EP1432897B1 - Method for protecting exhaust gas purification systems of internal combustion engines against thermal overload - Google Patents

Description

The invention relates to a method for operating an internal combustion engine, in particular a motor vehicle, with an exhaust system having an exhaust gas purification system, wherein an engine lambda value is set to a temperature-dependent engine lambda value in a manner different from normal operation as a function of a modeled or measured temperature at at least one critical point of the exhaust system an exhaust gas temperature is lowered when the determined temperature at the at least one location of the exhaust system exceeds a predetermined first temperature value, the engine lambda value is only then changed to lower the exhaust gas temperature from the value for normal operation in a temperature-dependent engine lambda value when the measurement temperature of the predetermined first temperature value has exceeded for a predetermined period, according to the preamble of claim 1.

Catalysts of internal combustion engines, for example in motor vehicles, age when exposed to high temperatures, whereby the light-off behavior deteriorates, ie a same conversion rate is achieved only at higher catalyst temperature, and / or the peak conversion rate, which in 3-way catalysts usually> 99% for HC, CO and NOx is decreasing. This process increases disproportionately with increasing aging rates.

In the exhaust system of an internal combustion engine very high exhaust gas and catalyst temperatures of possibly more than 1000 ° C may occur, which possibly damage within a short exposure time an exhaust gas catalyst in an inadmissible manner, so that emission limits are no longer met. This is the case in particular in exhaust-gas purification systems with at least one catalyst close to the engine (precatalyst or main catalytic converter) since only a small amount of heat is dissipated via the short non-adiabatic exhaust gas line.

It is known to limit the exhaust gas temperature at least almost full engine load by stoichiometric engine operation. The introduced into the combustion chamber fuel is not completely burned because of the lack of oxygen. As a result, the combustion chamber gases achieve a lower temperature with the same amount of energy supplied. In addition, the enthalpy of vaporization of the fuel cools the combustion chamber gases. Further, in this mode of operation, the residual oxygen in the exhaust gas is lowered, so that less exotherm is generated in the catalyst (s).

Usually, a maximum permissible catalytic converter temperature is predetermined and the engine lambda is set as a function of the deviation of the determined catalytic converter temperature from the maximum permissible catalytic converter temperature. It is also known to additionally monitor one or more exhaust gas or catalyst temperatures at different points of the exhaust gas purification 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 an undesirable increase in consumption is associated with these measures. Efforts are therefore being made to limit the additional consumption of these component protection measures as far as possible. For example, DE 196 09 923 describes a stepped phase-in of overheating protection measures. It is first taken a first, less pronounced measure whose success in terms of a Temperature reduction is checked, in case of insufficient temperature reduction, a second, more effective measure is taken. A disadvantage of this method is that a certain overloading of the exhaust gas purification is accepted in order to achieve a reduction in consumption.

From JP 63 045445 A a method for preventing the overheating of an exhaust system of an internal combustion engine is known in which based on a basic injection of fuel and a rotational speed, an initial value for a fuel scribing is set, the amount of fuel is reduced, if an exhaust gas temperature is longer than for exceeds a value of 800 ° C for a predetermined time.

The invention has for its object to improve a method of the type mentioned above in such a way that a reduction of the additional consumption is achieved by an exhaust gas and exhaust gas purification system conditioned engine lambda adjustment without overloading the exhaust gas purification.

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

For this purpose, it is provided according to the invention that the predetermined period of time is selected differently for different critical points of the exhaust system.

This has the advantage that short-term exceedances of the limit temperature for the exhaust gas purification system, which are compensated by subsequent cooling phases promptly again, can be detected, so that there is no deviation from the value of the motor lambda value is performed by normal operation, so that in the overall operation reduced fuel consumption result in less aggressive component protection measures by changing the engine lambda value. Furthermore, a different weighting of the different temperature-critical

Distances are made in the component protection-related adjustment of the engine lambda value and a distinction is made between short-term loads, for example during acceleration processes, and longer-lasting load, for example when driving at full throttle uphill. Different dynamics of the temperature change at different locations in the exhaust system can be taken into account.

For example, the predetermined period of time is chosen the longer the closer the critical point of the exhaust system to an engine block of the internal combustion engine.

The temperature is preferably determined at at least one critical point upstream, downstream and / or at a main catalyst and / or precatalyst.

In order to prevent irreversible damage to the exhaust gas purification system, the engine torque is converted from the normal operation value to the temperature-dependent engine lambda value before the predetermined period has elapsed, if the detected temperature exceeds a second predetermined temperature value within the predetermined time period which is greater than the predetermined one first temperature value.

