26 July 213
WIDE-RANGE AIR/FUEL SENSORS: FROM THE INSIDE OUT BY BERNIE THOMPSON The construction and operation of the six-wire, wide-range air/fuel ratio (WRAF) sensor are entirely different from a conventional oxygen sensor. This article will give you a better understanding of what a WRAF sensor is, how it works and what it can do. Photo courtesy Bosch The WRAF sensor is an amperometric (currentbased) oxygen pumping cell coupled with a potentiometric (voltagebased) oxygen sensing cell. The amperometric and potentiometric cells are made of zirconium dioxide with an yttria stabilizer. When two zirconium dioxide cells are used together, they can provide information to the microprocessor on a wide range of A/F ratios. The WRAF sensor can accurately detect an air/fuel mixture in the rich of approximately 11.5:1 and a mixture in the lean of approximately 24:1. It uses current flow across the pumping cell to detect the A/F ratio in a spark ignition internal combustion engine. The WRAF sensor is placed in the exhaust system, where it reads the exhaust gas stream. July 213 27
WIDE-RANGE AIR/FUEL SENSORS: FROM THE INSIDE OUT Fig. 1 ing Porous Electric Insulation Sensing Reference Spacer Ceramic Heater Electric Insulation Sensing Reference Spacer Fig. 2 Positive Diffusion Cell Atmospheric Cell Negative Now let s examine how the six-wire WRAF sensor is made and how it works. Its ceramic layered thick-film construction is referred to as a planar construction. The planar oxygen sensor heater element is layered into the sensor, allowing the sensor s temperature to be maintained in a range of 1112 to 1652 F with as little as 1 to 2 amps of current draw. Wiring the heater element into the sensor allows the sensor to obtain operational temperature in 1 to 2 seconds. This enables closed-loop operation to occur quickly, for total fuel control in cold-start conditions. An exploded view of the WRAF sensor is shown in Fig. 1 at right. The exhaust gases enter through an intake duct that s approximately.8mm in diameter and diffuse into a very narrow gap between the zirconium pumping cell and the zirconium sensing cell. The six-wire WRAF sensor is comprised of two zirconium dioxide oxygen sensors stacked one on top of the other, with a layer of electric insulation between them. The zirconium dioxide cell on the exhaust side is an amperometric oxygen pump, which pumps oxygen ions from one side of the pumping cell to the other. The pumping cell must be above 11 F for the oxygen ions to properly move through the zirconium dioxide. The pumping cell applies a potential (voltage) to the platinum electrodes. If one electrode is negatively charged and the other is positively charged, the oxygen ions will be pulled across the zirconium dioxide. The oxygen molecule will be catalytically disassociated on the negative platinum electrode, becoming an oxygen ion. It will then be pulled across the zirconium dioxide to the positive platinum electrode, where the oxygen ions will then associate with each other to form diatomic oxygen molecules. If the potential across the cell is reversed, the oxygen ions will be pumped in the opposite direction. The second zirconium dioxide cell on the atmospheric side is a potentiometric sensing cell. This cell operates just like a basic zirconium dioxide oxygen sensor. When the air/fuel mixture is lean, there s an abundance of oxygen at the outer cell. The oxygen in the inner cell is not attracted by the oxygen on the outer cell. In this condition, there s very little difference in the chemical balance, or equilibrium, of the oxygen between the atmospheric inner cell and the exhaust outer cell. Since the oxygen partial pressure is almost equal, there s very little pull on the oxygen ions, so very few will migrate across the zirconium oxide. In this lean condition, the oxygen sensor will produce a low voltage of less than.1 volt. When the A/F mixture is rich, there s a lack of oxygen at the outer cell. The oxygen in the inner cell is attracted by the hydrogen and carbon monoxide on the outer cell. In this condition, there s a much greater difference in the chemical balance of the oxygen between the atmospheric inner cell and the exhaust outer cell. This vast difference in the oxygen partial pressure increases the force on the oxygen ions so that a greater number of them will migrate across the zirconium oxide. In this rich condition, the oxygen sensor will produce a high voltage of greater than.8 volt. When the air/fuel ratio is stoichiometric (between.15 and.8 volt), the oxygen sensor s voltage becomes very sensitive. This is due to the sensor being balanced between rich and lean conditions. If the sensor is railed either rich or lean, there s time required to change the molecule level within the sensor. ing Gas Intake Magnesium Spinel Illustrations & photos: Bernie Thompson Amperometric Porous Electric Insulation Potentiometric Cell (Sensing Cell) Atmospheric Cell Ceramic Heater 28 July 213
