Patent Application
20240209767
The invention relates to a method for calculating reaction heat for modeling at least one exhaust temperature in an exhaust system, in particular an exhaust system in a motor vehicle, and to an arrangement for performing the method.
An exhaust system is used to gather the exhaust gases flowing from the cylinders and to clean pollutants from said gases. Furthermore, the exhaust system serves to reduce exhaust noises and direct exhaust gases outward from the interior of the vehicle. An exhaust system typically comprises a front system having an exhaust manifold, a cleaning system comprising, for example, particulate filters and at least one catalytic converter, connecting pipes, and a rear system having a muffler system and pipes.
Gas and material temperature calculations at any arbitrary location in the exhaust system of gasoline internal combustion engines in engine control unit software are regularly performed incrementally in the flow direction along the exhaust system. In so doing, the temperatures in the individual components, such as manifold, turbocharger, pipe section, and catalytic converter, are each calculated based on the temperature of the preceding element and the reaction heat arising in the element.
In light of this, a method and an arrangement are presented. Furthermore, a computer program as well as a machine-readable storage medium are presented. Embodiments arise from the dependent claims and from the description.
It has been found that to accurately model exhaust temperatures, a very accurate calculation of the reaction heat occurring in the exhaust system is needed. To this end, characteristic maps are plotted against load and engine speed, in which the quasi-stationary temperature increase relative to the preceding exhaust element due to catalytic reaction can be defined for the adjacent operating point. This model provides very good results for steady-state operating points, but start-stop operation or overrun cut-off are not sufficiently considered, for example.
The method presented is based on the insight that in dynamic operating conditions, additional heat arises in the catalysts due to exothermic oxygen storage and withdrawal processes. Conditions such as push pumps, i.e., short acceleration phases with push phases at different frequencies, lead to an actual temperature increase in the catalysts, which may be so high that the coatings in a catalytic converter are thermally damaged.
Particularly during an overrun shut-off and for start-stop phases, the storage reaction dominates the heat generation in the catalyst. It has been found that no additional maps are available to the existing model to replicate this additional heat generation. Because stored oil gases also react at the start of lean phases, the quasi-stationary temperature increases are not exclusively dependent on the current operating point, but also on their history, and therefore sometimes differ significantly. This is due to the fact that, depending on the previous operation of the engine, for example in the component protection range with a rich mixture, different amounts of oil gas are stored in the catalytic converter for reacting. Like the storage and withdrawal operations of oxygen, the storage state of oil gas is also not considered by the existing model.
The method is further based on the recognition that said temperature increase under dynamic operating conditions cannot be modeled or can only be insufficiently modeled with the previous model approach. The catalytic converters installed in the exhaust system can therefore only be reliably protected from thermal damage by component protection measures that are used very early or very severely. These measures increase CO2 emissions and should therefore be avoided.
In addition to the dynamic operating states, such as push pumps, the modeling of medium length, i.e., 10 to 30 s, and long phases of overrun shut-off of more than 30 s via a time filter in the existing model also present a challenge, as the storage state of the catalyst surface due to different past operating conditions is not considered. With the previous model approach, the temperature is not always sufficiently modeled. However, accurate modeling and reporting of the temperature in the catalytic converters is of high importance in order to protect them from cooling and to trigger measures for keeping the catalyst hot or cold-start catalyst heating more appropriately for the demand and with greater CO2 efficiency.
In order to be able to operate the catalytic converter as quickly as possible after lean operating points in the optimal, in particular stoichiometric, operating window, the previously stored oxygen is reacted by rich combustion. This is referred to as catalyst cleanup. Here too, a reaction heat arises, which depends not only on the operating point but also on the amount of stored oxygen. The accuracy of the existing model for determining the amount of heat released is also very inaccurate at these operating points and does not account for the storage state of the catalyst surface.
The model deviations resulting from the aforementioned points are particularly noticeable in a typical city trip. This is characterized by low loads, engine speeds, and mass flow rates, as well as frequent short phases of overrun shut-off and start-stop followed by catalyst cleanup. It is possible that model deviations of more than 100 K will occur. Because the temperature range is rather low in these driving situations, there is a risk that catalyst heating measures may be unnecessarily activated, or in the worst-case scenario, will not be activated even when necessary. Both cases negatively impact the emissions produced. In addition, these model deviations reduce the accuracy of catalytic converter diagnostics.
The method presented is for calculating the arising reaction heat to calculate at least one temperature in an exhaust system of an internal combustion engine using a model comprising a first model component and a second model component. The first model component refers to a calculation of exhaust components flowing from valves of the internal combustion engine; the second model component refers to the entire exhaust system. Total masses from the first model component are thereby divided along the exhaust system among the individual components of the exhaust system.
