TECHNICAL FIELD The present invention relates to a device in a silencer unit, according to the accom- panying claim 1. The invention is particularly intended for use with a silencer unit in an exhaust system of an automotive vehicle, for suppressing unwanted noise origi- nating from an exhaust gas flow through said exhaust system.

STATE OF THE ART In connection with today's vehicles, e. g. passenger cars, an exhaust system is used in a known manner for discharging an exhaust gas flow from the vehicle engine to the surroundings. The term"exhaust system"is used here to designate an arrange- ment comprising a number of tubular components conveying the exhausts out of an exhaust manifold of the engine, a silencer unit, and typically also a three-way cata- lytic converter. The silencer unit is utilise in a known manner for reducing the unwanted noise originating from the exhaust gas flow through the exhaust system, whereas the three-way catalytic converter is used for reducing the emission of harmful contaminants from the engine.

The above-mentioned silencer unit is functioning in a known manner to even out pulsation of the flowing exhaust gases and to make this pulsation as inaudible as possible. Due to requirements regarding good comfort for the vehicle passengers, today's silencer units are designed to reduce the noise from the exhaust gas flow to a level below a predetermined limit value. The term"limit value"will here also desig- nate a varying value (or a set of values) depending on the frequency of the noise. As a consequence of statutory requirements in various countries regarding the noise that can be perceived outside the vehicle, i. e. that influences the vehicle's exterior environment, it is furthermore necessary to limit the noise level so as to fulfil these requirements.

Consequently, the exhaust system has to be designed in such a way as to fulfil the comfort requirements as well as the statutory requirements with regard to accept- able noise levels.

A prior art type of silencer unit is intended for arrangement as a termination of the exhaust system, and comprises a chamber through which the exhaust gases are conveyed. The outlet from this chamber is a tubular so-called end pipe, through which the exhaust gases will flow. From the end pipe, the exhaust gases are passed into a further tubular member, positioned in a conventional manner so as to extend from the tail end of the vehicle.

According to the known art, the silencer unit is preferably filled, completely or in part, with mineral wool or a similar sound-absorbing material. Furthermore, the end pipe may be perforated, i. e. provided with a number of relatively small holes, distributed along the circumferential surface of the end pipe. In this way, the high-frequency energy in the exhaust gas flow can be vented out to the surroundings and be absorbed by the sound-absorbing material, which preferably encompasses the end pipe perforations. This will mean a reduction of the sound level from the exhaust gas flow, principal at relatively high frequencies. A reduction of the noise at relatively low frequencies is mainly achieved through a suitable sizing of the length and diameter of the end pipe.

The sound-absorbing properties of a certain silencer unit in a certain vehicle are determined by several factors, e. g. the type of engine in question, the flow proper- ties of the exhaust gases and the design and the dimensions of the exhaust system.

Not least, the sound-absorbing properties are influenced to a high degree by the configuration of the passenger compartment of the vehicle in question. In a certain type of vehicle, there may for example be a risk of the geometry and the dimensions of the passenger compartment contributing to a relatively high noise level at rela- tively low frequencies, more particularly in the order of 50-70 Hz, caused by station- ary wave conditions and resonance inside the compartment. This type of low- frequency noise is regarded as very unfavourable from a comfort aspect and will thus constitute a problem when exceeding levels corresponding to acceptable limits.

The problem of excessively loud low-frequency noise could as such be resolved by utilising a relatively long end pipe that is not equipped with perforations. This would then entail that the low-frequency noise could be reduced to a level that would be

acceptable from a comfort aspect, but would simultaneously have the effect of sub- stantially reducing the absorption of high-frequency noise. There would then arise a risk of the noise absorption in relatively, high frequencies, more particularly in the order of 150-200 Hz, being unsatisfactory. In unfortunate cases, this could lead to not being able to meet statutory requirements regarding exterior, high-frequency noise from the vehicle.

Consequently, there are contradictory requirements regarding the object of striving to suppress the low-frequency noise whilst at the same time meeting statutory requirements regarding the levels of the exterior high-frequency noise. Utilising an exhaust system having an increased volume could as such solve this problem.

However, this is not always possible, as any specific exhaust system always has to be sized and designed considering the laid-don requirements regarding its mount- ing space in the vehicle and its cost, weight and similar factors.

