With the development of automotive electronics technology, more and more sensors are being applied to automotive control systems. Automotive oxygen sensors are an important part of automotive electronic control systems that can effectively improve engine performance and overall vehicle economy. Understanding the working principle of the car's oxygen sensor and the cause of the failure of the car's oxygen sensor is of great help to the overall control of the car's control system. This article describes the working principle of automotive oxygen sensors and the reasons for their failure.
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1, the working principle of automotive oxygen sensor
Zirconium oxide (ZrO2) is a kind of solid electrolyte. It has a characteristic that oxygen ions are easy to move at high temperatures. This type of oxygen sensor sinters zirconia into a tubular shape and is coated with platinum (Pt) with the inner and outer layers. This is the action of an oxidizing catalyst. When the oxygen ions move, an electromotive force is generated, and the electromotive force is determined by zirconia. The platinum on both sides is exposed to oxygen, and the outermost layer is covered with a protective shell.
The inner layer of the platinum surface is in contact with the atmosphere, so the oxygen concentration is high, the outer layer of platinum is in contact with the exhaust gas, and the oxygen concentration is low. When the mixing is relatively high, the oxygen contained in the exhaust gas is relatively reduced. Therefore, the difference in oxygen contact between the platinum on both sides of the zirconia is large, and the electromotive force generated is relatively high (nearly 1 V); when the mixing ratio is lean After the combustion, the excess oxygen is more, and the platinum layer on both sides of the zirconia has a small oxygen drop, so the electromotive force generated is low (nearly 0V).
The engine control computer can detect the current mixing ratio by the voltage signal. However, the oxygen sensor must be used at a high temperature (400 ° C ~ 900 ° C), so when the engine is just starting, the oxygen sensor has not yet started to function, and it is necessary to wait until the temperature of the working temperature to start generating electromotive force, so after The oxygen sensor is modified into a heating type, as shown in the figure, that is, the ceramic heater is used to make the sensor quickly reach the normal working state, so the current model can be almost 30 seconds after the engine is started, the car oxygen sensor is It can supply the correct signal to the computer, and some models can even achieve lower time.
2, the cause of failure of automotive oxygen sensors
The main cause of automotive oxygen sensor failure is sensor element aging and poisoning. The main cause of aging of the oxygen sensor is that the surface temperature of the sensing element is too high. The phenomenon that the sensing element of the oxygen sensor is contaminated and fails is called poisoning. Oxygen sensor poisoning mainly refers to lead poisoning, silicon poisoning, and phosphorus poisoning.
2.1 Automotive oxygen sensor aging
In the process of closed-loop control of the engine using the 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, but after the engine is just started (especially after cold start) (or large In the load state operation), in order to quickly preheat the engine (or increase the engine output power), it is necessary to supply enough fuel, and the excess fuel in the exhaust gas will generate a combustion reaction on the surface of the oxygen sensor, on the one hand, carbon particles are formed. The protection of the surface of the oxygen sensor is peeled off, and on the other hand, the surface temperature of the sensing element is too high (more than 1000oC) to accelerate the aging of the sensor.
2.2 Lead poisoning
The lead ion in the fuel or lubricating oil additive chemically reacts 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, lead poisoning of oxygen sensors is inevitable due to the presence of various lead compounds in fuel or lubricant additives.
2.3 Silicon poisoning
Silicone sealants on the engine, silicone molded parts, silicon additives in the castings, etc. all have silicon ions, which contaminate the outer electrodes of the oxygen sensor, and the silicone rubber used for sealing the internal terminals of the oxygen sensor contaminates the inner electrodes. The phenomenon that the silicon ion reacts with the platinum electrode of the oxygen sensor to cause a decrease in the catalytic performance of the catalyst platinum is called silicon poisoning.
2.4 Phosphorus poisoning
On the surface of the sensor, phosphorus is rarely precipitated in a pure phosphorus state, but is precipitated in a certain compound state. These phosphides contaminate the oxygen sensor, which is called phosphorus poisoning. Phosphites are widely used as lubricants, rust inhibitors and cleaning agents. During engine running-in or after the piston ring wears, the phosphide in the engine oil additive will break into the cylinder and burn out with the exhaust. In the low temperature state, the phosphide precipitates in the state of microparticles and precipitates on the surface of the sensor protective layer to block the pores and cause poisoning of the sensor; at high temperature, the phosphide will adhere to the surface of the oxygen sensor and the three-way catalyst to cause contamination. .
Because of the aging and poisoning of oxygen sensors is inevitable. Therefore, when the car travels a certain distance (usually 80000Km), the oxygen sensor should be replaced.
Understand the working principle of the car oxygen sensor, often check whether the car oxygen sensor is invalid, and replace the failed car oxygen sensor in time, which is a guarantee for driving safety, and can also reduce vehicle pollutant emissions.
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