Catalytic converters have been an inevitable part of the exhaust system of every new vehicle for the last ten years. The reason lies in the prescribed environmental standards for the composition of the exhaust that vehicles must meet. The reasons for catalyst failure and their causes are explained in more detail in this article. Frequently asked questions about catalyst operation were also answered.
Motor vehicles cause air pollution from the exhaust gases in the exhaust system. The harmful gases are most often caused by incomplete combustion in the cylinder space. In addition to water (H20) and carbon dioxide (CO2), the exhaust contains the following gases:
CO - toxic gas without color and odor
HC - unburned hydrocarbons that contribute to the formation of smog
NOx - nitrogen oxides active in the formation of acid rain
Catalysts eliminate the presence of the listed gases in the exhaust gases, converting them into relatively harmless gases such as water and carbon dioxide. Therefore, we also call them three-stage catalysts.
The use of catalysts is the most effective method of removing harmful gases and their efficiency is about 90%.
The figure shows the dependence of the catalyst performance on the lambda factor.
The catalyst achieves maximum functionality when the engine is running with a fuel-air ratio in the mixture in a stoichiometric ratio of 14,7: 1 (lambda factor = 1) - or with 1 gram of fuel per 14,7 grams of air. Then the ratio of air and fuel is such that each molecule of fuel can be combined with the corresponding molecule of oxygen from the air, ie the possibility of the appearance of unburned gases in the exhaust gases is reduced. It also reduces the possibility of the appearance of excess oxygen molecules that combine with nitrogen from the air at high temperatures and pressures during the combustion process and create harmful nitrogen oxides.
The stoichiometric ratio of air to fuel is maintained by the control circuit in which they participate lambda probe and ECU vehicles.
NOTE: lambda factor during operation of diesel engine and otto engine with direct injection into the cylinder (newer Volkswagen FSI engines) are much larger than one in most work and the catalysts for such engines differ in construction.
Depending on the read lambda factor by lambda probes, the central computer controls the amount of fuel injected into the intake pipe and thus maintains the necessary stoichiometric fuel-air ratio necessary for maximum catalyst performance.
The figure shows the catalytic converter control circuit:
The catalytic converter is always placed closer to the engine in order to reduce the heating time to the required operating temperature (250 - 270 ° C min). Only at these temperatures does the chemical reactions in the active nucleus begin.
Catalysts are distinguished by the material of which the active catalyst core or monolith is made. There are two types of active core catalysts, and according to them catalysts are divided into:
METAL - core of metal coated with an active layer
CERAMIC - core of ceramic coated with an active layer
Metal catalysts are of better quality, more resistant to heat loads, create less resistance to the passage of exhaust gases, but they are also more expensive. The active surface is covered with a thin layer of platinum, rhodium or palladium which act as catalysts in chemical reactions. The monolith is welded to the load-bearing structure and thus more resistant to physical damage. The picture shows a cross section of the honeycomb structure of the metal core.
Ceramic catalysts are less expensive and are found on most smaller vehicles as standard catalytic converters. The monolith is brittle and sensitive to shock and is therefore wrapped in a protective sheath and inserted into the metal housing of the catalyst. The active surface is covered with active elements (AlMg - silicate). The square shape of the core cross section is visible in the figure.
A schematic illustration of the operation of the catalyst and its structure is shown in the following figure:
There are many reasons for catalyst failure. It is extremely important that the reason for canceling the catalytic converter be identified when replacing the catalytic converter in order to avoid an accelerated failure of the replacement catalytic converter.
Physical damage to the catalyst
Strong impacts on the catalyst housing can cause damage to the ceramic monolith. The ceramic monolith can break and, carried by the exhaust gases, become clogged in the rear parts of the exhaust system, thus making it difficult for the engine to operate. Physical damage is the most common cause of catalyst problems today. Metal monoliths are almost completely resistant to this form of damage.
Look for bumps and scratches on the catalyst housing or ringing inside the exhaust caused by broken parts. Also, defective exhaust system or engine mounts cause vibrations that can damage the monolith.
Installation of a new catalyst. Exhaust Bracket Replacement.
Catalyst clogged with lead
To avoid clogging of the catalyst with lead, it is necessary to use unleaded fuels. Namely, the lead creates a thin film on the active surface of the catalyst which prevents the reduction and oxidation processes from occurring. One or two full thin leaded fuels are already sufficient to significantly degrade the catalyst performance.
The level of clogging, or the effect of the catalyst, is proportional to the amount of lead deposited in the catalyst. With the help of special lead detectors, a clogged lead can be identified.
Installation of a new catalyst.
Runtime error Lambda probes
An error in the operation of the lambda probe leads to the formation of a mixture of fuel in which the deviation of air and fuel from the ideal stoichiometric ratio is used, in which the function of the catalyst is maximized. If the mixture is rich in fuel, the aforementioned problem of entering unburned fuel into the exhaust system is repeated, and if the fuel is too poor the vehicle loses its force. In this case, the catalyst does not perform its function satisfactorily and does not pass the eco-test (too high NOx composition in the exhaust).
