Use of gas analyzers. Why do you need a gas analyzer - what is it and how does the device work (85 photos)

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The purpose of gas analyzers is to measure and control the concentration of gases. In technological processes of metallurgical production, the concentration of various gases is controlled: flammable gases, combustion products, protective atmospheres, technological process gases, harmful and explosive impurities, etc. Monitoring the composition of gases in some cases makes it possible to judge the correctness of the technological process. For example, based on the composition of the top gas in a blast furnace, the smelting process is carried out; rate of carbon oxidation in a liquid bath...

Question 6. Purpose, operating principle and types of gas analyzers.

The purpose of gas analyzers is to measure and control the concentration of gases.

In technological processes of metallurgical production, the concentration of various gases is controlled: flammable gases, combustion products, protective atmospheres, process gases, harmful and explosive impurities, etc.

Monitoring the composition of gases in some cases makes it possible to judge the correctness of the technological process. For example, according to the composition of the top gas in a blast furnace, the smelting process is carried out; the rate of carbon oxidation in the liquid bath, which characterizes the progress of converter smelting, is determined based on gas analysis for the content of CO and CO2; Continuous monitoring of the combustion regime under operating conditions at modern thermal power plants is carried out using automatic gas analyzers for the content of O2 in combustion products (flue gases), etc.

An industrial gas analyzer usually consists of a sample preparation device, a receiver and a measuring device.

The sample preparation device is designed to take a sample of the analyzed gas mixture from a technological object, clean the sample from aggressive and mechanical impurities, and bring its parameters (temperature, pressure, etc.) to the values ​​​​normalized for the sample parameters at the input of the gas analyzer receiver.

The gas analyzer receiver is designed to generate unified output signals, the value of which is equivalent to the content (concentration) of the measured component in the gas mixture.

As a rule, standard measuring instruments are used as measuring devices.

Types of gas analyzers: thermoconductometric; thermomagnetic; electrochemical; absorption based infrared radiation: optical-acoustic and absorption.

Gas analyzers - instruments that measure the content (concentration) of one or more components in gas mixtures. Each gas analyzer is designed to measure the concentration of only certain components against the background of a specific gas mixture under standardized conditions. Along with the use of individual gas analyzers, systems are being created gas control, uniting dozens of such devices.

Gas analyzers are classified by type into pneumatic, magnetic, electrochemical, semiconductor, etc.

Thermoconductometric gas analyzers. Their action is based on the dependence of the thermal conductivity of the gas mixture on its composition.

Thermal conductometric gas analyzers do not have high selectivity and are used if the controlled component in thermal conductivity differs significantly from the rest, for example. to determine the concentrations of H 2, He, Ar, CO 2 in gas mixtures containing N 2, O 2, etc. The measurement range is from units to tens of percent by volume.

Thermochemical gas analyzers. These instruments measure the thermal effect of a chemical reaction in which the component being determined is involved. In most cases, oxidation of the component with atmospheric oxygen is used; catalysts - manganese-copper (hopcalite) or finely dispersed Pt deposited on the surface of a porous support. The change in temperature during oxidation is measured using metallic. or semiconductor thermistor. In some cases, the surface of a platinum thermistor is used as a catalyst. The value is related to the number of moles of the oxidized component and the thermal effect by the relation: , where k-coefficient, taking into account heat loss, depending on the design of the device.

Magnetic gas analyzers. This type is used to determine O2. Their action is based on the dependence of the magnetic susceptibility of a gas mixture on the concentration of O 2, the volumetric magnetic susceptibility of which is two orders of magnitude greater than that of most other gases. Such gas analyzers make it possible to selectively determine O2 in complex gas mixtures. The range of measured concentrations is 10 -2 - 100%. Magnetic mechs are the most common. and thermomagnetic gas analyzers.

Magnetic-mechanical gas analyzers measure the forces acting in a non-uniform magnetic field. field on a body (usually a rotor) placed in the mixture being analyzed.

Pneumatic gas analyzers. Their action is based on the dependence of the density and viscosity of the gas mixture on its composition. Changes in density and viscosity are determined by measuring the fluid mechanics. flow parameters.

Infrared gas analyzers. Their action is based on the selective absorption of IR radiation by molecules of gases and vapors in the range of 1-15 microns. This radiation is absorbed by all gases whose molecules consist of at least two different atoms.

Ultraviolet gas analyzers. The principle of their operation is based on the selective absorption of radiation by molecules of gases and vapors in the range of 200-450 nm. The selectivity of the determination of monatomic gases is very high. Di- and polyatomic gases have a continuous absorption spectrum in the UV region, which reduces the selectivity of their determination. However, the absence of a UV absorption spectrum for N 2, O 2, CO 2 and water vapor allows, in many practically important cases, to carry out fairly selective measurements in the presence. these components. The range of determined concentrations is usually 10 -2 -100% (for Hg vapor the lower limit of the range is 2.5-10 -6%).

Luminescent gas analyzers. Chemiluminescent gas analyzers measure the intensity of luminescence excited due to the chemical reaction of the controlled component with a reagent in the solid, liquid or gaseous phase.

Photocolorimetric gas analyzers. These devices measure the color intensity of selected products. relationships between the component being determined and a specially selected reagent. The reaction is carried out, as a rule, in solution (liquid gas analyzers) or on a solid carrier in the form of a tape, tablet, or powder (respectively, tape, tablet, powder gas analyzers).

Photocolorimetric gas analyzers are used to measure the concentrations of toxic impurities (for example, nitrogen oxides, O 2, C1 2, CS 2, O 3, H 2 S, NH 3, HF, phosgene, a number of organic compounds) in the industrial atmosphere. zones and in industrial air. premises. Portable intermittent devices are widely used to monitor air pollution. A large number of photocolorimetric gas analyzers are used as gas detectors.

Electrochemical gas analyzers. Their action is based on the relationship between the electrochemical parameter. system and the composition of the analyzed mixture entering this system.

