EP1432897A1

EP1432897A1 - Method for protecting exhaust gas purification systems of internal combustion engines against thermal overload

Info

Publication number: EP1432897A1
Application number: EP02782788A
Authority: EP
Inventors: Ekkehard Pott; Michael Zillmer; Michael Lindlau
Original assignee: Volkswagen AG
Current assignee: Volkswagen AG
Priority date: 2001-09-27
Filing date: 2002-09-06
Publication date: 2004-06-30
Anticipated expiration: 2022-09-06
Also published as: CN1561434A; DE50210083D1; JP2005504224A; EP1432897B1; WO2003029634A1; DE10147619A1; CN1327117C

Abstract

The invention relates to a method for operating an internal combustion engine, in particular of a motor vehicle, comprising an exhaust gas installation containing an exhaust gas purification system. According to said method, a motor lambda value is set, based on a modelled temperature or a temperature measured at at least one critical point in the exhaust gas installation, to a temperature-dependent motor lambda value, in a manner that differs from a normal operation to such an extent that an exhaust gas temperature is reduced, if the temperature that has been determined at the one or more points in the exhaust gas installation exceeds a predefined first temperature value. The motor lambda value is only modified to reduce the exhaust gas temperature from a value for a normal operation to a temperature-dependent motor lambda value, if the measured temperature has exceeded the predefined first temperature value for a predetermined period.

Description

Process for protecting exhaust gas purification systems of internal combustion engines against thermal overload

The invention relates to a method for operating an internal combustion engine, in particular a motor vehicle, with an exhaust gas system with an exhaust gas purification system, wherein an engine lambda value depending on a modeled or measured temperature at at least one critical point of the exhaust gas system is set to a temperature-dependent engine lambda value so that it deviates from normal operation an exhaust gas temperature is lowered when the determined temperature at the at least one point of the exhaust system exceeds a predetermined first temperature value, according to the preamble of claim 1.

Catalysts of internal combustion engines, for example in motor vehicles, age when exposed to high temperatures, with the starting behavior deteriorating, i.e. the same conversion rate is only reached at a higher catalyst temperature and / or the peak conversion rate, which is usually> 99% for HC, CO and NOx in the case of 3-way catalysts, decreases. This process increases disproportionately as the aging rate increases.

In the exhaust line of an internal combustion engine, very high exhaust gas and catalyst temperatures of possibly more than 1,000 ^° C. can occur, which may damage an exhaust gas catalyst in an inadmissible manner within a short exposure time, so that emission limit values are no longer maintained. This is particularly the case with exhaust gas purification systems with at least one catalytic converter close to the engine (pre-catalytic converter or main catalytic converter), since only a little heat is dissipated via the short non-adiabatic exhaust pipe.

It is known to limit the exhaust gas temperature at least almost full engine load by means of sub-stoichiometric engine operation. The fuel brought into the combustion chamber is not completely burned due to the lack of oxygen. As a result, the combustion chamber gases reach a lower temperature with the same supplied energy. In addition, the enthalpy of vaporization of the fuel cools the combustion chamber gases. Furthermore, the residual oxygen in the exhaust gas is lowered in this operating mode, so that less exothermic energy is generated in the catalyst or catalysts. A maximum permissible exhaust gas catalytic converter temperature is usually specified and the engine lambda is set as a function of the deviation of the determined exhaust gas catalytic converter temperature from the maximum permissible exhaust gas catalytic converter temperature. It is also known to additionally monitor one or more exhaust gas or catalyst temperatures at different points in the exhaust gas cleaning system for deviations from predetermined maximum temperatures and to adjust the engine lambda as a function of the most critical point.

The disadvantage is that these measures involve an undesirable increase in consumption. Efforts are therefore being made to limit the additional consumption of these component protection measures as far as possible. For example, a graded phase-in of overheating protection measures is described in DE 19609 923. First of all, a first, less pronounced measure is taken, the success of which is checked with regard to a temperature reduction, a second, more effective measure being taken in the event of insufficient temperature reduction. A disadvantage of this method is that a certain overload of the exhaust gas cleaning system is accepted in order to reduce consumption.

