Automotive Engine Performance

Oxygen Sensors

18-05 Explore oxygen sensor operation.

An oxygen (O₂) sensor measures the oxygen content in the exhaust system (FIGURE 18-36). Measuring the oxygen content is necessary for the PCM to adjust the fuel mixture to maintain the most efficient mixture in the cylinder as possible. The stochiometric mixture that should be maintained by the fuel system to promote complete combustion is 14.7:1. These sensors help the PCM increase the performance of the engine and also improve fuel economy. This process happens only when the engine reaches operating temperature and the O₂ sensor starts generating a voltage. Until that moment, the PCM is operating the engine in open-loop mode, where it is just running a preset program. To increase the rate at which the engine reaches closed-loop mode, some O₂ sensors have a heater built in so that when the key engine is operating it is heating up, the temperature of the sensor heats up faster than the engine could heat it up on its own (FIGURE 18-37).

FIGURE 18-36 The oxygen sensor is positioned to get the greatest amount of exposure to the exhaust fumes in the exhaust system without contamination.

FIGURE 18-37 The heated oxygen sensor is used when the sensor is located far away from the exhaust manifold, which may make it difficult to heat up the sensor.

The O₂ sensor also helps monitor catalyst efficiency. Taking a reading before and after the catalyst, the PCM can compare the oxygen content and determine whether the catalyst is cleaning the exhaust of the excess oxygen, hydrocarbons, and carbon monoxide. If the catalyst is working correctly, the output should consist of carbon dioxide and water. If the catalyst is not working correctly, there will still be unburned O₂ in the output. These sensors will not know what is causing the excess O₂ in the system but will alert the PCM that there is a fault. Using the O₂ sensor can aid the technician in accurately diagnosing the fault in the engine.

Zirconia Oxygen Sensors

Zirconia oxygen sensors use a zirconium dioxide metal that is bent into a bowl shape and placed on the engine of the sensor (FIGURE 18-38). The bowl has slits cut into it so that exhaust gas can flow into the cavity; take advantage of these slits to take a reading of the exhaust gases. The sensor allows outside air into the center that surrounds the inside of the metal, and the exhaust gas surrounds the outside of the metal. With these two dissimilar molecules on opposite sides of the zirconia, it generates a voltage based on the difference. If it is very different—i.e., if very little oxygen is in the exhaust gas—it will produce belowunder450 mV, which indicates a rich condition. This voltage is sent back to the PCM so that it can lean out the mixture to bring it closer to the center voltage of 450 mV. If the content of oxygen on both sides of the zirconia is nearly the same amount, the voltage will go above 450 mV and up to 1 volt. This indicates a lean condition, and it indicates that the engine is not burning all of the oxygen. The PCM will take this reading and increase fuel to the combustion chamber to richen up the mixture. These readings should be constantly changing as the PCM constantly tries to maintain the perfect mixture. If, for some reason, it gets stuck high or low, there may be another system affecting the operation of the O₂ sensor. When diagnosing an O₂ sensor fault, look at what could potentially cause the failure and investigate further. The problem could be something like a vacuum leak or a leaking injector. These two failures will flag an O₂ sensor code, but replacing the O₂ sensor will not fix the condition.

FIGURE 18-38 The zirconia oxygen sensor uses a metal that reacts to the difference in gases that touch it. When this reaction occurs, a voltage is produced based on the amount of exposure to those gases.

Wide-Band Oxygen Sensors

More precise control over the air-fuel mixture in an ICE is needed to meet ever-increasing emission standards. Measuring the oxygen content in the exhaust is a key component of meeting government regulations and increased performance. A conventional zirconia oxygen sensor is very simple and operates on the premise to just indicate that the exhaust is either rich or lean. This is fine to help the PCM adjust the fuel mixture to compensate, but is not precise enough to tell the PCM exactly what the air-fuel ratio is. A wide-band oxygen sensor and air-fuel sensors (A/F sensors) are more precise oxygen sensors that use a reference voltage to generate a signal voltage back to the PCM (FIGURE 18-39). This precise reading will allow the PCM to fine-tune the air-fuel mixture to create the most efficient combustion environment.

FIGURE 18-39 A wide-band oxygen sensor is a more precise oxygen sensor that the PCM can use to fine-tune the fuel mixture to increase performance and decreased emissions.

A word that will come up when diagnosing a wide-band oxygen sensor is lambda. Lambda is a calculation that determines for the PCM the actual oxygen content vs. the amount that should be there for perfect combustion. This calculation is more accurate and allows the PCM to interpret the oxygen content in the exhaust system instead of a generic rich or lean signal.

The internal components of a wide-band oxygen sensor start with a ceramic strip that includes a Nerst-effect oxygen pump and a diffusion gap that includes an oxygen-sensing element, all laminated together on the ceramic (FIGURE 18-40). As the exhaust gas enters the sensor through the gaps in the cover, it diffuses through the ceramic on to the element. This reaction causes the Nerst cell to generate voltage. This voltage is different from a conventional zirconia oxygen sensor, to generate this voltage the sensor must be much hotter than a conventional zirconia sensor. This sensor must be around 1,292°F–1,472°F (700°C–800°C), as opposed to 600°F (316°C) for a zirconia one. To get this sensor to that temperature faster, a heater must be incorporated. Wide-band oxygen sensors are becoming the new normal as emission standards become stricter and efficiency needs to be increased.