Patent Application
20060168938
This application claims the benefit of German Application No. DE 10 2005 004 651.7 filed Feb. 2, 2005.
1. Field of the Invention
The invention relates to an exhaust gas conveying apparatus of an internal combustion engine with an internal liner and a process for its fabrication.
The continuous reduction of the permissible pollutant emissions of motor vehicles requires continuous improvement in the treatment of exhaust gas. Special attention is paid to the exhaust gas treatment during the cold start phase. Immediately after the start relatively high quantities of unburned hydrocarbons are emitted. The reason for this, amongst others, is that the catalytic converter has not yet reached its light-off temperature, resulting in an insufficient conversion of the hydrocarbons.
Usually the catalytic converter reaches its light-off temperature about one minute after the start of the engine through the waste heat of the exhaust gas stream so that the catalytic conversion of the exhaust gas begins.
2. Description of Related Art
In DE 100 48 286 A1 it is proposed to buffer at least a part of the unburned hydrocarbons. For that, a part of the exhaust gas conduit is coated on its inside with an absorbing material, preferably on the basis of zeolite. This coating absorbs part of the unburned hydrocarbons during the cold start phase and releases them again at higher temperatures. Already during the cold start phase the coating of the exhaust manifold is exposed to exhaust gases with temperatures of up to 1050° C. This is a disadvantage since the coating rapidly heats up to it's desorption temperature and consequently it buffers only small amounts of hydrocarbon during the heat up period of the catalytic converter to its light off temperature. Furthermore, the coating which is directly applied to the internal walls of the exhaust manifold is exposed to high mechanical loads. It must withstand large thermal cycles and thermal stresses resulting from the different thermal expansion coefficients of the ceramic and the metal without delamination of parts of the coating which would go into the catalyst downstream.
In another approach stable porous inlays, typically high temperature resistant sintered ceramic bodies, are laid respectively glued into a part of the exhaust conduit. These inlays feature a better form-stability and, with regard to easily condensable exhaust gas constituents, better absorbing and/or adsorbing properties. A disadvantage of this approach is the unadjusted heat transfer or, as the case may be, heat removal. The porous inlays are usually very efficient thermal insulators, so that the exhaust gas is cooled to a much lesser degree by the surfaces of the exhaust gas conduit. Although this is an advantage during the cold start phase, since the catalytic converter reaches its light off temperature faster, this thermal insulation also results in a higher heat load for the catalytic converter during continuous operation, with the danger of overheating the catalyst material.
Since the known sintered ceramics as an inlay for exhaust gas conveying apparatuses or conduits usually feature only a low mechanical strength, the material of the conduit must essentially carry the entire mechanical load, so that the construction of the conduits cannot be designed lighter than in an apparatus without liners. The resulting weight gain of the overall system of the exhaust gas conduits is a disadvantage with respect to the desired lightweight construction.
The objective of the invention is to create an exhaust gas conveying apparatus as well as a process for its fabrication, which features a good buffer capacity for easily condensable exhaust gas constituents as well as a heat balance, which is adjusted to the cold start as well as the continuous operation phase, and with which the weight of the entire exhaust gas conveying apparatus may only be marginally increased.
According to the invention the objective is achieved by an exhaust gas conveying apparatus of an internal combustion engine with internal liner, as well as a process for the fabrication of an exhaust gas conveying apparatus of an internal combustion engine with internal liner.
In a first embodiment according to the invention an exhaust gas conveying apparatus of an internal combustion engine with internal liner is featured, in which the liner features capillary bores, which are open at least towards the exhaust gas conveying side of the liner. The bores can either open single-sided or they can penetrate the liner completely (through holes).
Such a modified structure of the liner of the exhaust gas conveying apparatus with bores as described above enlarges the active surface of the liner.
Here the active surface is referred to as the surface which acts absorptive or adsorptive on the easily condensable exhaust gas constituents such as light hydrocarbons (CHx). The active surface includes the outer as well as the inner surface of the liner which is constituted by the bores.
Such a modified structure facilitates the hydrocarbon condensation, in particular a hydrocarbon capillary condensation, and features in addition a capillary attraction which transports the hydrocarbon-condensate into the interior of the structure and/or into regions of lower exhaust gas concentration and/or retains the hydrocarbon-condensate until its desorption.
Possible embodiments of the invention are shown in the illustrations, which are not to scale. The illustrations are intended to help explain the principle of the invention in more detail and are not meant to impose any restrictions. They show:
The bores may be introduced into a metallic or ceramic liner. It is advantageous if the internal wall of the apparatus and the liner are made from materials with equal or similar thermal conductivity, e.g. both made from metal, so that even when exposed to large thermal cycles thermal stress is avoided or minimized. The bores may be introduced into a solid or porous liner.
In a preferred embodiment the liner features capillary bores with different depths and/or different diameters. The diameter of the bores is between 10 and 1000 μm, their depth is at least 10 μm.
Bores with a diameter which varies, particularly decreases, with the depth have been found to enhance condensation.
Preferably the liner features capillary bores with different depth and/or diameter, such that they exhibit a gradient of their active surface. For instance only part of the bores penetrates the liner deeply or even completely whereas the major part is located close to the surface of the liner side which is exposed to the exhaust gas. Thus a layer close to the surface features a larger surface than a layer further away from the surface.
Particularly advantageous is when the gradient is designed such that the active surface varies on average at least 20% across the overall length. The gradient may be a steady change or a change in steps.
