СО - carbon monoxide, carbon monoxide; CH - unburned hydrocarbons; NOx - nitrogen oxides. Engineers opposed this dangerous trinity to a very important device that is part of the exhaust system - the catalytic converter. In other words, the gases, passing through this device, from aggressive-toxic to relatively safe, neutral. In order for the neutralizer to effectively “ennoble” the gases entering it, the content of each component in them must fit into rather narrow frames corresponding to the combustion in the cylinders of a stoichiometric working mixture of fuel and air. Recall that its composition is characterized by the so-called coefficient of excess air l (sometimes - in Soviet literature, for example, instead of l they wrote another Greek letter - a). If l is greater than 1.0, the mixture is depleted, poor, etc. And vice versa - a mixture with l less than 1.0 - enriched, rich, etc. If there is exactly as much air as is required for complete combustion of the fuel, the mixture is called stoichiometric - in Fig. 1 is the range of l near 1.0.
But how to ensure such high accuracy and at the same time stability of fuel injection? It is known that carburetor motors, for all their simplicity, do not go through this item.
The goal was achieved with the advent of an electronic automatic control system with an oxygen sensor in the exhaust gas - in another way, a lambda probe. This sensor is the most important feedback element in the fuel injection system on modern cars, which allows stoichiometric composition to be maintained at steady-state engine operating conditions with an accuracy of ± 1%.
[caption id = "attachment_185708" align = "aligncenter" caption = "Fig. 2. Scheme of the zirconium oxygen sensor:
1 - pipe exhaust system;
2 - sensor housing; 3 - contact pads;
4 - ceramic protective layer; 5 - external and internal electrodes; 6 - ceramic base (ZrO2 and Y2O3). US - output voltage "]
On modern European cars, you can often see two types of oxygen sensors. The first one includes sensors based on zirconium dioxide (zirconium), the second - sensors based on titanium oxide (titanium). The zirconium probe is shown schematically in Fig. 2. The measuring element, placed in the exhaust gas stream, generates an emf depending on their composition. This dependence is illustrated in Fig. 3 - it has a “trigger” character. In other words, the EMF of the probe changes extremely sharply near the value l = 1.0 of the working mixture in the engine cylinder, reacting even to very slight fluctuations in the composition towards enrichment or depletion. The measuring element itself is a tube with one closed end (finger type - see Fig. 2) or a plate (planar type). The principle of operation is one, the difference is only in the design - in the future, in order not to get confused, we will mean the finger type.
Shown in fig. 2 measuring element (IE) has a deposition of a noble metal - platinum from the inside and outside. Inside, there is a “solid electrolyte” (ceramic) from a mixture of zirconium dioxide ZrO2 and yttrium oxide Y2O3. It works on the principle of a galvanic cell with a solid electrolyte: upon reaching a temperature of 300–350 ° С, ceramics begin to conduct oxygen ions. (It is useful to remember that this is the minimum possible operating temperature of the IE, whereas when the real engine is running, the temperature of the sensor is about 600 ° C. The maximum operating temperature is also limited - about 900–1000 ° C, depending on the type of sensor, overheating threatens to damage it.)
How does an oxygen sensor work? Obviously, when the engine is running, the oxygen concentration inside the exhaust system and outside it, in ambient air, is completely different. This difference makes oxygen ions move in the solid electrolyte, as a result of which a potential difference appears on the electrodes of the IE - the signal from the oxygen sensor.
The temperature dependence of the IE signal is shown in Fig. 4: as you can see, reactions to rich and poor mixtures differ very much, but when the temperature drops below 300 ° C the difference gradually decreases - this zone is already inoperative.
In order for the sensor to warm up faster after starting the engine, it is placed as close as possible to the motor, but still taking into account the restrictions on the maximum temperature. Especially “critical” is a long ride with full engine power.
Modern oxygen sensors are electrically heated, which is controlled by an electronic engine control unit, changing the heater current. (Accordingly, the computer also controls the health of the heater circuit, which is very important.)