Defective lambda factor can be determined on the results of eco-test, while defective lambda probe is checked by special apparatus for this purpose.
Replacing the catalytic converter will not affect the solution of this problem. The lambda probe needs to be replaced.
Oil and antifreeze entry into the combustion area
Oil and antifreeze that mistakenly enter the exhaust system can prevent the passage of exhaust gases in the catalyst by creating thick soot deposits inside the catalyst charge itself. Unburned carbon in the form of soot deposited in the catalyst creates the following problems. First, soot deposits prevent the primary function of the catalyst - reducing the amount of harmful gases (HC, CO, NOx) in the exhaust gases, and, secondly, clog the small holes in the catalyst charge and thus block the passage of exhaust gases, creating increased return pressure in the exhaust system. As a result, the engine's thermal loads are increased - thus a shorter service life, and the return of exhaust gases back to the cylinder, which significantly reduces the usefulness of the process that takes place in the cylinder and thus lower engine power with increased fuel consumption. This can be caused by worn piston links, worn valve seals or a faulty cylinder head gasket.
Replacing the catalytic converter is a temporary solution because it is necessary to remove the problem that causes the oil to enter the exhaust system.
Worn spark plugs or spark plug cables
Spark plugs that do not generate a spark or do so at the wrong time are the reason for unburned fuel to enter the exhaust system. Unburned fuel in the exhaust system in the catalyst burns, due to the high temperatures of the process in it, and even more increase the peak temperatures of the process, which affect the melting and decomposition of the catalyst itself. The correctness of spark plugs and spark plug cables should be regularly checked and changed in case of possible malfunctions.
Checking for spark plugs and cables.
Replacing the catalytic converter is a temporary solution, because it is necessary to replace defective spark plugs or cables in order to avoid re-failure of the catalyst.
Excess unburned fuel in the exhaust
The fuel that burns and thus gives engine power, is designed to burn completely in the cylinder. Excess fuel that does not burn in the cylinder enters the exhaust system, and burns when the exhaust gases reach the catalyst, where it is also the highest temperature in the entire exhaust system. This unwanted combustion additionally heats the catalyst to extremely high temperatures which lead to the melting of the ceramic catalyst charge.
Possible reasons for the incorrect ratio of air and gasoline in the fuel mixture can be the following: incorrect spark plug torque, corroded high spark plug cables, faulty lambda probe, faulty fuel injector or faulty engine sensors.
Engine malfunction, loss of power and torque, soot in the exhaust.
The catalytic converter should only be replaced after the primary source of malfunction has been removed.
Over the years of exploitation, catalysts have lost their effectiveness. Various additives of fuels and oils, metal and debris particles in the exhaust, and general fatigue of the catalyst material cause a decrease in operating efficiency and, consequently, an increase in the exhaust gas emissions.
Replacing the catalytic converter is the best solution to the problem, and regular maintenance of vehicles and the use of CAT-friendly additives is recommended as prevention.
How often is it necessary to change the catalytic converter?
Theoretically, the catalytic converter should exceed the vehicle's service life. However, due to the extreme thermal and mechanical loads to which the catalytic converter is exposed on the vehicle, it is necessary to replace the catalytic converter at least once during the vehicle's service life (every 100000 - 150000 km). In some vehicles, catalysts deteriorate much earlier due to ceramic monoliths.
Does the catalyst work during short driving intervals?
The catalyst needs to reach operating temperature in order to begin to perform the predicted chemical processes. Warm-up time is approximately 3 minutes during winter. Approaching the catalytic converter to the engine can further shorten the time required to reach operating temperature.
Why do catalyst exhaust sometimes smell like rotten eggs?
The stench of rotten eggs sometimes occurs due to the formation of hydrogen sulfur in the catalyst from unburnt elements of sulfur from the fuel. It usually occurs due to sudden changes in engine mode. This phenomenon most commonly occurs in newer vehicles and gradually disappears with age.
Can the catalyst be repaired?
A catalyst is an element that is not being repaired but is being changed. An adverse exhaust composition may also be present with the correct catalytic converter and the cause must then be diagnosed and removed.
Does the catalytic converter affect the performance of my vehicle?
No, because the operation of the vehicle is optimized for the construction of an exhaust system to which the standard and indispensable member is the catalyst. The catalytic converter does, however, cause additional subpressure in the exhaust system, but thanks to this the rear exhaust can be of less restrictive construction.
Are there catalysts for diesel vehicles?
Since. Their construction is so different that they are intended primarily for reduction reactions to reduce the amount of NOx, which is considerable in diesel engines. HC and CO emissions from diesel engines are negligible.
Can I use oil and fuel additives in vehicles with catalytic converters?
It is advisable to use additives that are declared as CAT friendly.
How do I know if my vehicle has or has a catalytic converter?
Presence lambda probe indicates that your vehicle has a catalytic converter.
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