In conductometric gas analyzers, the electrical conductivity of a solution is measured when it selectively absorbs the component being determined. The disadvantages of these gas analyzers are low selectivity and the time required to establish readings when measuring small concentrations. Conductometric gas analyzers are widely used to determine O 2, CO, SO 2, H 2 S, NH 3, etc.

Ionization gas analyzers. The action is based on the dependence of the electrical conductivity of gases on their composition. The appearance of impurities in the gas has an additional effect on the formation of ions or on their mobility and, consequently, recombination. The resulting change in conductivity is proportional to the impurity content.

All ionization gas analyzers contain flow ionization. a chamber in which a certain potential difference is applied to the electrodes. These devices are widely used to monitor microimpurities in the air, as well as as detectors in gas chromatographs.

In a modern diagnostic area, a gas analyzer is one of the basic devices.

Purpose of the gas analyzer

Unfortunately, in the minds of many car service specialists, a gas analyzer is still associated with carburetor adjustment. This is wrong.

Of course, monitoring the toxicity of exhaust gases (EG) is an important function of a car gas analyzer, but, nevertheless, it is far from the only one.

The device is capable of solving a wide range of tasks to study the condition of the engine and its systems, being a rich source of diagnostic information. It is safe to say that a gas analyzer is one of the main diagnostic tools.

Just as a doctor needs patient tests to make a diagnosis, a diagnostician needs “analysis” data to identify engine “diseases,” because the composition of the exhaust gas directly depends on its condition.

Evolution of the gas analyzer

The first samples of gas analyzers used to adjust the engine measured only the concentration of carbon monoxide CO from the entire set of exhaust gas components. The devices were single-component.

Analysis of CO concentration made it possible to draw a conclusion about the qualitative ratio of the fuel-air mixture and was used mainly when adjusting carburetors. Such gas analyzers had a arrow display of analysis results and worked on the principle of measuring the electrical conductivity of a platinum spiral in a carbon monoxide environment.

By the 70s of the last century, the need to control automobile toxic emissions became acute. The level of technological development of those years made it possible to create two-component automobile gas analyzers capable of measuring the concentration of another harmful component - unburned fuel, designated CH. These instruments worked on the principle of spectrometry of the gases under study in the infrared range, which is still used today.

Further development of automobile gas analyzers led to the emergence of three-, four- and even five-component devices that make it possible to measure the concentration of not only the above-mentioned carbon monoxide CO and hydrocarbons CH, but also carbon dioxide CO 2, oxygen O 2 and nitrogen oxides NO x, as well as calculate air-fuel ratio in the original fuel-air mixture.

Spectrometric gas analyzer unit: operating principle

The operating principle of the spectrometric unit of the gas analyzer is based on the effect of partial absorption of the energy of the light flux passing through the gas.

The molecules of each gas are a vibrational system capable of absorbing infrared radiation in a strictly defined wavelength range. If a stable infrared stream is passed through a flask with gas, then part of it will be absorbed by the gas. Moreover, only a certain part of the flow spectrum, called the absorption maximum of a given gas, will be absorbed. The higher the gas concentration in the flask, the greater the absorption will be observed.

The fact that different gases have different absorption maxima makes it possible to measure the concentration of gases in a mixture by measuring the absorption of the corresponding wavelength. In other words, the concentration of each gas in the exhaust gas can be determined by analyzing the decrease in the intensity of the light flux in the part of the spectrum corresponding to the absorption maximum of a given gas.

The spectrometric unit of the device is designed as follows

Pre-filtered exhaust gases are pumped through a measuring cell, which is a tube with ends covered with optical glass. On one side of the tube there is an emitter. It is a heated electric shock a spiral whose temperature is strictly stabilized. The emitter generates a stable stream of infrared radiation.

On the opposite side of the tube, light filters are installed, which isolate the required wavelengths from the entire flow, corresponding to the absorption maxima of the gases under study.

After passing through the light filters, the flow enters the infrared radiation receiver. The receiver measures the flow intensity and generates information about the concentration of gases in the mixture.

In this way, the concentration of CO, CH and CO 2 is determined. Subsequently, the mixture of gases from the measuring cell enters sequentially into electrochemical type sensors, which generate an electrical signal, the voltage of which is proportional to the concentration of oxygen O 2 and nitrogen oxides NO x.

In a modern device, the concentrations of CO, CH and CO 2 are measured using the described spectrometric method, and the concentrations of oxygen O 2 and nitrogen oxides NO x are measured using electrochemical sensors.

Processing of signals from sensors and the spectrometric unit in a modern gas analyzer is carried out electronic circuit, built on a microprocessor.

On the display of the device, information about the content of CO, CO 2 and O 2 is displayed as a percentage, and CH and NO x - in the so-called ppm (parts per million), “parts per million”. This designation is due to the extremely low concentration of the named components in the exhaust gas and the inconvenience of using percentages to indicate their quantity. The relationship between percentage and ppm looks like in the following way:

10,000 ppm = 1%

Therefore, the amount of, for example, CH in the exhaust gas of a typical engine would be about 0.001% -0.01%. It is difficult to work with such numbers; as a result, it is customary to use ppm.

Gas analyzer- the device is complex, and its quality is determined by the accuracy and reliability of the components, primarily the spectrometric unit.

Structurally and technologically, the spectrometric unit is so complex and specific that its production at the proper level in terms of quality has been mastered by only a few companies around the world.

Manufacturers of gas analyzers directly use ready-made spectrometric units, integrating them into their devices. This approach justifies itself, and in a device made in Russia, Italy or Korea you can find a spectrometric unit made in Japan or America.

Spectrometric unit - expensive device, which makes up a significant part of the cost of the device.

During operation, it is very important to ensure its durability. Mechanical particles, soot and moisture, settling on the walls of the block, lead to a significant drift in its readings and even to its complete inoperability.

