FR2708041A1 - Catalytic convertor with sheet-type catalyst support equipped with preheating means, and purification process - Google Patents

Abstract

The catalyst support is formed of a stack of flat metal sheets (2) held in the form of a block (1) by elongate elements slipped through holes (3) therein. The region (6) of the sheets swept firstly by the exhaust gases is separated from the remaining region (7) by slots (8) limiting the transfer of heat from one to the other. Heating tubes passing through the holes (3) of the first region (6) bring the catalysts present therein up to temperature quickly before the combustion engine is started. Application to motor vehicles.

Description

Catalytic converter with sheet catalyst support provided with preheating means and purification process
The invention relates to a method and a device for purifying the exhaust gases of a heat engine, in particular for a motor vehicle, the device comprising a support on which is deposited at least one catalyst for the chemical destruction of impurities in the exhaust gas, commonly known as a "catalytic converter". The function of such a device is to eliminate at least in part the polluting gases contained in the exhaust gases, in particular carbon monoxide, hydrocarbons and nitrogen oxides, by transforming them by reduction or 'oxidation.

FR-A-2 679 602 describes a device of this kind comprising a support in the form of superimposed sheets which are substantially planar and parallel maintained stacked by elongated elements extending in the direction perpendicular to the planes of the sheets, passing through holes made in them, and on which is deposited at least one catalyst for the chemical destruction of impurities in the exhaust gases.

This document provides in particular that certain elongated elements are tubes capable of playing the role of heat source for the preheating of the catalyst. Indeed, the reactions of destruction of polluting gases are only carried out with a suitable yield from a temperature threshold of the order of 200 to 250 "C. A catalytic converter is therefore ineffective when starting the engine cold, in the absence of preheating means.

The heating tubes can be distributed over the entire cross section of the block, and therefore pass through holes distributed over the entire surface of each sheet.

The temperature rise that they produce is then substantially homogeneous over the entire surface of the leaves, and it is necessary to wait until this entire surface is brought to the required temperature in order to be able to operate the heat engine without emission into the atmosphere of polluting discharges unacceptable.

An object of the invention is to reduce this waiting time, for a given heating power of the tubes.

The subject of the invention is a method for purifying the exhaust gases of a heat engine, in particular for a motor vehicle, in which said exhaust gases are circulated between superimposed sheets which are substantially flat and parallel on which is deposited. at least one catalyst for the chemical destruction of impurities in the exhaust gas, said sheets being kept stacked in the form of a block by elongated elements extending in the direction perpendicular to the planes of the sheets, passing through holes made in those -ci, and some of which form a first subset suitable for playing the role of heat source for the preheating of the catalyst.

According to a first aspect of the invention, said first subassembly is located in a first fraction of the cross section of the block, in which the entire stream of exhaust gas flows.

The heating is thus heterogeneous, the region of the sheets traversed by the tubes being heated alone, or at least faster than the remaining region. It therefore reaches the temperature necessary more quickly for the catalysts to be effective. The preheated part of the catalysts thus makes it possible to reduce the pollutant content of the exhaust gases, whereas this would not be affected if all the catalysts were heated for the same period, the temperature reached being insufficient.

Other characteristics, complementary or alternative, of the invention are set out below - The stream of exhaust gas first circulates in said first fraction, then in the remaining fraction of the cross section of the block.

- A second subset of said elongated elements is capable of removing heat for cooling the catalyst.

- Said second sub-assembly is located in the remaining fraction of the cross section of the block.

- Means are provided to limit the transfer of heat within each sheet between said first fraction and the remaining fraction.

- Said means are cutouts formed in the sheets, along lines separating the first fraction from the remaining fraction.

- The process includes the following steps
a) before the engine is started, it is determined whether the temperature of the catalyst is sufficient to purify the exhaust gases;
b) if the temperature is insufficient, said heat source is put into service for a suitable duration; and
c) if the temperature is sufficient, or at the end of said period, the engine is started.

- To determine if the temperature of the catalyst is sufficient, the temperature of an engine coolant is compared to a threshold, the temperature of the catalyst being considered sufficient if the temperature of the fluid is at least equal to the threshold.

- Said suitable duration is chosen as a function of said temperature of the cooling fluid.

