KR102467132B1 - Method for fault diagnosis of engine emission aftertreatment system - Google Patents

Abstract

A method for diagnosing an abnormality in an exhaust gas aftertreatment system is provided. The abnormal diagnosis method of the exhaust gas post-treatment system may include obtaining a first measurement value by measuring the flow rate, concentration, and temperature of the exhaust gas flowing into the catalyst, and converting the first measurement value and the injection amount of urea water into the SCR one-dimensional model. calculating the concentration and temperature of the exhaust gas at the rear end of the catalyst to obtain a model value, measuring the concentration and temperature of the exhaust gas discharged at the rear end of the catalyst to obtain a second measured value, and and diagnosing an abnormality in an exhaust gas aftertreatment system by calculating a difference between a model value and the second measurement value.

Description

Method for diagnosing abnormalities in exhaust gas post-treatment system {METHOD FOR FAULT DIAGNOSIS OF ENGINE EMISSION AFTERTREATMENT SYSTEM}

The present invention relates to a method for diagnosing an abnormality in an exhaust gas aftertreatment system.

Recently, as an issue related to environmental problems, various technologies for reducing nitrogen oxides (NOx), carbon monoxide (CO), and particle matters included in exhaust gas discharged from diesel engines have been developed. Generally installed close to the engine, DOC (Diesel Oxidation Catalyst), which performs the conversion function of non-methane hydrocarbons, DPF (Diesel Particulate Filter), which collects particulate matter, and SCR (which purifies nitrogen oxides (NOx) through reduction) Selective Catalytic Reduction) catalysts are used. Among them, the SCR catalyst is a technology that converts nitrogen oxides (NOx), one of air pollutants, into harmless nitrogen (N ₂ ) and water (H ₂ O) by using ammonia (NH ₃ ) as a reducing agent. The SCR catalyst exhibits high denitrification efficiency and is widely used as a technology for reducing nitrogen oxides (NOx) because it is easy to operate and maintain. However, if an abnormality occurs in the SCR system, there is a problem in that the reduction efficiency of nitrogen oxides decreases and the amount of pollutants discharged increases.

The present invention provides a method for diagnosing abnormalities in an exhaust gas aftertreatment system.

Other objects of the present invention will become apparent from the following detailed description and accompanying drawings.

A method for diagnosing an abnormality in an exhaust gas post-treatment system according to embodiments of the present invention includes obtaining a first measurement value by measuring the flow rate, concentration, and temperature of exhaust gas flowing into the catalyst, the first measurement value and the number of elements. Calculating the concentration and temperature of the exhaust gas at the rear end of the catalyst by applying the injection amount to the SCR one-dimensional model to obtain a model value, and measuring the concentration and temperature of the exhaust gas discharged at the rear end of the catalyst to obtain a second measurement. The method includes obtaining a value, and diagnosing an abnormality in an exhaust gas aftertreatment system by calculating a difference between the model value and the second measurement value.

The diagnosing the abnormality of the exhaust gas aftertreatment system may include calculating a difference between the model value and the second measured value to obtain a residual and comparing the residual with a threshold.

The SCR one-dimensional model may include an ammonia storage reaction, an ammonia oxidation reaction, a nitrogen monoxide oxidation reaction, and a nitrogen oxide reduction reaction (SCR reaction).

The SCR one-dimensional model may include the following R1 to R12 reactions.

In the above R1 to R12 reactions, S1 and S2 represent active sites of the catalyst.

The governing equation of the SCR one-dimensional model can be derived from the material balance and the energy balance.

According to embodiments of the present invention, it is possible to diagnose an abnormality in an exhaust gas aftertreatment system. Problems in the exhaust gas aftertreatment system can be identified, and pollutant emissions from diesel vehicles can be reduced by determining the replacement timing of the catalyst system or using it to change the control logic.

Figure 1 shows a one-dimensional model of element-SCR.
2 shows an exhaust gas aftertreatment system of a diesel engine vehicle.
3 shows a method for diagnosing an abnormality in an exhaust gas post-treatment system according to an embodiment of the present invention.
4 is a residual graph that appears when an abnormal urea injection scenario occurs.
5 is a residual graph that appears when a catalyst failure scenario occurs.

Hereinafter, the present invention will be described in detail through examples. Objects, features and advantages of the present invention will be easily understood through the following examples. The present invention is not limited to the embodiments described herein and may be embodied in other forms. The embodiments introduced herein are provided so that the disclosed contents may be thorough and complete and the spirit of the present invention may be sufficiently conveyed to those skilled in the art to which the present invention belongs. Therefore, the present invention should not be limited by the following examples.

