No modern internal combustion engine with all the power of its electronics would be worth almost a grain of grain without the electrical signals obtained from a tiny electro-mechanical element housed in the exhaust pipe of a car. Be sure to guess what element it is, it's a lambda probe…
The lambda probe is tasked to send a certain voltage signal electronic control unit (ECU) which recognizes the current composition of the air-fuel mixture. In order for the lambda probe to function properly, it must be preheated with the energy obtained from the current of hot, burnt gases to a certain temperature necessary for its proper functioning in the complete operating range of the engine.
The working principle
The lambda probe is placed in the exhaust stream and is designed so that the outer electrode is surrounded by the exhaust gas and the inner electrode is accessible to atmospheric air. The base of the lambda probe consists of a special ceramic element, the surface of which is coated with a porous platinum electrode. The probe operation is based on the fact that the ceramic material is porous and allows the diffusion (penetration) of oxygen present in the air. At higher temperatures it becomes conductive and if the oxygen concentration on one side is different from the oxygen concentration on the other then it creates stress between the electrodes. In the area of stoichiometric mixture of air and fuel (l = 1,00), there is a jump in the encoder output voltage curve. This voltage is a measuring signal.
The body of the ceramic lambda probe is housed in a hollow housing that has a protective cap and an electrical connection. The surface of the ceramic body of the lambda probe has a microporous platinum layer which on the one hand precisely affects the characteristic of the probe, while on the other hand it serves as an electrical contact. A highly adhesive and highly porous ceramic coating is applied over a platinum layer at the end of the ceramic body that is exposed to exhaust gases. This protective layer protects the platinum layer from erosion by solid particles from exhaust gases. On the side of the electrical connection (outside the exhaust pipe), a protective metal sheath is placed over the lambda probe, which is screwed into the housing. This sheath has an opening to provide pressure compensation inside the lambda probe, and also serves as a support for the disc spring. The connecting wires are twisted into the contact element and passed through the insulating sheath outside the lambda probe. To keep combustion deposits in the exhaust gas away from the ceramic body, the end of the lambda probe that penetrates the exhaust gas stream is protected by a special protective tube that has openings designed so that the exhaust gases and solid particles in it do not come into direct contact with ceramic (ZrO2 ) body.
In addition to the mechanical protection provided, the effective change of the temperature of the lambda probe during the transition from one working form to another has been successfully reduced.
The output voltage of the λ encoder, as well as its internal resistance, depends on the temperature. Reliable operation of the lambda probe is only possible at exhaust temperatures above 350 degrees Celsius (unheated version) and above 200 Celsius (heated version).
Heated lambda probe
The design of the heated lambda probe is largely identical to the design of the unheated lambda probe. The active ceramic of the lambda probe is heated from the inside by a ceramic heating element, which causes the temperature of the ceramic body to always remain above the functional limit of 250 degrees Celsius. The heated lambda probe is equipped with a protective cap that has smaller openings. Among other things, it protects the ceramic of the lambda probe from cooling when the exhaust gases are cold. Among the advantages of the heated lambda probe are: reliable and efficient control at low exhaust temperatures (eg at idle), minimal impact of changes in exhaust temperature, fast reaching the effect of lambda control after starting the engine, fast response of the encoder that prevents large deviations from ideal composition of exhaust gases, independence of the position of the encoder on the exhaust branch because it is independent of ambient heating.
Closed loop lambda control assembly
Lambda control with a closed loop represents, in fact, the existence of feedback from the lambda probe to the engine, ie to the control unit, and with its help the air-fuel ratio can be very precisely maintained at λ = 1,00. By using a closed-loop control assembly formed by means of said lambda probe, deviations from the specified air-fuel ratio can be established and corrected. This control principle is based on by measuring the oxygen content of the lambda probe in the exhaust.
