Oxygen sensor failure is a problem that often occurs in environmentally friendly engines. The cause of oxygen sensor failure has a lot to do with China's fuel, lubricants and road conditions. There are two main manifestations of automobile oxygen sensor failure: sensor element aging and poisoning. The main reason for the aging of the oxygen sensor is that the local surface temperature of the sensing element is too high. The phenomenon that the sensing element of the oxygen sensor becomes contaminated and fails is called poisoning. Oxygen sensor poisoning mainly refers to carbon monoxide poisoning, sulfur poisoning, lead poisoning, silicon poisoning, manganese lead poisoning, and phosphorus poisoning.
Oxygen sensor aging
During the closed-loop control of the engine using an oxygen sensor, the air-fuel ratio of the mixture is always controlled near the theoretical air-fuel ratio, and there is almost no excess fuel in the exhaust gas. The excess fuel in the exhaust gas will generate a combustion reaction on the surface of the oxygen sensor. On the one hand, the formation of carbon particles causes the protection of the surface of the oxygen sensor to peel off, and on the other hand, the local surface temperature of the sensor element is too high to accelerate. Sensor is aging.
poisoned by carbon monoxide
Because the mixed gas is too rich for a long time, the combustion is incomplete, and the carbon monoxide poisoning occurs in the oxygen sensor when there is too much carbon monoxide in the exhaust gas. Because platinum metal not only has an oxidation catalysis function, but also has a reduction catalysis function. Under long-term hypoxia conditions, adsorption Carbon monoxide on the platinum metal will be reduced to resolve to carbon, and carbon deposits will be formed on the platinum metal surface. When the carbon deposits cover more, the specific surface area of the platinum metal catalyst will be reduced and the activity will be reduced, resulting in that the oxygen sensor can no longer correctly feedback oxygen. Content, thereby poisoning loses effect.
The sulfur in the fuel is burned in the form of sulfur dioxide in the exhaust. The platinum metal catalyst active center has a strong affinity for sulfur dioxide. The sulfur dioxide is adsorbed by the active center and oxidized to become sulfur trioxide. The sulfur trioxide and the exhaust gas Sulfuric acid is combined with water to form sulfuric acid. Sulfuric acid reacts with metal oxides to form sulfates. It reacts with certain compounds in unburned fuels to form sulfuric acid colloids. Finally, sulfur chemical complexes are adsorbed on the active surface of platinum metal catalysts, preventing The catalyst adsorbs oxygen and carbon monoxide, weakens the sensitivity of the oxygen sensor to changes in air-fuel ratio, loses the function of storing and releasing oxygen, and causes the poisoning of the oxygen sensor. The effect of phosphorus and silicon in the lubricant on the oxygen sensor and sulfur Basically similar.
Lead ions in fuel or lubricating oil additives chemically react with the platinum electrode of the oxygen sensor, resulting in a decrease in the catalytic performance of the catalyst platinum, which is called lead poisoning. Although unleaded gasoline is now used, the chance of lead poisoning by oxygen sensors is greatly reduced. However, due to various lead compounds in fuel or lubricating oil additives, lead poisoning of oxygen sensors is also inevitable.
Silicon ions on the engine, silicone resin molded parts, and silicon additives in the casting have silicon ions. These silicon ions will contaminate the outer electrodes of the oxygen sensor, and the silicon rubber used for sealing the internal terminals of the oxygen sensor will contaminate the inner electrodes. The phenomenon that the silicon ion reacts with the platinum electrode of the oxygen sensor to reduce the catalytic performance of the catalyst platinum is called silicon poisoning.
Manganese and lead poisoning
Metals such as manganese and lead in fuels are usually deposited on the active surface of platinum metal catalysts in the form of metal salts and metal oxides. They have a strong affinity with the active center and will not decompose after forming a complex to make platinum metal. The catalyst's activity and selectivity decrease rapidly and cause poisoning. In particular, manganese salts and manganese oxides have the function of storing oxygen. Its deposition on the active surface of platinum metal catalysts will cause false alarms by the oxygen sensor and prevent the oxygen sensor from correctly feeding back the air-fuel ratio. .
On the sensor surface, phosphorus is rarely precipitated in the state of pure phosphorus, but is precipitated in the state of some compound. The phenomenon that these phosphides contaminate the oxygen sensor is called phosphorus poisoning. Phosphides are widely used as lubricants, rust inhibitors and cleaning agents. During engine running-in or after the piston ring wears, the phosphides in the engine lubricating oil additives will enter the cylinder to burn and discharge with the exhaust. At low temperature, the phosphide is precipitated in the form of particles and precipitates on the surface of the protective layer of the sensor to block the pores and cause the sensor to be poisoned. At high temperature, the phosphide will adhere to the surface of the oxygen sensor and the three-way catalyst to make it polluted .
Ageing and poisoning of oxygen sensors are inevitable. Therefore, the oxygen sensor should be replaced after the car travels a certain distance. Usually, you should always check whether the oxygen sensor of the car has failed and replace it in time if problems are found. This is also a guarantee for driving safety, and it can also reduce fuel consumption and reduce environmental pollution.