The invention has for its object to improve a method of the type mentioned in such a way that a reduction in the additional consumption by an exhaust gas and exhaust gas cleaning system temperature-related engine lambda setting is achieved without overloading the exhaust gas cleaning.

This object is achieved by a method of the above. Kind with the features characterized in claim 1 solved. Advantageous embodiments of the invention are specified in the dependent claims.

For this purpose, it is provided according to the invention that the engine lambda value is only changed from the value for normal operation to a temperature-dependent engine lambda value in order to lower the exhaust gas temperature when the determined temperature has exceeded the predetermined first temperature value for a predetermined period of time.

This has the advantage that short-term violations of the limit temperature for the exhaust gas purification system, which can be compensated promptly by subsequent cooling phases, can be detected, so that there is no deviation from the value of the engine lambda value from normal operation, so that in Overall operation results in reduced fuel consumption through less aggressive component protection measures by changing the engine lambda value. Furthermore, a different weighting of the various temperature-critical points is achieved in the setting of the engine lambda value due to component protection, and a distinction is made between short-term loads, for example during acceleration processes, and longer-lasting loads, for example when driving full throttle uphill.

In order to take into account different dynamics of the temperature change at different points in the exhaust system, the predetermined period of time is selected differently for different critical points in the exhaust system. For example, the predetermined period of time is selected the longer the closer the critical point of the exhaust system is to an engine block of the internal combustion engine.

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The temperature is preferably determined at at least one critical point upstream, downstream and / or on a main catalytic converter and / or pre-catalytic converter.

In order to prevent irreversible damage to the exhaust gas purification system, the engine lambda value is converted from the value for normal operation to the temperature-dependent engine lambda value before the end of the predetermined time period if the determined temperature exceeds a second predetermined temperature value within the predetermined time period, which is greater than the predetermined first temperature value ,

In order to take into account different dynamics of the temperature change at different points in the exhaust system, the predetermined second temperature value is selected differently for different critical points in the exhaust system. For example, the predetermined second temperature value is selected the higher the closer the critical point of the exhaust system to an engine block of the internal combustion engine.

The engine lambda value is expediently converted immediately or filtered from the value of normal operation into the temperature-dependent engine lambda value.

Further features, advantages and advantageous embodiments of the invention result from the dependent claims and from the following description of the invention with reference to the accompanying drawing. This shows in 1 is a graphical representation of a first temperature profile of the exhaust gas temperature upstream of a pre-catalytic converter and an engine lambda value over time with and without intervention in the engine lambda value according to the inventive method,

2 shows a graphical representation of a second temperature profile of the exhaust gas temperature upstream of a pre-catalytic converter and of an engine lambda value over time with and without intervention in the engine lambda value according to the inventive method and

3 shows a graphical representation of the temperature profile of the exhaust gas temperature upstream of a pre-catalytic converter and upstream of a main catalytic converter over time with and without intervention in the engine lambda value according to the method according to the invention.

According to the invention, an exposure time of the temperature or the exceeding of a predetermined first temperature is used as a criterion for the setting of a motor lambda value. For a predetermined period of time, a continuous load temperature limit can be exceeded by a predetermined temperature difference without specifying a temperature-dependent engine lambda value that deviates from normal operation. If the temperature overload lasts longer, the motor lambda value is immediately or filtered transferred to the temperature-dependent motor lambda value in order to avoid or reduce damage caused by permanent thermal stress.

Furthermore, the location of the occurrence of the temperature overshoot in the exhaust system is taken into account when determining the temperature-dependent engine lambda value. Close to the cylinder head, due to the low thermal inertia of the exhaust system up to this running distance, there is a temperature dynamic which follows the load very quickly and which further decreases downstream and in particular significantly behind the catalytic converter (s). This means that heating and cooling processes take place faster in front of a pre-catalytic converter close to the engine than in the middle of a large-volume main catalytic converter located away from the engine. Therefore, a temperature overload in the exhaust gas upstream of a first catalytic converter near the engine can be permitted for a longer period of time than a temperature overload at subsequent critical points in the exhaust system, since in the event of negative changes in load or speed or when one is set temperature-dependent engine lambda values at a location close to the engine with faster cooling and thus elimination of the critical situation can be expected.