DISCLOSURE OF INVENTION The object of the present invention is to provide an improved device for a silencer unit of an automotive vehicle, which, for reasons of comfort, provides a high degree of damping of low-frequency noise whilst at the same time, in response to existing statutory requirements, it provides a high absorption of high-frequency noise. This is achieved by an arrangement, the characteristics of which are disclosed in the accompanying claim 1.

The invention constitutes a device for a silencer unit of an automotive vehicle, for suppressing unwanted noise originating from an exhaust gas flow from the vehicle engine. According to the invention, the silencer unit is terminated by an end pipe, through which said exhaust gas flow is conveyed to the surroundings. The invention is characterised by the end pipe comprising at least one perforated section having a predetermined extension in the longitudinal direction of the end pipe and being posi- tioned downstream of a point along the end pipe where there will be substantially no turbulence in the exhaust flow. Through the invention, a high suppression of low- frequency noise as well as a good absorption of high-frequency noise is achieved. A further advantage of the invention is that it provides a passive silencing system that is simple, robust and cost-effective.

According to the prior art, there is generally a maximum allowable length for an end pipe. If the end pipe is made too long (which might be required for suppressing low- frequency noise) there is a risk of resonance occurring. This might lead to audible tones being generated, which would of course be a disadvantage. According to the present invention, however, such a maximum length can be exceeded without cre- ating unwanted tones, which is a further advantage of the invention.

Advantageous embodiments of the invention are defined in the subsequent, dependent claims.

DESCRIPTION OF DRAWINGS Fig. 1 shows a principle side view, in partial cross-section, of a silencer unit in which the present invention could be utilise, and Fig. 2 shows an end pipe configured according to the present invention.

PREFERRED EMBODIMENT Fig. 1 shows a side view, in partial cross-section, of a silencer unit 1 according to the present invention. According to a preferred embodiment, the silencer unit 1 is used in an automotive vehicle, preferably but not exclusively consisting of a passen- ger car. The silencer unit 1 is functioning to convey an exhaust gas flow from a (not shown) combustion engine of the vehicle in question to the vehicle's surroundings, and is to this end configured with an inlet 2, to which said exhaust gases flow from the engine. From the inlet 2, the exhausts are conveyed to a chamber 3 and on to a bend 4, via a straight tubular section 5. The flow direction of the exhaust is indicated by arrows in Fig. 1.

The bend 4 leads to an end pipe 6, preferably being cylindrical and straight, and extending through the chamber 3. By means of the end pipe 6, the exhaust gases are conveyed to the surroundings, preferably via a further pipe section 7 mounted on the end pipe 6 and being, in a known manner, intended for positioning so as to debouch at the rear end of the vehicle. The end pipe 6, together with said further

pipe section 7, thus defines a substantially straight, tubular termination of the pres- ent exhaust system.

According to the embodiment, the end pipe 6 is, at least partially, surrounded by a sound-absorbing material 8, preferabiy in the form of mineral wool or a similar sound absorbent. The invention is not, however, limited to this embodiment, but might in principe also be configured with the end pipe 6 surrounded by air.

According to what will be described in detail below, the end pipe 6 according to the embodiment is configured with a perforated section 6a, that is, a section being pro- vided with a large number of relatively small holes 9, made along a portion of the circumferential surface of the end pipe 6. The perforated section 6a extends along a predetermined portion of the total length of the end pipe 6. Through the configura- tion of the silencer unit 1 according to the invention, a suppressing is achieved of the unwanted noise occurring due to the exhaust gas flow through the exhaust system.

In this context it should be noted that the invention is based upon achieving a high suppression of low-frequency noise through a suitable adaptation of the lengths of the end pipe 6 and of the perforated section 6a, respectively, whilst at the same time obtaining a high absorption of high-frequency noise by means of the perforated sec- tion 6a. Consequently, according to the invention, silencing is achieved over a wide range of frequencies.

Fig. 2 illustrates a somewhat enlarged side view of the end pipe 6, from which it may be gathered that the end pipe 6 has a predetermined total length L, that could be described as being defined from the point where it is connected to the bend 4 (comp. Fig. 1) and up to the further pipe section 7 terminating the exhaust system.

