A variable mixture control system is a fuel injection system in which the fuel/air ratio can be continuously varied depending on operating conditions and the oxygen content of the exhaust gases.
The mixture formation device receives control commands from the oxygen concentration sensor (lambda probe), which is installed in front of the catalytic converter or in the exhaust manifold and is blown by the exhaust gas flow. The lambda probe is an electrical sensor that records the residual oxygen content in the exhaust gases and produces a corresponding electrical signal. The signal value allows us to draw a conclusion about the composition of the fuel-air mixture. In a fraction of a second, the lambda probe can produce a corresponding signal to the electronic control unit, due to which the mixture composition can be constantly adjusted. On the one hand, this is necessary, since operating conditions are constantly changing (e.g. full throttle, idle), on the other hand, because optimal afterburning occurs in the catalytic converter only if the exhaust gases contain a sufficient number of carbon particles (unburned gasoline).
Thus, in order for afterburning to occur at all in the catalytic converter at a temperature of 300–800°C, the exhaust gases must contain more fuel than is required for complete combustion in the engine cylinders. In this regard, when operating a car equipped with an engine with a catalytic converter, fuel consumption increases by approximately 5%. The catalytic converter is located in the engine compartment of the car in place of the front muffler. The converter consists of a ceramic honeycomb block coated with a noble metal catalyst - platinum or rhodium. An elastic heat-resistant wire mesh is used to attach the impact-sensitive ceramic block.
The catalytic converter used is a so-called three-way catalytic converter. This means that it simultaneously oxidizes carbon monoxide (CO) and hydrocarbons (CH) and reduces nitrogen oxides (NOx).
Operating Rules for Vehicles with a Catalytic Converter in the Exhaust System
To avoid damage to the oxygen concentration sensor (lambda probe) and the catalytic converter, it is imperative to follow the following instructions.
Be sure to use unleaded petrol.
If leaded fuel has been added by mistake, the exhaust manifold and catalytic converter must be replaced. Before installing new exhaust system components, the fuel tank must be completely filled with unleaded gasoline at least twice.
Starting a warm engine by pushing or towing is not allowed. Use an electric cable to start the engine. Unburned fuel, when ignited, can lead to overheating of the catalytic converter and its subsequent destruction.
Frequent cold starts following one another should be avoided. Otherwise, unburned fuel accumulates in the catalytic converter, which burns explosively when heated, damaging the converter.
If you have difficulty starting the engine, do not let the starter run for a long time, as fuel is injected during starting. Find and fix the problem, and then start the engine.
If there are interruptions in the operation of the ignition system, do not allow fuel injection when starting the engine until the cause of the malfunction has been determined.
Do not check for spark with the spark plug cap removed.
You cannot perform a balance test by disconnecting the high-voltage ignition wire of one of the cylinders. When disconnecting the high-voltage ignition wire of an individual cylinder, even with a special tester, unburned fuel will get into the catalytic converter.
If the ignition system malfunctions, avoid running the engine at high crankshaft speed. Eliminate the malfunction as soon as possible.
Do not park the vehicle on dry leaves or grass. The temperature of the exhaust system where the catalytic converter is installed is very high, and heat is emitted even after the engine is turned off.
When adding engine oil, make sure that the maximum oil level on the dipstick is not exceeded. Otherwise, excess oil may enter the catalytic converter and damage the coating or destroy it completely.
Useful information and tips
How to protect the neutralizer
Replacing a failed neutralizer will not be cheap, so it would be good to know in advance what the situation is with neutralizers on the auto parts market and what problems arise during their operation. Until recently, we knew only the following about the neutralizer: it is a thing that is not clear why it is needed, it is not clear how it works, our gasoline "kills" it, in general, only troubles. Cut it out - and no problems! But gradually we began to get used to the fact that the neutralizer is not a useless thing, at least thoughts about "surgical intervention" in the exhaust system visit fewer and fewer people.
In the spring, when the traffic police officers begin the "fight for clean air", another problem falls on us - we need to regulate CO. Owners of cars equipped with neutralizers do not even think about it, and they pass CO checkpoints without trembling in their knees and fear for their wallets. True, the same wallet can "lose weight" for another reason. Fines for exceeding the CO level will seem like a pittance compared to the costs of buying and replacing the neutralizer if it fails. That is why you need to know how to use it, and to do this, you first need to understand how it is designed and how it works.
How does a neutralizer work?
When the fuel-air mixture burns, a number of products harmful to human health are formed - carbon monoxide (CO), various hydrocarbons (CH) and nitrogen oxides (NO), etc. Despite the fact that these substances make up only 1% of the total exhaust volume (the rest is nitrogen, carbon dioxide and water vapor), they are very harmful and require neutralization. There are several ways to combat harmful exhaust components, such as leaning the mixture on which the engine operates or recirculating exhaust gases, but none of them can compare in efficiency with the result of the catalytic converter.
