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This photo shows one of the oxygen sensors (arrowed) used in a Holden VT Commodore. The VT’s V6 engine has two such sensors – one for each cyclinder bank. The 1997 Suzuki Vitara uses a 4-wire oxygen sensor – two for the heater, one for the signal and the other for ground. It’s mounted on the exhaust manifold. Narrowband & wideband oxygen sensors . . . how they work By JOHN CLARKE The oxygen sensor is an important component in your car’s engine management system. It monitors the oxygen content in the car’s exhaust, to indicate whether the mixture is too lean or too rich. Here’s a quick rundown on how the two basic types work. Y OUR CAR engine’s air/fuel ratio not only has a considerable bearing on its performance but also on fuel consumption and air pollution. If the mixture is too rich (ie, too much fuel), then fuel economy will suffer and the unburnt hydrocarbons will cause air pollution. Conversely, a lean mixture (ie, too much air) will give poor engine performance and produce more nitrous-oxide pollutants. A lean mixture can also cause ser­ ious engine damage under certain circumstances, particularly at high RPM or under heavy loads. To combat this, all modern cars use at least one exhaust gas oxygen (EGO) sensor which is mounted on the exhaust manifold. This monitors the resultant oxygen content in the exhaust and provides a voltage output which siliconchip.com.au indicates whether the mixture is rich or lean or at the “stoichiometric” point (ie, when there is just sufficient oxygen in the air-fuel mixture to give complete combustion). This information is fed to the engine management computer (ECU) which in turn controls the fuel injectors. It enables the ECU to continuously adjust the mixture to provide optimum power and economy, consistent with low exhaust emissions. In addition, your car’s catalytic converter has an important role to play in reducing emissions. This is also mounted in the exhaust system and converts combustion byproducts such as carbon monoxide (CO) to carbon dioxide (CO2), unburnt hydrocarbons to CO2 and H2O (water) and nitrous oxide (NO) to nitrogen (N2). Some cars include another EGO sensor after the catalytic converter, to monitor its performance. In practice, a catalytic converter works best when the air/fuel mixture is kept within a narrow range close to the stoichiometric ratio. This ratio varies according to the fuel used but is generally 14.7:1 for unleaded petrol; ie, the air mass must be 14.7 times the fuel mass. Lambda values Another way of specifying the air/ fuel ratio is by its “Lambda” (l) value. Basically, the Lambda value is the actual air/fuel ratio divided by the stoichiometric ratio. This means that the Lambda value is 1 at the stoichiometric point, while lean air/fuel ratios have a Lambda greater than 1 and rich November 2008  27  <1 O 2 SENSOR OUTPUT VOLTAGE (mV) 1000  =1  >1 Fig.1: this graph shows the output response from a typical narrowband oxygen sensor. Note the S-curve shape and the rapid variation either side of the stoichiometric (14.7:1) point. 800 600 400 200 0 12:1 14.7:1 16:1 AIR/FUEL RATIO EXHAUST GAS HIGH-PRESSURE SEAL SLITS OUTSIDE AIR – V + INTERIOR PLATINUM ELECTRODE HOUSING ZIRCONIA SENSOR SENSOR SHIELD EXTERIOR PLATINUM ELECTRODE EXHAUST MANIFOLD Fig.2: what’s inside a narrowband zirconia oxygen sensor. It consists of a zirconia ceramic sensor element with thin platinum electrodes on both sides. air/fuel ratios have a Lambda that’s less than 1. Narrowband sensors Virtually all cars (with a few except­ ions) are fitted with what a known as “narrowband” oxygen sensors. This type of sensor is generally only accurate around the stoichiometric point but that doesn’t matter for use in a car engine since it is only required to indicate whether the mixture is rich or lean. In operation, a typical narrowband oxygen sensor outputs a voltage rang28  Silicon Chip ing from just 0-0.9V. A stoichiometric measurement gives an output of 0.45V and varies sharply either side of stoichiometric. As a result, the sensor’s output varies from about 0.2-0.8V over a very narrow band. Fig.1 shows the output response from a typical narrowband zirconia oxygen sensor. Note the steep voltage changes about the stoichiometric point and the tapering off of the response at the rich and lean ends. This response is often referred to as an “S” curve. The result is that for rich mixtures, the sensor varies from just 0.8V to 0.9V The Bosch LSU 4.2 wideband sensor is used in conjnction with a wideband controller (eg, the Innovate Motosports LC-1). (ie, 100mV), while for lean mixtures the voltage range is usually less than 200mV. Fig.2 shows how a narrowband zirconia oxygen sensor is made. It’s typically about the same size as a spark plug and is threaded into the exhaust system so that the sensor is exposed to the exhaust gasses. The assembly is protected using a shield that includes slots so that the exhaust gasses can pass through into the sensor. The sensor itself is made from a zirconia ceramic material that has a thin layer of porous platinum on both sides. These platinum coatings form electrodes to monitor the voltage produced by the zirconia sensor as the exhaust gas passes through it. The device operates by measuring the difference in oxygen content between the exhaust gas and the outside air. The oxygen content of the air (about 21%) serves as the reference. In operation, a voltage is produced