I. Oxygen Sensor Principle
Oxygen sensors are standard equipment in automobiles. They utilize ceramic sensing elements to measure the oxygen potential in the exhaust pipe and calculate the corresponding oxygen concentration based on chemical equilibrium principles. This allows for monitoring and controlling the air-fuel ratio during combustion, ensuring product quality and compliance with exhaust emission standards. Oxygen sensors are widely used in atmosphere control for various coal-fired, oil-fired, and gas-fired furnaces. They are currently the optimal method for measuring combustion atmosphere, offering advantages such as simple structure, rapid response, easy maintenance, convenient use, and accurate measurement. Using this sensor for combustion atmosphere measurement and control can stabilize and improve product quality, shorten production cycles, and save energy.
The working principle of an oxygen sensor in a car is similar to that of a dry cell battery, with the zirconium oxide element in the sensor acting like an electrolyte. Its basic working principle is: under certain conditions, the difference in oxygen concentration on the inside and outside of the zirconium oxide sleeve generates a potential difference, and the greater the concentration difference, the greater the potential difference. The oxygen content in the atmosphere is 21%. Exhaust gas from the combustion of a rich air-fuel mixture actually contains no oxygen, while exhaust gas from the combustion of a lean air-fuel mixture or due to misfire contains a relatively high amount of oxygen, but still much less than the oxygen in the atmosphere. Under high temperature and platinum catalysis, negatively charged oxygen ions are adsorbed on the inner and outer surfaces of the zirconium oxide sleeve. Because there is more oxygen in the atmosphere than in the exhaust gas, the side of the sleeve open to the atmosphere adsorbs more negative ions than the side open to the exhaust gas, and the concentration difference of the ions on both sides generates an electromotive force.
When the oxygen concentration on the exhaust side of the fuel filter is low, a high voltage (0.6–1V) is generated between the electrodes of the oxygen sensor. This voltage signal is sent to the vehicle's ECU for amplification and processing. The ECU interprets the high voltage signal as a rich mixture and the low voltage signal as a lean mixture. Based on the oxygen sensor's voltage signal, the computer dilutes or enriches the mixture according to a theoretically optimal air-fuel ratio as close as possible to 14.7:1. Therefore, the oxygen sensor is a key sensor for electronic fuel metering. The oxygen sensor's characteristics are only fully realized and it can output voltage at high temperatures (above 300°C at the tip). It reacts fastest to changes in the air-fuel mixture at approximately 800°C, while this characteristic changes significantly at low temperatures.
II. Function of Oxygen Sensor
To achieve high exhaust purification efficiency and reduce the levels of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) in exhaust gases, fuel-injected vehicles must utilize a three-way catalytic converter. However, for the three-way catalytic converter to function effectively, the air-fuel ratio must be precisely controlled to remain close to the stoichiometric air-fuel ratio. The catalytic converter is typically installed between the exhaust manifold and the muffler. The oxygen sensor has a characteristic where its output voltage changes abruptly near the stoichiometric air-fuel ratio (14.7:1). This characteristic is used to detect the oxygen concentration in the exhaust and feeds it back to the ECU to control the air-fuel ratio. When the actual air-fuel ratio is higher, the oxygen concentration in the exhaust increases, and the oxygen sensor notifies the ECU of a lean mixture (low electromotive force: 0 volts). When the air-fuel ratio is lower than the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust decreases, and the oxygen sensor's state (high electromotive force: 1 volt) notifies the ECU.
The ECU determines whether the air-fuel ratio is too low or too high based on the difference in electromotive force from the oxygen sensor and controls the injection duration accordingly. However, if the oxygen sensor malfunctions and causes an abnormal output electromotive force, the ECU cannot accurately control the air-fuel ratio. Therefore, the oxygen sensor can also compensate for errors in the air-fuel ratio caused by wear and tear on mechanical components and other parts of the electronic fuel injection system. It can be said to be the only "intelligent" sensor in the electronic fuel injection system.
The sensor's function is to measure whether there is excess oxygen in the exhaust gas after engine combustion, i.e., the oxygen content, and convert the oxygen content into a voltage signal that is transmitted to the engine computer, enabling the engine to achieve closed-loop control with the excess air factor as the target; ensuring that the three-way catalytic converter has the maximum conversion efficiency for the three pollutants in the exhaust gas: hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), and maximizing the conversion and purification of pollutants in the emissions.