Therefore, before entering the measuring unit, the exhaust gases undergo preparation, which is usually carried out in several stages:

• rough cleaning of exhaust gases. This is done with a filter installed at the inlet of the device or in the handle of the sample collection probe. Large mechanical particles and soot are filtered out.
• moisture separator. He may be the most various designs. The purpose is to separate moisture droplets condensing on the internal surfaces of the probe and connecting hose from the gas flow and remove them. Removal is carried out automatically or manually by the operator by periodically draining the condensate from the storage tank.
• fine filter. With its help, final filtration from the smallest mechanical particles is carried out. Several filters can be installed sequentially one after another.

What you need to know when using gas analyzers

The design feature of the device affects its operation and recommendations for its care. As a rule, operating a car gas analyzer is not very difficult and is performed by one operator.

Before performing measurements, it is necessary to correct the zero of the device, for which you press the corresponding button on the front panel. Some gas analyzers perform zero correction automatically after a specified period of time; in this case, operator intervention is not required.

To take readings, you need to install the probe into the exhaust pipe of the car to a depth of at least 300 mm and secure it with a clamp. Such a significant depth is required in order to prevent suction into the probe atmospheric air and obtaining false evidence.

Next, you need to start the measurement and wait for the steady readings on the instrument display. The duration of the process of setting the readings usually ranges from 15 to 45 seconds and depends on the length of the hose and the design of the pneumatic path, which can vary significantly between devices from different manufacturers.

Based on many years of experience in operating gas analyzers, the following recommendation can be made.

After each measurement, disconnect the hose with the probe from the device and blow it in the opposite direction compressed air to remove condensate. Most often, a very significant release of moisture is observed. Of course, the built-in moisture separator performs its function, but, nevertheless, following this recommendation seems to be a measure that increases the likelihood of trouble-free operation of the device.

Maintenance of the gas analyzer comes down mainly to periodic replacement of fine and coarse filters. Recommendations for replacing them are given in the operating instructions for the specific device.

It is very important to pay attention to the following point: fine filters used in gas analyzers differ from gasoline filters and the use of the latter in gas analyzers is unacceptable.

It is also important to ensure that the filters are dry. Wet filters must either be dried by blowing air in the direction opposite the arrow marked on the housing, or replaced.

Exhaust gas composition analysis

The most important thesis that needs to be voiced before presenting the methodology for analyzing the composition of exhaust gases is the following.

For a competent and correct analysis, an absolute understanding of where this or that component comes from in the composition of exhaust gases is required.

It is necessary to clearly understand the flow of processes in the cylinders and exhaust tract of the engine, the chemical transformations that occur during this process, and be based on this understanding.

With this approach, the diagnostician begins to think and competently analyze the composition of the exhaust gas, seeing cause-and-effect relationships. An approach like “if the composition of the exhaust gas is such and such, then such and such a defect occurs” does not seem constructive and will not be considered.

First of all, let us remember the composition of atmospheric air from a school chemistry course. This will be required for a correct understanding of the processes occurring in the cylinders and in the exhaust tract of the engine.

The remaining gases, mostly inert, are present in small quantities and in our case do not play a big role, just like argon. Figures very close to those given can be seen on the gas analyzer display if you start the measurement “in the fresh air”.

So, the working mixture burns in the engine cylinders. The oxidation reaction of fuel hydrocarbons occurs according to the following scheme:

CH + O 2 => CO 2 + H 2 O.

Let us recall that the composition of the mixture is usually assessed by the coefficient of excess air λ. It represents the ratio of the actual amount of air entering the cylinders to the theoretical amount required for complete combustion of the fuel. Mixtures in which the amount of air coincides with the theoretically required one are called stoichiometric. In this case λ=1. If the amount of air is more than necessary, then the mixture is usually called lean, and the coefficient is in the range λ=1.0...1.3. A leaner mixture stops igniting. If there is less air than necessary, then the mixture is called rich. Such a mixture is characterized by the value λ=0.8...1.0.

It would seem that during the combustion of a stoichiometric mixture, the exhaust gases should consist of carbon dioxide CO 2, water vapor H 2 O and nitrogen N 2. But in practice, everything happens differently. Under the influence of high temperature in the engine cylinder, nitrogen and oxygen react, resulting in the formation of nitrogen oxides. The combination of these oxides is designated NO x and is displayed by five-component gas analyzers. The formation of NO x increases greatly with increasing gas temperature and oxygen concentration. The main component in the mixture of nitrogen oxides is NO monoxide. After leaving the engine cylinders, it is oxidized in the atmosphere to NO 2 dioxide, which is much more toxic and, when combined with water vapor in the atmosphere, forms acid rain.

In addition, exhaust gas always contains CH hydrocarbons. They represent original or decayed fuel molecules that did not take part in combustion, as well as breakdown products of motor oil. Hydrocarbons appear in the exhaust gas as a result of flame extinguishing near the relatively cold walls of the combustion chamber, in pinched volumes such as the space between the piston and the cylinder above the upper compression ring.

Part of the CH is emitted as a result of the fact that during the intake and compression strokes of the combustible mixture, fuel vapors are absorbed by the oil film on the cylinder walls. At the stroke and release stroke, they are released from the film. A similar effect of absorption of fuel vapor is observed on soot covering the walls of the combustion chamber.

Further, the exhaust gas must contain a product incomplete combustion fuel - carbon monoxide CO (carbon monoxide). It is formed mainly during the combustion reaction with a lack of oxygen, so the main influence on the formation of CO in gasoline engines is the composition of the mixture: the richer it is, the higher the concentration of CO.

It should be noted that this component is perhaps the most dangerous in terms of its effects on the human body. Carbon monoxide is colorless and odorless, but when inhaled it combines with hemoglobin in the blood and can be fatal in high concentrations.

Of course, the exhaust gas will inevitably contain oxygen that has not reacted. It should be noted that oxygen may end up in the exhaust gas not from the engine cylinders, but from atmospheric air entering through leaks in the exhaust tract.

Catalytic converter

Numerous studies have shown that improving the combustion process, optimizing the control of the mixture composition and ignition timing do not reduce exhaust toxicity to at least a level that ensures compliance with Euro II standards, not to mention higher requirements.

To solve the problem, it was proposed to use additional exhaust gas treatment in the engine exhaust tract. The devices that perform this treatment are called catalytic converters.

