FR2720782A1 - Internal combustion engine exhaust system - Google Patents

Abstract

The exhaust system for an internal combustion engine is designed to purify the exhaust gases. It consists of an exhaust collector (2), which is connected to the exhaust manifold (21, 22, 23 & 24), which leads the exhaust gases from the engine cylinders. A catalytic pot (4) presents a closed envelope to the outlet from the collector, and for the treatment of the exhaust. A short link pipe (3) links the collector to the pot and both the link pipe and the pot are fitted with the device (6) for cooling, which uses the ambient air flowing around the outside as the cooling medium.

Description

EXHAUST DEVICE FOR A MOTOR
INTERNAL COMBUSTION
The present invention relates to a device for exhausting combustion gases from an internal combustion engine fitted in particular to a motor vehicle. The present invention relates more particularly to an exhaust device using a device for purifying the exhaust gases by catalytic conversion.

 In general, the standards concerning the pollution of internal combustion engines fitted to motor vehicles are becoming more and more severe every day in all industrialized countries. The automotive industry is therefore today fully mobilized to find technical solutions to meet these constraints, without penalizing either the performance of the engines or their cost price.

 To fight against the emission of polluting gases, the automobile industry has adopted the use of exhaust devices treating by catalytic conversion the harmful components of the exhaust gases. These devices, also called catalytic converters, in fact allow the oxidation of unburnt hydrocarbons HC Qt of carbon monoxide CO as well as the reduction of nitrogen oxides NOx. These reactions are in fact all greatly accelerated by the presence of catalysts so that they can be accomplished during the short passage time of the exhaust gases through the pot.

 However, the catalytic conversion of pollutants requires precise control of the exhaust gas temperature in order to function properly. In fact, if the exhaust gases are too cold, such as when the engine is started, the catalytic reactions cannot start; it therefore follows a more or less long phase during which the pollutants emitted by the engine are not or insufficiently treated. It can thus be estimated that 80 to 90% of the particles are emitted during the first two minutes of engine operation. If, on the contrary, the exhaust gases are very hot, such as for example during operation at full load, they then generate accelerated aging of the catalyst.

Two types of exhaust systems were then developed. The first place the catalytic converter away from the engine so that the temperature of the gases is lowered enough not to damage the catalyst, but it is then necessary to provide means for raising the temperature of the catalytic converter during periods of engine start-up. We can cite exhaust air injection systems to cause the post-combustion of exhaust gases and accelerate the temperature rise of the catalytic converter. We can also cite the electrical or electromagnetic heating or preheating means catalytic converters as described in patent applications WO-A-90/14507,
WO-A-92/02714 or even EP-A-465.184. The document
WON90 / 02250 provides for the use of a so-called heated starting catalyst used only as long as the exhaust gases have not reached a sufficient temperature, current deflector means directing the exhaust gases as a function of their temperature to the starting catalyst or in a bypass.

 The devices of the second type have them, the catalytic converter as close as possible to the engine and the combustion chambers to accelerate the rise in temperature of the catalyst by limiting the thermal losses and by maintaining a high temperature for the exhaust gases entering the pot. catalytic, but it is then necessary to provide means for reducing the temperature of the catalytic converter during other periods of operation and in particular during periods of operation at full load. One can thus cite catalytic converters cooled by liquid from a tapping operated on the engine cooling circuit.

 These different solutions have the drawbacks of involving expensive embodiments, requiring the use of controlled heat exchanger means operating selectively according to the operating conditions of the engine and requiring more or less significant energy inputs.

 The object of the present invention is therefore to remedy the drawbacks of the prior art by proposing an efficient, simple and inexpensive catalytic exhaust device, requiring no control system.

 The exhaust device for purifying the exhaust gases of an internal combustion engine according to the invention comprises an exhaust manifold having exhaust pipes attached to the outlet of the corresponding cylinders of the engine, a catalytic converter having an envelope containing at least one monolith for the treatment of exhaust gases and at least one short connecting conduit connecting the manifold to the catalytic converter, so as to have the latter as close as possible to the engine.

 According to the invention, the exhaust device for an internal combustion engine is characterized in that the connection duct and / or the catalytic converter are provided with cooling means operating continuously and using ambient air as cooling fluid.

 According to another characteristic of the exhaust device for an internal combustion engine which is the subject of the invention, the cooling means are formed directly by the external walls of the connecting duct and / or of the catalytic converter which are adapted to offer large areas heat exchange with the ambient air.

 According to another characteristic of the exhaust device for an internal combustion engine which is the subject of the invention, the external walls of the connection duct and / or of the catalytic converter are adapted to provide heat exchange surfaces at least twice as large as the heat exchange surfaces of the interior walls of the connecting duct and / or of the catalytic converter in contact with the exhaust gases.

 According to another characteristic of the exhaust device for internal combustion engine object of the invention, the outer walls of the connecting pipe and / or the catalytic converter are provided with fins.

The aims, aspects and advantages of the present invention will be better understood from the description given below of an embodiment of the invention, given by way of nonlimiting example, with reference to the attached drawing, on which ones
- Figure 1 shows a schematic view in partial longitudinal section of an exhaust device according to the invention.