However, if the measuring temperature already exceeds a predetermined second temperature value within the predetermined time period, which is higher than the predetermined first temperature value for this measuring point, i.e. in other words, if the temperature difference between the measuring temperature and the predetermined first temperature value becomes greater than a predetermined value, it makes sense to set the temperature-dependent engine lambda value before the expiry of the predetermined period in order to rule out irreversible catalyst damage.

1 and 2 graphically illustrate a dynamic component protection according to the invention. The time is plotted on a horizontal axis 10, an exhaust gas temperature upstream of a pre-catalytic converter on a first vertical axis 12 and an engine lambda value on a second vertical axis 14. Value 16 on axis 14 corresponds to an engine lambda value of 1, line 18 corresponds to the predetermined first temperature value (in this example 900 ^° C.) and line 20 corresponds to the predetermined second temperature value (in this example 940 ^° C.). Graph 22 shows the temperature profile of the exhaust gas temperature over time without component protection intervention, Graph 24 shows the temperature profile of the exhaust gas temperature over time with component protection intervention according to the prior art, and Graph 26 shows the temperature profile of the exhaust gas temperature over time with component protection intervention according to the inventive method. Graph 28 shows the course of the engine lambda value over time without component protection intervention, graph 30 shows the course of the engine lambda value over time with component protection intervention according to the prior art, and graph 30 shows the course of the engine lambda value over time with component protection intervention according to the inventive method. Reference numeral 34 denotes a first time TO, reference numeral 36 denotes a second time T1, reference numeral 38 denotes a third time T2, reference numeral 40 denotes a fourth time T3 and reference numeral 42 denotes a fifth time T4. In Fig. 2, reference numeral 44 denotes a sixth time TKR. The time difference between the third and fourth times 38 and 40 corresponds to the predetermined time period 46.

Fig. 1 graphically illustrates a load jump partial load full load at time TO 34, for example when entering a longer, steep incline. The Exhaust gas purification system includes, for example, a pre-catalytic converter near the engine and a main catalytic converter arranged further downstream, FIG. 1 illustrating the course of the exhaust gas temperature (axis 12) upstream of the pre-catalytic converter. The exhaust gas temperature rises upstream of the pre-catalytic converter after time TO 34 as a result of the jump in load at time TO 34 and approaches time T1 36 a critical temperature threshold in the form of the predetermined first temperature value 18 at 900 ° C. In the prior art (graphs 24, 30), the engine lambda (graph 30) is set to values <1 from time T1 36 (graph 24) in order to reliably rule out exhaust gas temperatures> 900 ^° C. In the novel process it is first checked between T2 T3 38 to 40, whether the temperature difference threshold of 40K and the predetermined second temperature value of 940 ^Q C is exceeded within a time interval 46th In the example according to FIG. 1, this is not the case, so that after the time interval 46 of, for example, 5 seconds has elapsed, by gradually (or immediately) setting a corresponding engine lambda value (graph 32) after the time T3 40, the exhaust gas temperature (graph 26) falls below the permanent load threshold 18 is lowered. This means that the additional consumption resulting from the change in the engine lambda value only begins at a later point in time. Overall, the exhaust gas temperature (graph 26) for the interval T2 38 to T4 42 is above the continuous load limit 18.

In the alternative example according to FIG. 2, the temperature difference threshold (hard threshold) or the predetermined second temperature value 20 of 940 ° C. is exceeded within the time interval 46 at the time TKR 44 and the engine lambda value (graph 32) is reduced to the temperature gradient The value below the permanent exposure limit 18 is set. The interval T238 to T442 is thus shorter than in the example according to FIG. 1.

