Catalyst
- 1 - body;
- 2 - seal;
- 3 - honeycomb catalyst
Elements entering the catalyst and products leaving the catalyst
All vehicles are equipped with catalytic converters to reduce the amount of harmful substances emitted into the atmosphere through exhaust gases.
The exhaust gas catalyst has a stainless steel housing to which the inlet and outlet cones with pipes and mounting flanges are welded, as well as heat reflectors holding the ceramic lining. The entire internal volume of the housing is occupied by a ceramic porous monolith fixed with rings or a stainless steel mesh. The structure of the monolith is a regular filter, but the entire internal and external surface of the pores in contact with gases is covered with a very thin molecular layer of an alloy containing platinum, rhodium and palladium. The porous monolith has a large surface area covered with this alloy of very expensive metals, which is mainly what determines the high price of the catalysts.
The catalytic converter causes a reaction that converts potentially hazardous hydrocarbons and carbon monoxide in the exhaust gases into harmless gases and water vapor.
The engine emits into the atmosphere together with the exhaust gases the products of complete combustion (water vapor H2O, nitrogen N₂, etc.) and incomplete (carbon monoxide CO, as well as CnHm, nitrogen oxides NOX) combustion of fuel. The total number of components contained in these gases exceeds several hundred, and most of them are harmful to human health.
The exhaust gases, penetrating the porous surface of the monolith, firstly heat it, and secondly, oxidize. CO ₂ is obtained from CO, that is, non-toxic carbon dioxide, CnHm in several stages passes into CO ₂ and H2O, NOX turns into molecular N₂, which is contained in ordinary air, and into water. In short, rather complex chemical reactions occur in the catalyst, due to the high temperature and the presence of a special coating of expensive metals.
The main positive effect of the catalyst is the complete neutralization of three components – CO, CnHm, NOX, – which are more abundant in exhaust gases than other harmful substances. And it is achieved not only due to the presence of platinum, rhodium and palladium. The temperature, maintained within the range of 300–800°C, plays an important role. If it drops to 250°C, chemical reactions of neutralization of CO, CnHm, NOX, despite the presence of metal catalysts, will not occur. And at a temperature of about 900°C, the catalytic film begins to melt and disintegrate.
Petrol engines
On all models with a catalytic converter, the fuel supply system is closed-loop, i.e. based on the signal from the oxygen sensor installed in the exhaust system, the control unit constantly optimizes the composition of the fuel-air mixture. Depending on the oxygen content in the exhaust gases, the oxygen sensor induces a voltage of 0.1 V (high oxygen, lean mixture) up to 0.9 V (low oxygen, rich mixture). Based on this data, the engine control unit changes the opening time of the fuel injectors and changes the fuel ratio in the fuel-air mixture. The metric ratio of the fuel mixture, at which its complete combustion occurs and there are no harmful substances in the exhaust gases, is 14.7 weight parts of air to 1 part of fuel.
The catalyst operates effectively at temperatures from +300°C to +800°C.
Diesel engines
In vehicles with diesel engines, exhaust gas cleaning is performed by an unregulated oxidation catalyst.
The percentage of nitrogen oxides in the exhaust gases of a diesel engine is reduced by installing an exhaust gas re-burning (EGR) system on the vehicle. The EGR valve of the exhaust gas re-burning system is installed in the knee of the exhaust system and is controlled by a vacuum regulator. The EGR valve on a warmed-up engine directs part of the exhaust gases into the combustion chambers of the engine, which reduces the temperature and at the same time reduces the percentage of nitrogen oxides in the exhaust gases.
Operation of vehicles with a catalytic converter
The catalytic converter is a reliable and simple device that does not require any maintenance, but there are some facts that the owner should know to ensure maximum service life of the catalytic converter.
Do not use leaded gasoline in a vehicle equipped with a catalytic converter, as the lead will coat the precious metals in the catalytic converter grid, reducing their conversion efficiency and eventually causing failure of the catalytic converter.
Always keep the ignition and fuel system in good order.
If misfiring is observed while the engine is running, do not operate the vehicle until the problem is corrected.
If the engine control indicator lamp lights up while the engine is running, you should contact a service station as soon as possible. If there is damage to the engine control, then in addition to increased fuel consumption and reduced engine performance, the catalytic converter may fail.
Do not tow start the engine.
Do not turn off the ignition at high engine speeds.
Do not use fuel or motor oil additives as the additives may contain substances that can damage the catalyst.
Do not operate the vehicle if blue smoke is visible from the exhaust pipe.
After the engine has stopped, the exhaust system continues to emit a significant amount of heat for a long time, so you should avoid parking the car on grass and dry leaves. Remember that the catalytic converter is fragile, so avoid impacts when removing the catalytic converter or exhaust system.
A catalytic converter used on a well-adjusted vehicle should last from 100 to 160 thousand kilometers.
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