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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Lambda probe. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices Since March 1, 2006, the Euro-2 standard has been introduced in Russia. According to the new requirements, the CO content in the exhaust gases should not exceed 0,5% at idle and 0,3% at high speeds. In Europe, this standard has been introduced since January 1996 (today the leading manufacturers are already preparing for the transition to Euro-5). This standard imposes rather stringent requirements on the operation of all elements of the engine power system, the quality of the engine itself and the quality of the fuel. Even if the engine is perfectly serviceable, it is necessary to maintain the air / fuel mixture ratio very accurately. For optimal combustion of 1 kg of fuel, 14,7 kg of air is required. This ensures the lowest content of toxic substances in the exhaust, maximum efficiency and "power" characteristics of the engine. This composition of the mixture is called "stoichiometric", and the ratio of the stoichiometric composition of the mixture to the existing one is usually denoted by the letter λ. If λ (excess air) - enriched. During the technical inspection of vehicles, this parameter is controlled (should be within 0,97 ... 1,03). For more than 20 years abroad, to maintain the stoichiometric composition, devices for the operational control of the combustion process (by combustion products) with the organization of feedback have been used. This became possible with the advent of electronic control systems for injection and ignition timing. Today, almost every car offered on the market of developed countries is equipped with such a control system. For this system to work, a special oxygen sensor is placed in the engine exhaust manifold, which is called a "lambda probe" (Fig. 1).
This improvement was a milestone in the history of the automotive industry. The working element of the sensor is a porous ceramic material based on zirconium dioxide, coated with platinum by sputtering. Exhaust gases flow around the work surface. The sensor reacts to the difference between the level of oxygen in the exhaust gases and in the atmosphere, producing the corresponding potential difference at the output. The first "lambda probes" were resistive, ie. change their resistance. Modern sensors work as threshold elements: the output voltage (Fig. 2) can take either a high logic level (rich mixture) or low (lean mixture). The reaction rate is so high that the duration of the signal front can be ignored.
The electronic control device (ECU) of the engine with a certain frequency (at least 2 times per second at idle and bowl with an increase in engine speed and a number of other parameters) removes the signal from the "lambda probe" and compares it with the value stored in the memory. If the current If the value differs from the set value (optimal, corresponding to the selected operating mode), then the controller issues a command to increase or decrease the duration of fuel injection by injectors, i.e. the engine operation mode is fine-tuned to the current situation with the achievement of maximum fuel economy and minimization of harmful emissions. The sensor can only work if the temperature of the sensitive elements is not lower than 300...350°C (otherwise it does not generate a signal), and the limiting temperature can reach 950°C. The first modifications of the "lambda probe" tried to be placed as close as possible to the exhaust manifold to ensure the fastest warm-up and enable the sensor to work. Modern probes are equipped with a special heating element, and the installation location has become less critical. The sensor device is shown in Fig.3.
Until recently, the potential difference was taken between the signal wire and the ground. Such sensors were screwed into the exhaust pipe using a special conductive lubricant. Over time, the probability of losing contact increased. Recent modifications are devoid of this shortcoming. Now the "mass" is brought out by a separate wire (Fig. 4).
The signal levels produced by most sensors are approximately the same, although there are exceptions (!). A low signal level corresponds to a voltage in the range of 0,1 ... 0,2 V. A high level corresponds to 0,8 ... 0.9 V. Proprietary sensors are quite reliable. Their service life is 80 ... 160 thousand km of car run. As the sensor ages, the voltage corresponding to a low logic level rises (more than 0,25 V), and the voltage corresponding to a high one decreases (less than 0,65 V). The “response” of the sensor also increases. If its “response” is more than 250 ms with a lean combustible mixture and more than 450 ms with an enriched one, then the sensor is considered faulty. These are averaged data. car, but such information, unfortunately, is published very little. In a car service, special motor testers are used to control the "lambda probe", but at home you can use a conventional oscilloscope. The receptacle for connecting to the sensor is usually located on the main wiring harness running under the deflectors to drain water from the windshield. The sensor contains very fragile and sensitive components. When servicing the exhaust system, it must not be subjected to impacts, nor washed with any detergents or solvents. When installing the sensor, do not use sealants that polymerize at room temperature, as well as those containing silicone (special contact grease has already been applied to the threads of new sensors). Premature failure of the "lambda probe" is most often caused by:
Possible signs of a failed oxygen sensor are:
The oxygen sensor is not an independent device. It works "in conjunction" with an exhaust gas catalytic converter (many car owners are painfully "bitten" by its price: 400 ... 600 dollars). A catalytic converter is a three-component device designed to oxidize toxic substances (carbon monoxide, hydrocarbons and nitrogen oxides) to carbon dioxide, nitrogen and water as a result of a catalytic reaction. Optimal operation of the catalyst (neutralization of approximately 80% of all components) is achieved only when the engine operates at stoichiometry for a century of mixture composition (Fig. 5).
The loss or distortion of the signal from the "lambda probe" leads to an increase in the content of toxic substances in the combustion products, and consequently, to a sharp reduction in the service life of the catalyst. It is worth noting that, until recently, the dismantling of faulty "lambda probe" and catalyst, followed by the installation of an insert pipe, was quite often practiced. Thus, a large number of foreign cars were "finalized" due to the impossibility of carrying out expensive repairs. But with the introduction of new rules, when passing a technical inspection, the presence and operability of all equipment will be required if the car was equipped with it regularly (during manufacture). Literature
Author: I.Nagovitsyn, Talagi-Neftebaza, Arkhangelsk region.
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