In an advantageous embodiment the liner features capillary bores which are arranged perpendicular to the exhaust gas conveying surface of the liner. Such perpendicular bores in the liner can easily be fabricated.
In an alternative or additive advantageous embodiment the liner features capillary bores which are inclined with respect to the exhaust gas conveying surface of the liner. The bores maybe inclined away from or towards the direction of the exhaust gas stream. Advantageous angles between the bores and the surface are between 30° and 60°.
The inclination of the bore affects the quantity and the speed with which the gas is absorbed, adsorbed and condensed as well as the flow behavior of the gas. Hence it is advantageous to locally incline the bores differently. For instance in regions where an undisturbed flow of the gas is important the bores are inclined towards the flow to reduce turbulence and back-mixing of the gases. In regions with a more tranquil flow the bores are inclined with the direction of the flow to ensure an efficient penetration of the gas. In some regions it is advantageous to locally design the bores with different inclination as a network (bores not connected) or as a tunnel system (bores connected).
In a further embodiment the internal wall of the exhaust gas conveying apparatus and the liner are connected by adhesive bond and form fit, in which a transitional layer is formed in which the bores of the liner are partly filled by the material of the internal wall of the exhaust gas conveying apparatus. The material of the liner is quasi transitioning through a two-phase structure connecting layer into the material of the exhaust gas conveying apparatus.
One advantage of the liner according to the invention is that the bore structure exhibits a significantly higher stiffness compared to the usually used porous material. In particular this is true with a closed surface on the outside of the liner. Thereby the wall thickness of the exhaust gas conveying apparatus, e.g. the exhaust manifold of a motor vehicle engine, can be reduced.
Through this it is also possible to create an air gap insulated exhaust gas conduit, in particular an air gap insulated exhaust manifold, by designing the liner with a closed outer surface. Between this closed outer surface of the liner and the internal surface of the exhaust gas conveying apparatus at least partially an air gap is located, which is typically a few millimeters thick. Through the thermal insulation of the air gap an advantageous improvement of the cold start properties of the internal combustion engine is achieved. There is no necessity of an additional air gap insulated exhaust manifold, which is advantageous in terms of the overall weight.
Exhaust gases can reach the air gap through the bores which penetrate the liner completely, and are buffered there in gaseous and/or liquid form, which increases the buffer capacity even further.
As material for the liner, composite metals and/or ceramics are suitable. Among the preferred suitable alloys are high temperature and oxidation resistant alloys on the bases of Ni, NiCr and/or NiCrAl as well as high-alloyed steels. Among the well suitable ceramics are refractory oxides like Al2O3, ZrO2, fireclay and such, as well as SiC or Si3N4.
A preferred variant of the metallic liner consists of two different metal alloys, in which the first consists of a Ni containing alloy and the second of a catalytically active noble metal containing alloy.
Another preferred metallic variant consists of two metals or, as the case may be, alloys, which exhibit a large difference in their thermal conductivity.
For mixed structures consisting of ceramic and metallic material layers it is advantageous to locate the ceramic layers on the outside of the liner or to sandwich the ceramic layers between metallic layers. Thereby loose ceramic particulates are trapped and cannot be carried away by the exhaust gas stream.
An advantage of this combined design is that the outer layers are shielded by the inner material layers, at least during the start phase, e.g. the cold start phase, of an internal combustion engine. This delays the heating up of the outer regions thus prolongating the absorptive effect in this cooler region. Thus it is for instance possible to sustain the absorptive effect of porous zeolite or zeolite coated material in the exhaust stream of a motor vehicle almost until it reaches the CAT light off temperature (starting temperature of the catalytic converter).
The gradient in the structure and material of the liner, especially in connection with the combined materials, is preferably adjusted such that during the start phase thermal conductivity, thermal transfer and heat capacity result in a minimal liner heating up and a maximal heating up of the catalyst up to its CAT light off temperature. Thus a high buffering capacity for easily condensable exhaust gas constituents, particularly CHx, and a rapid availability of the catalytic converter are ensured.
During the operation the liner heats up and releases the adsorbed matter such that they can be catalytically converted by the catalytic converter which is now above its CAT light off temperature. The liner dissipates its surplus heat through the surface of the exhaust gas conveying apparatus such that a possible overheating of the catalytic converter is prevented.
Another aspect of the invention relates to a process for the fabrication of an exhaust gas conveying apparatus of an internal combustion engine with internal liner in which the liner is fabricated with capillary bores.
The fabrication of a liner without bores as well as the fitting and fastening of a liner with bores is done by well known processes according to the state of the art. Advantageous methods for the fabrication of the bores are sink erosion or laser drilling which are suitable for variable diameter bores.
The liner is preferably inserted into the exhaust gas conveying apparatus such that at least a partial air gap remains between the outer wall of the liner and the internal wall of the exhaust gas conveying apparatus. Particularly preferred, the majority of the outer wall of the liner is separated from the internal wall of the exhaust gas conveying apparatus by an air gap.
The mechanical connection between the porous liner and the exhaust gas conveying apparatus can be done by metallic or ceramic spacers, which are for instance bonded or welded in place. Furthermore the liner or, as the case may be, the internal parts, of an air gap insulated exhaust manifold can be fabricated by means of powder metallurgy.
According to the embodiment of the invention shown in
According to the second embodiment of the invention shown in
According to a third embodiment of the invention, which is not shown in the illustrations, bores with a diameter decreasing from the surface going into the liner are machined into said liner.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 004 651.7 | Feb 2005 | DE | national |
