Brief Analysis of Wide Range Oxygen Sensor for EFI Vehicles

1. Defects of ordinary oxygen sensors

Ordinary oxygen sensors generally have 4 wires, 2 of which are heating wires, the third is a signal wire, and the other is a ground wire. It is a zirconium dioxide coating attached to both sides of the ceramic body, which can conduct oxygen ions at a temperature of 350°C or higher. The concentration difference of oxygen on both sides of the sensor causes a potential difference between the two surfaces, and the working curve is very steep. When the mixture is close to the theoretical air-fuel ratio, it outputs a voltage of 0.45V. When the exhaust gas is slightly rich, the output voltage suddenly changes to 0.6V~0.9V; on the contrary, when the exhaust gas becomes thinner, the output voltage suddenly changes to 0.3V~0.1V (Figure 1). Let's analyze: If the exhaust gas is further enriched, will the voltage of the oxygen sensor increase again? The output voltage of 0.9V has been capped. In addition, if the exhaust gas becomes thinner, will the voltage of the oxygen sensor decrease again? The output voltage of 0.1V is already the bottom. From the above analysis, it can be seen that ordinary oxygen sensors cannot measure excessively rich and thin exhaust gas. The two-state voltage signal of 0.1V to 0.9V is no longer sufficient for controlling automobile emissions. It can only detect the oxygen content of the exhaust gas within a relatively narrow range after the mixture is burned at a theoretical air-fuel ratio of 14.7:1, so this is the defect of ordinary oxygen sensors.

2. Structure and operation of wide-range oxygen sensor

In order to overcome the defects of ordinary oxygen sensors, a new generation of wide-range oxygen sensors was born. The structure of the wide-range oxygen sensor is shown in Figure 2. It consists of an ordinary narrow-range concentration voltage type oxygen sensor (zirconium oxide reference cell , a limit current type oxygen sensor, and an aluminum oxide pump cell) and a diffusion hole and a diffusion chamber. It requires a specially designed sensor controller to control its normal operation. In Figure 2, the sensor controller is represented by A and B. The exhaust gas enters the diffusion chamber through the diffusion hole. The exhaust gas may be a rich mixture rich in oil or a lean mixture rich in oxygen. After the aluminum oxide reference cell senses the concentration of the exhaust gas, it generates a voltage Us. Depending on the concentration of the exhaust gas, the rich mixture rich in oil will generate a Us higher than the reference voltage UsRef. The sensor controller will generate a pump current Ip in one direction. The pump current Ip pumps oxygen into the diffusion chamber for chemical decomposition reaction, producing water and carbon monoxide and some oxides in the exhaust gas, which are attached to the surface of the pump oxygen element. In the chemical reaction, excessive hydrocarbons are decomposed, thereby reducing the concentration of exhaust gas and restoring the diffusion chamber to the equilibrium state of exhaust oxygen concentration with a Us voltage of 0.45V. On the contrary, the oxygen-rich lean mixture will produce a Us lower than the reference voltage UsRef, and the sensor controller will generate a reverse pump current Ip, which pumps oxygen out of the diffusion chamber. When the HC fuel or oxygen is neutralized, the voltage Us generated by the reference battery is equal to the reference voltage UsRef. At this time, the pump current IP reflects the concentration of the exhaust gas. The sensor controller converts the pump current IP into an output voltage Uout. By changing the polarity (current flow direction) and size of the pump current, the oxygen content of the exhaust gas in the diffusion chamber can be balanced. How to control this changing pump current again The adjustment of the injection time of the injector by the engine ECU is crucial. There is a DSP (digital signal processor) circuit in the control loop. The circuit has two outputs. One converts the changing pump current signal into a linear voltage by amplifying the analog-to-digital conversion. This voltage changes continuously from 0V to 5V to control the air-fuel ratio adjustment of the engine ECU. The other outputs a pulse width modulation signal to control the on and off time of the COM field effect switch transistor, providing current to the heater to heat the oxygen sensor.

From Figure 3 we can see the characteristics of the wide-range oxygen sensor. The working curve is smooth and can continuously detect the air-fuel ratio from 10 to 20, which is equivalent to a wide range of excess air coefficient from 0.686 to 1.405. When the linear voltage is 2.5V, the control of the theoretical air-fuel ratio of 14.7 is achieved.

When testing a wide-range oxygen sensor, you cannot use a multimeter voltage range or an oscilloscope to directly measure the port harness voltage of the oxygen sensor. You can only use a related dedicated tester to analyze the data flow. The air-fuel ratio (AFR) sensor installed upstream of the three-way catalytic converter in new Honda models has a detection signal of current (mA) (Figure 4).

3. Test method of wide-range oxygen sensor

There is only one item for the single-piece inspection of wide-range oxygen sensor: Terminals 3 and 4 are heaters, which should not be open circuited, and the voltage applied to them is 12V. Terminal 1 is the signal output, terminals 5 and 6 are the reference voltage, and terminal 2 is the pump current input. Some wide-range oxygen sensors have terminals 5 and 6 as the same terminal output.

The method of the maintenance station is to analyze by reading the data stream. Take the Bora as an example: the engine control unit converts the current signal of the wide-range oxygen sensor into a voltage value and displays it. The voltage specification value of the wide-range oxygen sensor is 1.0V~2.0V. When the voltage value is greater than 1.5V, the mixture is too lean (more oxygen), and when the voltage value is less than 1.5V, the mixture is too rich (less oxygen). When the voltage value is constant at 0V, 1.5V, and 4.9V, it means that the oxygen sensor circuit is faulty. The voltage peak observed with an oscilloscope may reach 4.9V, which is normal.

The voltage specification value of the aluminum oxide oxygen sensor is 0.0V~1.0V. When the voltage value is greater than 0.45V, the mixture is too rich; when the voltage value is less than 0.45V, the mixture is too lean; when the voltage value is 0V, 0.4V~0.5V, or a constant value of 1.1V, it indicates that there is a fault in the oxygen sensor circuit.