The catalytic converter

The modern ‘cat’ is highly efficient, but it does have disadvantages. It must run at the right temperature and is very costly to replace. Jake Venter explains.

The modern three-way catalytic converter (cat) can reduce unwanted exhaust emissions by more than 90%. It changes deadly carbon monoxide into the gas we exhale (carbon dioxide), harmful oxides of nitrogen into nitrogen (found freely in the atmosphere), and unburnt hydrocarbon compounds into carbon dioxide and water.

These conversions occur most efficiently where the air:fuel ratio is chemically correct: between 14,6 and 14,8 parts of air to one part of fuel by mass. This ratio is controlled by one or two lambda sensors; these measure the oxygen level in the exhaust before it enters the cat and after it has exited, and transmits these values to the engine control unit (ECU). The sensors start operating at an exhaust gas temperature of 350°C. This means that when an engine is cold, the ECU has to calculate the correct air:fuel ratio from the mass-flow and air temperature sensors. Above 350°C, the ECU uses the oxygen content in the exhaust to gauge the air:fuel ratio and adjust it accordingly.

Operating temperature plays a significant part in the cat’s functioning. Little pollutant conversion takes place when the engine is cold, but the rate gathers pace when the exhaust gas temperature (EGT) exceeds 300°C. At normal operation, the EGT is between 400°C and 800°C, which is ideal for high catalytic conversion rates and long life.

If a rich mixture raises the EGT to over 800°C, the cat starts to degrade due to thermal ageing; that is, the catalysts begin breaking up. When the temperature rises to over 1 000° C, the cat can become useless very quickly.

A cat’s life can be shortened by the following:

never park a cat- equipped car on grass; you could start a fire

• ‘Catalyst poisoning’

This can occur when the cat is exposed to substances that coat the working surfaces. Lead is a major culprit, which is why cars equipped with converters should use only unleaded fuel. Silicon is another cat poison; it usually enters the unit via coolant additives that leak into the combustion chamber due to a faulty cylinder head gasket.

• Meltdown

Any condition that raises the cat’s temperature to high levels increases the risk of a meltdown of the interior, which will result in catalytic deactivation. The cat may even become partially blocked so that the engine suffers a performance loss. This often happens when excessive unburnt petrol reaches the cat, such as when push-starting an engine. If the engine does not start within 10 seconds, the cat may overheat when the engine finally starts.

An engine misfire also feeds unburnt fuel into the cat and can cause the internal temperature to rise beyond 1 400°C, but modern cars are equipped with a ‘check engine’ light to warn of a serious misfire condition.

fire hazard

Some owner’s manuals warn that a converter-equipped car can cause a fire when parked on dry grass, as the unit gets so hot.

Cats are so expensive to replace that when they fail, many motorists consider replacing them with a length of piping. In this case, the lambda sensor will no longer function correctly and the engine will therefore seldom be supplied with the correct air:fuel ratio. This is illegal and causes pollution, and should be sorted out by a competent workshop.

If your fuel consumption suddenly increases, acceleration is poor, starting becomes difficult, or the ‘check engine’ light comes on, you may have a failed catalytic converter.

• Jake Venter is a journalist and a retired engineer and mathematician. Email him at jacobventer77@gmail.com. Subject line: Auto engineering.