The device and principle of operation of the oxygen sensor

Oxygen sensor - a device designed to record the amount of oxygen remaining in the exhaust gases of a car engine. It is located in the exhaust system near the catalyst. Based on the data received by the oxygen generator, the electronic engine control unit (ECU) corrects the calculation of the optimal proportion of the air-fuel mixture. The excess air ratio in its composition is indicated in the automotive industry by the Greek letter lambda (λ), due to which the sensor received a second name - lambda probe.

Excess air factor λ

Before disassembling the design of the oxygen sensor and the principle of its operation, it is necessary to determine such an important parameter as the excess air ratio of the fuel-air mixture: what it is, what it affects and why it is measured by the sensor.

In the theory of ICE operation, there is such a concept as stoichiometric ratio - this is the ideal proportion of air and fuel, at which complete combustion of fuel occurs in the combustion chamber of the engine cylinder. This is a very important parameter, on the basis of which fuel delivery and engine operating modes are calculated. It equals 14,7 kg of air to 1 kg of fuel (14,7: 1). Naturally, such an amount of the air-fuel mixture does not enter the cylinder at one point in time, it is just a proportion that is recalculated for real conditions.

Excess air ratio (λ) Is the ratio of the actual amount of air entering the engine to the theoretically required (stoichiometric) amount for complete combustion of the fuel. In simple terms, it is “how much more (less) air entered the cylinder than it should have”.

Depending on the value of λ, there are three types of air-fuel mixture:

• λ = 1 - stoichiometric mixture;
• λ <1 - “rich” mixture (excretion - soluble; deficiency - air);
• λ> 1 - "lean" mixture (excess - air; lack - fuel).

Modern engines can run on all three types of mixture, depending on the current tasks (fuel economy, intensive acceleration, reduction of the concentration of harmful substances in the exhaust gases). From the point of view of optimal values ​​of engine power, the coefficient lambda should have a value of about 0,9 (“rich” mixture), the minimum fuel consumption will correspond to the stoichiometric mixture (λ = 1). The best results for cleaning exhaust gases will also be observed at λ = 1, since the efficient operation of the catalytic converter occurs with a stoichiometric composition of the air-fuel mixture.

Purpose of oxygen sensors

Two oxygen sensors are used as standard in modern cars (for an in-line engine). One in front of the catalyst (upper lambda probe), and the second after it (lower lambda probe). There are no differences in the design of the upper and lower sensors, they may be the same, but they perform different functions.

The upper or front oxygen sensor detects the remaining oxygen in the exhaust gas. Based on a signal from this sensor, the engine control unit “understands” what type of air-fuel mixture the engine is running on (stoichiometric, rich or lean). Depending on the oxygenator readings and the required operating mode, the ECU adjusts the amount of fuel supplied to the cylinders. Typically, the fuel delivery is adjusted towards the stoichiometric mixture. It should be noted that when the engine warms up, the signals from the sensor are ignored by the engine ECU until it reaches operating temperature. The lower or rear lambda probe is used to further adjust the composition of the mixture and monitor the serviceability of the catalytic converter.

The design and principle of operation of the oxygen sensor

There are several types of lambda probes used in modern cars. Let's consider the design and the principle of operation of the most popular of them - the oxygen sensor based on zirconium dioxide (ZrO2). The sensor consists of the following main elements:

• Outer electrode - makes contact with exhaust gases.
• Internal electrode - in contact with the atmosphere.
• Heating element - used to heat the oxygen sensor and bring it to operating temperature more quickly (about 300 ° C).
• Solid electrolyte - located between two electrodes (zirconia).
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• Tip guard - has special holes (perforations) for exhaust gases to enter.

The outer and inner electrodes are platinum-coated. The principle of operation of such a lambda probe is based on the occurrence of a potential difference between platinum layers (electrodes), which are sensitive to oxygen. It occurs when the electrolyte is heated, when oxygen ions move through it from atmospheric air and exhaust gases. The voltage at the sensor electrodes depends on the oxygen concentration in the exhaust gases. The higher it is, the lower the voltage. The oxygen sensor signal voltage range is 100 to 900 mV. The signal has a sinusoidal shape, in which three regions are distinguished: from 100 to 450 mV - lean mixture, from 450 to 900 mV - rich mixture, 450 mV corresponds to the stoichiometric composition of the air-fuel mixture.

Oxygenator resource and its malfunctions

The lambda probe is one of the most quickly worn out sensors. This is due to the fact that it is constantly in contact with exhaust gases and its resource directly depends on the quality of the fuel and the serviceability of the engine. For example, a zirconium oxygen tank has a resource of about 70-130 thousand kilometers.

Since the operation of both oxygen sensors (upper and lower) is monitored by the OBD-II on-board diagnostics system, if any of them fails, a corresponding error will be recorded, and the “Check Engine” indicator lamp on the instrument panel will light up. In this case, you can diagnose a malfunction using a special diagnostic scanner. From the budget options, you should pay attention to the Scan Tool Pro Black Edition.

This Korean-made scanner differs from analogues in its high build quality and the ability to diagnose all components and assemblies of a car, and not just the engine. He is also able to track the readings of all sensors (including oxygen) in real time. The scanner is compatible with all popular diagnostic programs and, knowing the permissible voltage values, you can judge the health of the sensor.

When the oxygen sensor is working properly, the signal characteristic is a regular sinusoid, showing a switching frequency of at least 8 times within 10 seconds. If the sensor is out of order, then the signal shape will differ from the reference one, or its response to a change in the mixture composition will be significantly slowed down.

The main malfunctions of the oxygen sensor:

• wear during operation (sensor “aging”);
• open circuit of the heating element;
• pollution.

All these types of problems can be triggered by the use of low-quality fuel, overheating, the addition of various additives, the ingress of oils and cleaning agents into the operating area of ​​the sensor.

Oxygenator malfunction signs:

• Malfunction warning light indication on the dashboard.
• Loss of power.
• Poor response to the gas pedal.
• Rough engine idling.

Types of lambda probes

In addition to zirconia, titanium and broadband oxygen sensors are also used.

• Titanium. This type of oxygen chamber has a titanium dioxide sensitive element. The operating temperature of such a sensor starts from 700 ° C. Titanium lambda probes do not require atmospheric air, since their principle of operation is based on a change in the output voltage, depending on the concentration of oxygen in the exhaust.
• The broadband lambda probe is an improved model. It consists of a cyclone sensor and a pumping element. The first measures the concentration of oxygen in the exhaust gas, recording the voltage caused by the potential difference. Next, the reading is compared with the reference value (450 mV), and, in the event of a deviation, a current is applied, provoking the injection of oxygen ions from the exhaust. This happens until the voltage becomes equal to the given one.

The lambda probe is a very important element of the engine management system, and its malfunction can lead to difficulties in driving and cause increased wear of other engine parts. And since it cannot be repaired, it must be immediately replaced with a new one.