Device
The exhaust gas catalytic converter has a stainless steel casing 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 case is occupied by a ceramic porous monolith, fixed with stainless steel rings or mesh. The structure of the monolith is similar to an ordinary filter, but the entire inner and outer surface of the pores in contact with gases is covered with a very thin molecular layer of an alloy that contains 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 neutralizers.
Principle of operation
The engine emits into the atmosphere together with exhaust gases products of complete (water vapor H2Oh nitrogen N2 and so on.) and incomplete (carbon monoxide CO and also CnHm, nitrogen oxides NOx) fuel combustion. The total number of components contained in these gases exceeds several hundred, and most of them are harmful to human health.
Exhaust gases, penetrating through the porous surface of the monolith, firstly, heat it, and secondly, are additionally oxidized. CO is made from CO2, i.e. non-toxic carbon dioxide, CnHm passes into CO in several stages2 and H2Oh nox turns into molecular N2, which is contained in ordinary air, and in water. In a word, quite complex chemical reactions take place in the converter due to the high temperature and the presence of a special coating of expensive metals that serve as catalysts.
So, the main positive effect of this device is the complete neutralization of the three components - CO, CnHm, NOx - which are more in the exhaust gases than other harmful substances. And it is achieved not only due to the presence of platinum, rhodium and palladium. An important role is played, as I have already said, by the temperature maintained within the limits of 300-800°C. If it decreases to 250°С, chemical reactions of neutralization of CO, CnHm, NOx, despite the presence of catalyst metals, will not occur. And at a temperature of about 900°C, the catalytic film begins to melt and break down.
Exploitation
In order not to disable a serviceable converter, you must strictly adhere to the rules for its operation. First of all, leaded gasoline should not be used. It is enough to spend 20-30 liters of this gasoline - and the active surface of the catalyst will be covered with lead, which will lead to breakdown.
It is necessary to constantly and carefully monitor the technical condition of the engine. The converter does not withstand significant deviations in the composition of the fuel mixture. When the engine is running on rich and very rich mixtures, it quickly becomes clogged with soot and other deposits, and on poor and very poor mixtures, it overheats until the active layer melts. To maintain the composition of the mixture within the required limits, on vehicles with converters, an oxygen sensor - a lambda probe - is required to be installed - at the signal of which the electronic control unit measures the composition of the fuel mixture. Serviceability and correct operation of the control system largely determine the durability of the converter. Therefore, in case of any malfunctions in the operation of the engine due to the ignition system or difficult starting, it is necessary to find and eliminate the malfunction.
It is forbidden to start the engine from a tow, to turn it for a long time with a starter, or to try to start it cold often at short intervals. In addition, in order to prevent the appearance of cracks, you should avoid hitting road obstacles. The cause of mechanical damage due to a sharp temperature drop can be a car falling into a deep pothole with water and a sharp cooling of the converter heated to 800-900°C.
It should be noted that recently they began to produce catalytic converters with a metal monolith, which increases their strength. They are much more reliable due to their greater resistance to various mechanical and thermal influences, but therefore they are more expensive than their ceramic counterparts.
As already noted, the converter with a ceramic monolith is very sensitive to shock, so you can not drop it or use an impact tool when working with the system. And to avoid the deposition of fats, it is forbidden to touch the working surface of the oxygen sensor with your hands. You can not check the presence of a spark by removing the tip from any spark plug or use non-standard spark plugs, as well as check the efficiency of the cylinders by turning off the spark with a motor tester. These actions can cause unburned fuel to enter the catalyst, the consequences of which are discussed above.
Advantages and disadvantages
The first and main advantage of this device is the almost complete neutralization of the most harmful (taking into account their total concentration and toxicity) exhaust gas components. The second plus is that automakers, without additional costs for improving serial engines, by installing a converter and fuel-supply control systems with a lambda probe, can continue to produce cars that fully meet stringent exhaust gas toxicity standards, for example, Euro-2 and even Euro 3.
But the neutralizer also has disadvantages. One of its significant drawbacks is low reliability, and not only because of the fragility of the ceramic monolith, but also due to failure due to violation of the rules of operation, and even when these rules are not violated. According to surveys, catalysts break down faster in city driving conditions with frequent engine starts and short runs. And also - with stable high-speed traffic on highways. In the first case, the converter is simply clogged with fuel and soot, because at each start it must warm up to operating temperature, and this takes time. In the second - from overheating.
We have a rare car that will work for a long time with a neutralization system, given the condition of the roads, the availability of unleaded gasoline and the quality of this gasoline, as well as the professional and environmental awareness of drivers and repairmen. This one is expensive but not "long-playing" the system operates in the car until the first problems appear. And then, as a rule, the monolith is knocked out of the body and simply thrown away.
The logic of similar "measures" understandable: firstly, it is much cheaper, faster and easier than looking for, negotiating and waiting for a new neutralizer to be brought; secondly, these actions will not entail any negative consequences for the car owner, because almost all foreign cars with a working gasoline injection system fit into the current GOST (which, unfortunately, cannot be said about many CIS cars).
This is how we do it, but how "them", where the toxicity standards are more stringent, and gas analytical control is established, and the people have money, and the roads are in order, and the service is up to par? There, if the converter is out of order, it is simply replaced in order to avoid too high fines for increased toxicity. By the way, replacement is also not always justified, because not every service station has a complete set of necessary equipment and relevant parts. Consumer dissatisfaction arising from this increases the demand for cars without converters. These are cars with gas, diesel and gas-diesel engines, as well as with direct injection of gasoline, and others that fit into Euro-3 standards without converters.
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