The main parts of the catalytic converter are:

• heat-resistant stainless steel housing
• block carrier, which is a honeycomb structure made of ceramics or corrugated foil with a thickness of 0.1..0.5 mm
• layer with a porous structure made of aluminum oxide
• active catalytic layer

The carrier block consists of several thousand thin channels through which exhaust gases flow. The channels of the ceramic or metal carrier block are covered with a very porous layer. This increases the useful surface area of ​​the catalytic converter by approximately 7,000 times, which ensures the necessary mass transfer between the exhaust gas and the active catalyst. A catalytically active layer is applied to the interlayer.

The three-way catalytic converter has a catalytically active layer of platinum (Pt), rhodium (Rd) and palladium (Pd). The name “three-way catalytic converter” means that three chemical conversion reactions occur simultaneously and in parallel in one housing.

For the normal course of these reactions, it is necessary to maintain a high temperature in the neutralizer within the range of 400...800°C. With more low temperatures The effectiveness of the neutralizer is low, and at temperatures above 1000°C, thermal destruction of the active layer occurs and even sintering of the honeycomb of the carrier block occurs.

Without going into details of the chemical reactions occurring on the surface of the active layer, we can only give simplified final results:

• NO x are reduced to pure nitrogen N 2 with the release of free oxygen O 2
• CO is oxidized to CO 2, which consumes oxygen O 2
• CH hydrocarbons are oxidized to CO 2 and H 2 O, and oxygen O 2 is also consumed

A distinctive feature of the three-component catalytic converter is that for its full operation the engine must operate on a stoichiometric fuel-air mixture. This is explained as follows. Only at λ = 1 is an exhaust gas composition obtained in which the free oxygen released during the reduction of nitrogen oxides is sufficient for the complete oxidation of CO and CH to CO 2 and H 2 O.

This fact is so important that it bears repeating: Full functioning of the catalytic converter is only possible if the engine runs on a stoichiometric mixture.

The literature even uses the term “catalysis window,” which refers to the range of values ​​of λ at which the neutralizer is able to perform its function. Strictly speaking, this range is shifted from stoichiometry towards a rich mixture, and is approximately within the range λ = 0.98..0.99. Maintaining the mixture composition in a given range is entrusted to the engine control system, for which it includes an oxygen concentration sensor in the exhaust gas.

It is also necessary to mention engines with direct fuel injection. Such engines in some modes can operate on ultra-lean mixtures, which leads to a significant increase in the proportion of nitrogen oxides NOx. Therefore, to neutralize NO x, another catalyst, the so-called storage type, is installed in the exhaust tract.

To better understand the operation of the catalytic converter, the following experiment was conducted.

A VAZ 2112 car was taken, equipped with a VS5.1 ECU with firmware V5D07X09, which supports fuel supply adjustment from diagnostic equipment.

1. The neutralizer is present. Readings of CO, CO 2 , O 2 , CH and λ were recorded when the adjustment coefficient changed in the range from −0.250 to +0.250.
2. An insert pipe was installed instead of the neutralizer, and the measurements were repeated.

The results are displayed in graphs. The solid line corresponds to measurements with a neutralizer, the broken line - without it.

The graphs were constructed manually, with slight interpolation. One nuance should be noted - for some reason the device showed incorrect CO 2 values ​​when measured with a neutralizer. This probably happened due to long work engine at low speed and, accordingly, reducing the temperature of the converter. With this caveat, you can pay attention to the results obtained and analyze them:

The first thing that catches your eye is that the value of λ in both cases practically coincided.

In the range of rich mixtures, the points generally formed one line; in the range of poor mixtures, a discrepancy is observed at the level of measurement error. And only on the poorest mixtures the difference is noticeable, but, probably, in that range it is simply impossible to correctly calculate λ.

Conclusion: Regardless of the presence or absence of a neutralizer, the calculated value of λ remains the same. Actually, it couldn’t be any other way, because the value of λ characterizes only the operation of the engine, no matter with or without a neutralizer.

The CH value behaves very curiously. Without a neutralizer, a classic dependence is observed. With the neutralizer the picture is more interesting. It has a strong effect in the lean range. Near stoichiometry, a characteristic depression is observed, corresponding to the catalysis window. Moreover, with a slight enrichment of the mixture relative to stoichiometry, a very sharp jump in the CH value occurs, and then it is almost compared with the value obtained without the neutralizer.

The same can be said about CO graphs. The range in the stoichiometric region, where the efficiency of the neutralizer is maximum, is clearly visible, and the graphs accordingly vary as much as possible.

CO 2 graphs also have an academic look. The amount of CO 2 in the exhaust gas is greater in the case of a neutralizer. This is explained by the fact that the latter converts hydrocarbons and carbon monoxide contained in the exhaust gas into CO 2. When deviating from stoichiometry, both in the direction of depletion and in the direction of enrichment of the mixture, the amount of CO 2 decreases.

This is very important point: maximum amount CO 2 in the exhaust gas approximately corresponds to the stoichiometric mixture.

Calculated coefficient λ

The excess air coefficient λ deserves a separate discussion. It should be clearly understood that the λ value displayed on the device display is not a real, but a calculated coefficient. It is calculated by the gas analyzer processor based on the amount various components as part of the OG. The calculation is made using the so-called Bertschneider formula:

The formula is given as reference material and we will not go into detail.

The calculated value of λ will correspond to the real value only if the engine exhaust tract is completely sealed and the measuring elements of the gas analyzer are calibrated. If the exhaust tract is leaky (there are atmospheric air leaks), then the calculated value of λ may not only be incorrect, but also exceed all reasonable limits. This is explained by the fact that the Bertschneider formula uses the oxygen content in the exhaust gas, and any appearance of excess oxygen leads to a significant error in calculating this coefficient.

