More about catalytic converters

More about catalytic converters

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With the world’s largest fleet of motor vehicles, the USA also runs the risk of having the most polluted air, its huge land area notwithstanding. Had there been no serious efforts to minimise the toxicity of the exhaust gases that are by-products of the internal combustion process, it has been estimated that the country could have a blanket of carbon monoxide (at toxic levels) 150 metres thick.

But, fortunately, legislation was introduced in the 1970s which, among other requirements, made the catalytic converter mandatory in cars. This device chemically converts the toxic elements in exhaust fumes into harmless water vapour and carbon dioxide before the fumes get into the air we breathe.
According to the US Environmental Protection Agency, vehicles now sold in the US emit 98% less hydrocarbons, 96% less carbon monoxide, and 90% less oxides of nitrogen than vehicles sold in the early 1970s, thanks to the catalytic converter.

Catalytic converters typically consist of a ceramic or metal honeycombed monolith substrate composed of precious metal catalysts. The coated substrate is wrapped in an intumescent mat that expands when it is heated, thus both securing and insulating the substrate. The substrate is packaged in a stainless steel shell and installed along the engine exhaust system.

As exhaust gases pass over the catalysts, there are chemical reactions that convert the pollutants into harmless gases and water. Hydrocarbons combine with oxygen to become carbon dioxide; oxides of nitrogen react with carbon monoxide to produce nitrogen and carbon dioxide; and with hydrogen to produce nitrogen and water vapour.

The catalyst formulation is usually a mixture of the noble metals platinum, palladium, and rhodium. Modern catalytic converters for petrol engines have oxygen injection to hydrocarbons and carbon monoxide and remove oxygen from oxides of nitrogen simultaneously. But they can only perform both functions at the same time if the combustion mixture, and exhaust gas composition, is held within a narrow band around the stoichiometric air/fuel ratio, usually 14.7 to 1. This calls for accurate and efficient engine management. A catalyst also works best when it is heated up, which is why manufacturers position the catalytic converter close to the hot exhaust manifold.

It should be noted that a catalytic converter is not a ‘stand-alone cure-all’ that can minimise emissions; it is part of an engine management sub-system, an integrated set of specific-purpose emission control components. For example, exhaust oxygen sensors measure the percentage of oxygen in the exhaust stream and signal the engine’s onboard computer so that it can continuously adjust the air/fuel mixture to the proper ratio.

Other devices that go into an emissions control system include early fuel evaporation systems which heat intake manifolds to help vaporize fuel and lower hydrocarbon and carbon monoxide emissions during a cold start. There are also exhaust gas recirculation systems that lower temperatures in the combustion chamber and reduce formation of poisonous oxides of nitrogen. Air meters, which precisely measure air flowing into an engine, and positive crankcase ventilation valves, which control hydrocarbon emissions from the crankcase.

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