Lambda probe

Where R is the gas constant, T is the temperature, and F is the Faraday constant. Caused by the logarithmic correlation between the gas concentration and the voltage signal, the potentiometric measurement is best suited for measurements of small amounts of oxygen. A well-known application of this principle has been realized in the so called lambda-probe for automotive applications where they are used to control the lambda value within a small interval around 1 = 1. The lambda-value is defined by the relation between the existing air/fuel ratio and the theoretical air/fuel ratio for a stoichiometric mixture composition ... 

Potentiometric lambda-probes are not suitable for measuring the oxygen concentration at 1 3> 1. 

In this section we will have a closer look at the sensors which are used for measuring substances, the so-called chemical sensors. A very well-known example of such a sensor in a chemical laboratory is the pH-meter used to measure the acidity of solutions. In particular the so-called Lambda-probe, abbreviated l-probe, which is used in cars to optimize the combustion process will be discussed in more detail. 

Electrolytes are used in electrochemistry to ensure the current passage in -> electrochemical cells. In many cases the electrolyte itself is -> electroactive, e.g., in copper refining, the copper(II) sulfate solution provides the ionic conductivity and the copper(II) ions are reduced at the - cathode simultaneous to a copper dissolution at the - anode. In other cases of -> electrosynthesis or - electroanalysis, or in case of - sensors, electrolytes have to be added or interfaces between the electrodes, as, e.g., in case of the -> Lambda probe, a high-temperature solid electrolyte. 

The cell reaction is the transfer of oxygen from one side to the other. (See also Lambda probe). In the case of an - electrochemical equilibrium (subentry of -> equilibrium) the measured -> open circuit potential (subentry of - potential) or -> equilibrium potential (subentry of -> potential) Ueq or E (emf) can be calculated by the -> Nernst equation ... 

Lambda probe — The so-called lambda probe (lambda sensor) is a potentiometric oxygen - gas sensor that is used to monitor the partial pressure of oxygen po2 (oxygen concentration), e.g., in the exhaust gas of cars in order to control the air/fuel ratio that is going to the internal combustion engine. The term lambda originates from the stoichiometry number A which describes the composition of the air/fuel mixture, as follows ... 

The Lambda probe consists of a solid-state electrolyte (yttrium-stabilized zirconia) that is covered with porous platinum - electrodes on the inside (1) and outside (2) (see Fig.). 

The Lambda probe is usually shaped like a sparking plug where the solid-state electrolyte forms a closed-end cylinder that it inserted into the exhaust gas. The inside of the cylinder is in contact with ambient air with a constant partial pressure of oxygen, po2,i> and the outside is in contact with the exhaust gas in which the partial pressure of oxygen, po2,2, is monitored by measuring the potential difference between the two platinum electrodes... 

Oxygen sensors — Device for measurement of oxygen concentration or activity. See - Clark cell, - lambda-probe. 

Many - gas sensors based on - solid electrolytes operate under potentiometric conditions [iii]. The sensors for oxygen use oxide -> conductors, such as ZrC>2 -based ceramic, those for halogens use halide conductors (e.g., KAg s), while -> hydrogen sensors use protonic conductors. There are sensors for C02, N02, NH3, S03) H2S, HCN, HF, etc. (see -> lambda probe). 

This working principle is the one in the lambda probes used to control combustion in engines. If the two electrodes are under different oxygen partial pressures, then an electromotive force is produced following the Nemst law (Equation [16.4]). In practical devices, one of the electrodes is in permanent contact with a reference atmosphere (see Fig. 16.2). 

Comment the lambda value measured with the help of lambda probes in cars is nothing but a degree of titration, here the degree of oxidation of the petrol by the oxygen of the air. The burning of petrol can be considered a redox titrationi... 

If two such electrodes are separated by a thin layer of only zirconia, the application of a potential will lead to the pumping of oxygen from the cathode to the anode. This device can be used as an amperometric sensor for oxygen if a diffusion barrier restricts the flux of oxygen to the cathode. Note that similar devices are also often used as potentiometric sensors according to the Nernst equation (i.e., the lambda-probe in cars with catalytic converters). In this case one side of the cell has to act as a reference, e.g., by using ambient air. 

Lambda Probe Readout for Carburetor Tuning Improving the Combustion Efficiency by Lambda Sensor... 

The length of time over which a chemical sensor can be expected to function reliably can be remarkably great (in the range of years), as in the case of glass-membrane or solid-state membrane electrodes, the lambda probe (Section 28.2.2.3), and the Taguchi gas sensor (see Section 28.2.2.2). On the other hand, a biosensor that depends upon a cascade of enzymes to... 

Figure 10. Schematic representation of a lambda probe operated as a potentiomelric sensor...

Galvanic cells can be set up with solid electrolytes rather than electrolytic solutions. Such a cell is the basis for a well-known potentiometric gas sensor, the lambda probe. The latter is designed to determine the oxygen content of combustion gases, e.g. in motor vehicles. The lambda probe can operate in two different modes, either potentiometrically or amperometrically. 

The so-caUed Lambda probe to this day is the most successful and most widespread chemical sensor. It is used in all modern motor vehicles, where it serves to control the oxygen content in combustion gas. The combustion process in the motor is regulated to minimize the output of poisonous carbon monoxide. 

The Lambda probe differs from all electrochemical sensors discussed so far in that it works with a soUd electrolyte rather than an electrolyte solution. The sample is in the gaseous state, and the working temperature is ca. 500°C. 

A schematic view of a lambda probe is given in Fig. 7.12. A hollow part in the form of a finger made of zirconium dioxide ceramics protrudes into the studied gas compartment. The ceramic body is covered on either side by a gas-permeable platinum layer. Its inner volume is in contact with ambient air. The emf E is measured between the inner and outer platinum layers. The oxygen partial pressure in the compartment studied is calculated by means of Eq. (7.11). 

Figure 7.12. Lambda probe. Left functional scheme, right technical...

The signal of the lambda probe in the exhaust pipe is fed into an electronic control circuit which keeps the actual lambda value close to 1, i.e. so that nearly complete combustion is achieved. The fuel-air composition under these conditions is a rich rather than a lean mixture. The exhaust gas then contains CO, NO and residues of hydrocarbons. These components are converted by means of the three-way catalytic converter into the less harmful gases N2, H2O and CO2. If the lambda value is higher or lower than the optimum, then the poisonous gases canot be converted properly. The optimum APR is ca. 14.7, i.e. 14.7kg of air are needed to burn 1kg of fuel. 

---