Technical field The present invention relates to an combustion engine comprising an exhaust pipe and a particle filter located in said exhaust pipe in accordance with the preamble of claim 1.

Particularly, the invention relates to a combustion engine comprising an exhaust pipe, a particle filter located in said exhaust pipe, and means for cleaning said filter. Furthermore, the patent application relates to a method for cleaning a particle filter located in an exhaust pipe associated with a combustion engine.

Background of the invention Today's vehicle industry strives to utilise combustion engines with higher and higher demands for purity of the emissions produced by combustion processes taking place inside the combustion engines. Particularly where combustion processes in diesel engines are concerned, soot and ash particles are one type of such emissions. In order to reduce the occurrence of soot and ash particles in the exhaust gases which are finally emitted into the ambient environment, particle filters are installed in the exhaust pipe. The particle filters are constituted of mechanical filters and, in principle, function as a screen which allows passage of certain exhaust gas components, such as e. g. carbon dioxide and water ;-but prevents passage of soot particles and ash particles. This means that the soot particles and the ash particles will gradually form a deposit on the filter, which makes passage of the exhaust gases more difficult and results in an increase of the back pressure, and thereby also an increase of the fuel consumption of the vehicle. Should the process be allowed to proceed, the particle quantity finally becomes so large that the engine becomes useless. A traditional method for ensuring that'the back pressure remains at an acceptable level is constituted by replacing the filter at regular intervals. In a combustion engine having a cylinder volume of 7 litres, approx. 1 litre of particles is produced during 70000 km of

driving when operating on normal fuels. This means that filter replacement has to take place at relatively frequent intervals, something which might be perceived as awkward since it its desirable that the vehicle service intervals are not too frequent.

Therefore, in order to increase the service life of the filter, it is desirable that the filter be cleaned in some way without having to dismantle the filter from the exhaust pipe. In this connection, trials have been performed with catalytic filters which are intended to perform catalytic combustion of the soot particles. This type of process, however, does not remove the ash deposit from the filter surface. Furthermore, as a result of the heat generated in the catalytic combustion process, there is a risk that the ash forms a cement on the filter surface.

Summary of the invention An object of the invention is to provide a combustion engine comprising an exhaust pipe and a particle filter located in said exhaust pipe, wherein cleaning of the particle filter from soot and ash particles is possible without having to remove the filter from the exhaust pipe.

This object is achieved by means of a combustion engine according to the characterising portion of claim 1. By means of connecting a tone generator to said exhaust pipe, a means for cleaning the particle filter by means of an oscillating air stream generated by the tone generator is obtained. The oscillating air stream releases soot and ash particles from said filter which subsequently can be transported away from the filter.

A second object of the invention is to provide a method for cleaning a particle filter located in an exhaust pipe associated with a combustion engine. This object is achieved by means of a method according to the characterising portion of claim 11. As a result of the tone generator generating an oscillating air stream, soot and ash particles are released from said filter by an infrasonic sound, and can be transported away from the filter.

Brief description of the drawings In the following, the invention will be described in greater detail by means of an embodiment, and with reference to the attached drawings, in which

Fig. 1 is a schematic representation of a combustion engine comprising an exhaust pipe and a filter located in said exhaust pipe, Fig. 2 shows a detail of the design of a membrane present in the particle filter, Fig. 3 shows an embodiment of the invention where the switching-on of the tone generator takes place based on of information concerning the pressure drop across the filter.

Detailed description of preferred embodiments Fig. 1 is a schematic representation of a combustion engine 1 comprising an exhaust pipe 2 and a particle filter 3 located in said exhaust pipe. The combustion engine itself is of a previously known type, and will therefore not be described in any greater detail. Whenever applicable, the exhaust pipe comprises an exhaust manifold 4 which connects the outlets from the respective cylinders (not shown) in the combustion engine 1. The exhaust pipe 2 itself is of a conventional type and, whenever apllicable, can be equipped with one or several exhaust gas turbines (not shown) for operating one or several compressors (not shown) located in the inlet port (not shown) of the combustion engine. Furthermore, whenever applicable, catalytic reaction chambers (not shown) can be placed in the exhaust pipe.

