A variable carburetor is a carburetor or fuel injection system in which the ratio of fuel to air can constantly change depending on the driving conditions and the oxygen content in the exhaust gases. This cannot be done with a traditional carburetor, as it is not equipped with the appropriate control unit. For this reason, they return to either an electronically controlled carburetor or a fuel injection system.
The carburetor receives control commands from an oxygen sensor, which is installed in the exhaust pipe before the catalytic converter or in the exhaust manifold and is washed here by the exhaust gas flow. The oxygen sensor is a sensitive element and signals the residual oxygen content in the exhaust gases through voltage fluctuations and contributes to the issuance of a conclusion on the composition of the air-fuel mixture. In a fraction of a second, the oxygen sensor can transmit the appropriate electrical signals to the carburetor control unit and, thanks to this, the fuel-air ratio is constantly changing. On the one hand, this is required, since in any case, operating conditions are constantly changing (idle, full throttle), and, on the other hand, also because only in this case afterburning occurs on the catalyst, since there is still a sufficient proportion of gasoline in the exhaust gases of the engine.
Therefore, in order for afterburning to occur on the catalyst at temperatures between 300 and 800°C, the combustible mixture must have a higher percentage of fuel than would be required for clean combustion. Thus, when using a catalyst, it is necessary to reckon with increased fuel consumption, reaching from one to five percent.
The catalyst is installed under the base of the car body instead of an additional muffler. The catalyst consists of a porous ceramic monolith coated with a layer of platinum or rhodium. An elastic and heat-resistant metal fabric is used to install the shock-sensitive ceramic body.
In this case, we are talking about the so-called three-functional catalyst. This means that on this catalyst, based on the data received from the oxygen sensor, the oxidation of carbon monoxide is carried out (SO) and hydrocarbons (NS) to CO2 and H2O, as well as the simultaneous reduction of nitrogen oxides (NOx) to nitrogen.
Caring for vehicles equipped with a catalytic converter
To avoid damage to the oxygen sensor and catalyst, be sure to follow these guidelines:
- As a matter of principle, fill your vehicle with unleaded petrol only. If you mistakenly filled up with leaded gasoline, you must replace the exhaust pipe in front of the catalytic converter, as well as the catalytic converter. Before installing new catalyst parts, at least two fuel tanks filled with unleaded gasoline must be driven out. Starting the engine by pushing or towing can only be done when the engine starts immediately. Unburned fuel, when ignited, can lead to overheating of the catalyst and thus to its destruction. Use an auxiliary battery starter.
- Avoid frequent, consecutive starts of a cold engine. In this case, unburned fuel is collected on the catalyst, which, upon subsequent heating, suddenly ignites, and the catalyst is damaged.
- If you have difficulty starting the engine, do not press the starter for a long time. During starting, fuel is continuously injected. Determine and eliminate the cause of the malfunction.
- Only use approved Mercedes spark plugs.
- Do not test for spark at spark plug wire ends.
- If intermittent sparking is observed, avoid high engine speeds and repair the defect immediately.
- Do not park over dried leaves or grass. The exhaust system becomes very hot in the catalyst area and radiates residual heat even after the engine has stopped.
- When filling or topping up engine oil, pay special attention to ensure that the maximum mark on the oil level indicator is not exceeded in any case. Otherwise, excess oil, due to incomplete combustion, gets on the catalyst and can damage the noble metal or completely destroy the catalyst.