This story was part of Counterman's annual Technical Sales Seminars, which was published in the April 2009 issue.
Oxygen sensors have been around a long time, but in recent years some new designs have appeared that are quite different from traditional O2 sensors. They are called Wide Ratio Air Fuel (WRAF) sensors. The basic function is still the same, which is to monitor oxygen levels in the exhaust so the Powertrain Control Module (PCM) can maintain the best air/fuel ratio for emissions. But the way they work is much different.
A conventional zirconia-type O2 sensor switches its output voltage to indicate a rich or lean exhaust condition. When the engine is running rich, there is less unburned oxygen in the exhaust and the sensor’s output voltage is high (up to 0.9 volts). When the engine is running lean, there is more unburned oxygen in the exhaust and the sensor’s output drops down to about 0.2 volts.
When the PCM receives a rich or lean signal from the O2 sensor, it commands the fuel mixture to do just the opposite: a lean signal from the O2 sensor causes the PCM to command more fuel, while a rich signal from the O2 sensor causes the PCM to cut back on the fuel. So while the engine is running, the sensor’s output is constantly flip flopping back and forth from rich to lean as the PCM keeps readjusting the fuel mixture for the best average mixture.
The main limitation with the older-style O2 sensors is that they can’t accurately indicate the exact level of oxygen in the exhaust. All they do is give a rich (high voltage) or lean (low voltage) signal.
With today’s ultra-low emission requirements, a more accurate feedback signal is required from the oxygen sensor to precisely control the air/fuel mixture. A wide ratio air/fuel sensor measures the actual amount of oxygen in the exhaust, from extremely rich all the way down to extremely lean (even straight air!). This ability allows the PCM to control fuel mixtures much more accurately to reduce emissions and improve fuel economy.
WRAF’S ARE FASTER
WRAF sensors also react much faster than ordinary O2 sensors, which allows them to monitor the fuel mixture from individual cylinders as each puff of exhaust blows by the sensor element. The PCM can then adjust the mixture for each cylinder individually to reduce emissions and optimize fuel economy.
WRAF sensors also don’t generate a voltage signal like a traditional zirconia O2 sensor. They produce a linear signal that changes in direct proportion to oxygen levels in the exhaust. So the sensor’s output is more like that from a rheostat rather than a toggle switch.
The output signal from a WRAF sensor is also preprocessed internally by its own built-in circuitry. The voltage signal from the sensing element is converted into a variable current signal that can travel in one of two directions (positive or negative). The signal gradually increases in the positive direction when the air/fuel mixture becomes leaner. At the “stoichiometric” or “lambda” point when the air/fuel mixture is perfectly balanced (14.7 to 1), the current flow stops and there is no current flow in either direction. When the air/fuel ratio becomes progressively richer, the current reverses course and flows in the negative direction.
The PCM sends a control reference voltage (typically 3.3 volts on Toyota applications, 2.6 volts on Bosch sensors) to the WRAF sensor through one pair of wires, and monitors the sensor’s output current through a second set of wires. The sensor’s output signal is then processed by the PCM, and can be read on a scan tool as the air/fuel ratio, a fuel trim value and/or a voltage value depending on the application and the display capabilities of the scan tool.
Like ordinary oxygen sensors, WRAF sensors also have an internal heater circuit to help them reach operating temperature quickly. To work properly, WRAF sensors require a higher operating temperature: 1,292 to 1,472 degrees F versus about 600 degrees F for ordinary oxygen sensors. Consequently, if the heater circuit fails, the sensor may not put out a reliable signal and set a fault code.
LONG SERVICE LIFE
WRAF sensors are designed for a service life of up to 240,000 kilometers (150,000 miles) under normal driving conditions. But like ordinary O2 sensors, WRAF sensors are vulnerable to contamination and aging. They can become sluggish and slow to respond to changes in the air/fuel mixture as contaminants build up on the sensor element. Contaminants include phosphorus and zinc from motor oil (from worn valve guides and rings), silicates from antifreeze (leaky head gasket or cracks in the combustion chamber that leak coolant), and even sulfur and other additives in gasoline.