 Only the elements of the exhaust device necessary for understanding the invention have been shown.

 In accordance with FIG. 1, the exhaust device presented equips an internal combustion engine referenced 1, with four in-line cylinders arranged for example transversely in the front compartment of a motor vehicle.

 The exhaust device comprises at its upstream end, in the direction of the flow of the burnt gases, a manifold 2 having four pipes respectively 21, 22, 23, 24 ensuring the evacuation of the burnt gases from each of the cylinders of the engine 1 These pipes, which are directly fixed to the cylinder head of the engine by means of a fixing flange, are shaped to lead jointly into a common metallic connection conduit 3 leading the gases to the inlet of a reactor or pot. catalytic 4, the gases then being discharged into the atmosphere at the outlet of the exhaust line. The connecting pipe 3 is as short as possible so as to bring the catalytic converter closer to the engine to accelerate the initiation of the chemical reactions when the engine is started.

 The catalytic converter 4 comprises a metal casing defining a passage extending between the tapered inlet and outlet ends respectively divergent and convergent. In this passage is housed one (or more) treatment monolith also known as the honeycomb body, which conventionally consists of an oblong block occupying the entire width of the passage, for example made of stainless steel sheet. or porous ceramic, this body being formed of a multitude of channels arranged longitudinally of square or trapezoidal section whose walls are covered with catalyst to allow the treatment of exhaust gases.

 According to the invention, the outer walls of the connecting duct 3 and of the catalytic converter 4 are adapted to form large surfaces for heat exchange with the ambient air. The increase in exchange surfaces is obtained by corrugated walls 6 or also by suitable fins obtained by foundry or forging with said walls or also welded to said outer walls.

 Thus, thanks to these outer walls having large heat exchange surfaces 6, the exhaust device according to the invention naturally operates conduction cooling thanks to the only air flow generated by the movement of the vehicle and passing through the engine compartment of the duct 3 and of the catalytic converter 4 and therefore that of the exhaust gases and of the monolith during engine operation and in particular during engine full load operations, which makes it possible to avoid despite the proximity of engine 1 excessive temperature of the catalyst.

 These heat exchange surfaces 6, however, have no effect on the temperature rise phase of the catalyst during start-ups.

In fact, during cold start transients and in particular during the first minutes, the heat losses are mainly due to energy transfers from the gases to the surface of the interior walls of the conduits traversed. The heat exchanges towards the outside are then minor and consequently the external heat exchange surfaces of these conduits play only a secondary role.

 Therefore, the increase in the external surface, object of the invention, does not modify the temperature of the gases at the inlet of the monolith during the starting phases of the engine 1 and therefore does not delay the initiation of the catalytic reactions .

On the other hand, after these phases are complete, this increase in the external surface increases the heat losses between the outputs of the engine and the inlet of the monolith, which has the effect of lowering the level of the temperature of the exhaust gases.

 This device, which is particularly simple and economical to produce, therefore allows both rapid priming of the catalyst and limitation of the temperature of the gases during engine operation, which delays the aging of the catalyst.

 The shape, dimensions and materials used to make the exhaust device according to the invention which has just been described are determined and optimized on the test bench to promote heat transfers, in order to take account in particular of the level of the admissible temperature at the inlet of the catalytic converter, temperatures at the engine outlet, the position of the catalytic converter on the exhaust line or the ease with which air is able to circulate along the duct 3 or of the catalytic converter 4.

 Of course, the invention is in no way limited to the embodiments described and illustrated which have been given only by way of examples.

 On the contrary, the invention includes all the technical equivalents of the means described and their combinations if these are carried out according to the spirit.

 Thus, the adaptation of the outer walls to promote heat exchange can be limited to the single connecting duct 3 or to the single catalytic converter.

Claims (4)

[1] Exhaust device for purifying the exhaust gases of an internal combustion engine, comprising an exhaust manifold (2) having exhaust pipes (21,22,23,24) fixed to the outlet corresponding cylinders of the engine, a catalytic converter (4) having an envelope containing at least one monolith for the treatment of exhaust gases and at least one connecting conduit (3) of short length connecting said manifold (2) to said catalytic converter (4), characterized in that said connecting duct (3) and / or said catalytic converter (4) are provided with cooling means (6) operating continuously and using ambient air as cooling fluid. [2] Exhaust device for internal combustion engine according to claim 1, characterized in that said cooling means (6) are formed directly by the outer walls of said connecting conduit (3) and / or said catalytic converter (4 ) which are suitable for providing large areas of heat exchange with the ambient air. [3] Exhaust device for internal combustion engine according to claim 2, characterized in that the outer walls of said connecting conduit (3) and / or said catalytic converter (4) are adapted to provide heat exchange surfaces at least twice as large as the heat exchange surfaces of the interior walls of said connecting duct (3) and / or of said catalytic converter (4) in contact with the exhaust gases. [4] Exhaust device for internal combustion engine according to any one of claims 2 to 3, characterized in that the outer walls of said connecting duct (3) and / or said catalytic converter (4) are provided with fins (6).