DE19510606A1 - Reducing IC engine exhaust counter pressure - Google Patents

Abstract

The flow through the individual catalyser cells (7) takes place from outside inwards, i.e. across the long axis of the catalyser while the exhaust gases flow out parallel to the long axis of the catalyser. The individual exhaust pipes are so fitted in front of the catalyser that a precisely defined swirl flow with superimposed axial component results. There is a low-pressure loss flow in the exhaust pipes in spite of the pulsations in the flow. There is a uniform flow through the catalyser cells, and a high recovery of the dynamic pressure of the high velocity exhaust flow from the individual pipes.

Description

State of the art

It is known to use catalysts for cleaning the exhaust gases of internal combustion engines clog.

It is also known that the catalyst until a minimum temperature is reached causes no or only a greatly reduced reduction in exhaust gas emissions.

In order to shorten the time to reach the working temperature, it has therefore in the In the past, there was no lack of trials thanks to the Kataly being installed close to the engine sators to quickly start the exhaust gas detoxification.

Despite good results in terms of exhaust emissions in the legal test cycles but is currently dispensed with in many places, the catalyst in the immediate vicinity to arrange the engine, since durability tests are particularly practical Driving operations have resulted in great uncertainties.

As an alternative, catalyst installation is currently relatively far away from Engine preferred, with appropriate insulation of the exhaust pipes and through a reduction in the heat-active mass with the exhaust gas in front of the catalyst Contacting standing pipe sections is attempted to heat up the pipe as quickly as possible Ensure catalyst.

This has u. a. to the well-known version of exhaust manifold and exhaust manifold in double-walled welded construction, which is not only with considerable costs connected, but also only conditionally brings similar results as a close-coupled engine Installation of the catalyst.

For this reason, intensive efforts are currently underway to find a suitable one heat the catalyst electrically or via an additional burner for one to ensure rapid ignition of the catalyst.

As an alternative to preheating, efforts are also known to use an intermediate spoke exhaust gases from the cold start until the catalyst ignites the required To achieve emission limit values.

The costs for preheating the catalyst or for temporarily storing the Exhaust gases are enormous, but the question of durability is still an issue for these processes has not been fully clarified.

To the cost of the alternatives to the close-coupled catalytic converter bypass, some manufacturers use the close-coupled catalyst in conjunction with extreme mixture enrichment at full throttle or in the upper torque range. The excess amount of fuel lowers via the heat of evaporation and the increased specific heat capacity the exhaust gas temperature and thus protects the catalytic converter Overheating. A post-reaction takes place in the catalyst due to the lack of oxygen  only very limited. As a result, the catalyst is overheated protected, but there are disadvantages for fuel consumption and emissions hand.

Whether the temporary emission of such large amounts of unburned hydrocarbons in The future will still be legally permissible, is very questionable. At least it is however, it is expected that vehicles with such fuel waste and Pollution of a negative publicity and in extreme cases a real rush campaigns will be exposed to the press.

This is especially true for low powered vehicles, which are relatively common at high Load and thus operated with extreme mixture enrichment.

In addition to the thermal stress on the catalyst material lies with the known Applications of the close-coupled catalyst in comparison to conventional underbody Catalyst a negative reaction to the charge change before. This does not apply to the possibility of a specific coordination of gas dynamic effects in the Exhaust pipes a targeted increase in torque in individual speed bands to achieve, on the other hand, the maximum drops due to the increased exhaust gas back pressure power.

Task

From this, the task can be derived for internal combustion engines with catalytic exhaust gas aftertreatment to develop a process based on today's available catalyst materials an installation of the catalyst close to the engine without Restriction regarding the aging behavior of the catalyst allows.

In addition to the indispensable requirement for thermal aging resistance continue the task of the engine-near catalyst with as little negative off enable effects on maximum torque and maximum engine power. This is particularly the demand for the lowest possible exhaust back pressure significant, since this not only affects the torque and power curves, but also in wide map areas on fuel consumption and pollutant emissions sion.