In the aforementioned document, the heating tubes are tubular sheaths containing electric heating resistors. These electrical resistors being used before the engine starts, the energy they consume comes from the vehicle battery, which can be annoying when the latter is weakly charged.

A second aspect of the invention aims to avoid this drawback.

According to this second aspect of the invention, which can be implemented independently of the first or jointly with the latter, the elongated elements of the first sub-assembly are tubes, one upstream end of which is connected to a source of hot gas.

In this second aspect, it can also be provided that - The hot gas source comprises a burner supplied with fuel and combustion air.

- The burner is supplied with fuel by a pump from the vehicle's fuel tank, and / or with combustion air by a fan.

- The hot gas, after having passed through said tubes, enters between the sheets in the space reserved for the circulation of the exhaust gases.

- Each of said tubes has the shape of a U, the two branches of which are engaged in respective wells formed by aligned holes in the sheets, the well which receives the downstream branch extending beyond the free end thereof. .

- The two wells and the upstream branch of the tube extend over the entire length of the block and the downstream branch over only a fraction of it.

The invention also relates to a purification device for implementing the method defined above, comprising superimposed sheets that are substantially flat and parallel on which is deposited at least one catalyst for the chemical destruction of impurities in the exhaust gases. , said sheets being kept stacked in the form of a block by elongated elements extending in the direction perpendicular to the planes of the sheets, passing through holes formed in them, and some of which form a first sub-assembly suitable for playing the role of heat source for the preheating of the catalyst, which is located in a first fraction of the cross section of the block.

The invention further provides for this device the following optional characteristics - The block is of substantially parallelepiped shape, said first fraction and the remaining fraction of the cross section of the block being adjacent respectively to two opposite faces thereof.

- It comprises tubes constituting the elongated elements of the first sub-assembly, a burner, a pump for supplying fuel to the burner from the vehicle's fuel tank, and a fan for supplying the burner with combustion air and for sending the gas burned in the tubes.

- It includes means for igniting the burner.

- It includes means for prohibiting the circulation of exhaust gases through the tubes towards the fan when the latter is not in use.

The characteristics and advantages of the invention will be explained in more detail in the description below, with reference to the accompanying drawings, in which - Figure 1 is a perspective view of a stack of catalyst support sheets of a device according to the invention, for the purification of exhaust gases from the heat engine of a motor vehicle; and - Figure 2 is a partial view in longitudinal section of this stack, also showing a preheating tube and, schematically, a source of hot gas connected to this tube.

The figures illustrate a stack 1 produced according to the well-known technology of tube and fin block heat exchangers. It comprises a multiplicity of sheets 2 of stainless steel with a thickness of approximately 0.03 to 0.06 mm, provided with perforations 3 and threaded on elongated tubes each of which passes through perforations of all the sheets, thus forming a block of general parallelepiped shape in which the leaves are spaced a few tenths of a millimeter apart.

The sheets 2 are covered, by known means, with one or more catalysts based on precious metals such as platinum, palladium and rhodium or on metal oxides.

When the heat engine of the vehicle is running, the exhaust gases leaving it penetrate into the intervals between the sheets 2 through one face 4 of the block, to come out of it through the opposite face 5, circulating according to the arrows F. They sweep the faces of the sheets covered with catalysts, causing the chemical reaction of the polluting gases.

Each of the sheets 2 is divided into two zones 6 and 7, respectively adjacent to the faces 4 and 5, by several cutouts 8 in the form of slots, elongated and mutually aligned along a line parallel to these same faces. The holes 3 are arranged in several rows also parallel to the faces 4 and 5, one of these rows being in zone 6 and the others in zone 7. In the example shown, each row comprises four holes. The four holes arranged in the zone 6 receive two U-shaped preheating tubes, one of which 9 is shown in FIG. 2. This tube comprises an upstream branch 10 which crosses the block 1 over the entire length thereof, a bottom 11 located outside the block and a downstream branch 12 which crosses the block over most of its length and then stops, so that on the remaining part the well 13 formed by the alignment of the holes 3 of the different sheets communicate freely with the intervals between them. This well 13 is closed in a sealed manner by a plate 14 parallel to the sheets 2, disposed at the end of the block and crossed by the branch 10 of the tube.

The upstream end of the branch 10 is connected to a source of hot gas 15. This comprises a combustion chamber 16 in which there is an injector 17 supplied by a pump 18 from the vehicle fuel tank 19.

