DE9204220U1 - Exhaust system for combustion engines - Google Patents

Description

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Description:

The invention relates to exhaust systems for internal combustion engines, which, in addition to a main catalyst, have at least one starting catalyst, according to the preamble of claim 1. Such an exhaust system is known from EP-A-417 412.

When building exhaust systems with catalytic converters for internal combustion engines, especially in motor vehicles, the designer is faced with a major problem: if he places the catalytic converter as close as possible to the engine, the catalytic converter very quickly reaches the minimum temperature required for the catalytic conversion of the exhaust gases; the so-called starting behavior is excellent. During the main operating phase, on the other hand, the catalytic converter easily overheats and ages quickly; its fatigue strength is poor. If the catalytic converter is placed further away from the engine, the fatigue strength is good, but the starting behavior is poor. In practice, the catalytic converter is placed a considerable distance from the engine, since the main operating phase covers a significantly larger percentage of the total operating time than the starting phase.

Many suggestions have already been made to improve the starting behavior. It has already been suggested to heat the catalyst body using external energy, for example by means of a fuel-fed burner in the catalyst housing or by means of an electric resistance heater.

Other proposals intend to concentrate the exhaust gases on a partial volume of the catalyst during the start-up phase in the hope that this part of the catalyst will then

reaches its operating temperature (e.g. DE-A-36 29 945). However, these suggestions are useless because the concentration of the exhaust gases on a partial cross-section increases the flow rate of the exhaust gases, which reduces the residence time of the exhaust gas molecules in the catalytically active catalyst part I accordingly and reduces the conversion of the harmful exhaust gas components accordingly.

It has also been suggested that two parallel catalysts should be used and that the exhaust gases should only be directed to one catalyst during the start-up phase. In this case, the catalyst that is only effective during the start-up phase will reach its operating temperature more quickly; however, as soon as the second catalyst is put into operation, a second start-up phase occurs during which the exhaust gases are released into the environment uncleaned. In addition, considerable thermal stresses and deformations occur in the exhaust pipes.

The latest proposed solution to the dilemma described is the exhaust system described in EP-A-417 412 mentioned at the beginning. This uses the main catalyst, which has proven itself for decades and is installed far enough away from the engine, for the main operating phase.

For the starting phase, however, separate starting catalysts are used, which are positioned as close to the engine as possible. The optimal solution is to integrate these starting catalysts into the cylinder head.

During the start-up phase, the exhaust gases flow through the start-up catalysts, which in this way reach their operating temperature very quickly. At the same time, they gradually heat up the main catalyst. As soon as the main catalyst has reached its operating temperature, a flap provided in the start-up catalysts is opened.

which opens up a central bypass channel. This is intended to ensure that the starting catalyst bodies do not overheat during the main operating phase.

However, this goal cannot be achieved with the design disclosed in EP-A-417. Since the exhaust gases leaving the cylinder pulsate very strongly, exhaust gas will enter the starting catalyst body even when the bypass is open. At the same time, the exhaust gases flowing through the bypass channel heat up the starting catalyst body due to radiation and convection. Further problems arise from the flap or its actuating axis, which must pass through two housing walls of the starting catalyst. This axis passage must not only remain permanently operational at the extremely high temperatures of 900 degrees C and more at this point, but also, in particular, remain permanently gas-tight. Otherwise, ambient air will enter the exhaust system, causing the lambda sensor to measure an incorrect air ratio.

The present invention is based on the object of improving the exhaust system of the type mentioned at the outset in such a way that the starting catalyst body is subjected to less thermal stress in the main operating phase than before.

This object is achieved by a generic exhaust system with the features according to the characterizing part of claim 1.

In the present invention, the central channel and the ring channel of the starting catalyst are extended by the concentric exhaust pipes, so that the elements required for control, such as switching flaps, etc., are located at a considerably greater distance from the engine.

can be positioned where they are subjected to far less thermal stress. In addition, the starting catalyst body is relieved of some of the exhaust gas pulsations.

