Device
The catalytic converter of exhaust gases has a stainless steel housing, to which the inlet and outlet cones with pipes and fastening flanges, as well as heat reflectors holding the ceramic lining, are welded. 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 similar to a conventional 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 the converters.
Operating principle
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, are oxidized. From CO we get CO ₂, that is, non-toxic carbon dioxide, CnHm in several stages it turns into CO ₂ and H ₂ O, NOx is converted into molecular N ₂, which is contained in ordinary air, and into water. In short, rather complex chemical reactions occur in the neutralizer 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 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. An important role is played, as I have already said, by the temperature, which is maintained within the limits of 300-800°C. 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 break down.
Operation
In order not to damage a serviceable neutralizer, you must strictly adhere to the rules for its operation. First of all, you cannot use leaded gasoline. 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 a breakdown.
It is necessary to constantly and carefully monitor the technical condition of the engine. The neutralizer 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 lean and very lean mixtures, it overheats until the active layer melts. To maintain the mixture composition within the required limits, cars with neutralizers must be equipped with an oxygen sensor - a lambda probe - by the signal of which the electronic control unit measures the composition of the fuel mixture. The serviceability and correct operation of the control system largely determine the durability of the neutralizer. Therefore, if there are any malfunctions in the engine due to the ignition system or difficult starting, you need to find and fix the malfunction.
It is prohibited to tow the engine, crank it for a long time with the starter or try to start it unheated with short intervals. In addition, to prevent cracks from appearing, avoid hitting road obstacles. Mechanical damage due to a sharp temperature drop can be caused by the car falling into a deep pothole with water and a sharp cooling of the neutralizer heated to 800-900°C.
It should be noted that recently they have started producing catalytic converters with a metal monolith, which increases their strength. They are much more reliable due to their greater resistance to various mechanical and temperature effects, but that is why they are more expensive than ceramic analogues.
As already noted, the ceramic monolith neutralizer is very sensitive to impacts, so it must not be dropped or an impact tool must not be used when working with the system. And to avoid fat deposits, it is prohibited to touch the working surface of the oxygen sensor with your hands. You must not check for a spark by removing the tip from any spark plug or use non-standard spark plugs, or 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.
(Material was taken from a resource opelbook.ru)
Pros and cons
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) components of exhaust gases. The second advantage is that car manufacturers, without additional costs for upgrading serial engines, by installing a neutralizer and fuel supply control systems with a lambda probe, can continue to produce cars that fully comply with strict exhaust gas toxicity standards, such as Euro-2 and even Euro-3.
But the neutralizer also has its downsides. One of its significant drawbacks is low reliability, not only because of the fragility of the ceramic monolith, but also because of failure due to violation of operating rules, and even when these rules are not violated. According to surveys, catalytic converters break down faster in city driving conditions with frequent engine starts and short runs. And also - with stable high-speed movement on highways. In the first case, the neutralizer simply gets clogged with fuel and soot, because with each start it must warm up to operating temperature, and this takes time. In the second - from overheating.
It is rare for a car to work for a long time with a neutralization system, given the state 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 expensive, but not "long-lasting" system operates in the car until the first problems appear. And then, as a rule, the monolith is knocked out of the body and is simply thrown away.
The logic of such "measures" is clear: firstly, it is much cheaper, faster and easier than searching, negotiating and waiting for a new neutralizer to be delivered; secondly, these actions will not entail any negative consequences for the car owner, because almost all foreign cars with a working gasoline injection system comply with the current GOST (which, unfortunately, cannot be said about many CIS cars).
This is how we do it, but what about "theirs", where toxicity standards are stricter, gas analysis control is well-established, and people have money, and the roads are in order, and the service is top-notch? There, if the neutralizer fails, it is simply replaced to avoid too high fines for increased toxicity. By the way, they do not always replace it justifiably, because not every service station has a full set of the necessary equipment and the corresponding parts. The dissatisfaction of consumers that arises in connection with this increases the demand for cars without neutralizers. These are cars with gas, diesel and gas-diesel engines, as well as with direct gasoline injection, and others that meet Euro-3 standards without neutralizers.
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