The method presented utilizes a new model of oxygen balancing for the catalysts, which manages to substantially increase the accuracy of modeling the exothermic reaction in the catalysts, especially under dynamic operating conditions. Conditions, such as push pumps, that can actually lead to a sharp temperature increase in the catalysts and can even thermally damage the coatings of the catalysts, are modeled and predicted with sufficiently high model quality. The new function therefore has the advantage that both the trigger time and the severity of the countermeasure can be designed in a more need-based manner and thus CO2 can be saved.
In addition, the new modeling approach improves modeling of medium-length phases, i.e., 10 to 30 s, and long phases of overrun shut-off of more than 30 s. Accurate modeling and reporting of the temperature in the catalysts are of high importance in order to ensure that the catalysts are always kept in an optimum temperature window for converting emissions in order to also comply with future exhaust gas regulations, such as EU7.
With the increased accuracy, it is possible to trigger the catalytic maintenance measures or the catalytic heating for cold starts more closely when the catalysts cool off and thus save CO2 in this way.
The advantages of the new model approach are in modeling the incompletely burned rich exhaust constituents, such as hydrocarbons, carbon monoxide, soot, and lean exhaust constituents, such as oxygen, nitrogen oxides, the combustion specific to an engine cylinder, as well as their storage and/or transport in the gas volumes of the individual exhaust components, as well as the storage and withdrawal into the surface of the catalysts. In the second step, the total heat generated is calculated from the stored oxygen mass and the rich and lean exhaust constituents reacting with each other. This is provided to the exhaust temperature model, which calculates the temperature change for the respective exhaust element or catalyst.
Due to the modeling of the storage capacity of the catalyst surface for rich and lean components, a link between the operating history and the reaction heat currently being released is established for the first time.
Previously, catalytic converter cleanup, overrun shut-off, and other dynamic operating conditions and their catalytic response have been described by individual delta temperatures. In contrast, in the new physical modeling approach, only the storage of exhaust components and their reaction with each other are considered. Thus, the modeling of the exothermic reaction in the catalysts for stationary and, above all, under dynamic operating conditions, is carried out in a much more precise, physical, and reliable manner. This is closer to the real-world process. Instead of writing a 10 K increase in temperature in a map, an amount of heat resulting in a 10 K increase in temperature is calculated from a defined mass of oxygen and oil gas along with an associated reaction enthalpy. Instead of adding 10 K, as in the empirical model variant, the physics and chemistry behind the effect, the temperature increase, are depicted.
The described arrangement is used to carry out the presented method and is implemented in hardware and/or software, for example. The arrangement may also be integrated in a control unit of a motor vehicle or configured as such.
Further advantages and configurations of the invention arise from the description and the accompanying drawings.
It goes without saying that the aforementioned features and the features yet to be explained below can be used not only in the particular specified combination, but also in other combinations or on their own, without leaving the scope of the present invention.
The invention is illustrated schematically in the drawings on the basis of embodiments and is described in detail below with reference to the drawings.
Output variables are control unit signals:
Input variables are:
Further variables are:
Output variables are:
Output variable is a control unit signal:
Variables are:
The model is thus divided into two model components 10, 150 and areas. The first area relates to the calculation of exhaust components flowing out of the valves of the engine. Here, the incompletely reacted exhaust components, as well as the air flowing directly into the exhaust system, are calculated individually for each cylinder. The amount of air, the mixture, and the ignition timing must be observed. By cylinder-specific calculation from the current engine operating variables, special modes of operation, such as overrun shut-off, cylinder suppression, half-engine operation, scavenging, and purging and cylinder balance, are automatically covered.
Output from the first model range is the sum of the mass flows of the reactive residual gas components across all cylinders of an exhaust bank.
The second area refers to the entire exhaust system. Here, the total masses from the first model part are distributed along the exhaust system to the individual components, such as manifold, turbocharger, catalytic converter, particulate filter. In catalysts and catalytically coated particulate filters, storage of a portion of the residual gases into the catalyst surface is also modeled depending on an applied adsorption efficiency.
Because the catalytic surfaces have limited storage capacity, the storage capability values of the individual catalysts are read from the catalytic converter diagnostic functions; alternatively, a fixed value may be specified. The heat generated by the exothermic reaction of storing oxygen in the catalytic surfaces is calculated.
Depending on the amount of rich and lean portions present in the gas volume and on the surface, the reaction heat is modeled by the reaction of rich and lean exhaust components to carbon dioxide and water depending on an applied reaction efficiency. Unreacted portions from the surface are considered again in the next calculation step. Components remaining in the gas are passed to the subsequent exhaust element.
Finally, the proportions of heat generated from the adsorption and reaction per exhaust element are added and used to calculate the modeled exhaust temperature in that exhaust element.
In
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2022 214 359.0 | Dec 2022 | DE | national |
