An oxygen sensor is an electronic component that detects the concentration of oxygen molecules in the air or a gas matrix such as in a combustion engine exhaust gas.
For automotive applications, an oxygen sensor is referred to as a lambda sensor, where lambda refers to the air–fuel equivalence ratio, usually denoted by λ. It was developed by Robert Bosch GmbH during the late 1960s under the supervision of Günter Bauman. The original sensing element is made with a thimble-shaped zirconia ceramic coated on both the exhaust and reference sides with a thin layer of platinum and comes in both heated and unheated forms. The planar-style sensor entered the market in 1990 and significantly reduced the mass of the ceramic sensing element, as well as incorporating the heater within the ceramic structure. This resulted in a sensor that started sooner and responded faster.
The most common application is to measure the exhaust-gas concentration of oxygen for internal combustion engines in automobiles and other vehicles in order to calculate and, if required, dynamically adjust the air-fuel ratio so that catalytic converters can work optimally, and also determine whether the converter is performing properly or not. An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust.
Scientists use oxygen sensors to measure respiration or production of oxygen and use a different approach. Oxygen sensors are used in oxygen analyzers, which find extensive use in medical applications such as anesthesia monitors, respirators and oxygen concentrators.
Divers use oxygen sensors (and often call them ppO2 sensors) to measure the partial pressure of oxygen in their breathing gas. Open circuit scuba divers test the gas before diving as the mixture remains unchanged during the dive and partial pressure changes due to pressure are simply predictable, while mixed gas rebreather divers must monitor the partial pressure of oxygen in the breathing loop throughout the dive, as it changes and must be controlled to stay within acceptable bounds.
Oxygen sensors are also used in hypoxic air fire prevention systems to continuously monitor the oxygen concentration inside the protected volumes.
There are many different ways of measuring oxygen. These include technologies such as zirconia, electrochemical (also known as galvanic), infrared, ultrasonic, paramagnetic, and very recently, laser methods.
Automotive applications
Automotive oxygen sensors, colloquially known as O2 sensors, make modern electronic fuel injection and emission control possible. They help determine, in real time, whether the air–fuel ratio of a combustion engine is rich or lean. Since oxygen sensors are located in the exhaust stream, they do not directly measure the air or the fuel entering the engine. Still, when information from oxygen sensors is combined with that from other sources, it can be used to determine the air–fuel ratio indirectly. Closed-loop feedback-controlled fuel injection varies the fuel injector output according to real-time sensor data rather than operating with a predetermined (open-loop) fuel map. In addition to enabling efficient electronic fuel injection, this emissions control technique can reduce the amounts of both unburnt fuel and oxides of nitrogen entering the atmosphere. Unburnt fuel is pollution in the form of air-borne hydrocarbons. At the same time, oxides of nitrogen (NOx gases) are a result of combustion chamber temperatures exceeding 1300 kelvin, due to excess air in the fuel mixture, thereby contributing to smog and acid rain. Volvo was the first automobile manufacturer to employ this technology in 1976, along with the three-way catalyst used in the catalytic converter.
The sensor does not actually measure oxygen concentration, but rather the difference between the oxygen content of the exhaust gas and that of the air. Rich mixture causes an oxygen demand. This demand causes the voltage output to rise as oxygen ions are transported through the sensor layer. A lean mixture causes low voltage because there is an oxygen excess.
Modern spark-ignited combustion engines use oxygen sensors and catalytic converters to reduce exhaust emissions. Information on oxygen concentration is sent to the engine management computer or engine control unit (ECU), which adjusts the amount of fuel injected into the engine to compensate for excess air or excess fuel. The ECU attempts to maintain, on average, a specific air-fuel ratio by interpreting information from the oxygen sensor. The primary goal is a compromise between power, fuel economy, and emissions, and in most cases is achieved by an air–fuel ratio close to stoichiometric. For spark-ignition engines (such as those that burn gasoline or autogas / liquefied petroleum gas (LPG), as opposed to diesel), the three types of emissions modern systems are concerned with are: hydrocarbons (which are released when the fuel is not burnt completely, such as when misfiring or running rich), carbon monoxide (which is the result of running slightly rich) and NOx (which dominate when the mixture is lean). Failure of these sensors, either through normal aging, the use of leaded fuels, or fuel contaminated with silicones or silicates, for example, can lead to damage of an automobile's catalytic converter and expensive repairs.
