Auto Manual of VW Passat B5

6.2. The catalyst

THE GENERAL DATA

The catalyst

1 – the case, 2 – consolidation, 3 – honeycombs of catalyst

The elements entering into the catalyst and products, leaving the catalyst


On all cars catalysts for reduction of quantity of the harmful substances which are thrown out in atmosphere with exhaust gases are established.

The catalyst of the fulfilled gases has the case from stainless steel to which welding attaches inlet and final cones with branch pipes and fixing flanges, and also теплоотражатели, keeping a ceramic covering. All internal volume of the case is occupied by the ceramic porous monolith fixed by rings or a grid from stainless steel. The monolith structure represents the usual filter, but all internal and external surface of the time contacting to gases, is covered by very thin molecular layer of an alloy which contains platinum, rhodium and a palladium. Porosity of a monolith has the big area of the surface covered with this alloy from very expensive metals that basically and defines the high price of catalysts, fig. the Catalyst see.

In the catalyst there is a reaction in which result potentially dangerous hydrocarbon and окись carbon in exhaust gases turn to safe gases and water steam.

The engine throws out in atmosphere together with the fulfilled gases products full (water steam Н₂О, nitrogen N₂, etc.) and incomplete (оксид carbon WITH, and also CnHm, оксиды nitrogen NOx) fuel combustion. Total number of components which contain in these gases, exceeds some hundreds, and the majority of them are unhealthy the person.

The fulfilled gases, getting through a porous surface of a monolith, first, heat up it, and secondly, доокисляются. From WITH it turns out СО₂, that is nontoxical carbonic gas, CnHm in some stages passes in СО₂ and Н₂О, NOx turns in molecular N₂ which contains in usual air, and in water. In a word, in the catalyst there are difficult enough chemical reactions, thanks to a heat and presence of a special covering from expensive metals.

The main positive effect of the catalyst – full neutralisation of three components – WITH, CnHm, NOx, – which in the fulfilled gases more than other harmful substances. Also it is reached not only thanks to presence of platinum, rhodium and a palladium. The important role is played by the temperature maintained in borders 300–800 by S.Esli she decreases to 250 With, chemical reactions of neutralisation WITH, CnHm, NOx, despite presence of metals-catalysts, will not occur. And at temperature nearby 900 With begins плавиться and to collapse каталитическая a film.

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 gauge of the oxygen established in an exhaust system the block of management constantly optimises structure mix fuel-air. Depending on the oxygen maintenance in exhaust gases the oxygen gauge induces pressure from 0,1 In (the high maintenance of oxygen, a poor mix) to 0,9 In (the low maintenance of the oxygen, a rich mix). On the basis of this data the engine management block changes time of opening of fuel atomizers and changes a fuel parity in mix fuel-air. A metric parity of a fuel mix at which there is its full combustion and in exhaust gases there are no harmful substances, is 14,7 weight parts of air on 1 part of fuel, fig. the Elements entering into the catalyst and products, leaving of the catalyst see.

The catalyst works effectively at temperatures 300 With – 800 With.

Diesel engines

On cars with diesel engines clearing of exhaust gases is made by not adjustable oxidising catalyst.

Reduction of percentage of oxides of nitrogen in exhaust gases of the diesel engine is reached at the expense of installation on the car of system of repeated burning of a part of the fulfilled gases (EGR). Valve EGR of system of repeated burning of the fulfilled gases is established in a knee of an exhaust system and copes a vacuum regulator. Valve EGR on the heated-up engine directs a part of the fulfilled gases to chambers of combustion of the engine therefore the temperature decreases and thus the percent of oxides of nitrogen in exhaust gases decreases.