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December 15, 2009 | Reply More

A simplified schematic of an air/fuel ratio sensor.

We all want clean air. We thrive in it. We breathe it in, turn it into power and exhale it. Our vehicles do exactly the same thing. The process of internal combustion is not 100 percent efficient, however. Not all the fuel is burned, and some is only partially burned. Totally unburned fuel is measured as hydrocarbons (HC), and partially burned fuel is measured as carbon monoxide (CO). If gas temperatures are high, say from a lean mixture, other gases that are produced as a by-product of combustion are oxides of nitrogen (NOx). These gases are all hazardous in their own way, and they are emitted from any vehicle’s tailpipe (O2 and CO2 are emitted with them, but only the latter has possible long-term environmental effects). Maximizing the efficiency of the combustion process will not only reduce air pollution, but also increase fuel mileage.

The Path to Enlightenment

One way to accurately control combustion is to monitor the contents of the exhaust and adjust fuel quantity and ignition timing accordingly. Monitoring the exhaust is now a time-honored tradition — we have been doing it since the mid-1970s. Within those 30+ years, the technology used has evolved. We started the journey with the single-wire zirconium dioxide O2 sensor (also known as a Lambda sensor or probe). The concept this sensor was based on is called the Nernst principle. Basically, this sensor generates its own signal voltage the strength of which depends on the oxygen content in the exhaust stream compared to that in ambient air. The signal is relatively high — 600 to 900 millivolts — in a mixture richer than the stoichiometric 14.7:1. The signal is less than 400 millivolts when the mixture is leaner than 14.7:1. The stoichiometric ratio theoretically yields the best optimization of combustion gases that allows a catalytic converter to both oxidize HC and CO, and reduce NOx to harmless nitrogen and oxygen.

This original type of oxygen sensor can’t generate a voltage signal until it reaches 350 deg. C. A typical engine does not produce this level of exhaust heat until it reaches normal operating temperature, or is run at high rpm. So, the electrically-heated version appeared in the early 1980s (sometimes referred to as a HEGO for Heated Exhaust Oxygen Sensor). The heater element brings the sensor up to 400 deg. C. rapidly, and keeps it there at idle so that closed-loop operation is assured.

The next development was the planar type, which uses micro-thin layers of galvanic material incorporated with the heating element to provide even faster operation. BMW also used NGK “resistive jump” sensors that are similar to the Titanium type in that they require a reference voltage from the control unit of five volts that the sensor pulls to ground according to the O2 level. These sensors work the opposite of what we are used to. Sensor signal voltages above 2.5 volts (half the reference voltage) reflect lean mixtures, and those below 2.5


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Category: BMW TechDrive

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