbate the combustion knocking problem.
NO MORE KNOCKING
It took years but eventually they developed a robust test method (the octane rating method), discovering that a tiny amount of lead was extremely effective in the suppression of combustion knocking.
They had also worked out how to make high-octane petrol without using lead, but it involved complex refinery processes that were much more expensive than the lead additive. From then on, all petrol sold at forecourts contained lead.
Why was lead taken out of petrol?
The answer is smog. During the 1960s, air pollution in the world’s big cities was appalling. Cleaning up the toxic vehicle exhaust fumes necessitated the introduction of catalytic converters, but lead was poisonous to the catalyst material. Refineries had to invest millions of rand to make high-octane petrol without the use of lead.
Another challenge was that, while most car engines had aluminium cylinder heads with hardened steel inserts for the valve sealing faces, there were certain vehicles that still used cast-iron cylinder heads, which usually didn’t have valve seat inserts. Lead in the petrol had been inadvertently performing another beneficial function – to act as a dry lubricant for the valves’ seats (like the Teflon coating on non-stick frying pans).
Because the valve spring has a helical form, it imparts a bit of a twist to the valve as it opens and shuts. At high engine speed, the twisting causes the valves to spin fast, causing the cast-iron to wear out. This process is known as valve-seat recession.
There was a need to find an alternative solution that would not harm the catalytic converters but would protect the valve seats of the older vehicles.
A few parts per million of manganese/potassium rectified the issue and this is what can be found in lead-replacement petrol. The number of old cars that need lead-replacement petrol has decreased drastically, but many people still ask for lead replacement petrol anyway – just playing safe perhaps?
Most informed people changed to unleaded when it became available because spark-plugs and exhaust systems last much longer.
So, as a pinch of salt can make a difference to the taste of your meal, trace compounds in your fuel can have a disproportionately large effect on your engine’s performance.
We’ll take a look at sulphur in diesel in our next article. – Sasol Technical Team