The catalytic converter looks like a silencer. It contains a ceramic structure, consisting of a huge number of small cells. The surface of these cells is treated with a thin layer of precious metals. Diesel engine: the catalyst acts on both harmful substances (carbon monoxide and hydrocarbon compounds). In the catalyst, up to 50% of these substances are oxidized to harmless compounds such as carbon and water vapor. Of particular importance is the reduction of hydrocarbons, since (although the catalytic converter is not a soot filter) it helps to reduce the amount of soot in the exhaust. The reason for this is that the gaseous hydrocarbon settles on the solid soot particles and thereby increases the amount of soot emitted. If the hydrocarbon is oxidized in the catalyst, then it can no longer combine with soot.
Gasoline engines: The most efficient way to treat exhaust gases on gasoline-powered vehicles today is with a triple-acting catalytic converter.
The adjustment takes place in the range from l = 0.8 to l = 1.2 in a rapidly changing order: increasing the air supply for the combustion of hydrocarbons, decreasing the air supply to reduce the emission of nitrogen oxides. The gaseous wastes formed from this mixture enter the catalyst, where almost completely (with a new catalyst up to about 90%) are converted into harmless compounds such as carbon dioxide, water vapor and nitrogen. While the engine is warming up, the device does not work, and then, turning on, maintains the average value of l. The same thing happens when the throttle is fully open.
Operating temperatures
Before the catalyst starts to work, it must reach a temperature of about 300°C. This temperature is reached in 25-270 seconds. The lambda probe reaches its operating temperature a little faster, as it is additionally heated by electricity.
The catalyst and lambda probe are very sensitive to overheating. If the temperature of the catalyst exceeds 900°C, then its wear increases greatly, starting from a temperature of 1200°C and above, the catalyst stops working. The same applies to the lambda probe. When the catalyst overheats, its ceramic structure is destroyed and it begins to hang out in its body. This can be determined by the characteristic knocks in the exhaust system. In this case, the catalyst must be replaced.
Precautions for vehicles with a catalyst
The owner's manual has a lot of recommendations for cars with a catalyst. Especially dangerous is the unburned working mixture, which is oxidized in a hot catalyst, which can lead to a critical temperature rise when the engine is started.
Below we provide the most important recommendations:
- If there are interruptions in the ignition, check its serviceability.
- In summer, at high air temperatures, with a long absence of rain, be careful, because when parking the car after a long trip, dry grass or foliage can catch fire from the hot catalyst case.
- Regularly check the condition of the vehicle's heat shields.