A selective catalytic reduction (SCR) system is modeled with the MATLAB program using the material balance and energy balance equations as the governing equations. A total of 12 reactions occurred in the selective reduction catalyst were calculated, and they were largely subdivided into NH ₃ storage reaction, NH ₃ oxidation reaction, NO oxidation reaction, and SCR reaction. Based on the model whose accuracy has been verified, the catalytic parameters are found using a genetic algorithm (GA, Genetic Algorithm) and particle swarm optimization (PSO, Particle Swarm Optimization). The genetic algorithm is an optimization technique using the concepts of natural selection and genes. Each individual is evaluated by an objective function in each generation, and elite individuals are selected through selection, and the remaining unselected individuals are evaluated. is removed. The selected individual generates the next generation through crossover and mutation, and evolves into an individual more suitable for the given objective function through continuous evaluation, selection, crossover, and mutation. The particle swarm optimization algorithm is modeled after the movements of animals that live in groups such as fish and birds. Initially, particles are randomly generated, and each particle is evaluated according to a given objective function. At this time, the objective function of the global optimization is to minimize the difference between the actual experimental measurement value and the model predicted value. Rather than simply comparing the nitrogen oxide conversion efficiency, the concentrations of all measurable substances (NH ₃ , NO, NO ₂ , etc.) are compared to find reaction parameters that well simulate the actual system. Reliability is higher because it can reflect various experimental conditions by utilizing Rig evaluation data used for actual catalyst evaluation. The reaction occurring in the SCR catalyst can be largely divided into NH ₃ storage reaction, NH ₃ oxidation reaction, NO oxidation reaction, and SCR reaction. In order to find a specific type of reaction parameter, the number of parameters to be found is reduced by using the Rig evaluation data, and the accuracy of the reaction parameter is reduced. can increase

[SCR 1D model]

Figure 1 shows a one-dimensional model of element-SCR. The SCR catalyst system is a cylindrical monolith with several channels. A one-dimensional model for a single channel is used to simulate the element-SCR system. This modeling approach takes much less computational time than 2D or 3D models with little loss of model accuracy.

Referring to Figure 1, the bulk gas of the diesel engine enters a single channel of the module into which the urea solution is injected. After the bulk gas diffuses into the pores of the washcoat, the reactants are adsorbed on the catalyst surface. Chemical species are discharged out of the channel through a series of reactions. The governing equations of the SCR one-dimensional model based on the differential reactor volume are derived from the mass balance and energy balance.

mass balance

[Equation 1] Bulk Gas:

[Equation 2] Surface:

[Equation 3] Intermediate:

energy balance

[Equation 4] Bulk Gas:

[Equation 5] Surface:

Table 1 shows chemical reactions and reaction rate equations. The reaction rate constant k _i , excluding the desorption reaction, follows the Arrhenius equation in Equation 6.

[Equation 6]

[Table 1]

Reaction R1 to R4 is an ammonia storage reaction, in which ammonia is adsorbed and desorbed from the two active sites of the catalyst. Ammonia is adsorbed on the catalyst as well as oxidized by the hot exhaust gas. The reaction rate is proportional to the ammonia storage capacity ψ of each active site. As shown in Equation 7, in the case of the ammonia desorption process, the coverage fraction θ affects the activation energy.

[Equation 7]

From the R5 reaction to the R7 reaction, ammonia is oxidized through the ammonia oxidation reaction to produce nitrogen oxide. The R8 reaction is a nitric oxide oxidation reaction, which is an equilibrium reaction between nitric oxide oxidation and nitrogen dioxide reduction. Kp in Equation 8 is the equilibrium constant for the R8 reaction.

[Equation 8]

From the R9 reaction to the R12 reaction, the nitrogen oxide is removed by the adsorbed ammonia through the nitrogen oxide reduction reaction (SCR reaction). There are three types of SCR reactions: standard SCR, fast SCR and NO ₂ SCR. Reaction R9 is a standard SCR that removes only NO, reaction R10 is a high-speed SCR that removes both NO and NO ₂ , and reactions R11 and R12 are NO ₂ SCRs using only NO ₂ . All reactions are inserted as r _i into equations 1 to 5 of the governing equations. The parameters estimated in the present invention are A and E _a for 12 reactions, and γ and ψ for two active sites.

An explanation of the symbols of the above equation and the reaction rate equation is as follows.