Fig. 3 illustrates the importance of different time intervals (predetermined period) for the approval of thermal overload for two different positions or measuring points in the exhaust system. The temperature is plotted on a vertical axis 48 and the time is plotted on a horizontal axis 40. Line 52 denotes a maximum permissible temperature for the pre-catalytic converter and line 54 denotes a maximum permissible temperature for the main catalytic converter. Graph 56 shows the course of the exhaust gas temperature upstream of the pre-catalyst without component protection intervention and Graph 60 shows the course of the exhaust gas temperature upstream of the pre-catalyst with component protection intervention according to the invention. Graph 62 shows the course of the exhaust gas temperature upstream of the main catalytic converter without component protection intervention, Graph 64 shows the course of the Exhaust gas temperature in front of the main catalytic converter with component protection intervention according to the prior art and graph 66 shows the course of the exhaust gas temperature in front of the main catalytic converter with component protection intervention according to the invention.

While a change in the setting of the engine lambda value (graph 64) very quickly leads to a lowering of the temperature in front of the pre-catalytic converter (graph 60) due to the low thermal inertia of the exhaust system, there is a measuring point in the middle of the main catalytic converter even when a temperature-related low engine lambda value is set after exceeding a critical temperature threshold 54 can only be expected with a slow decrease in the component temperature. Long time intervals would increase the risk of thermal overload.

At the exhaust gas temperature upstream of the pre-catalytic converter (graph 60), the continuous load threshold 52 is exceeded after a jump in load at time TA 68 and component protection is initiated at time TB 70. At time TC 72, the exhaust gas temperature is again below the continuous load threshold 52.

At the temperature in the main catalytic converter (graph 64) after a similar load step, component protection is initiated at time TA '74. The time interval from time TA '74 to time TB ¹ 76 corresponds to the time interval from time TA 68 to time TB 70. Because of the higher thermal inertia of the upstream exhaust system, the temperature rises more slowly than the exhaust gas temperature upstream of the pre-catalyst. For this reason, however, the cooling takes longer and overall, the component-critical interval from time TA ¹ 74 to time TC 78 (graph 64) is longer than the interval from time TA 68 to time TC 72. In particular, temperature-sensitive NOx storage catalysts are caused by the long exposure time is severely damaged, even if the temperature peak exceeds the continuous load value 54 less than the exhaust gas temperature (graph 60) upstream of the pre-catalytic converter. In the method according to the invention according to graph 66, the temperature peak and the duration of the temperature overshoot (interval from the time TA '74 to the time TC "80") are significantly lower.

Claims

PATE NTANS P RÜCH E

1.Method for operating an internal combustion engine, in particular a motor vehicle, with an exhaust gas system with an exhaust gas purification system, wherein an engine lambda value depending on a modeled or measured temperature at at least one critical point in the exhaust gas system is set to a temperature-dependent engine lambda value in such a way that normal exhaust gas temperature is adjusted is lowered when the determined temperature at the at least one point of the exhaust system exceeds a predetermined first temperature value, characterized in that the

The engine lambda value is only changed to lower the exhaust gas temperature from the value for normal operation into a temperature-dependent engine lambda value when the measurement temperature has exceeded the predetermined first temperature value for a predetermined period of time.

2. The method according to claim 1, characterized in that the predetermined period is selected differently for different critical points of the exhaust system.

3. The method according to claim 1 or 2, characterized in that the predetermined period is chosen the longer, the closer the critical point of the exhaust system to an engine block of the internal combustion engine.

4. The method according to at least one of the preceding claims, characterized in that the determined temperature is determined at least at a critical point upstream, downstream and / or on a main catalyst.

5. The method according to at least one of the preceding claims, characterized in that the determined temperature is determined at least one critical point upstream, downstream and / or on a pre-catalyst.

6. The method according to at least one of the preceding claims, characterized in that the engine lambda value is converted from the value for normal operation into the temperature-dependent engine lambda value before the expiry of the predetermined period if the determined temperature exceeds a second predetermined temperature value within the predetermined period, which is greater than the first predetermined temperature value.

7. The method according to claim 6, characterized in that the predetermined second temperature value is selected differently for different critical points of the exhaust system.

8. The method according to claim 6 or 7, characterized in that the predetermined second temperature value is chosen the higher, the closer the critical point of the exhaust system to an engine block of the internal combustion engine.

9. The method according to at least one of the preceding claims, characterized in that the engine lambda value is converted immediately or filtered from the value of normal operation into the temperature-dependent engine lambda value.