Composition of exhaust gas from a serviceable engine

Taking into account all of the above, it is necessary to announce the composition of the exhaust gases of a working engine. It should be noted in advance that in the future we will talk about working with a four-component device, since five-component devices, which display, among other things, the amount of NO x, are practically not used in diagnostic areas due to their high price. The figures that will be given below are obtained from many years of experience in using gas analyzers.

Before naming them, let us focus on the following point.

The vast majority of modern gasoline engines are equipped with a catalytic converter. Therefore, the exhaust gas compositions of such an engine and an engine not equipped with a converter will differ significantly. Based on this consideration, it seems most correct to consider the composition of the exhaust gas in the exhaust tract before and after the converter. These numbers are the standard from which all subsequent conclusions are drawn; one might say, this is the basis of gas analysis. They need to be remembered and constantly kept in mind. So,

The composition of the exhaust gas of a serviceable engine, warmed up to operating temperature, running on a stoichiometric mixture in the exhaust tract before the catalytic converter is as follows: (Table 1)

The composition of the exhaust gas of a serviceable engine, warmed up to operating temperature, running on a stoichiometric mixture, with a serviceable and warmed-up catalytic converter, in the exhaust tract after the converter is as follows: (Table 2)

The lower values ​​of CO and CH in the second case are explained by the course of chemical reactions in the neutralizer. The percentage of oxygen also decreased due to its consumption in oxidation reactions. The amount of carbon dioxide CO 2 increased due to the oxidation of CO.

Here we do not see nitrogen oxides NOx, but we must not forget that in the neutralizer they were reduced to pure nitrogen and lost their harmful effect on the environment. Note that the value of λ is 1 in both cases.

The considered gas analysis parameters are reference ones; this is what will appear on the instrument display with a fully operational, warmed-up engine running on a stoichiometric mixture. Now let's talk about deviations that occur in practice and about the analysis of the composition of the exhaust gas in these cases.

Exhaust tract leaking

We should not forget that the movement of gases in the exhaust tract is of a complex wave nature, and pressure zones alternate with rarefaction zones.

When the leak in the duct is in a pressure zone, the exhaust gases rush out with a characteristic sound (the tract “cuts”), and when it is in a vacuum zone, atmospheric air enters the exhaust tract. Now let's remember its composition. Even if the leakage is insignificant, the O 2 content in the exhaust gas will increase very much, because in the air it is almost 21%, and in the exhaust gas about 0.5%. At the same time, there is little CO 2 in the air, and the amount of this gas in the exhaust gas will not change so significantly. The same can be said about the content of CO and CH.

In the case of air leaks into the exhaust tract, there is an unnaturally large amount of O 2 in the exhaust gas. It can be argued that the first parameter that needs to be assessed when analyzing the composition of exhaust gases is precisely the oxygen content. If it exceeds 1.5..2%, then there is suction of atmospheric air into the exhaust tract.

Further analysis makes no sense without eliminating tract defects. It must be noted that a large amount of oxygen in the exhaust gas will also be observed during misfires, but they are characterized by a large amount of unburned fuel, and it is almost impossible to confuse these two situations.

Of course, in the presence of suction, it is simply pointless to analyze other parameters of the exhaust gas composition. We only note that the calculated coefficient λ in such a situation acquires prohibitive values. Indirectly, they also indicate the described defect.

Rich mixture

In this case, λ ‹ 1, there is less air in the mixture than necessary for complete combustion. It is easy to come to the conclusion that with a lack of oxygen, combustion does not occur completely and the exhaust gas contains more CH than with a stoichiometric mixture. The CO content will increase for the same reason. The amount of CO 2 will be less than when working on a stoichiometric mixture, because the fuel did not burn in an optimal way. Therefore, the exhaust gas composition of an engine running on a rich mixture without a converter looks approximately like this: (Table 3)

It should be noted that in the presence of a catalytic converter, a slight enrichment of the mixture in terms of exhaust gas composition may not be detected, but any serious deviation will lead to exit from the catalytic window and a clear deviation of the exhaust gas composition from the norm. In this case, the numbers on the device display will be similar to those given above.

In relation to modern engines, the causes of a rich mixture include increased fuel pressure, drift of the mass air flow sensor characteristics, and fuel flow through a leaky membrane of the vacuum pressure regulator (on systems with reverse fuel drainage).

The cause may also be a faulty DTOZH; such a defect is easily detected by scanner readings. Separately, mention should be made of such a tricky defect as air leaks into the exhaust tract in front of the oxygen signal sensor. In such a situation, atmospheric oxygen is detected by the sensor, which leads to a significant enrichment of the mixture and even the appearance of a corresponding fault code.

Another source of excess fuel in the mixture is motor oil.

Here we should make a small digression. The fact is that the oil film on the cylinder surface plays an important role in the formation of the working mixture and the processes occurring in the combustion chamber. If for some reason the engine runs for a long time on a too rich mixture or simply does not start the first time, which very often happens in winter, then gasoline gets into the oil.

It can be assumed that unburned gasoline flows down the cylinder walls or simply penetrates through the piston ring locks. One way or another, gasoline gets into the oil, and we must accept this as reality. In what ways does it subsequently enter the combustion chambers? There are two assumptions. Gasoline vapors, together with crankcase gases, move through the crankcase ventilation system and mix with air in the intake manifold. But, as practice shows, if you disconnect the crankcase ventilation hoses from the intake manifold, the mixture becomes slightly leaner. However, after changing the engine oil, everything returns to normal.

From this it becomes possible to conclude: Fuel molecules enter the combustion chamber from the oil film on the cylinder walls. After all, the walls are lubricated by splashing, and with each stroke of the piston, the oil film is renewed. The described phenomenon should in no case mislead the diagnostician: if, after an unsuccessful winter start attempt, a rich mixture or a low fuel supply correction coefficient is observed, then this is an absolutely normal phenomenon. In such a situation, it makes sense to recommend replacing the engine oil in order to avoid increased mechanical wear of the engine and a decrease in the performance properties of the oil itself.