A tone generator 5 is installed in connection with the exhaust pipe 2. The tone generator 5, which is of a type well lcnown to the skilled person, generates an oscillating air stream having a low frequency which is guided into the exhaust pipe, preferably in a direction towards the filter. In a preferred embodiment, as shown in Fig. 1, the tone generator 5 comprises a loudspeaker 6a which communicates with a resonance chamber 6 from which a waveguide 7 communicates with the exhaust pipe. In another preferred embodiment, a reflector 8 is installed in the exhaust pipe in order to direct the air stream or sound even more towards the filter. In order to ensure that the sensitive loudspeaker of the tone generator is not exposed too much to the heat generated in the exhaust pipe in normal

operation, the waveguide 7 can be cooled down, for example by means of cooling flanges installed around the waveguide. In order to prevent the reflector 8 from guiding particles into the waveguide 7, the reflector 8 can be provided with cavities or slits close to the attachment of the reflector to the outlet from the waveguide 7 or, depending on the installation of the reflector, to the wall of the exhaust pipe. In this way, particles are able to fall down along the reflector 8, then pass through said slits or cavities, and finally be collected in the collecting vessel. In a preferred embodiment, said reflector is constituted of a valve which is intended to open and close the waveguide 7, wherein an undisturbed flow inside the exhaust pipe 2 is possible in normal operation.

In a preferred embodiment, a collecting vessel 9 for released soot and ash particles is placed in said exhaust pipe 2 upstream and in direct connection with said filter 3. In this respect,"upstream"means inside the exhaust pipe and in a direction towards the combustion engine. When the particles are released from the filter 3, the particles are transported from the filter surface to the collecting vessel. In a preferred embodiment of the invention, in order to allow transport from the filter 3 to the collecting vessel 9, the filter 3 is placed in a portion 10 of said exhaust pipe 2 exhibiting a local direction of extension 11 which forms an angle with the horizontal plane which exceeds 60°. In another preferred embodiment, the local direction of extension extends at between 85° and 90° in relation to the horizontal plane. The measurement is intended to be performed when the exhaust pipe is installed in a condition intended for use and, in case the combustion engine is installed in a vehicle, when the vehicle is placed on level ground. Positioning the filter in said portion will allow particles which have been released from the filter to be transported towards the collecting vessel by the influence of gravity. In order to ensure that the particles fall down into the collecting vessel 9, the vessel is placed in connection with the continuation downstream of the filter of the local direction of extension.

Since the particle generator generates an oscillating air stream, the tone generator will also contribute to transporting the particles away when they have been released from the filter surface. An oscillating air stream does not exhibit any average transport direction when forming a time average for one or several time periods. However, the concentration which

prevails at the filter wall when initiating the cleaning process will, in real time very rapidly, gradually be equalized so that the particle concentration, with the exception of effects resulting from the formation of stationary waves in the system, is substantially homogenous within the region where the oscillation takes place. This results in a transport of particles away from the filter surface 3 in a direction towards the collecting vessel 9 by means of the tone generator.

Accordingly, in a preferred embodiment of the invention, the collecting vessel is positioned in such a way that a main portion of the region 12 located downstream of the collecting vessel 9 exhibits a local direction of extension which forms an angle with the horizontal plane which exceeds 60°, and which is preferably is between 85° and 90°.

In one embodiment of the invention, as shown in Fig. 3, the outlet 13 from the tone generator 5 is placed in direct connection with the collecting vessel 9 and behind a perforated structure 14, for example a screen, which allows passage of the particles into the vessel. The perforated structure 14 reduces the risk of the exhaust gas flow carrying particles from the collecting vessel 9 in normal operation and thereby returning them to the filter 3. By positioning the outlet 13 from the tone generator behind the perforated structure 14, a more protected location of the tone generator 5 is obtained, as is a smaller influence by the tone generator is obtained in normal operation of the combustion engine.