Oxygen in the exhaust is a measure of the composition of the mixture of air and fuel that prima engine. The lambda probe works by sending information (electrical impulses) whether the mixture is richer or poorer than λ = 1,00. In case of deviation from this value, the voltage of the encoder output signal changes abruptly. This change is handled in a central computer unit (ECU) equipped with a closed loop control for this purpose.
Fuel injection into the engine is controlled by the injection control system, and according to the information of the lambda probe on the composition of the air-fuel mixture. This control is such that an air-fuel ratio of λ = 1 is achieved. The voltage of the lambda probe is actually a measure of correcting the amount of fuel in the mixture of air and fuel entering the cylinder.
Before giving a reliable signal, the lambda probe must reach a temperature above 350 degrees. Until that temperature is reached, the closed-loop control is suspended, and the mixture of fuel and air is formed at the middle level by the open-loop control, ie without feedback. One question logically arises here, is the value of the lambda coefficient, after reaching the operating temperature, in the entire operating mode of the engine always equal to one? Of course it is not. Depending on the current wishes and needs of the driver, this value can range from 0,8 to 1,2. If, for example, sudden and sharp acceleration is required, the central computer unit switches the fuel injection to the open loop mode and injects as much fuel as needed to achieve the desired engine operation (λ <1). The same applies in cases where engine braking is required, which is characteristic of long descents, the engine will then be injected with less fuel than usual for a certain number of revolutions (λ> 1).
Although the lambda probe operates with very high precision of ± 1%, the tolerances and aging of the motor have no effect on the closed-loop lambda control.
Prepared by: Dušan Ković
Retrieved from: www.motorna-vozila.com
Who invented and which car first used the lambda probe
The oxygen sensor was invented in 1975 by Robert Bosch engineers, in response to U.S. environmental requirements for controlling car emissions. Initially, lambda probes were only installed on gasoline vehicles with injection systems.
The first generation of the lambda probe has withstood 20.000 kilometers. And the first car in which the probe was installed in 1977 was the Volvo Model 244.
The second generation of the lambda probe appeared in 1982. These sensors have already withstood higher temperatures and longer life.
Largest lambda probe manufacturers: Bosch (Germany), Denso (Japan), NGK (Japan), Delphi (UK)…
Lambda probe service life
It depends on the ceramic tip material, the presence of the heating probe, and other factors. On average, a modern lambda probe has a life span of about 60 to 000 km, but experts advise that they should be checked every 80 km.
Defective lambda probe - causes and how to understand
The Lambda probe is one of the most sensitive sensors in the car.
How to understand that the lambda probe is faulty:
- increase in fuel consumption;
- "check engine" label;
- violation of engine idle stability;
- jerking in motion and deteriorating car acceleration dynamics;
- black smoke and an unusual smell of exhaust
However, these are pretty vague symptoms because the check engíne indicator lights up when there are many different failures in the computer, including poor quality fuel. Only the on-site diagnostics can give the correct answer, which should not be delayed. The fact is that a faulty lambda probe can significantly reduce the catalyst resource and disable other components and parts. As a result, repairs will be more expensive.
Reasons for lambda sensor failure:
- workforce completed;
- low quality fuel with high lead content;
- exhaust system overheating;
- hit the top of the probe - water, dirt or similar liquids (oil, antifreeze, brake fluid);
- improper installation of sensors;
- damage to wiring.
Defective lambda probe:
- This spare part may not be cheap (depending on the model and age of the car), and without throwing money away, it is better to let the master tell you about the lambda probe malfunction.
- Do not attempt to repair the sensor yourself - there is no generally accepted technology for repairing faulty oxygen sensors. And although sensor cleaning services are offered, their resource and performance after repair are unpredictable.
- Don't buy a used sensor - you don't know its usability or the remaining resource;
- only buy the original lmbda probe for a specific car model.
- There are universal sensors with adapters, but they are also intended for a specific model list.
- Do not try to change the sensor yourself - you will not have the necessary equipment and special grease.
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