WIDE-RANGE AIR/FUEL SENSORS: FROM THE INSIDE OUT Fig. 3 Fig. 4 When the sensor is in a balanced condition (neither rich nor lean), the sensor can change states very quickly. The WRAF sensor takes advantage of this rapid voltage change between lean and rich air/fuel ratios. It targets.45 volt, which will change quickly if any change in the A/F ratio is sensed. The WRAF sensor tries to keep the sensing tank at.45 volt by using the oxygen pumping cell to move oxygen into and out of the diffusion chamber, which is located right above the sensing electrode (see Fig. 2 on page 28). The sensing tank reads the exhaust gas content contained in the diffusion chamber. The WRAF sensor controls the exhaust gases in the diffusion chamber by pumping oxygen into the chamber in the rich and out of the chamber in the lean. By controlling the amount of oxygen in the diffusion chamber, sensing tank voltage can be maintained at approximately.45 volt. The air/fuel ratio can be accurately calculated by measuring the pumping current required to keep the sensing tank at.45 volt. One example of 3 July 213 Circle #18
this is if three tanks are used with a pumping system to maintain the sensing tank at a given level (Fig. 3). If the pumping system is disabled and more fluid is dumped into the exhaust tank, the volume of all three tanks will increase, and the levels in the tanks will rise (Fig. 4). If less fluid is dumped into the exhaust tank, the volume of all three tanks will decrease, and the levels will fall (Fig. 5). When the pumping system is activated, it can maintain the sensing tank to a given Fig. 5 level regardless of how much fluid is dumped into the exhaust tank. To keep the level in the sensing tank from dropping, the fluid from the exhaust tank is pumped into the diffusion tank (Fig. 6 on page 34). This will keep the sensing tank at a given level. The less fluid entering the exhaust tank, the more fluid will need to be pumped from the exhaust tank into the diffusion tank. By measuring the amount of fluid used to maintain the sensing tank s level, the amount of fluid entering the exhaust can be calculated. To keep the sensing tank level from rising, the fluid from the diffusion tank is pumped into the exhaust tank (Fig. 7). This in turn will keep the sensing tank at a given level. The more fluid entering the exhaust tank, the more fluid will need to be pumped from the diffusion tank into the exhaust tank. By measuring the rotation of the pump, the gallons pumped into or out of the diffusion tank will be known. This will give the precise total amount of fluid entering the exhaust tank. This is similar to how the WRAF sensor and its circuit work to calculate the A/F ratio of an internal combustion engine (Fig. 8). In the rich there s very little oxygen in the diffusion chamber. This rich A/F mixture causes the sensing cell s voltage to rise above the.45-volt equilibrium point. The voltage produced from the voltage regulator is put on the exhaust side of the sensing cell. This regulated voltage of 2.55 volts is the reference at which the sensing cell s voltage starts. When the sensing cell s voltage pro- Circle #19 July 213 31
WIDE-RANGE AIR/FUEL SENSORS: FROM THE INSIDE OUT RICH LEAN Fig. 6 duces an equilibrium voltage of.45 volt, it s added to the regulation voltage of 2.55 volts, producing a total of 3. volts. The sensing cell output (VS) is fed to the negative leg of the differential amplifier. As the sensing cell s voltage starts to rise above 3. volts, it s compared to the positive leg of the differential amplifier, which is connected to the voltage reference (VR). This reference voltage is maintained at approximately 3 volts. Applying this reference voltage to the positive input of the differential amplifier causes the voltage out (VO) to be inverted from the voltage sensed (VS) input. The amplification of VS to VO is multiplied many times and is set by the type of differential amplifier. VS changes by just several milivolts, VO by several volts. In the rich, VS rises, causing the VO to drop. In turn, this lowers the voltage to the exhaust platinum pumping electrode to below 2.55 volts. This electrode now becomes more negative than the diffusion pumping electrode. is Fig. 7 now pumped from the exhaust into the diffusion chamber, maintaining the sensing cell at approximately 3. volts. In the lean, the voltage sensed drops. This differential between VS and VR causes the VO to rise. The differential amplifier raises the voltage to the exhaust platinum pumping electrode above 2.55 volts. This electrode now becomes more positive than the diffusion pumping electrode. is now pumped out of the diffusion chamber to the exhaust, maintaining the sensing cell at approximately 3. volts. The microprocessor uses a shunting resistor placed in the pumping circuit between the differential amplifier output and the exhaust platinum pumping electrode. The voltage drop across the shunting resistor is proportional to the current flowing through the pumping circuit. During the production of the planar WRAF sensor, the sintering process with green sheet thick-film technology causes the diffusion chamber s