VW Passat B5

since 1996 release

Repair and operation of the car

The catalyst of exhaust gases has the case from stainless steel to which welding attached inlet and final cones with branch pipes and fixing flanges, and also the heatreflectors holding a ceramic covering. All internal volume of the case is busy with the ceramic porous monolith recorded by rings or a grid from stainless steel. The structure of a monolith represents the ordinary filter, but all internal and external surface of the time contacting to gases is covered with very thin molecular layer of alloy which contains platinum, rhodium and palladium. Porosity of a monolith has big surface area, covered with this alloy from very expensive metals that generally and determines the high price of catalysts, see rice. Catalyst.

In the catalyst there is a reaction as a result of which potentially dangerous hydrocarbon and carbon monoxide in exhaust gases turn into safe gases and water vapor.

The engine releases into the atmosphere together with exhaust gases products full (_{H2O water vapor}, _{N2 nitrogen}, etc.) and incomplete (SO carbon oxide, and also CnHm, NOx nitrogen oxides) fuel combustion. Total number of components which contain in these gases exceeds several hundreds, and most of them are unhealthy the person.

Exhaust gases, getting through a porous surface of a monolith, first, heat it, and secondly, dookislyatsya. From WITH _CO2 turns out, that is nontoxical carbon dioxide, CnHm in several stages passes into _{CO2 and H2O}, NOx turns into molecular _N2 which contains in usual air, and in water. In a word, in the catalyst there are quite difficult chemical reactions, thanks to high temperature and existence of a special covering from expensive metals.

The main positive effect of the catalyst – full neutralization of three components – WITH, CnHm, NOx, – which in exhaust gases is more, than other harmful substances. Also it is reached not only thanks to availability of platinum, rhodium and palladium. An important role is played by temperature which is maintained in borders of 300-800 °C. If it decreases to 250 °C, chemical reactions of neutralization WITH, CnHm, NOx, despite availability of metals catalysts, will not occur. And at a temperature about 900 °C the catalytic film begins to melt and collapse.

Petrol engines

On all models with the catalyst the power supply system fuel of the closed type, that is on the basis of a signal from the sensor of the oxygen installed in the exhaust system the control unit constantly optimizes composition of fuel-air mix. Depending on the content of oxygen in exhaust gases the sensor of oxygen induces tension from 0,1 B (high content of oxygen, poor mix) to 0,9 B (low content of oxygen, rich mix). On the basis of these data the control unit of the engine changes time of opening of fuel nozzles and changes a fuel ratio in fuel-air mix. A metric ratio of fuel mix at which there is its full combustion and in exhaust gases there are no harmful substances, are 14,7 weight parts of air on 1 part of fuel, see rice. The elements entering the catalyst and products which are going out of the catalyst.

The catalyst works effectively at temperatures of +300 °C – +800 °C.

Diesel engines

On cars with diesel engines purification of exhaust gases is made not by the adjustable oxidizing catalyst.

Reduction of percentage of oxides of nitrogen in exhaust gases of the diesel engine is reached due to installation on the car of system of repeated burning of a part of exhaust gases (EGR). The EGR valve of system of repeated combustion of exhaust gases is installed in a bend of the exhaust system and is run by the vacuum regulator. The EGR valve on the heated-up engine directs a part of exhaust gases to engine combustion chambers therefore temperature decreases and at the same time the percent of oxides of nitrogen in exhaust gases decreases.