To account for different dynamics of the temperature change at different locations in the exhaust system, the predetermined second temperature value is selected differently for different critical locations of the exhaust system. By way of example, the closer the critical point of the exhaust system is to an engine block of the internal combustion engine, the higher the predetermined second temperature value is selected.

Fig. 1

eine graphische Darstellung eines ersten Temperaturverlaufes der Abgastemperatur vor einem Vorkatalysator sowie eines Motorlambdawertes über die Zeit mit und ohne Eingriff in den Motorlambdawert gemäß dem erfindungsgemäßen Verfahren,

Fig.2

eine graphische Darstellung eines zweiten Temperaturverlaufes der Abgastemperatur vor einem Vorkatalysator sowie eines Motorlambdawertes über die Zeit mit und ohne Eingriff in den Motorlambdawert gemäß dem erfindungsgemäßen Verfahren und

Fig. 3

eine graphische Darstellung des Temperaturverlaufes der Abgastemperatur vor einem Vorkatalysator und vor einem Hauptkatalysator über die Zeit mit und ohne Eingriff in den Motorlambdawert gemäß dem erfindungsgemäßen Verfahren.

Other features, advantages and advantageous features of the invention will become apparent from the dependent claims, as well as from the following description of the inven tion with reference to the accompanying drawings. This shows in

Fig. 1

a graphical representation of a first temperature profile of the exhaust gas temperature before a pre-catalyst and a motor lambda value over time with and without intervention in the engine lambda value according to the inventive method,

Fig.2

a graphical representation of a second temperature profile of the exhaust gas temperature before a pre-catalyst and a motor lambda value over time with and without intervention in the engine lambda value according to the inventive method and

Fig. 3

a graphical representation of the temperature profile of the exhaust gas temperature before a pre-catalyst and before a main catalyst over time with and without intervention in the engine lambda value according to the inventive method.

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

Furthermore, the location of the occurrence of the temperature exceeding in the exhaust system is taken into account in the determination of the temperature-dependent engine lambda value. Close to the cylinder head, because of the thermal inertia of the exhaust system, which is still very short of the load, due to the still low thermal inertia of the exhaust system, the temperature decreases considerably further downstream and in particular behind the catalyst (s). Thus, heating and cooling processes take place more quickly in front of a pre-catalyst close to the engine than in the center of a large-volume main catalytic converter arranged further away from the engine. Therefore, a temperature overload in the exhaust upstream of a near-engine first catalyst can be allowed for a longer period than a temperature overload at subsequent critical points of the exhaust system, as in negative load or speed changes or setting a Temperature-dependent engine lambda value at a location close to the engine with a faster cooling and thus remedy the critical situation can be expected.

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

Figures 1 and 2 graphically illustrate a dynamic part protection according to the invention. In this case, the time is plotted on a horizontal axis 10, an exhaust gas temperature in front of a precatalyst on a first vertical axis 12 and a motor lambda value on a second vertical axis 14. Value 16 on axis 14 corresponds to a motor lambda value of 1, line 18 corresponds to the predetermined first temperature value (900 ° C in this example) 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 prior art component protection intervention 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 progression of the engine lambda over time with no component protection intervention. Graph 30 shows the history of the engine lambda over time with prior art component protection intervention and Graph 30 shows the progression of the engine lambda over time with component protection intervention in accordance with the method of the present invention. Reference numeral 34 denotes a first time T0, 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 period 46.

FIG. 1 graphically illustrates a load jump at partial load full load at time T0 34, for example when entering a longer, steep grade. The Exhaust gas purification system includes, by way of example, a close to the engine primary catalyst and a further downstream arranged main catalyst, wherein in Fig. 1, the time course of the exhaust gas temperature (axis 12) is illustrated before the precatalyst. The exhaust gas temperature rises rapidly before the pre-catalyst after time T0 34 as a result of the load jump at time T0 34, and approaches a critical temperature threshold at time Tl 36 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 already set at values <1 (graph 24) from time T1 to 36 to safely exclude exhaust gas temperatures> 900 ° C. According to the method of the invention, it is first checked in a time interval 46 between T2 38 to T3 40 whether the temperature difference threshold of 40K or the predetermined second temperature value of 940 ° C is exceeded. In the example of FIG. 1, this is not the case, so that after lapse of the time interval 46 of, for example, 5 seconds by gradual (or immediate) setting a corresponding motor lambda value (graph 32) after the time T3 40, the exhaust gas temperature (graph 26) below the continuous load threshold 18 is lowered. Thus, the resulting from the change in the engine lambda value more consumption only at a later date. Overall, the exhaust gas temperature (graph 26) for the interval T2 38 to T4 42 is above the endurance limit 18.