size to change; it will vary from 1 to 5 microns. This size variation changes the gas rate determining diffusion. At stoichiometry, there s very low current flow, so the size variation of the diffusion chamber will have little influence. However, in the rich or lean, as current through the pumping circuit increases, the sensor s dispersion can be inaccurate by 15%, thus changing the total pumping current. This is compensated for by adding a parallel circuit to the pumping circuit, which has one fixed resistor in the PCM and one adjustable resistor in the WRAF sensor s connector. A parallel circuit leg is added to the pumping circuit to divide the pumping current between both circuits. Manufacturers use an adjustable resistor built into the WRAF sensor s connector (Fig. 9) to alter the total pumping current. By adjusting this calibration resistor, the WRAF sensor can be adjusted to read the air/fuel ratio accurately. The calibration resistor can hold the 2.55V Regulated 5V VR VS 12V VO 3V Regulated 5V PCM 5V WRAF Calibration Resistor Amplifier A/D Converter Fig. 8 Fig. 9 34 July 213
WIDE-RANGE AIR/FUEL SENSORS: FROM THE INSIDE OUT dispersion to within 1%. The value of the calibration resistor can vary between 3 and 3 ohms, depending on the diffusion chamber in each individual WRAF sensor. The microprocessor measures the voltage drop across the shunting resistor in the pumping circuit with an analog-to-digital converter. This voltage drop is equal to the air/fuel mixture in the engine. When connecting an oscilloscope to the WRAF sensor, there are six possible connections two to the heater circuit, two to the sensing cell and two to the pumping cell. Of the two wires that connect to the heating circuit, one will have battery power and the other will be a controlled square wave being pulled to ground. This will control the temperature of the heating element. The heater circuit is very important on the sensor; if there s any problem with it, the WRAF sensor will not function properly. The heater circuit should draw 1 to 2 amps, depending on the heater element temperature and bat- Fig. 1 Circle #22 36 July 213
tery voltage supplied. If the WRAF sensor is a very early design, the sensor is not a planar design, but a bulb-in-bulb design. These types of WRAF sensor heater circuits will pull about 1 amps. Of the two wires that connect to the sensing cell, one carries a reference voltage of approximately 2.55 volts. This voltage will raise the sensor s reference above ground voltage. The other sensing cell wire produces.45 volt once the sensing cell is operational. The total sensing voltage will be 3 volts (the 2.55 volts from the reference plus the.45 volt produced from the sensing cell). The sensing cell s voltage will change approximately 25mV, depending on the A/F ratio that s contained in the diffusion chamber. Some manufacturers will vary the sensing cell s reference voltage. This reference voltage may range from 2.4 to 2.7 volts, but is commonly 2.55 volts. Of the two wires that connect to the pumping cell, one connects to the shunting resistor circuit and the other connects to the calibration resistor circuit. At low pumping current levels, the two pumping voltages will follow each other very closely. As soon as the pumping current is increased, the two pumping voltages will vary from one another (Fig. 1 on page 36), due to the different resistance values of the two circuits. If the engine is running lean, the pumping voltage will be greater than the 2.55 reference voltage at the pumping cell; the greater the voltage, the leaner the A/F ratio. If the engine is running rich, the pumping voltage will be less than the 2.55 reference voltage at the pumping cell; the lower the voltage, the richer the A/F ratio. If the A/F ratio is stoichiometric (14.7:1), the pumping cell voltage will be crossing the 2.55 reference voltage. This will be a cyclical waveform of rich-lean-leanrich at a frequency of 1 to 2Hz. Be aware that this pumping voltage is the reverse of a conventional zirconium dioxide oxygen sensor. If the voltage is high (above 2.55 volts), the system is lean; if the voltage is low (below 2.55 volts), the system is rich. The sensing cell voltage should be very stable at approximately 3 volts and the pumping cell voltage should be actively crossing the 2.55 voltage reference. This shows that the sensor is working correctly. To check the operation of the WRAF sensor, drive the system rich with propane, then drive the system lean by creating an air leak. Make sure the pumping cell voltage in the rich condition drops below 1.85 volts. Make sure the pumping cell voltage in the lean condition exceeds 3.5 volts. To check the calibration of the WRAF sensor, use an exhaust gas analyzer. If the pumping cell voltage is cycling across the 2.55- volt reference and the tailpipe gases do not show a lambda of 1, the sensor is out of calibration, the PCM has a problem or the exhaust has false oxygen entering the exhaust system. This article can be found online at www.motormagazine.com. July 213 37