This applies in particular if, according to an advantageous development of the invention, the inner exhaust pipe is pulled through the central channel of the starting catalyst up to its front surface.

Preferably, the wall of the central channel or the corresponding section of the inner exhaust pipe is thermally insulated. This has two advantages. In the start-up phase, the starting catalyst body gives off less heat to the central pipe, so it heats up more quickly. In the main operating phase, less heat is given off by the exhaust gases to the starting catalyst body, which therefore stays cooler and ages more slowly.

Two possible solutions are conceivable for this. On the one hand, an additional insulating tube can be provided, and on the other hand, an area of the starting catalyst body can be inactivated by placing an annular diaphragm on its front surface.

According to a preferred embodiment of the invention, an exhaust gas supply pipe section is guided in a funnel shape into the central channel or the inner exhaust pipe, so that a throttle point is created between them. This throttle point is intended to dampen the exhaust gas pulsations during the main operating phase, so that they only reach the starting catalyst body in a weakened form, if at all. Since this then no longer receives any exhaust gases to be catalytically converted, its own temperature does not rise.

Preferably, the wall thickness of the inner exhaust pipe is less than that of the outer one. This is easily

possible because the shape and strength are ensured by the outer exhaust pipe. The thinner pipe has a lower heat capacity due to its lower mass, so it requires less heat energy to heat up, which ultimately causes the main catalyst to heat up more quickly. Since the exhaust gases flow through the annular space between the outer and inner exhaust pipe during the start-up phase, both pipes are heated evenly; thermal distortion problems are eliminated. During the main operating phase, however, the annular space between the outer and inner exhaust pipe acts as thermal insulation, so that on the one hand more heat is supplied to the main catalyst and on the other hand the thermal load on the vehicle chassis is reduced.

According to an advantageous embodiment of the invention, a flap is again provided to close the central channel during the start-up phase, but this is now positioned at the end of the inner exhaust pipe or a corresponding extension pipe. There it is subjected to considerably less thermal stress.

According to an advantageous development of the invention, a flap is also provided to close the ring channel during the main operating phase, namely at the end of the outer exhaust pipe or a corresponding extension of the same. It is naturally recommended to open and close the two flaps alternately using a suitable coupling.

Thanks to the second flap, it is ensured that no more exhaust gases can enter the starting catalyst body during the main operating phase, even if the exhaust gases pulsate strongly.

According to an advantageous development of the invention, a pipe switch is positioned at the end of the concentric exhaust pipes. This pipe switch leads the concentric exhaust pipes into two parallel single exhaust pipes, in each of which one of the flaps is then located. If it is a multi-pipe exhaust system with two or more starting catalytic converters, this pipe switch also leads all the inner pipes and all the outer pipes together.

The flaps can be controlled based on the engine map and/or exhaust back pressure. In addition, it is possible to include the concentric exhaust pipes in the acoustic tuning of the exhaust system. Since exhaust pipes with a small cross-section are acoustically advantageous at low engine power, the annular space between the inner and outer exhaust pipes is always opened in this operating state and not just during the actual start-up phase. At high engine power, on the other hand, pipes with a large cross-section are acoustically more advantageous. Finally, it is also possible to dimension the concentric exhaust pipes as lambda-quarter l lines, which are switched on and off as required using flaps.

The invention will be explained in more detail in the form of an embodiment using the drawing. It shows:

Fig. 1 an exhaust system for a three-cylinder engine,

Fig. 2 shows a cross-section through the exhaust system of Fig. 1 in the area of the switchgear in an enlarged view,

Fig. 3 an exhaust system for a six-cylinder engine,

Fig. 4 shows a variant of the exhaust system of the

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Fig.3,

Fig. 5 a longitudinal section through a first starting catalyst and

Fig. 6 a longitudinal section through a second start catalyst.

In Fig. 1 you can see an exhaust system for a three-cylinder engine with engine manifold 1. Directly behind the engine manifold 1 there is a starting catalyst 2 with a central bypass channel 3 and an annular channel 4 containing the starting catalyst body 2'.