As experts say, a catalytic converter is a simple device in which a complex chemical process takes place. Inside the stainless steel housing is a ceramic or metal "brick" with a honeycomb structure. This monolith has a huge surface area, and all of it is covered with a thin layer of a special alloy - the catalyst itself, containing platinum, rhodium and palladium. These rare metals are responsible for the wonderful properties of the converter, and they also determine its high cost.
The exhaust gases "wash" the surface of the monolith, and when the temperature reaches the "critical" value of +270°C, a catalytic reaction begins. Carbon monoxide turns into dioxide (carbon dioxide), hydrocarbons - into water and again into carbon dioxide, and nitrogen oxides - into water and nitrogen. All this is less harmful to the environment.
Catalytic converters are capable of reducing exhaust toxicity quite effectively, while they have virtually no effect on fuel consumption and engine power. With a catalytic converter, exhaust back pressure increases slightly, causing the engine to lose 2-3 hp, but this is almost the entire "price" for cleaning exhaust gases. However, installing a catalytic converter is not an ideal solution. Theoretically, it should last forever, since the above-mentioned metals serve only as a catalyst, which, as is known, is not consumed during a chemical reaction. In practice, the service life of a catalytic converter has a limit...
Causes of Neutralizer Failure
Catalytic converter failure can occur for several reasons, although it is usually a gradual process that cannot be detected without special equipment.
The "core" of most catalytic converters is made of ceramics, a material known for its fragility. A car can hit a pothole at speed, hit something, or even just "scratch" the catalytic converter body on a stone, which can cause the catalytic "brick" to crack. After that, it's only a matter of time before the "core" loses its working qualities.
New generation neutralizers containing a metal monolith are not so vulnerable in this regard. Of course, it is possible to break them, but in any case it is not so easy.
In addition to physical destruction, there is another common reason for the neutralizer to fail - fuel. It is extremely sensitive to the fuel composition. If the gasoline is leaded, then the tetraethyl lead contained in it is deposited on the active surface of the catalytic "brick" and quickly "contaminates" it, which is why all reactions stop. It seems that at gas stations they put hose tips of different sizes, and they paint the dispensers in different colors, and they write about it on every corner, but still consumers sometimes get confused and fill up with the wrong gasoline. But it is enough to "burn" half a tank of such gasoline to "kill" the neutralizer.
But it is not only leaded gasoline that is the enemy of the neutralizer. It can be ruined by unleaded gasoline, if the engine management system is faulty, the fuel-air mixture does not burn completely, or the engine is heavily worn out.
Triple catalytic converters ("triple" because the catalyst is a combination of three metals) are installed only on those cars whose engines are equipped with a closed exhaust control system. An oxygen concentration sensor is installed in front of the converter, which evaluates the composition of the exhaust gases and transmits this data to the central unit of the electronic engine control system. Depending on the oxygen content in the gases, the central unit regulates the composition of the combustible mixture and ignition so that their optimal values are maintained. This serves as the main protection of the converter and ensures fuel economy, high engine efficiency. The converter does not tolerate large deviations in one direction or another in the mixture. A poorly adjusted engine with an increased content of hydrocarbons in the exhaust simply ruins the converter. Too lean a mixture can cause a sharp overheating of the converter, which will again damage the monolith, only this time "physically". Thus, the "life" of the converter depends on the serviceability of the engine control system.
Much also depends on the serviceability of the oxygen sensor itself. With "age" it becomes "lazy" or completely fails, which affects the composition of the working mixture and, accordingly, the serviceability of the neutralizer.
The neutralizer can also be damaged by the exhaust of a heavily worn engine burning oil. The latter, getting into the neutralizer together with the exhaust, is "baked" on the surface of the monolith, like varnish, and does not allow the neutralizer to work.
There are other harmful factors. For example, spark plugs. Unsuitable spark plugs will not provide complete combustion, which can cause a destructive melting reaction in the neutralizer.
Be careful when using gasoline or motor oil additives. Most drivers don't think about it, but additives can also have a harmful effect on the catalytic converter. If the product doesn't say "compatible with catalytic converter", it's better not to risk it.
Another dangerous case is starting the engine by towing. In this case, pure gasoline may enter the neutralizer. This poisons the neutralizer and can also cause an immediate reaction and even an explosion.
Also, watch where you are driving, try not to get into deep puddles. The operating temperature of the neutralizer is about +900°C. Its sudden entry into water can be fatal.
In general, it has been noted that the service life of the neutralizer is also affected by operating conditions. Neutralizers suffer more in cars operated in urban mode, when the engine is often started and stopped. However, during long-term driving at high speed outside the city, the neutralizer also suffers, but from overheating.
Finally, you would be wise to inspect your exhaust system regularly. If the brackets are broken or the rubber mounts on the mufflers have fallen off, the exhaust pipe will vibrate, putting unnecessary stress on the converter.