A fan 20 sends air into the combustion chamber. Non-return valves 21 and 22 are provided for the fuel and for the air upstream of the combustion chamber, and in this is arranged a filament 23 which can be heated by an electric current supplied by the vehicle battery, for lighting the flame.

In addition, chemical reactions of transformation of impurities give off a large amount of heat, which can cause the catalytic converter to turn red. To lower the temperature thereof and avoid degradation of the materials, and also to recover the heat of reaction, the invention provides for circulating a coolant in tubes constituting at least some of the elongated elements which pass through the holes 3 zone 7 of the sheets, these tubes being connected for this purpose to an external circuit not shown, as known in the art of heat exchangers.

The device described operates as follows.

Before starting the engine of the vehicle, it is checked that the temperature of the catalysts is sufficient to allow the transformation of the pollutants contained in the exhaust gases. For this purpose, the temperature of the engine coolant can be used as a control. In fact, after the engine has stopped, the temperatures of the catalysts and of the coolant decrease in parallel. By way of example, it can be considered that the temperature of the catalysts is sufficient if the temperature of the cooling fluid is at least equal to 80 "C.

If the catalyst temperature is sufficient, the engine can be started immediately. Otherwise, the pump 18 and the fan 20 are put into service so as to burn fuel in the combustion chamber 16 and to send the combustion gas into the tube 9. The filament 23 is temporarily energized so to ensure the ignition of the combustion flame. The combustion gas flows through the branch 10 then the branch 12 of the tube 9, thus supplying heat to the sheets 2 of the block 1, which carry the catalysts, via the wall of the tube. Beyond the free end of the branch 12, the gas flows directly into the well 13, from where it enters the intervals between the sheets 2, thus continuing to transfer heat to them. It is then evacuated to the atmosphere by the path provided for the exhaust gases.

The gas flowing in the tubes 9 gives priority to the heating of the zone 6 of the sheets 2. The narrow bridges remaining between the openings 8 allow only a reduced heat transfer between zone 6 and zone 7, this which limits the time required for zone 6 to reach the desired temperature. This time having elapsed, the hot gas source 15 is put out of service and the engine can start. This duration can be chosen as an increasing function of the temperature of the cooling fluid, which, as indicated earlier, is representative of the initial temperature of the catalysts. Stopping the fan 20 causes the non-return valve 22 to close, preventing the exhaust gases from the heat engine from passing through the tube 9 from the intervals between the sheets 2 and from the well 13.

During the operation of the heat engine, the exhaust gases circulate through block 1 according to the arrows
F of FIG. 1. They come into contact with zone 7 of the sheets 2 after having been partially purified by the catalysts deposited on zone 6. The catalysts in zone 7 are gradually heated by the exhaust gases and can supplement the 'purification of these.

After a certain heating of the block 1, a coolant is circulated in the tubes passing through the zone 7 of the sheets. This circulation has a direct cooling effect on zone 7, and a limited effect on zone 6, due to the openings 8. However, it is possible to choose the dimensions of the openings 8 and the flow rate of the cooling fluid so as to maintain both zone 6 at a temperature low enough to avoid any degradation and zone 7 at a temperature high enough to ensure the effectiveness of the catalysts.

The position of the openings 8 can be modified with respect to that shown in FIG. 1, for example so that the two zones each have two rows of holes. In general, the number of rows of holes in each zone, and the number of holes per row, can be freely chosen. The holes are not necessarily arranged in regular rows. The heating tubes, which are two in number in the illustrated embodiment, but which may be more numerous, can be connected in parallel to the same source of hot gas such as source 15.

It is also possible to replace the slots 8 by the complete separation of each sheet into two independent juxtaposed sheets, further improving the concentration in the area 6 of the heat provided by the heating tubes. This solution is however only possible when excessive heating of the catalytic converter is not to be feared, as it does not allow the cooling of zone 6.

The method according to the invention can use known electronic components for the various functions such as comparison, timeout, etc. To accelerate the starting of the engine, the steps for evaluating the temperature of the catalysts and starting up the heat source can be initiated before the driver enters the vehicle, for example by the remote control for unlocking the doors.