In one embodiment of the invention, as exemplified in Fig. 3, the transport of released particles away from the filter wall is constituted by an evacuating duct 19, connected to the collecting vessel in which a negative pressure is created by means of e. g. a fan 30.

Preferably, the collecting vessel 9 is provided with a device 15 which provides combustion of the soot particles in the collecting vessel. This device, for example, can be constituted by an electric coil.

Furthermore, Fig. 1 shows a control computer 16, which is intended to control switching- on and switching-off of the tone generator 5 and, whenever applicable, opening and

closing of valves 8 arranged to open and close a communicating duct between tone generator 5 and exhaust pipe 2. In one embodiment of the invention, the tone generator is activated as a function of the mileage of a vehicle driven by the combustion engine. In an alternative embodiment, the pressure drop across the filter 3 is measured, for example by means of pressure sensors 17,18 located upstream and downstream of the filter 3.

Figs. 2a and 2b show the structure of the filter in detail. The filter 3 comprises a membrane which is permeable to gas components generated by a combustion process in the combustion engine, but impermeable to soot and ash particles generated by the same combustion process. The membrane is constituted of a first set of channels 21, each exhibiting an open inlet 22, side walls 23 and a closed end wall 24, and a second set of channels 25, each exhibiting an open outlet 26, side walls 23 and a closed end wall 27, wherein the exhaust gas flow from the combustion engine is intended to be guided from the first set of channels to the second set of channels via said side walls.

Said first set of channels preferably is installed with a local direction of extension 27 which forms an angle with the horizontal plane which exceeds 60°. In another preferred embodiment of the invention, the local direction of extension 27 of the first set of channels forms an angle with the horizontal plane which is between 85° and 90°.

In an alternative embodiment of the invention, the tone generator is constituted solely by a resonance circuit resonated to the desired frequency. In this case, the resonance circuit is driven by means of the exhaust gas flow of the combustion engine. Thereby, the resonance circuit has to be disconnected from the exhaust pipe in normal operation, and has to communicate with the exhaust pipe during the process of cleaning the filter. For this reason, a resonance chamber 6 is connected to the exhaust pipe via a waveguide 7.

Furthermore, the exhaust pipe, alternatively the waveguide, exhibits a valve which allows the duct between resonance chamber 6 and exhaust pipe 2 to be closed or opened.

The control unit 16 also comprises an input channel 28 for receiving information about the operational condition of the combustion engine and, whenever applicable, about the

vehicle driven by the combustion engine. In one embodiment of the invention, the tone generator is activated only under predetermined operational conditions of the vehicle, preferably, when idling or when the engine is switched off. In order to ensure that the particles are brought from the filter 3 to the collecting vessel 9, it is of importance that the volume flow in the exhaust pipe is not too large. Should an electrical tone generator be utilised, the cleaning process preferably takes place with the engine switched off, and should the tone generator be constituted of a resonance chamber activated by means of the exhaust gas flow, the cleaning process preferably takes place when idling.

Preferably, the cleaning process takes place with an oscillating air stream having a period below 100 Hz, and in another preferred cleaning process having a period below 50 Hz.

Preferably, the cleaning process takes place at an acoustic pressure between 50 and 150 dB.

Furthermore, in a preferred embodiment, the collecting vessel 9 is provided with a service hatch 29 through which ash, and whenever applicable soot particles, are removed. Also the resonance chamber 6 can be provided with such an inspection hatch.

The invention is not limited to the above described embodiments, but may be varied within the scope of the appended claims. For example, the pressure sensors, the evacuating duct and the location of the outlet from the waveguide into the exhaust pipe, mentioned in connection with Figs. 1 and 3, can be freely combined. Furthermore, the tone generator can be placed downstream the particle filter.