between the electrodes because the zirconia sensor has a high conductivity for oxygen ions at high temperatures. Generally, the accuracy from this type of oxygen sensor at the rich and lean ends is poor and it cannot be relied on to give consistent air/fuel ratio readings. In fact, the accuracy at the rich end is particularly variable because it changes markedly with temperature. To overcome this problem, some sensors include a resistive heating element to ensure that they operate within their correct temperature range. This reduces the errors under rich mixture conditions when the engine is cold. In addition, Bosch manufactures a sensor that is relatively accurate over a wider range of air/fuel ratios than other narrowband sensors. This is designated the LSM11 and is used in some air/fuel mixture display units but siliconchip.com.au PUMP CELL SENSOR CELL DIFFUSION GAP EXHAUST Rcal Ip A O2 CONTROLLER Ip CURRENT 61.9 OUTPUT ZrO 2 O 2– O2 ZrO 2 ZrO 2 V O 2– O2 Ip SENSE Vs 21% O 2 Vs SENSE COMPARATOR 450mV REFERENCE Vh REFERENCE AIR HEATER (HEATER CONTROL NOT SHOWN) Fig.3: the basic scheme for a wideband oxygen sensor and its associated controller circuit. Ip (mA) 1.500 PUMP CURRENT 1.000 LEAN MIXTURE 0.500 0.000 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40  –0.500 –1.000 –1.500 RICH MIXTURE –2.000 BOSCH LSU 4.2 WIDEBAND SENSOR Fig.4: pump current vs lambda value for a typical wideband sensor. It’s converted to a linear response by the wideband controller. it is still a narrowband sensor. The LSM11 is sometimes called a “wideband” sensor because it can provide a wider measurement of air/ fuel ratio in the lean region than typical narrowband sensors. However, its response is still an “S” curve and its accuracy is still compromised beyond the stoichiometric region (ie, in the rich and lean regions). Wideband sensor & controller The wideband sensor and its associated controller circuit was developed in order to obtain an output that is far more linear with respect to air/fuel mixture. This not only gives much improved accuracy but this type of sensor also covers a wider range of values in the rich and lean regions. The wideband sensor design is based on the narrowband zirconia oxygen sensor but includes a clever method to obtain a more linear response. It’s based on the fact that the narrowband siliconchip.com.au zirconia oxygen sensor is very good at detecting the stoichiometric mixture, ie, where there is no oxygen remaining after the combustion process and no excess unburnt fuel. By adding an oxygen pump cell to the sensor, oxygen ions can be fed either into or out of the sensor so that it is always measuring at the stoichiometric point. This means that if the mixture is lean, then excess oxygen is detected by the oxygen sensor. The pump cell then drives oxygen ions out of the sensor until the stoichiometric point is reached. Similarly, if the mixture is rich, oxygen ions are pumped into the sensor cell until a stoichiometric reading is obtained. As a result, the current applied to the pump cell can be either positive or negative, depending on whether oxygen is pumped into or out of the sensor cell. Fig.3 shows the basic scheme. The voltage from the oxygen sensor cell is Vs, while the current into the pump cell is Ip. At stoichiometric, Vs is 450mV and this voltage is compared against a 450mV reference. If Vs is higher than the 450mV reference, the mixture is rich and the Vs sense comparator output goes low. This “informs” the controller that Ip needs to go negative to pump oxygen ions into the sensor cell in order to regain a stoichiometric measurement. Note, however, that this oxygen pumping has no effect on the actual air/fuel ratio of the exhaust mixture. It only changes the sensor response. Similarly, if Vs is lower than the 450mV reference, the exhaust mixture is lean and the comparator goes high. As a result, the controller changes the Ip current direction to pump oxygen out of the sensor cell. In operation, the circuit is continuously controlled so that Vs is maintained at 450mV. The actual current required to maintain stoichiometric readings from the sensor cell is proportional to the air/fuel ratio. Fig.4 plots current against lambda for a typical wideband sensor. Note that the current with respect to lambda is far more linear than the output of a narrowband sensor. The wideband controller converts this response into a 0-5V output that represents the air/ fuel ratio as a linear scale. Calibration resistor Note also that Ip is sensed by measuring the voltage across a 61.9W resistor that is also in parallel with a calibration resistor (Rcal) – see Fig.3. The Rcal resistor is adjusted in parallel with the 61.9W resistor during the manufacture of each wideband sensor so that the current versus lambda curve is accurate when connected to a con­ troller, even if the sensor is replaced. Apart from controlling the oxygen pump, the wideband controller also controls a heater element so that the sensor’s temperature is maintained at a constant 750°C. In fact, the sensor doesn’t provide accurate readings until this temperature is reached. The controller determines the sensor current by measuring the impedance of the sensor cell which is 80W at 750°C. So that’s basically how oxygen sensors work. Elsewhere in this issue, we describe a Wideband Oxygen Sensor Display unit, so that you can monitor the air/fuel ratio as you drive. You’ll SC find it on page 58. November 2008  29