Two concentric pipes 6, 7 are welded to the outlet funnel of the starting catalyst 2, in such a way that the inner exhaust pipe 7 is connected to the central channel 3, and the outer exhaust pipe 6 is connected to the ring channel 4. The inner exhaust pipe 7 is led through the central channel 3 up to the front face of the starting catalyst body 2'. An exhaust gas supply pipe section 5 is led from the inlet funnel of the starting catalyst 2 in a funnel-like manner into the central channel 3 or the inner exhaust pipe 7, so that a throttle point is created between the two, which is intended to throttle the pulsations contained in the exhaust gases coming from the engine before they reach the starting catalyst body 2'.

The end of the inner exhaust pipe 7 remote from the starting catalyst 2 can be closed by means of a flap 11 during the starting phase. This flap 22 is subjected to little thermal stress.

Behind the flap 11, the exhaust gases are led through a single exhaust pipe 13 to the main catalytic converter 14, which

can be positioned sufficiently far away from the engine as required for a long service life.

During the start-up phase, the switching flap 11 is set so that the exhaust gases coming from the exhaust manifold 1 pass through the large opening between the central channel 3 and the funnel 5 into the annular space 4 and to the starting catalyst body 2', where they are catalytically cleaned. The cleaned exhaust gases then flow through the annular space between the outer and inner exhaust pipes 6, 7. Both pipes 6, 7 heat up evenly so that thermal distortion problems do not occur. The amount of heat required to heat up the pipes 6, 7 can be reduced by making the inner exhaust pipe 7 considerably thinner-walled than the outer exhaust pipe 6. The exhaust gas flows are then brought together behind the flap 11, as already mentioned.

After the main catalytic converter 14 has also reached its operating temperature, the switch 11 is switched. As a result, the exhaust gases now flow directly through the central bypass channel 3 and the inner exhaust pipe 6. The starting catalytic converter body 2' is no longer flowed through. It can cool down, which benefits its service life. At the same time, the annular space between the outer and inner exhaust pipes 6, 7 acts as air insulation, so that the exhaust pipes 6, 7 radiate less heat into the environment. This not only reduces the thermal load on the vehicle chassis, for example, but also ensures improved heating of the main catalytic converter 14.

The additional space required by using concentric pipes is considerably less than when using two parallel pipes, which would inevitably result in thermal distortion.

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Fig. 2 shows a cross section along the line II-II in Fig. 1. It shows a bracket 18 with radial struts in the annular space between the outer and inner exhaust pipe 6, 7.

In Fig. 3 you can see an exhaust system for a six-cylinder engine with two engine manifolds 1, two starting catalysts 2 and two concentric exhaust pipes 6, 7. The latter are led at their ends remote from the starting catalysts 2 into a pipe switch 10, where the inner exhaust pipes 7 and the outer exhaust pipes 6 are brought together and branched into two parallel pipes 8, 9. This branching is necessary in order to alternately block one or the other exhaust duct using the simplest and most inexpensive flaps 11, 12.

As shown in Fig. 3, the outer exhaust pipes 6 can be provided with a perforation 17 in the area of the pipe switch 10. This causes the exhaust gases to flow into the outer area of the pipe switch 10 and then to the flap 12.

However, there are other ways of influencing the acoustic tuning of the exhaust system. It is known that exhaust pipes with a small cross-section are acoustically advantageous at low engine power, while exhaust pipes with a large cross-section are acoustically advantageous at high engine power. This can be taken into account by directing the exhaust gases through the ring channel between the outer and inner exhaust pipes 6, 7, not only during the actual start-up phase, but whenever a low engine power is being delivered. In doing so, they naturally also flow over the start-up catalyst body 2', which is tolerable, however, since the exhaust gases are not so hot in this operating state.

For high engine power, both flaps 11, 12

open in order to acoustically activate the maximum pipe cross-section.

Finally, there is also the possibility of adjusting the lengths of the inner and outer exhaust pipes 6, 7 so that they act as quarter-wave pipes for certain frequencies.

Fig. 4 shows a section of the exhaust system in the figure in the area of the pipe switch 10. Here the perforation 17 is missing, the acoustic tuning is achieved solely by the switching flaps 11, 12.