Tampering with or modifying the signal that the oxygen sensor sends to the engine computer can be detrimental to emissions control and can even damage the vehicle. When the engine is under low-load conditions (such as when accelerating very gently or maintaining a constant speed), it is operating in "closed-loop mode". This refers to a feedback loop between the ECU and the oxygen sensor(s), in which the ECU adjusts the amount of fuel and expects to see a resulting change in the oxygen sensor's response. This loop forces the engine to operate both slightly lean and slightly rich on successive loops, as it attempts to maintain a mostly stoichiometric ratio on average. Suppose modifications cause the engine to run moderately lean. In that case, there will be a slight increase in fuel efficiency, sometimes at the expense of increased NOx emissions, much higher exhaust gas temperatures, and sometimes a slight increase in power that can quickly turn into misfires and a drastic loss of power, as well as potential engine and catalytic-converter (due to the misfires) damage, at ultra-lean air–fuel ratios. Suppose modifications cause the engine to run rich. In that case, there will be a slight increase in power up to a point (after which the engine starts flooding from too much unburned fuel), at the cost of decreased fuel efficiency and increased unburned hydrocarbons in the exhaust, which can cause overheating of the catalytic converter. Prolonged operation at rich mixtures can cause catastrophic failure of the catalytic converter (see backfire). The ECU also controls the spark engine timing and the fuel-injector pulse width, so modifications that cause the engine to operate either too lean or too rich may result in inefficient fuel consumption when the fuel is ignited too soon or too late in the combustion cycle.
When an internal combustion engine is under high load (e.g. wide open throttle), the output of the oxygen sensor is ignored, and the ECU automatically enriches the mixture to protect the engine, as misfires under load are much more likely to cause damage. This is referred to as an engine running in "open-loop mode". Any changes in the sensor output will be ignored in this state. In many cars (except for some turbocharged models), inputs from the air flow meter are also ignored, as they might otherwise lower engine performance due to the mixture being too rich or too lean, and increase the risk of engine damage due to detonation if the mixture is too lean.
Function of an O2 sensor O2 sensors provide feedback to an engine control unit (ECU). Where applicable, gasoline, propane, and natural gas engines are fitted with three-way catalysts to comply with on-road vehicle emissions legislation. Using the O2 sensor signal, the ECU can operate the engine with an air–fuel ratio very close to 14.7:1, which is the ideal operating mixture for a three-way catalyst to be effective. Robert Bosch GmbH introduced the first automotive lambda sensor in 1976, and it was first used by Volvo and Saab in that year. The sensors were introduced in the US from approximately 1979 and were required on all car models in many European countries by 1993.
The probe
The sensor element is a ceramic cylinder, plated on the inside and outside with porous platinum electrodes; a metal gauze protects the whole assembly. It operates by measuring the difference in oxygen between the exhaust gas and the external air, generating a voltage or changing its resistance accordingly.
The sensors only begin to work effectively when heated to approximately 316 °C (600 °F), so most newer lambda probes have heating elements encased in the ceramic that bring the ceramic tip up to temperature quickly. Older probes without heating elements would eventually heat up from the exhaust. Still, there is a time lag between when the engine is started and when the components in the exhaust system reach thermal equilibrium. The time required for the exhaust gases to bring the probe to temperature depends on the ambient air temperature and the geometry of the exhaust system. Without a heater, the process may take several minutes. There are pollution problems attributed to this slow start-up process, including a similar issue with the catalytic converter's working temperature.
The probe typically has four wires attached: two for the lambda output and two for the heater power. However, some automakers use the metal case as ground for the sensor element signal, resulting in three wires. Earlier non-electrically-heated sensors had one or two wires.
Operation of the probe
Zirconia sensor
The zirconium dioxide, or zirconia, lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere.
An output voltage of 0.2 V (200 mV) DC represents a "lean mixture" of fuel and oxygen, where the amount of oxygen entering the cylinder is sufficient to fully oxidize the carbon monoxide (CO), produced in burning the air and fuel, into carbon dioxide (CO2). An output voltage of 0.8 V (800 mV) DC represents a "rich mixture", which is high in unburned fuel and low in remaining oxygen. The ideal setpoint is approximately 0.45 V (450 mV) DC. This is where the quantities of air and fuel are in the optimal ratio, which is ~0.5% lean of the stoichiometric point, such that the exhaust output contains minimal carbon monoxide.
The voltage produced by the sensor is nonlinear with respect to oxygen concentration. The sensor is most sensitive near the stoichiometric point (where λ = 1) and less sensitive when either very lean or very rich.
The ECU is a control system that uses feedback from the sensor to adjust the fuel/air mixture. As in all control systems, the time constant of the sensor is important; the ability of the ECU to control the fuel–air ratio depends upon the response time of the sensor. An aging or fouled sensor tends to have a slower response time, which can degrade system performance. The shorter the time period, the higher the so-called "cross count""Zirconia sensors" in Spark Plug 411 , at sparkplugs.com. and the more responsive the system.
The sensor has a rugged stainless-steel construction internally and externally. Due to this the sensor has a high resistance to corrosion, allowing it to be used effectively in aggressive environments with high temperature/pressure.