In the alternative example according to FIG. 2, within the time interval 46 at the time TKR 44 the temperature difference threshold (hard threshold) or the predetermined second temperature value 20 of 940 ° C. is exceeded and the motor lambda value (graph 32) is set to fall below the steep gradient Continuous load limit 18 required value. Thus, the interval T2 38 to T4 42 is shorter than in the example of FIG. 1.

Fig. 3 illustrates the meaning of different time intervals (predetermined period) for the admission of thermal overload for two different positions or measuring points in the exhaust system. On a vertical axis 48 is the temperature and on a horizontal axis 40, the time is plotted. Line 52 indicates a maximum allowable temperature for the pre-catalyst and line 54 denotes a maximum allowable temperature for the main catalyst. Graph 56 shows the profile of the exhaust gas temperature upstream of the precatalyst without component protection intervention and graph 60 shows the profile of the exhaust gas temperature upstream of the precatalyst 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 Exhaust gas temperature upstream of the prior art component protection intervention catalyst and Graph 66 shows the exhaust gas temperature progression prior to the main component protection intervention catalyst of the present invention.

While before the pre-catalyst (Graph 60) due to the low thermal inertia of the exhaust system, a change in the setting of the motor lambda value (Graph 64) causes a very rapid lowering of the temperature, is at a measuring point in the middle of the main catalyst even with immediate adjustment of a temperature-induced low engine lambda value after exceeding a critical temperature threshold 54 only to be expected with a slow decrease in the component temperature. Long time intervals would increase the risk of thermal overload here.

At the exhaust gas temperature upstream of the precatalyst (graph 60), the endurance threshold 52 is exceeded after a load step at time TA 68 and the component protection is initiated at time TB 70. At time TC 72, the exhaust gas temperature is again below the steady state load threshold 52.

At the temperature in the main catalytic converter (Graph 64) after a similar load step, component protection is initiated at TA '74. The time interval from time TA '74 to time TB' 76 corresponds to the time interval from time TA 68 to time TB 70. The temperature rises more slowly than the exhaust gas temperature before the pre-catalyst because of the higher thermal inertia of the upstream exhaust system. For this reason, however, the cooling takes longer and overall, the component critical interval from time TA '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 catalytic converters is greatly damaged by the long exposure time, even if the temperature peak exceeds the steady-state load value 54 less than the exhaust gas temperature (graph 60) before the pre-catalyst. In the method according to the invention according to graph 66, the temperature peak and the duration of the temperature excess (interval from time TA '74 to time TC "80) are much lower.

Claims (7)

1. Method for the operation of an internal combustion engine, in particular of a motor vehicle, comprising an exhaust system with exhaust emission control system, an engine lambda value being set, as a function of a modelled temperature or a temperature measured at one or more critical points in the exhaust system, to a temperature-dependent engine lambda value, in a manner that differs from the normal operation to such an extent that an exhaust gas temperature is reduced if the temperature that has been determined at one or more points in the exhaust system exceeds a predefined temperature value, the engine lambda value being adjusted in order to reduce the exhaust gas temperature from the value for normal operation to a temperature-dependent engine lambda value only if the measured temperature has exceeded the predefined first temperature value for a predetermined period, characterized in that the predetermined period selected is different for various critical points in the exhaust system.
2. Method according to Claim 1, characterized in that the predetermined period selected is longer the closer the critical point in the exhaust system lies to an engine block of the internal combustion engine.
3. Method according to at least one of the preceding claims, characterized in that the temperature determined at one or more critical points is determined upstream of, downstream of and/or at a main catalytic converter.
4. Method according to at least one of the preceding claims, characterized in that the temperature determined at one or more critical points is determined upstream of, downstream of and/or at a pre-catalytic converter.
5. 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 to the temperature-dependent engine lambda value before the predetermined period has elapsed if, within the predetermined period, the temperature that has been determined exceeds a second predefined temperature value, which is greater than the first predefined temperature value.
6. Method according to Claim 5, characterized in that the second predefined temperature value selected is different for various critical points in the exhaust system.
7. Method according to Claim 5 or 6, characterized in that the second predetermined temperature value selected is higher the closer the critical point in the exhaust system lies to an engine block of the internal combustion engine.

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