As little or no mixture enrichment as possible should be used for cooling the catalyst be used. This means that the engine if possible only at stoichiometric fuel air ratio (lambda = 1) is operated. Beyond that engine operating points with slight mixture enrichment for the purpose of torque and performance enhancement are chosen, not least depends on the legal framework conditions.

In particular, the method according to the invention is also intended to reduce the Ge mixed enrichment can be ensured that, for example, in the USA wrote exhaust test after 100,000 miles no problems from excessive aging of the catalyst are to be expected.  

This method is particularly mentioned as an inexpensive alternative to the one already mentioned Process for heating the catalyst or for temporarily storing the cold start to see emissions. Often, the full potential of the electrical does not have to be Catalyst heating can be achieved to meet the required statutory emissions to meet restrictions.

In a further step, the task is the costly isolation or double-walled design of the exhaust pipes in front of the catalytic converter to one if possible restricting small areas or even making them superfluous.

solution

According to the invention, a method for reducing the Exhaust gas back pressure in internal combustion engines with exhaust gas catalytic converter, in particular for the reduction of the exhaust gas back pressure with a catalytic converter installed extremely close to the engine proposed, the flow through the individual catalyst cells from the outside to inside, d. H. transversely to the longitudinal axis of the catalyst, and the exhaust gases approximately parallel flow out of the catalytic converter to the longitudinal axis of the catalytic converter is that the individual exhaust pipes are arranged in front of the catalyst are that there is a precisely defined swirl flow with a superimposed axial component results in a low pressure loss through despite the pulsation of the exhaust gas flow of the exhaust pipes, a uniform flow through all catalyst cells and in particular the highest possible recovery of the dynamic pressure from the Exhaust gas escaping from the individual exhaust pipes at high speed.

Such a flow leads to a strong swirl flow local and temporal equalization of the flow rate until on occurs in the catalyst cells. By the radial supply of the exhaust gas to the Individual catalyst cells have a large inlet cross-sectional area, verbun the one with a low flow rate inside the cells.

The large inlet cross-sectional area or the reduced flow velocity is in turn coupled with a drastic improvement in pressure loss. This effect is additionally reinforced by the fact that with the same cell density and the same volume of the Monoliths, d. H. of the catalytically active area, inevitably a reduction in Length of the individual catalyst cells is present.

Since the heat and mass transport and sometimes also the catalytic conversion in the front area of the individual cells takes place much more intensely than further downstream, The increase in zones with thin boundary layers results in a further ver improvement of the catalytic conversion. The reduction of local ones does work Exhaust gas velocities within the catalyst cells somewhat opposed, but should for the catalytic effectiveness of the former influence may be dominant.

Normally, the savings in pressure loss will be so great that drastic Increasing the cell density is possible. At the reduction of the flow cross section is in turn coupled an improvement in catalytic effectiveness, which is an additional Liche reduction of the catalyst volume, in particular the cell length.  

It should be obvious that the savings in catalytically active material in particular to the savings in precious metals, significant cost reductions pelt are.

The swirl of the flow in particular leads to a reduction in the pressure pulsations of the Exhaust gas and thus to a lower thermal and mechanical load on the Catalyst cells. In particular, the ex emerging at high speed hits Do not jet the gas directly onto the catalyst cells.

At the same time, the relative speed between those from the one is due to the swirl individual pipes emerging exhaust gas jets and the eddy flow in the Catalyst prechamber or in the exhaust manifold reduced, which has a positive effect on the off puffing process from the respective combustion chamber and thus also on the amount of fresh gas affects.

As a particularly cheap variant, a method is proposed in particular, which is characterized in that the mouth of the individual exhaust pipes in one defined distance from the outer surface of the catalyst or the front arranged exhaust gas collector is arranged to flow into a region as possible allow low static pressure.

This process variant leads to the fact that due to the centrifugal forces increasing radial pressure in the radial direction does not act as back pressure on the exhaust operation affects. The exhaust gas flows at high speed and high swirl - achieved by a special arrangement of the exhaust pipe with an offset in the direction Center line - in the front catalytic converter area, is due to the centrifugal forces to the outside ousted and delayed at the same time. That means it becomes part of the dynamic pressure recovered from the exhaust process. It is obvious that this is positive affects the pressure loss balance.