Lean mixture

Such a mixture is characterized by a value of λ › 1 and an excess amount of air. It is easy to come to the conclusion that if there is an excess of air in the mixture, the amount of residual oxygen in the exhaust gas will increase. The amount of CH will change slightly, because one of the reasons for the appearance of fuel vapors in the exhaust gas is the extinguishing of the flame in pinched volumes, and this does not depend on the composition of the mixture. The CO value will noticeably decrease. This is primarily due to excess oxygen and the oxidation of CO to CO 2 . Despite this, the percentage of CO 2 relative to the stoichiometric mixture will decrease due to the overall increase in the amount of gases. Of course, the calculated coefficient λ will be higher than 1. The exhaust gas composition of an engine running on a lean mixture and not equipped with a converter is given below (Table 4):

The reasons for a lean mixture in a modern engine include, first of all, air leaks into the throttle body. There are many ways: this is the vacuum brake booster, and the destruction of the sealing gaskets of the intake manifold, wear of the throttle valve axle-bushing pair, aging of the rubber seals of the injectors and regulator idle move. The location of such a defect can be localized using a smoke generator.

In addition to air leaks, the cause of a lean mixture may be low fuel pressure due to wear of the fuel pump or clogging of the fuel filter and line, decreased performance of injectors, and incorrect readings of the mass air flow sensor.

Detecting lean operation in an engine equipped with a catalytic converter is quite difficult. The fact is that when leaving the catalytic window towards depletion, the converter continues to have a significant effect on the composition of the exhaust gas. In this case, it is necessary to use the CO 2 value and evaluate the overall combustion efficiency.

High CH content

For an engine without a converter, the normal value of this parameter is 100..200 ppm. If on the instrument display we see CH equal to 300..400 or more, this is a reason to look for the reason why gasoline simply does not burn, in other words, there are misfires.

There are many reasons for such omissions. Worn or faulty spark plugs, high voltage wires, defective ignition module, unadjusted valve clearances, low compression, faulty injector. Moreover, all this - both in one and in several cylinders. Another reason for the increased content of fuel vapors in the exhaust gas is a leaky exhaust valve or one that is starting to burn out. In this case, during the compression stroke, part of the fuel charge is simply pushed into the exhaust tract. In this case, the engine can operate quite normally, and other gas analysis parameters will be normal. The table below contains an example of the exhaust gas composition of a real engine without a converter, which has defective spark plugs (Table 5):

All other engine systems are obviously in in perfect order. Let's analyze the data obtained.

An increased content of fuel vapors in the exhaust gas indicates that the latter simply does not burn.

The high oxygen content, coupled with the high amount of CH, allows us to assume that there are omissions. Where does oxygen come from? From the engine cylinders, which, when misfired, simply emit atmospheric air mixed with fuel vapor. CO 2 is below normal, which also indicates abnormal combustion. Well, the calculated coefficient λ - the device calculates it based, among other things, on the oxygen content. It was misfires that were observed on the engine under study; they were clearly audible at the end of the exhaust pipe.

In the case of an engine without a converter, if misfires occur, there are no special problems, other than increased emissions of toxic substances. But on engines equipped with a converter, misfires lead to unacceptable heating. Unburned fuel vapor mixed with atmospheric oxygen reacts on the surface of the carrier block, causing the release large quantity warmth. The temperature of the carrier block and the neutralizer body increases to values ​​of 1000°C or more. This phenomenon is very dangerous and can lead, for example, to fire of dry grass under the bottom of the car or damage to elements adjacent to the neutralizer.

In practice, the melting of the sound insulation of the cabin and the destruction of the insulation adjacent to the body have been repeatedly observed. electrical wires and a short circuit in them.

But first of all, misfires and subsequent overheating of the converter lead to the destruction of the latter. In the ceramic carrier block, honeycombs are sintered, causing an increase in the gas-dynamic resistance of the exhaust tract.

If the carrier block is made of steel foil, as a rule, sintering does not occur, but the catalytic active layer is destroyed and the neutralizer ceases to perform its function. One way or another, misfire in an engine equipped with a converter is a very dangerous phenomenon.

Due to this modern system The engine control system monitors misfires and, if detected, turns off the faulty cylinder.

Analysis of the amount of CO 2

As mentioned above, this exhaust gas component is the product of the most complete combustion of fuel. The better the fuel burns in the engine cylinders (and “burns out” in the converter), the higher the amount of CO 2 in the exhaust gas will be.

This statement may not be fair when applied to an engine with direct injection of gasoline into the cylinders when running on an ultra-lean mixture. But at the moment we are talking about more mass-produced engines with manifold injection. A serviceable engine not equipped with a converter will contain approximately 14% CO 2 in its exhaust gas, while an engine equipped with it will contain 16%. By naming these numbers, it is difficult to say that they are exactly what you will see on the display of your device. It is best to see what the readings will be from the device you are using and use them in your work. But this will not change the general principle of analysis.

Having received the CO 2 value, you should evaluate it

If it approximately coincides with the maximum achievable value for a given type of engine (see Tables 1.2), then we can conclude that there are no problems with the fuel supply and the formation of the fuel-air mixture. On the contrary, a decrease in the amount of CO 2 should alert you, because it is a sign of a problem.

Of course, the gas analyzer will not indicate a faulty sensor or element, but it will tell you the direction to look for the defect or at least indicate its presence.

In the author’s practice, there were cases when, in all respects, engine operation seemed to be within normal limits, but the amount of CO 2 in the exhaust gas indicated a problem. As a result, the defect was discovered and measures were taken to eliminate it. It is this criterion that allows one to evaluate the operation of an engine equipped with a neutralizer without referring to the values ​​of CO and CH, which in this case are close to zero and do not carry information.

Monitoring the condition of the catalytic converter

Modern electronic engine control units monitor the condition of the converter and set the corresponding fault code if its effectiveness decreases. However, it does not seem reasonable to sentence a very expensive unit to replacement based only on the code displayed by the block.

You need to make sure the diagnosis is correct, and in this case, a gas analyzer is the only device that can help you. The method for assessing the performance of a neutralizer is based on the principle of its operation. Since it begins to perform its function only when there is sufficient high temperature and the engine is running on a stoichiometric mixture, it is necessary to warm up the engine before turning on the fan and, using a scanner, make sure that the loop feedback the oxygen sensor is closed. Then the composition of the exhaust gas is analyzed.

