Device
The catalytic converter of exhaust gases has a housing made of stainless steel, 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 body is occupied by a ceramic porous monolith, fixed with rings or a mesh made of stainless steel. 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 is what mainly determines the high price of neutralizers.
Operating principle
The engine emits into the atmosphere together with the exhaust gases the products of complete combustion (water vapor H₂ O, nitrogen N₂, etc.) and incomplete (carbon monoxide CO, as well as 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.
The exhaust gases, penetrating through the porous surface of the monolith, firstly heat it up, and secondly, oxidize. CO is converted into CO₂, i.e. non-toxic carbon dioxide, CnHm in several stages turns into CO₂ and H₂ O, NOx turns into molecular N₂, 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 this 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 decreases 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 use 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 neutralizer cannot 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 to the point of melting the active layer. To maintain the mixture composition within the required limits, cars with neutralizers are required to have an oxygen sensor - a lambda probe - installed, using the signal from 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 problem.
It is prohibited to start the engine by towing, to crank it for a long time with the starter, or to try to start it when it is not warmed up frequently with short intervals. In addition, to prevent cracks from appearing, you should avoid hitting road obstacles. Mechanical damage due to a sudden temperature change can be caused by the car falling into a deep pothole with water and the sudden cooling of the neutralizer heated to 800-900°C.
It should be noted that recently they have begun 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 temperature influences, but that is why they are also more expensive than their ceramic counterparts.
As already noted, the ceramic monolith neutralizer is very sensitive to impacts, so it cannot be dropped or impact tools used when working with the system. To avoid fat deposits, it is forbidden to touch the working surface of the oxygen sensor with your hands. You cannot check for a spark by removing the tip of any spark plug or using 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 catalytic converter, the consequences of which are described above.
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, without additional costs for upgrading production engines, by installing a neutralizer and fuel management systems with a lambda probe, car manufacturers 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 due to the fragility of the ceramic monolith, but also due to 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 mileage. And also - during stable high-speed movement on highways. In the first case, the neutralizer simply becomes 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.
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).
That's how we do it, but what about "theirs", where toxicity standards are stricter, and gas analysis control is well-established, and people have money, and roads are in order, and service is top-notch? There, if the neutralizer fails, it is simply replaced to avoid excessively high fines for increased toxicity. By the way, they also do not always replace them justifiably, because not every service station has a full set of necessary equipment and corresponding parts. The resulting consumer dissatisfaction is driving up demand for cars without catalytic converters. These are cars with gas, diesel and gas-diesel engines, as well as with direct petrol injection, and others that meet Euro-3 standards without neutralizers.
Article source on the website hondabook