= Gas layer fraction of cell (gas layer volume)/(gas + washcoat layer volume)

= Gas layer fraction of cell (gas layer volume)/(gas + washcoat layer volume)

= Washcoat layer fraction of solid layer (washcoat layer volume)/(washcoat + substrate layer volume)

= Washcoat layer fraction of solid layer (washcoat layer volume)/(washcoat + substrate layer volume)

= Washcoat porosity

= Washcoat porosity

= Mole fraction of species j in gas phase

= Mole fraction of species j in gas phase

= Mole fraction of species j in washcoat phase

= Mole fraction of species j in washcoat phase

= Bulk gas velocity [

]

= Bulk gas velocity [

]

= Mass transfer coefficient of species j [

]

= Mass transfer coefficient of species j [

]

= Geometric surface area to catalyst volume ratio [

]

= Geometric surface area to catalyst volume ratio [

]

= Temperature of monolith [

]

=Temperature of monolith [

]

= Temperature of bulk gas [

]

= Temperature of bulk gas [

]

= Gas constant, [

]

= Gas constant, [

]

= Reaction coefficient of species j in ith reaction

= Reaction coefficient of species j in ith reaction

= Reaction rate of ith reaction [

]

= Reaction rate of ith reaction [

]

= Coverage fraction of kth site

= coverage fraction of kth site

= Storage capacity of kth site, mole of active site per unit reactor volume [

]

= Storage capacity of kth site, mole of active site per unit reactor volume [

]

= Gas density [

]

= Gas density [

]

= Heat capacity of gas [

]

= Heat capacity of gas [

]

= Overall heat transfer coefficient between bulk gas and surface gas [

]

= Overall heat transfer coefficient between bulk gas and surface gas [

]

= Heat capacity of monolith [

]

= Heat capacity of monolith [

]

= Enthalpy of formation of species j [

]

= Enthalpy of formation of species j [

]

= Equilibrium constant

= equilibrium constant

= Inhibition factor

= inhibition factor

= Reaction rate constant of ith reaction

= reaction rate constant of ith reaction

= Pre-exponential factor of ith reaction

= Pre-exponential factor of ith reaction

= Activation energy of ith reaction

= Activation energy of ith reaction

= Parameter for surface coverage dependence for kth site

= Parameter for surface coverage dependence for kth site

The SCR one-dimensional model requires the following assumptions.

- The shape and size of a single channel are uniform and the gas distribution between channels is uniform

- Pressure drop is negligible

- Plug flow with no radial gradient

- Gas is incompressible, so the density is constant and the gas velocity in the axial direction is constant

- Enthalpy change is due solely to the reaction

- Heat transfer by radiant heat is insignificant and the gas temperature of the pores of the washcoat is the same as that of the washcoat

- The conduction period is omitted

- Heat transfer due to convection between the catalyst module and the outside air is omitted

[diagnosis of exhaust gas post-processing system abnormalities]

In the present invention, an abnormality in a catalyst system is diagnosed using a one-dimensional SCR model. When the controller determines the amount of urea injection and injects the urea solution to the injection unit in front of the catalyst, the injected urea solution is converted into ammonia due to the heat of the exhaust gas. Thereafter, ammonia is adsorbed on the catalyst by an ammonia storage reaction, and due to the high temperature of the exhaust gas, an ammonia oxidation reaction occurs and is oxidized and consumed. Nitrogen monoxide and nitrogen dioxide in the exhaust gas entering the catalyst react with the adsorbed ammonia and are reduced to nitrogen. In the abnormal diagnosis method of the present invention, a model value is obtained based on a virtual SCR one-dimensional model including a series of reaction equations, and the abnormality of the catalyst system is diagnosed through the difference between the model value and the actual measurement value measured by the sensor. .

2 shows an exhaust gas aftertreatment system of a diesel engine vehicle.

Referring to FIG. 2, the exhaust gas aftertreatment system is installed close to the engine and performs a conversion function of non-methane hydrocarbons through a DOC (Diesel Oxidation Catalyst), a DPF (Diesel Particulate Filter) that collects particulate matter, and a reduction action. It consists of a SCR (Selective Catalytic Reduction) catalyst that purifies nitrogen oxides (NOx). A method for diagnosing an abnormality in an exhaust gas post-treatment system according to embodiments of the present invention measures the flow rate, concentration, and temperature of exhaust gas entering the SCR catalyst and applies it to the SCR one-dimensional model, based on the calculated model value. Diagnose system errors.

3 shows a method for diagnosing an abnormality in an exhaust gas post-treatment system according to an embodiment of the present invention. The method for diagnosing an abnormality in the exhaust gas aftertreatment system uses a one-dimensional model reflecting characteristics of a catalyst.

