A catalyst is a substance which speeds up a chemical change, without being altered itself. The catalytic converter is a steel canister containing a ceramic honeycomb material coated with catalyst. The exhaust gases pass freely over the honeycomb, where the catalyst speeds up the change of the harmful gases into harmless gases and water vapour.
To avoid damaging the catalyst, the engine must be properly turned, and certain petrol additives (mainly those containing lead) should not be used.
On early vehicles with a catalytic converter, the converter works independently, and it relies on a well-maintained to be effective. This type of catalytic converter system is called “open-loop”.
On modern vehicles, an oxygen (or “Lambda”) sensor is fitted to the exhaust system. This is used by the engine management system to control the fuel/air mixture. If the mixture is kept within certain limits, the catalytic converter can work at its maximum efficiency. The Lambda sensor sends the engine management system details of how much oxygen is in the exhaust gas, and this is used to automatically control the fuel/air mixture. This type of catalytic converter system is called “closed-loop”.
After a number of years, the catalytic converter will have to be renewed, because the catalyst inside will deteriorate with age. This can be expensive because of the precious metals used to make the catalyst.
Catalytic converters have certain side-effects. First, they only work properly once they heat up to an optimum working temperature, and when they’re cold, they hardly reduce pollution at all. If you’re following a car with a catalytic converter, you might notice a strong “rotten egg” smell when the car accelerates hard, or when it’s under a heavy load climbing a hill – this is due to hydrogen sulphide gas. Another side effect is that cars with a catalytic tend to produce more water from the exhaust, especially when the engine’s cold, or on a short run – this means that the exhaust system tends to rust more quickly.