Fig. 1 shows an exemplary embodiment of the inventive method. The gases flow according to the firing order one after the other through lines 1 - 4 of the exhaust manifold into the catalytic converter housing 5 welded to the exhaust manifold in the present case. The inflow takes place on the end face, the mouths of the exhaust pipes being arranged in such a way that a defined swirl flow results with a superimposed axial component. At the same time, the orifices are offset somewhat from the jacket of the catalytic converter in the direction of the center line in order to flow into a zone of reduced static pressure if possible.

(Whether this the diameter of the casing tube something must be greater than in Figure 1, in order to use the full potential in terms of minimal pressure losses must be clarified on the basis of experiments, the diameter of the Abgasleitun gen. 1 -. 4 and the catalyst casing tube 5 , are subject to certain constraints anyway depending on the cost and weight or depending on the installation space and also heat loss to the environment.)

The flow is deflected outwards by the swirl and the cover plate 6 and flows — still with swirl — along the casing tube and is distributed over the individual catalyst cells 7 . The catalytically cleaned exhaust gases are collected in the interior of the catalytic converter and then guided axially out of the catalytic converter to the soundproofing system, which is coupled to the flange 8 .

In this application, the swirl does not only result in damping the pressure pulsations but also on an even distribution of the exhaust gas over the individual catalyst cells. At the same time, the exhaust gas is mixed intensively. this has This is particularly advantageous if the engine is driven with a lambda of 1 overall the individual cylinders but different fuel air ratios and thus have a fuel air ratio not equal to 1. For catalysts with only one Directly flowed through monoliths would be only a limited effectiveness here given.

Even when using two flow-throughs arranged in quick succession Monoliths would still be a disadvantage here.

By mixing the exhaust gas before it flows into the individual catalyst cells is also the short-term exposure to extreme temperature peaks on the strongly jagged and thus highly sensitive catalyst surface avoided, like it is present in the previously known direct flow in the longitudinal direction.

At the same time, the swirl flow results in good heat transfer to the catalyst jacket, which also leads to a damping of temperature peaks. This advantage with regard to the temporary temperature peaks first appears through one certain disadvantage with regard to the rapid heating in the cold start. This however, i. a. not too, because on the one hand a monolith of less mass is heated must, on the other hand, not over the entire length directly with the outer jacket in There is thermal contact.

In particular, the type of catalyst according to FIG. 1 is characterized by a significantly more homogeneous temperature load than previously known types of catalyst. This fact is underlined by the fact that the monolith for this application can be made of metal without cost disadvantages - and thus with high temperature conductivity and extreme cell density - which offers further advantages with regard to a uniform temperature load.

In order to keep the heat losses as low as possible, a process variant is proposed in FIG. 2, in which the catalyst is insulated by an additional jacket 9 , which together with the inner jacket 5 , which is now provided with a minimal heat-active mass, forms an insulating gap 10 .

It is obvious that such an application can be implemented without too much effort and in no way with the effort for a double-walled design of all Exhaust pipes is comparable. The results regarding rapid heating are however much better.

Due to the low heat-active mass in this application is also compared to Conventional design near-engine catalytic converter no longer a disadvantage for rapid heating.  

It is particularly advantageous if the insulating gap 10 is formed by a vacuum, an air gap or other radiation-permeable insulating material. The dependency of the heat transfer by heat radiation on the fourth power of the wall temperature then leads to the particularly advantageous property that there is a good insulation effect through the inner wall 5 in the heating phase, while in the case of a very hot catalytic converter there is a noticeable relief from the heat radiation to the surroundings. This can be further improved by appropriate cooling of the outer wall 9 .

As a further improvement according to the invention, the catalytic converter according to FIG. 3 has a two-part catalytic converter area 7 a and 7 b with an air gap 7 c. This embodiment has the advantage that the block 7 a heats up extremely quickly due to the lack of heat conduction into the further downstream areas. Furthermore, it is b connected by thorough mixing prior to entry into the portion 7 with thin boundary layers in the inlet 7 in b. This in turn is linked to an improved catalytic conversion, combined with a reduced risk of local overheating inside the catalyst cells.