Fig. 5 shows a longitudinal section through a starting catalyst 2. The central channel 3 is surrounded by an insulating tube 15, the insulation being achieved essentially by an air gap between the two tubes. This significantly reduces the heat transfer between the central bypass channel 3 and the starting catalyst body 2' .

Fig. 6 also shows a longitudinal section through a starting catalyst 2 with insulation between the central bypass channel 3 and the starting catalyst body 2'. In this case, the area of the starting catalyst body 2' directly adjacent to the central bypass channel 3 is inactivated by an annular diaphragm 16 placed on the front surface.

Claims (18)

Sc hat &zgr;claims: 1. Exhaust system for internal combustion engines, comprising a main catalyst (14), at least one starting catalyst (2) and at least one exhaust pipe (6, 13) in between, the starting catalyst (2) being positioned as close as possible to the engine and having a central channel (3) that can be closed during the starting phase and a ring channel (4) surrounding it and containing at least one starting catalyst body (2'), characterized in that two concentric exhaust pipes (6, 7) are provided behind the starting catalyst (2) and that the central channel (3) opens into the inner exhaust pipe (7), the ring channel (4) opens into the annular space between the concentric exhaust pipes (6, 7). 2. Exhaust system according to claim 1, characterized in that the inner exhaust pipe (7) is pulled through the central channel (3) of the starting catalyst (2) up to its front face. 3. Exhaust system according to claim 1 or 2, characterized in that the wall of the central channel (3) or the corresponding section of the inner exhaust pipe (7) is thermally insulated. 4. Exhaust system according to claim 3, characterized in that an insulating pipe (15) enclosing the central channel (3) is provided. 5. Exhaust system according to claim 3, characterized in that an annular diaphragm (16) surrounding the central channel (3) is placed on the end face of the starting catalyst body (2'). 6. Exhaust system according to one of claims 1 to 5, characterized in that an exhaust gas supply pipe section (5) is funnel-shaped and is guided into the central channel (3) or the inner exhaust pipe (7), so that a throttle point is created between them. 7. Exhaust system according to one of claims 1 to 6, characterized in that the wall thickness of the inner exhaust pipe (7) is smaller than that of the outer exhaust pipe (6). 8. Exhaust system according to one of claims 1 to 7, characterized in that the flap (11) for closing the central exhaust channel (3) is positioned at the end of the inner exhaust pipe (7) or a corresponding extension pipe (9). 9. Exhaust system according to one of claims 1 to 8, characterized in that a flap (12) is provided for closing the annular channel (4) and is positioned at the end of the outer exhaust pipe (6) or a corresponding extension pipe (8). 10. Exhaust system according to claim 8 and 9, characterized in that the two flaps (11, 12) open and close alternately. 11. Exhaust system according to one of claims 1 to 10, characterized in that a pipe switch (10) is positioned at the end of the concentric exhaust pipes (6, 7). 12. Exhaust system according to one of claims 1 to 11, characterized in that a time-dependent control device is provided for the flaps (11, 12). 13. Exhaust system according to one of claims 1 to 11, characterized in that an engine map-dependent control device is provided for the flaps (11, 12). 14. Exhaust system according to one of claims 1 to 11, characterized in that an exhaust back pressure dependent
Control device for the flaps (11, 12) is provided. 15. Exhaust system according to one of claims 1 to 11, characterized in that a speed-dependent control device is provided for the flaps (11, 12). 16. Exhaust system according to one of claims 1 to 15, characterized in that the cross section of the annular channel between the inner and outer exhaust pipe (6, 7) is designed for acoustic
is dimensioned for effectiveness. 17. Exhaust system according to one of claims 1 to 16, characterized in that the cross section of the inner exhaust pipe
(7) is dimensioned for acoustic effectiveness. 18. Exhaust system according to one of claims 1 to 17, characterized in that the inner and/or the outer
concentric exhaust pipes (6, 7) as acoustically effective
Lambda quarter L-pipe is dimensioned.