An oxygen sensor is an electronic device used to measure the oxygen content
in exhaust gasses. In the automotive industry it is also known as a lambda sensor,
and is used to regulate the air/fuel mixture and exhaust gas emissions in the
internal combustion engine. The lambda sensor is used to indicate whether the
fuel mixture is rich or lean. The
levels of oxygen are determined by exposing one electrode to the outside air and
another to the exhaust gas. The difference in oxygen content causes a flow
of electrons through a ceramic element that generates a voltage potential between
two boundary layers. The voltage created is directly related to the level of
oxygen content in the exhaust gas.
The lambda sensor is very sensitive to
temperature. The temperature of the ceramic element will determine its ability
to conduct the oxygen ions and essentially influence the response time of the
sensor. Most sensors are manufactured with an integrated electrical
heating element to sustain the response time at low
exhaust temperatures. This function assures that the emissions of the vehicle
are controlled over a wider range of engine operating temperatures, especially
during cold starts.
There are two different types of oxygen sensors that differ in the output signal. The step change sensor produces a significant "jump" in the signal voltage around lambda = 1 while the wideband sensor provides a signal over a wider range for lambda.
The Step Change Sensor
The function of the step change sensor is based on an electrochemical cell called a Nernst cell (Figure 1). A solid electrolyte made of zirconium dioxide doped with yttrium dioxide transports oxygen ions when the temperature of the ceramic exceeds 662 degrees Fahrenheit (350 degrees centigrade). The voltage that is generated between the two platinum electrodes is the sensor's output signal. The value of the voltage depends on the relative amount of oxygen in the exhaust gas. Due to the sudden change in voltage around lambda = 1, this sensor only facilitates the control the air/fuel mixture by swinging between rich and lean. Thus, this sensor type can only be used in gasoline engines that are supposed to run with lambda = 1.
The Wideband Sensor
Wideband sensors, also known as wide range sensors, have an additional ceramic cell (Figure 2). The exhaust gas partially diffuses through the diffusion barrier. The air/fuel ratio of the exhaust gas in the chamber is measured with the Nernst cell. Depending on whether the air/fuel ratio in the chamber is rich or lean, a control circuit applies a voltage to the electrodes of the pump cell. Oxygen ions are transported from the inner to the outer electrode so that the air/fuel ratio in the chamber becomes lambda = 1. The electrical current, Ip, generated is the signal. There is a specific range of current corresponding to lambda from 0.7 to infinity and the curve is linear. The signal is zero when the air/fuel ratio of the exhaust gas is lambda = 1. The output curve permits steady control with a predetermined nominal value for lambda. Because the wideband sensor can also measure lean air/fuel mixtures, it is frequently used for diesel engines and gasoline engines with direct injection.
Properties of Modern Oxygen Sensors
The heating elements of oxygen sensors are typically controlled in an open loop with a pulse width modulated voltage, though modern sensors often have heating elements that are controlled in a closed loop. The measured resistance of the ceramic indicates the temperature, so the energy needed to hold the temperature constant can be readily calculated. Closed loop control assures a more reliable signal in different environmental conditions.
In addition, many modern oxygen sensors do not need outside air as a reference. Rather, a reference pump current is applied to the Nernst cell, which simulates the influence of the air. In these sensors, a gap in the element for the reference air is not required. Therefore, the sensor element requires less volume and heating it up requires less time and energy. Furthermore, operating without reference air makes the sensor less sensitive to contamination.