First of all, the exhaust gas composition is checked when the engine is running at idle speed. If the converter is working properly, the exhaust gas composition will correspond to the above reference for an engine with a converter (Table 2). If there is an increased content of CO (0.1%...0.6%) and CH (50...200 ppm), as well as a reduced amount of CO 2, the neutralizer has lost its functionality.

If there are no problems and the numbers on the display correspond to the standard, you should increase the rotation speed to approximately 4000 rpm and take the gas analyzer readings again.

The idea of ​​the technique is this. With a low exhaust gas flow, characteristic of low rotation speed, the converter has time to fully process harmful components. With a large flow at high speed, its efficiency may not be sufficient. That's why The criterion for the serviceability of the converter can be considered its ability to provide the standard parameters of the exhaust gas composition at high rotation speeds.

Just for fun, you can do the following experiment. We connect the gas analyzer to the exhaust pipe of a cold engine, start the engine and monitor the composition of the exhaust gas. You can clearly track the initial operation of the engine on a rich mixture, then the gradual change in parameters towards the stoichiometric mixture and, finally, the shift of parameters towards the reference ones for an engine with a neutralizer.

Such experiments are very useful, since they clearly connect the theory of operation of the engine and control system with the practical results of its operation, observed using instruments.

Gas analysis and diagnostics: brief summary

A creative approach is required when working with a gas analyzer.

No algorithms can be used here. You need to critically evaluate the numbers on the device display and think about why they are exactly the way they are and where this or that component came from.

We have considered the most basic, basic points of analyzing the composition of exhaust gases; now it’s a matter of practice and gaining your own experience.

Analysis of gaseous media is mandatory event in the operation of chemical plants, as well as in many industrial enterprises. Such studies are procedures for measuring one or another component in a gas mixture.
For example, in mining enterprises, knowledge of the characteristics of the air in a mine is a safety issue, and environmentalists thus determine the concentration of harmful elements.
Such analyzes are not often used for domestic purposes, but if such a task arises, then a gas analyzer can also be used.
This measuring device, allowing you to determine the composition of the gas mixture.

The main tasks of gas analyzers:
control of the working area atmosphere (safety);
control of industrial emissions (ecology);
control of technological processes (technology);
control of air pollution in residential areas (ecology);
control of vehicle exhaust gases (ecology and technology);
control of the air exhaled by a person (alcohol);
Separately, we can mention the control of gases in water and other liquids.

Classification of gas analyzers:
by functionality (indicators, leak detectors, alarms, gas analyzers);
by design (stationary, portable, portable);
by the number of measured components (single-component and multi-component);
by the number of measurement channels (single-channel and multi-channel);
for its intended purpose (to ensure work safety, to control technological processes, to control industrial emissions, to control vehicle exhaust gases, for environmental control.

- are designed to solve a number of problems in the field of environmental monitoring and control of air pollution and air pollution in the working area, as well as for some other purposes it is necessary to take measurements at various points of the enterprise, which are not always equipped with power outlets.

In these cases, portable ones become indispensablegas analyzers (portable gas analyzers)!

Unlike stationary gas analyzers, such devices are distinguished by their compactness, mobility and ease of use, as well as short preparation time for operation and a wide range of operating conditions.

Application area portable gas analyzers:
In closed vessels and rooms (tunnels, wells, chimneys, pipelines, etc.);
At plants for the extraction and processing of various petroleum products;
At water settling tanks, sewage and filtration pumping stations;
In the automotive industry;
IN chemical laboratories and others production processes associated with the release of various pollutants;
In addition to the above purpose, portable gas analyzers are used for calibration and verification of stationary gas analyzers.

Advantages of portable gas analyzers:
Low cost;
Mobility;
Ease of operation;
Wide range of detectable gases and pollutants;
High sensitivity of sensors, which allows you to determine even the smallest fractions harmful substances;
Ability to connect electrochemical, thermocatalytic or optical sensors;
Big the lineup;
Speed ​​of the microprocessor unit;
Instant detection of the presence of explosive vapors;
Can act as a calibration device for stationary gas analyzers;
Compact size and light weight;
Measurements are taken of both the qualitative and quantitative composition of the air or gas mixture;
Allows you to simultaneously control the content of up to several gases in the air of the working area;
Ability to configure and program device response thresholds;
Availability of interfaces (IR, Wi-Fi, Bluetooth, Ethernet, etc.) for connecting to a computer or printer;
Availability of memory for recording results, time and dates of measurements.

- designed for permanent installation in work area industrial plants and factories, chemical laboratories, oil refining and gas production enterprises and other industries.

These are effective and high-precision devices that have the appropriate degree of protection, are highly reliable and can be retrofitted with an automation system for removing poisonous, toxic and flammable gases from various rooms!

Stationary gas analyzers are used in cases where it is necessary to make constant and fairly frequent periodic measurements of the concentration of pollutants and oxygen in an industrial area to maintain the required level and to organize technological control of production processes.

Scope of application of stationary gas analyzers:
Boiler houses;
Refrigeration units;
GRP premises (gas distribution points);
Work areas of industrial enterprises;
Laboratories;
Diesel and turbine units;
Sewage systems;
Kilns, etc.

The main advantages of stationary gas analyzers:
Reliability;
Acceptable price;
High accuracy measurements;
Ability to control several gases at once;
Long service life;
Possibility to equip the premises automatic system exhaust ventilation;
Remote control air mixture composition;
High degree of device protection.

Despite the many design variations of the device, there is a set of basic components that are present in each model. First of all, this is the housing, which contains all the working elements of the gas analyzer.
The fact is that such devices require high degree protection, therefore serious requirements must be placed on the outer shell.
Almost every device requires power supply - accordingly, the battery can also be considered as an essential part of the device.
Next we should move on to a more critical component. This is the primary transducer, that is, the gas analyzer sensor or sensing element, providing direct data for measurement.
It must be said that there are several types of such sensors, including thermocatalytic, infrared, electrochemical, and optical. The task of this element is to transform the required component gas composition into an electrical signal.