Referring to FIG. 3 , a first measurement value is obtained by measuring the flow rate, concentration, and temperature of the exhaust gas flowing into the front of the catalyst using a sensor in front of the catalyst. A model value is obtained by applying the first measured value and the injection amount of urea water to the SCR one-dimensional model to calculate the concentration and temperature of the exhaust gas at the rear end of the catalyst. A second measured value is obtained by measuring the concentration and temperature of the exhaust gas discharged from the rear end of the catalyst using the line direction at the rear end of the catalyst. A difference between the model value and the second measurement value is calculated, and the value is defined as a residual. If the residual is greater than the threshold, it can be determined that there is an abnormality in the catalyst system. The threshold indicates a limit value at which the exhaust gas aftertreatment system can be considered to be free of abnormalities and is predetermined.

The abnormal scenarios of the Selective Catalytic Reduction (SCR) include urea water injection abnormality and catalyst abnormality. A problem may occur in the urea water injector itself or the inlet of the injection hole may be narrowed, so that less urea water may be injected than the control command. In addition, the catalyst may be poisoned by the sulfur component in the exhaust gas, and the catalyst function may be deteriorated due to the heat of the exhaust gas. At this time, the active site of the catalyst is reduced, and the ammonia storage capacity is lowered, and the exhaust gas purification capacity is lowered.

4 is a residual graph appearing when an abnormal urea water injection scenario occurs, and FIG. 5 is a residual graph appearing when an abnormal catalyst scenario occurs. An anomaly occurred 10 minutes after the simulation run (red dotted line), and the black dotted line represents the threshold.

Referring to FIG. 4 , after the occurrence of the abnormality, the nitrogen oxide concentration gradually increases and the residual exceeds the threshold, but the ammonia concentration slightly decreases without a significant change. When the injection amount of urea water is smaller than that of the urea water injector, the amount of ammonia is reduced accordingly, and the reduction reaction of nitrogen oxide is reduced, thereby increasing the amount of nitrogen oxide discharged.

Referring to FIG. 5, immediately after the occurrence of the abnormality, both the nitrogen oxide concentration and the ammonia concentration appear high. When the ammonia storage capacity of the SCR decreases, the amount of ammonia that can be adsorbed to the catalyst decreases, and accordingly, the reduction reaction of nitrogen oxide decreases, increasing the amount of nitrogen oxide emission. In addition, since ammonia converted from urea water is not stored and discharged as it is, the amount of ammonia discharged after the catalyst also increases.

In this way, by comparing the model value calculated using the SCR one-dimensional model with the measurement value measured by the sensor, it is possible to detect an abnormality and diagnose which part of the exhaust gas aftertreatment system has occurred.

So far, we have looked at specific embodiments of the present invention. Those skilled in the art to which the present invention pertains will be able to understand that the present invention can be implemented in a modified form without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments should be considered from an illustrative rather than a limiting point of view. The scope of the present invention is shown in the claims rather than the foregoing description, and all differences within the equivalent scope will be construed as being included in the present invention.

Claims (5)

Obtaining a first measured value by measuring the flow rate, concentration and temperature of the exhaust gas flowing into the catalyst;
calculating the concentration and temperature of the exhaust gas at a rear end of the catalyst by applying the first measurement value and the injection amount of urea water to a one-dimensional SCR model to obtain a model value;
obtaining a second measured value by measuring the concentration and temperature of the exhaust gas discharged from the rear end of the catalyst; and
Diagnosing an abnormality in an exhaust gas aftertreatment system by calculating a difference between the model value and the second measured value;
The SCR one-dimensional model,
A method for diagnosing an abnormality in an exhaust gas aftertreatment system, comprising the following R1 to R12 reactions.

(In the above R1 reaction to R12 reaction, S1 and S2 represent the active site of the catalyst) According to claim 1,
The step of diagnosing the abnormality of the exhaust gas post-treatment system,
obtaining a residual by calculating a difference between the model value and the second measured value; and
A method for diagnosing an abnormality in an exhaust gas aftertreatment system, comprising the step of comparing the residual with a threshold. According to claim 1,
The SCR one-dimensional model,
A method for diagnosing an abnormality in an exhaust gas aftertreatment system comprising an ammonia storage reaction, an ammonia oxidation reaction, a nitrogen monoxide oxidation reaction, and a nitrogen oxide reduction reaction (SCR reaction). delete According to claim 1,
The method for diagnosing abnormality in an exhaust gas aftertreatment system, characterized in that the governing equation of the SCR one-dimensional model is derived from the material balance and the energy balance.

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