Three-way catalyst
A device fitted to the exhaust of a petrol car that turns three kinds of harmful component in the exhaust gas into harmless gases at the same time.
Exhaust contains unburnt hydrocarbons, carbon monoxide and nitrogen oxides. On the surface of precious metals such as platinum, palladium and rhodium, the first two are oxidised and the nitrogen oxides reduced, giving water, carbon dioxide and nitrogen. All three can be treated at once only within a narrow band where fuel and air are in just the right proportion, so the catalyst is paired with electronic control that measures oxygen in the exhaust and finely adjusts the fuel[1].
What brought it into use was the US Clean Air Act of 1970. Limits were set before any means of meeting them existed, and that forced the catalyst into production. Lead poisons the catalyst, so its introduction also led to the end of leaded petrol. It works poorly just after a cold start, and much of a car's emissions come out in that interval, a problem that persists.
Related terms
- Tetraethyl leadA lead compound added to petrol to suppress knocking. It spread lead into the air with the exhaust and was banned worldwide once the harm to health became clear.
- Internal combustion engineAn engine that burns fuel inside its cylinders and uses the pressure to push pistons. Petrol and diesel engines are both of this kind.
- KnockingAbnormal combustion in an engine in which the fuel-air mixture explodes before the spark ignites it, with a metallic knocking sound.