After this, a measuring and display device comes into operation, which processes this signal and demonstrates its indicators in the form of an indication or display.
The operating principle of a thermochemical (thermocatalytic) sensor is based on the direct dependence of the heat obtained during combustion of the detected gas on the concentration of this gas.
In electrochemical sensors, the component being tested interacts with a sensitive layer directly on the electrode or in a layer of conductive electrolyte solution near it.

An electrochemical cell (ECC), as a rule, has two or three electrodes for performing an electrochemical reaction.

Electrochemical sensors have the following advantages when compared with conventional analytical equipment:
- small dimensions;
- high selectivity;
- Ease of use;
- simplicity of design;
- high reliability;
- significant work resource;
- relatively low cost.

The following electrochemical sensors are distinguished:
coulometric, potentiometric, amperometric (voltammetry), conductometric.

Optical sensors record changes in the optical density of the gas mixture under study at a certain wavelength.
The following are distinguished: optical sensors: spectrophotometric, luminescent.

Checking gas analyzers
All gas analyzers, in accordance with the law, are periodically verified or calibrated. Verification is carried out once a year, the frequency of calibration is set by the owner of the gas analyzer.

When performing verification, the following operations are performed:
♦ External inspection
♦ Determination of electrical insulation resistance, leak testing gas system
♦ Determination of metrological characteristics.
♦ Determination of the main reduced error of the gas analyzer.
♦ Checking the measurement range alarm using a unified output signal

Unfortunately, it is impossible to create one universal gas analyzer, with the help of which it would be possible to solve all problems of gas analysis, for the reason that none of the known methods does not allow measurements to be made with equal accuracy over the widest possible range of concentrations.
Monitoring of different gases, in different concentration ranges, is carried out different methods and ways. Therefore, manufacturers design and produce devices to solve specific tasks measurements.

To summarize, it must be said that gas analyzers are irreplaceable devices that are used both in production and at home and allow you to determine the qualitative and quantitative composition of pollutants in the work area or any other room where there are dangerous factors for the leakage of harmful substances and gases.

Thank you for reading this article.
We also inform you that in our online store you can purchase a gas analyzer of any type at favorable price, and our company’s specialists will answer all your questions and help you choose a device that meets your requirements both in terms of technical and price characteristics.

Gas analyzers are equipment that helps accurately measure the qualitative and quantitative composition of gas. The operating principle of a gas analyzer is not very complicated, but each type of equipment has its own characteristics. These points can best be reflected in the diagram of a gas analyzer. In this article we will look at both the general principle of operation and the operation of some models of gas analyzers.

General operating principle

The principle of operation is based on the absorption of constituent substances by special reagents. This happens in a special sequence. If the operating principle is automatic, then the measurement occurs continuously, which means there are no interruptions. This is convenient in that the physicochemical parameters of the gas mixture are recorded accurately, which is also possible when interacting with individual components of the substance.

Analysis of various gas mixtures is used by enterprises in the metallurgical, chemical and heat-generating industries. Data that makes it clear about the quantity of certain components is needed to control the process in order to subsequently optimize it and debug its operation.

Gas measuring equipment includes models different types. They differ from each other in some parameters and operating principle.

Their work is based on the fact that the thermal conductivity of a gas mixture depends on which components are included in its composition. This gas analyzer has the following main parts:

1. The measuring cell is in the form of a cylindrical channel, which is made of a material with high thermal conductivity and filled with the analyzed gas.
2. A heating element, which is located inside the channel and is powered by a voltage source.

The cell is filled with air. If the current value is stable, then a heating element will have a certain temperature, in which case the heat received by the element and the heat it transfers to the channel material will be equal to each other.

If the channel is filled not with air, but with gas, which differs in thermal conductivity, the heating element will have a different temperature. If the thermal conductivity of the gas exceeds the thermal conductivity of the air, the temperature of the element will be lower, but if it does not exceed, but becomes lower, the temperature of the element will increase.

Optical devices

The basis of operation of this type of device is that the radiation flux is absorbed by various gases in a selective way. In the infrared part of the spectrum, a change in selective absorption is usually carried out, since it is in this place that selectivity of absorption is observed.

This gas analyzer has:

1. Infrared radiation source;
2. Cameras of two optical channels, which differ only in internal content: the comparison chamber is filled clean air, and the working chamber constantly blows through a controlled gas mixture; a stream of infrared radiation enters these cameras.
3. Filtration chambers.

The radiation flux, when passing through the volume of the second, working chamber, loses part of its energy. This does not happen when passing through the comparison chamber. Both radiation streams then enter the filter chambers, where the unmeasured components of the gas mixture are located. At this point, the energy corresponding to the spectrum is completely absorbed.

Thermochemical gas analyzers

Such devices determine the energy of heat released when a chemical reaction takes place in a mixture of gases. The operating principle is based on the oxidation process of gas components. However, additional catalysts are used, such as finely divided platinum and manganese-copper catalyst.

A special thermistor helps measure the resulting temperature. This device changes its resistance, which depends on temperature, which contributes to a change in the passing current.

Electrochemical gas analyzers

This model is designed to detect toxic gases. Its special feature is that it can be used in hazardous areas. This device is compact, energy-saving and insensitive to mechanical stress.

The basis for the operation of these gas analyzers is the phenomenon of electrochemical compensation. This means that a special reagent is released that reacts with a specific component of the mixture. There are several types of electrochemical gas analyzers:

• potentiometric; their purpose is to measure the field strength ratio;
• electrical conductometric; they respond to changes in voltage and current;
• galvanic; sensitive to changes in electrical conductivity.

As you can see, the principle of operation of gas analyzers is not complicated, however, one type of device differs from another, as it pursues different goals. Gas analyzers – useful devices, allowing you to determine the current state of gas in the room, which will help maintain human health at an acceptable level.