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The working principle of automobile oxygen sensor and its failure reason

  • Time of issue:2021-02-25 14:00

The working principle of automobile oxygen sensor and its failure reason

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  • Time of issue:2021-02-25 14:00
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With the development of automotive electronics technology, more and more sensors are applied to automotive control systems. Automobile oxygen sensor is an important part of automobile electronic control system, which can effectively improve engine performance and vehicle economy. Understanding the working principle of the automobile oxygen sensor and the failure reason of the automobile oxygen sensor is of great help to the overall understanding of the automobile control system. This article introduces the working principle of automobile oxygen sensor and the reasons for its failure.

1. Working principle of automobile 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 tube, and coats the inner and outer layers with platinum (Pt). This is the role of oxidation catalyst. When oxygen ions move, it will generate electromotive force, and the magnitude of electromotive force depends on zirconia. The platinum on both sides depends on the oxygen that it comes into contact with, and the outermost layer is covered with a protective shell.

The inner platinum surface is in contact with the atmosphere, so the oxygen concentration is high, and the outer platinum surface is in contact with the exhaust gas, so the oxygen concentration is low. When the mixing ratio is high, the oxygen contained in the exhaust gas is relatively reduced. Therefore, the oxygen level difference between the platinum on both sides of the zirconia is large, and the generated electromotive force is relatively high (nearly 1V); when the mixing ratio is lean , There is more excess oxygen after combustion, and the oxygen drop of the platinum layer on both sides of zirconia is small, so the generated electromotive force is low (near 0V).

The engine control computer can detect the current mixing ratio from the voltage signal. However, the oxygen sensor must be at a high temperature in order to function normally (400℃~900℃). Therefore, when the engine is just started, the oxygen sensor has not yet started to function. It has to wait until it reaches its operating temperature before it starts to generate electromotive force. The oxygen sensors are all improved into heating type, as shown in the figure, that is, the ceramic heater is used to make the sensors reach the normal working state quickly. Therefore, the current model can almost be 30 seconds after the engine starts, the car oxygen sensor is It can provide the correct signal to the computer, and some models can even reach a lower time.

2. Reasons for failure of automobile oxygen sensor

The main reason for the failure of automobile oxygen sensor is the aging and poisoning of the sensing element. 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 lead poisoning, silicon poisoning, and phosphorus poisoning.

2.1 Aging of automobile oxygen sensor

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. There is almost no excess fuel in the exhaust, but the engine has just started (especially cold start) after (or large). When working under load), in order to quickly warm up the engine (or increase the output of the engine), it is necessary to supply enough fuel. The excess fuel in the exhaust will produce a combustion reaction on the surface of the oxygen sensor. On the one hand, it forms carbon particles. This causes the protection of the oxygen sensor surface to peel off. On the other hand, the local surface temperature of the sensing element is too high (over 1000oC) to accelerate the aging of the sensor.

2.2 Lead poisoning

The lead ions in the fuel or lubricant additives chemically react with the platinum electrode of the oxygen sensor, resulting in a decrease in the catalytic performance of the platinum catalyst, which is called lead poisoning. Although unleaded gasoline is used now, the probability of lead poisoning of the oxygen sensor is greatly reduced. However, because the additives of fuel or lubricating oil contain a variety of lead compounds, lead poisoning of oxygen sensors is inevitable.

2.3 Silicon poisoning

The silicon sealant on the engine, silicon resin molding parts, and silicon additives in the castings all have silicon ions. These silicon ions will contaminate the outer electrode of the oxygen sensor, and the silicone rubber used for sealing at the internal terminal of the oxygen sensor will contaminate the inner electrode. The phenomenon that silicon ions react with the platinum electrode of the oxygen sensor to reduce the catalytic performance of the catalyst platinum is called silicon poisoning.

2.4 Phosphorus poisoning

On the surface of the sensor, phosphorus rarely precipitates as pure phosphorus, but as a certain compound. The phenomenon that these phosphides contaminate the oxygen sensor is called phosphorus poisoning. Phosphates are widely used and can be used as lubricants, rust inhibitors and cleaning agents. During the engine running-in period or after the piston ring is worn, the phosphide in the engine lubricating oil additive will escape into the cylinder for combustion and be discharged with the exhaust gas. In the low temperature state, the phosphide precipitates in the state of fine particles and precipitates on the surface of the sensor protection layer, blocking the pores and causing the sensor to be poisoned; in the high temperature state, the phosphide will adhere to the surface of the oxygen sensor and the three-way catalyst and cause it to be contaminated .

Because, the aging and poisoning of oxygen sensor is inevitable. Therefore, the oxygen sensor should be replaced after the car has driven a certain mileage (generally 80,000 km).

Understand the working principle of automobile oxygen sensor, check whether the automobile oxygen sensor is invalid, and replace the failed automobile oxygen sensor in time, which is a guarantee for driving safety and can also reduce automobile pollutant emissions.

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Hubei TianBang is one-stop provider for automotive exhaust solutions, focusing on high-tech R&D and automated production of sensors. Mainly products including: Oxygen sensor, exhaust temperature sensor, nitrogen oxygen sensor, fuel injector, VVT valve, intake pressure sensor, crankshaft position sensor, camshaft position sensor, ABS sensor and so on.

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