From the 1920s until the 1970s, most gasoline cars in the USA were using fuel that had lead mixed into it. The reason for this was to reduce the engine knocking effect from abnormal combustion in internal combustion engines of the time. While lead — in the form of tetraethyllead — was effective at this, even the 1920s saw both the existence of alternative antiknock agents and an uncomfortable awareness of the health implications of lead exposure.
We’ll look at what drove the adoption of tetraethyllead, and why it was phased out once the environmental and health-related issues came into focus. But what about its antiknock effects? We’ll also be looking at the alternative antiknock agents that took its place and how this engine knocking issue is handled these days.
It’s a Matter of Octane
In an internal combustion engine (ICE), ideally the air-fuel mixture that gets injected into a cylinder will detonate at the perfect moment where the flame front will travel outwards from the point of ignition, with every bit of the air-fuel mixture burning up fully. This will allow for maximum use of the energy in the fuel mixture, while causing a clean stroke of the piston.
In reality, however, pockets of this fuel-air mixture will detonate before the flame front reaches them. These so-called ‘cool flames’ occur because of the compression by the piston combined with slight unevenness in the mixture, causing additional pressure waves in the cylinder. This raises the cylinder pressure and causes the typical metallic pinging noise that is indicative of engine knocking. Depending on how many of these pockets detonate outside of the spark plug’s flame front, the result may be increased wear on components, or even their outright destruction.
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