Contents: Petrol engines ↓ Diesel engines ↓
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 porosity of the monolith has a large surface area covered with this alloy of very expensive metals, which mainly determines the high price of catalysts, see Fig. Catalyst.
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 H 2 O, nitrogen N 2 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 turns into CO ₂ and H ₂ O, 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, see Fig. Elements included in the catalyst and products leaving the catalyst.
The catalyst operates effectively at temperatures of +300°C – +800°C.
Diesel engines
In vehicles with diesel engines, exhaust gas cleaning is performed by a non-regulated 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.
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