First, the role of oxygen sensors
In the 1990s, automobile emissions pollution has become a hot topic of concern. With the gradual regulation of China's automobile emission regulations and the importance of society's control of vehicle emission pollutants, EFI engines have become popular in our country. After nearly two decades of development, EFI engine technology has become increasingly mature, and vehicle emissions pollution has also been gradually controlled, which is an important part of the engine - oxygen sensor inseparable.
Specifically, the EFI engine uses a mixture of closed-loop control and three-way catalytic reactor combination of the use of technology, which is so far the most effective way to purify the exhaust gas. The three-way catalytic converter can effectively purify three kinds of harmful gases such as CO, HC and NOx, but its purification efficiency is strictly dependent on the concentration of the mixture must be kept within the narrow range near the theoretical air-fuel ratio (14.7: 1). Once the concentration of the mixed gas deviates from this range, the ability of the three-way catalytic converter to purify the exhaust pollutants is drastically reduced. It is because the change of the air-fuel ratio causes a corresponding change in the oxygen concentration in the exhaust gas, so that an oxygen sensor is provided in the exhaust pipe.
Oxygen sensor at any time to detect the oxygen concentration in the exhaust, and at any time to the car's electronic control unit feedback signal. The electronic control unit adjusts the fuel injection quantity according to the feedback signal promptly, if the signal reflects the thickening of the mixed gas, reduces the fuel injection time; on the other hand, increases the fuel injection time, so that the air-fuel ratio of the mixed gas is always kept near the theoretical air-fuel ratio. This is the so-called engine closed-loop control.
Oxygen sensor is an essential component to achieve this closed-loop control, which plays an indispensable role in engine emission control. Modern EFI engines are typically equipped with two oxygen sensors, three-way catalytic converter efficiency monitoring, must be used in the ternary catalytic converter rear of the second oxygen sensor. When the three-way catalytic converter is working normally, the number of variations of the oxygen sensor located in front of the ternary catalyst should be higher than that of the rear oxygen sensor. The monitor compares the number of times of the front / rear oxygen sensor to determine the aging of the three-way catalytic converter no.
Second, the type of oxygen sensor and working principle
The current use of oxygen sensors are zirconia and titanium oxide two, which is the most widely used zirconia oxygen sensor.
1, zirconia oxygen sensor
Zirconia oxygen sensor is the basic component of zirconia ceramic tube, also known as zirconium tube, as shown in Figure 2. The wrong tube is fixed in a fixed sleeve with a mounting thread. The inner and outer surfaces are covered with a porous lead film whose inner surface is in contact with the atmosphere and the outer surface is in contact with the exhaust gas. The terminal of the oxygen sensor has a metal sheath with a hole for the inner cavity of the misalignment of the cavity and the atmosphere; the wire leads the inner surface of the misalignment of the inner surface of the lead tube from the terminal.
Zirconium in the temperature exceeds 300 ℃ before the normal work. Most of the cars use oxygen sensors with heaters, the sensor has an electric heating element, the engine can be started within 10-30s after the oxygen sensor quickly heated to the operating temperature.
2, titanium oxide oxygen sensor
Titanium oxide oxygen sensor is the use of titanium dioxide material with the oxygen content of the exhaust gas changes in the characteristics of the change made, it is also known as resistance type oxygen sensor.
The shape of the titanium dioxide oxygen sensor is similar to that of the zirconia oxygen sensor, and is a titanium dioxide thick film element in the front cover of the sensor. Pure titanium dioxide at room temperature is a high resistance of the semiconductor, but the surface once the lack of oxygen, its character will be defects, resistance decreases. Since the resistance of the titanium dioxide varies with the temperature, there is also an electric heater inside the titania type oxygen sensor to keep the temperature of the oxidized oxygen sensor constant during the engine operation.