It is to be expected that the measures according to the invention largely relate to a Mixture enrichment for cooling the catalyst can be dispensed with. Yet it can be used in particular to achieve a high nominal output or to achieve a high torque may be advantageous to allow a certain amount of mixture enrichment. The reduction in displacement made possible by this measure can be done in daily driving operations often lower fuel consumption and u. Sometimes even less Cause emissions than a power engine with the same capacity and without large displacement Mixture enrichment.

Despite the stoichiometric mixture control, it can be caused by the control fluctuations and especially in unsteady driving that occur on fuel enrichment a phase with strong mixture emaciation follows.

It is special for the protection of the catalytic converter for the above-mentioned operating states advantageous to temporarily make a significant mixture enrichment in order to avoid that the catalyst by the exothermic reaction during the mixture enrichment attached reaction partners overheated in the emaciation phase. Therefore, it is proposed according to the invention that in phases of extreme mixture leaning, especially in unsteady driving, additional fuel is supplied, to reduce the weight of the mixture with its particularly negative effects on the To limit catalyst aging.

Especially when the engine brakes heavily from operating conditions with mixture enrichment it is proposed that additional fuel be injected briefly, to lean the mixture from substoichiometric to far overstoichiometric with its particularly negative effects on catalyst aging.

Claims (11)

1.Procedure for reducing the exhaust gas back pressure in internal combustion engines with an exhaust gas catalytic converter, in particular for reducing the exhaust gas back pressure in the case of a catalytic converter installed extremely close to the engine, the flow through the individual catalytic converter cells from the outside inwards, that is to say transversely to the catalytic converter longitudinal axis, and the exhaust gases being approximately parallel to the catalytic converter longitudinal axis from the Flowing out the catalytic converter, characterized in that the individual exhaust gas pipes are arranged in front of the catalytic converter in such a way that there is a precisely defined swirl flow with a superimposed axial component and, in spite of the pulsation of the exhaust gas, a low-pressure flow through the exhaust gas pipes, a uniform flow through all catalyst cells and, in particular, the highest possible Recovery of the dynamic pressure from the exhaust gas emerging at high speed from the individual exhaust pipes results. 2. The method according to claim 1, characterized in that the individual exhaust pipes of the exhaust manifold into the front of the catalytic converter housing or in front of it arranged exhaust manifold. 3. The method according to claim 1, characterized in that the individual exhaust pipes of the exhaust manifold radially, d. H. on the outer surface, in the catalytic converter housing or enter the flue gas collector in front of it. 4. The method according to any one of the above claims 1-3, characterized in that the mouth of the individual exhaust pipes at a precisely defined distance from the outside Shell surface of the catalytic converter or the exhaust gas collector arranged in front of it is arranged is to keep an inflow in a region of the lowest possible static pressure enable. 5. The method according to any one of claims 1-4, characterized in that the Catalyst block in the flow direction of the individual cells has at least one interruption of several millimeters. 6. The method according to any one of claims 1-5, characterized in that the Catalyst jacket an additional insulated inner jacket of low heat-active mass has, in particular during the heating phase, the heat losses to the environment to minimize. 7. The method according to claim 6, characterized in that the insulation over a Air gap, a radiation-permeable material or by vacuum. 8. The method according to any one of claims 6 and 7, characterized in that special Measures for good cooling of the outer jacket, e.g. B. via a corresponding Air circulation, to be taken. 9. The method according to any one of claims 6-8, characterized in that the isolated Inner jacket is equipped with a catalytically active surface. 10. The method according to any one of claims 1-9, characterized in that in  Phases of extreme mixture leaning, especially in unsteady driving, additionally Fuel is supplied to lean the mixture with its particular to limit the negative effects on catalyst aging. 11. The method according to claim 10, characterized in that especially in strong Braking the engine briefly from operating conditions with mixture enrichment Additional fuel is injected to lean the mixture from substoichiometric after far more than stoichiometric with its particularly negative effects limited to catalyst aging.