The length L, is selected in dependence of a. o. the type of vehicle and type of engine in question, in order to provide a high suppression of the noise from the exhaust gas flow at relatively low frequencies. The perforated section 6a is designed with an extension or length L2 in the longitudinal direction of the end pipe 6, repre- senting a predetermined portion of the total length L, of the end pipe 6. Furthermore, the perforation preferably extends around the entire circumference of the end pipe 6.

The perforated section 6a is further located so as to extend from a point along the end pipe 6, positioned at a predetermined distance L3 from the connection of the

end pipe 5 to said bend 4, i. e. measured from the upstream end portion of the end <BR> <BR> <BR> pipe 6. The last-mentioned distance L3 is preferably selected so as to correspond to that distance between the upstream end portion and that downstream point along the end pipe 6, at which the turbulent flow, existing in the exhaust gases through the bend 4 (and having been created due to bent pipe sections, deflections and area changes inside the silencer unit 1), has substantially ceased and been transformed to a non-turbulent flow in the end pipe 6.

In a normal application, the length Li of the end pipe 6 may be in the order of 250- 300 mm, whereas the distance L3 between the connection of the end pipe 6 to the bend 4 and the perforated section 6a according to the invention will be in the order of 70-100 mm. However, variations may of course occur within the scope of the invention, depending on the current type of engine, flow conditions of the exhaust gases, and similar factors. A basic principle of the invention is, however, that the last-mentioned distance L3 is selected so that there will be substantially no turbulent flow in the end pipe 6 at that point from which the perforated section 6a has its extension.

Preferably, the holes 9 are arranged in a regular pattern, according to what is shown in Fig. 2, and are configured in such a way that each hole 9 has a diameter selected so that the high-frequency energy from the exhaust gas flow can be vented out and converted to heat, through friction against the absorbing material 8 surrounding the perforated section 6a. In this way, an efficient absorption of high-frequency noise is achieved. In a normal application, the size of the holes 9 is preferably in the order of 3-5 mm, most preferably about 3.5 mm. The spacing between two adjacent holes is preferably in the order of 3-8 mm, most preferably about 5-6 mm.

As was discussed above, the length L2 of the perforated section 6a of the end pipe 6 corresponds to a predetermined portion of the total length L, of the end pipe 6. More <BR> <BR> <BR> particularly, the length L2 of the perforated section 6a is sized as a balance between the requirement for suppression at relatively low frequencies and the requirement for suppression at relatively high frequencies. The longer the perforated section 6a is made, the worse the low-frequency suppression will be, and the shorter it is made, the worse the high-frequency absorption will be. The invention is based upon the

principe that the length L2 is selected to a value corresponding to fulfilling predeter- mined requirements for comfort-directed low-frequency suppression as well as statutory requirements for high-frequency absorption.

In a preferred embodiment, the length L2 of the perforated section 6a is selected at about 10-20 % of the total length L, of the end pipe 6. In a normal application, where the length of the end pipe 6 is in the order of 250-300 mm, the length L2 of the perfo- rated section 6a would be in the order of 25-60 mm, preferably about 35-45 mm.

The invention is not limited to the embodiment described above, but could be varied within the scope of the accompanying claims. For example, the invention may in principe be utilise in passenger cars, buses and load-carrying vehicles. Besides this, the dimensions of for example the end pipe 6 and the rest of the silencer unit 1 may vary, as they are influence by various factors such as the current engine type, the calculated mass flow of the exhaust, and the available mounting space. In gen- eral, it could for example be stated that the diameter of the end pipe 6 must be suffi- ciently large to allow the expected mass flow of exhaust gases from the engine in question. Further, the cross section of the end pipe 6 could be circular or altema- tively oval, or of another suitable shape.

The invention is not limited to the type of silencer unit shown in Fig. 1, but can be utilise in other configurations and types of silences.

Furthermore, the perforated section can be positioned around the entire circumfer- enbal surface of the end pipe, or altematively along a portion of said circumferential surface.

According to an altemative embodiment, an end pipe according to the invention can be configured with two ore more, smaller perforated sections instead of one larger perforated section (as described above). Also according to this altemative embodi- ment, the total length or extension of the perforated sections are selected to a value corresponding to fulfilling predetermined limit values regarding suppression at rela- tively low frequencies, as well as relatively high frequencies, of the noise from the exhaust flow.