On petrol cars, a catalytic convertor has become a standard part of the car exhaust. Catalytic convertors – ‘cats’ for short – originated in America. Like many other items, cats soon went transatlantic and later made their way into European legislation. Unsurprisingly, they wound up in the UK.
Let’s begin with the nuts and bolts of the matter. In chemistry, a ‘catalyst’ is a substance that causes or accelerates a chemical reaction without itself being affected. Fair enough but what has this to do with car exhausts? Still talking at nuts-and-bolts level, a catalytic converter is a device that uses a catalyst to convert three harmful compounds in car exhaust gases into harmless chemicals.
Again, fair enough but what are these three compounds that are so harmful? They are to be found in the exhaust gas of any petrol car and consist of…
Hydrocarbons (incompletely burnt fuel)
Carbon Monoxide (formed when petrol burns)
Nitrogen Oxides (made by combustion heat making atmospheric nitrogen combine with oxygen.)
So, just how scary are these components in car exhaust gases? Carbon Monoxide, a.k.a. CO, is the nastiest one, being poisonous to any organism that breathes air. Nitrogen oxides (NO2) lead to the formation of both smog and acid rain. Relatively innocent Hydrocarbons (HC) just produce smog but are particulate matter, which makes matters worse.
A catalytic convertor, looking rather like a silencer, forms part of a car exhaust system. The catalyst used is liable to consist of precious metals such as rhodium, platinum and palladium. These coat a ceramic honeycomb or ceramic beads inside the ‘cat’. Carbon monoxide is converted into the less harmful carbon dioxide (CO2) and hydrocarbons are converted into carbon dioxide (CO2) and water (H2O). As for the nitrogen oxides, these are converted back into nitrogen and oxygen.
Though they don’t have moving parts, ‘cats’ can suffer malfunctions. If the catalytic convertor in a car exhaust stops working, you won’t necessarily know about this until your next MOT test. A failed cat may just be worn out or its substrate (the metal-coated pellets or ceramic honeycomb inside) may have broken down. Your car may run perfectly well – it just won’t be able to pass the emissions test part of an MOT.
Often, diagnosing what’s awry with the cat in a car exhaust is tricky. If something has gone wrong with the car’s fuel system and it allows raw fuel into the cat, the unit will run red hot. This is usually pretty easy to detect, which is just as well, as the substrate can shatter at such temperatures. Fouled sparking plugs, incorrect ignition timing and failure of the engine’s air pump and/or check valve can also cause overfuelling.
Should the cat’s innards go into meltdown, or shatter through thermal shock or overheating, the result will mimic a blocked car exhaust. A sluggish throttle response is the major clue. Modern electronics mean that a fault light will flash to verify there’s a problem but detecting exactly what the problem is likely to involve diagnostics – on a laptop computer. In any event, should the problem lie within the ‘cat’, a good tyre and exhaust bay will be able to offer you a replacement unit.
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