Claims (20)

Claims 1. A method of purifying the exhaust gases of a heat engine, in particular for a motor vehicle, in which said exhaust gases are circulated between superimposed substantially flat and parallel sheets (2) on which is deposited at least a catalyst for the chemical destruction of impurities from exhaust gases, said sheets being kept stacked in the form of a block (1) by elongated elements extending in the direction perpendicular to the planes of the sheets, passing through holes (3 ) formed therein, and some of which (9) form a first sub-assembly suitable for playing the role of heat source for preheating the catalyst, characterized in that said first sub-assembly is located in a first fraction ( 6) of the cross section of the block, in which the entire stream (F) of exhaust gas flows. 2. Method according to claim 1, characterized in that the exhaust gas stream first circulates in said first fraction (6), then in the remaining fraction (7) of the cross section of the block (1). 3. Method according to one of claims 1 and 2, characterized in that a second subset of said elongated elements is capable of removing heat for the cooling of the catalyst. 4. Method according to claim 3, characterized in that said second sub-assembly is located in the remaining fraction (7) of the cross section of the block. 5. Method according to one of the preceding claims, characterized in that means (8) are provided for limiting the transfer of heat within each sheet (2) between said first fraction and the remaining fraction. 6. Method according to claim 5, characterized in that said means are cutouts (8) formed in the sheets, along lines separating the first fraction from the remaining fraction. 7. Method according to one of the preceding claims, characterized by the following steps  a) before the engine is started, it is determined whether the temperature of the catalyst is sufficient to purify the exhaust gases;  b) if the temperature is insufficient, said heat source (9) is put into service for a suitable duration; and  c) if the temperature is sufficient, or at the end of said period, the engine is started. 8. Method according to claim 7, characterized in that, to determine whether the temperature of the catalyst is sufficient, the temperature of an engine coolant is compared to a threshold, the temperature of the catalyst being considered to be sufficient if the temperature of the fluid is at least equal to the threshold. 9. Method according to claim 8, characterized in that said suitable duration is chosen as a function of said temperature of the cooling fluid. 10. Method according to one of the preceding claims, characterized in that the elongated elements of the first subset are tubes (9), one upstream end of which is connected to a source of hot gas (15). 11. Method according to claim 10, characterized in that the source of hot gas comprises a burner (17) supplied with fuel and combustion air. 12. Method according to claim 11, characterized in that the burner is supplied with fuel by a pump (18) from the fuel tank (19) of the vehicle, and / or with combustion air by a fan (20). 13. Method according to one of claims 10 to 12, characterized in that the hot gas, after having passed through said tubes (9), enters between the sheets in the space reserved for the circulation of exhaust gases. 14. The method of claim 13, characterized in that each of said tubes has the shape of a U whose two branches (10,12) are engaged in respective wells formed by aligned holes (3) of the sheets, the well (13) which receives the downstream branch (12) extending beyond the free end thereof. 15. Method according to claim 14, characterized in that the two wells and the upstream branch (10) of the tube extend over the entire length of the block (1) and the downstream branch (12) over only a fraction thereof. this. 16. Purification device for implementing the method according to one of the preceding claims, comprising substantially flat and parallel superimposed sheets (2) on which is deposited at least one catalyst for the chemical destruction of impurities from the gases d exhaust, said sheets being kept stacked in the form of a block (1) by elongated elements extending in the direction perpendicular to the planes of the sheets, passing through holes (3) formed therein, and some of which (9 ) form a first sub-assembly suitable for playing the role of heat source for the preheating of the catalyst, which is located in a first fraction (6) of the cross section of the block. 17. Device according to claim 16, characterized in that the block is of substantially parallelepiped shape, said first fraction (6) and the remaining fraction (7) of the cross section of the block being adjacent respectively to two opposite faces (4,5 ) of it. 18. Device according to one of claims 16 and 17, characterized in that it comprises tubes (9) constituting the elongated elements of the first sub-assembly, a burner (17), a pump (18) for supplying the burner in fuel from the fuel tank (19) of the vehicle, and a fan (20) for supplying the burner with combustion air and for sending the burnt gas into the tubes (9). 19. Device according to claim 18, characterized in that it comprises means (23) for igniting the burner. 20. Device according to one of claims 18 and 19 for implementing the method according to claim 13, characterized in that it comprises means (22) for preventing the circulation of the exhaust gases through the tubes ( 9) towards the fan (20) when the latter is not in use.