State of the art A known practice for reducing particle emissions from vehicles powered by combustion engine, particularly diesel-powered heavy vehicles, is post-treatment of exhaust gases from the engine with a view to reduction of particle content. This is done for example by particle filters comprising filter elements made of ceramic material. Such filters are typically regenerated spontaneously during operation of the vehicle by accumulated particles, mainly soot, being allowed to burn so that the filter is kept clean and can therefore continuously perform its filtering function.

In an ideal case, the burning clean takes place passively in such a way that N02 reacts with carbon in the particles gathered in the filter so as to form carbon dioxide and NO.

This process works well within a limited temperature range and subject also to the NO2/particle ratio not exceeding a certain value.

In most operating situations of diesel-driven heavy vehicles, there is some likelihood of the necessary regeneration conditions occurring during an operating period.

However, if a vehicle equipped with a particle filter is driven in such a way as to operate for a long time outside the range within which the filter would be subject to the temperature required for the regeneration process, and/or if the NO2/particle ratio is low, no regeneration takes place, which means that an excess of soot particles may accumulate in the filter.

The whole or at least part of the particle filter may then contain a supercritical amount of soot resulting in risk of the particle filter being damaged when the particles in the filter eventually ignite, since their combustion may either throughout or locally become uncontrolled, leading to excessively high temperatures.

Objects and most important characteristics of the invention One object of the present invention is to eliminate or at least alleviate the problems of the state of the art and indicate a device for particle filters which increases the certainty of protecting filter devices against such damage.

This object is achieved according to the invention with a method of the kind mentioned in the introduction by the features in the characterising part of claim 1. Hence the accumulation in the filter of an excess of soot particles, which might result in damage to the filter from eventual ignition, is prevented.

According to the invention, when the filter has accumulated so many particles as to cause a certain predetermined counterpressure, exhaust gases are instead led past the filter so that the latter will receive a limited amount of particles before regeneration takes place, i. e. before the operating conditions become such as to cause spontaneous ignition and combustion of the particles.

As mentioned above, normal operation also sometimes includes operation resulting in low exhaust temperatures. This kind of operation may occur during no-load running, running lightly laden, running with additional equipment for heavy vehicles such as cranes, lifts etc. Long periods of such operating conditions with exhaust cleaning by particle filter according to the state of the art may result in the accumulation of soot particles in the filter becoming supercritical, with consequent risk of the aforesaid damage.

Another typical form of operation which is risky in this context is urban passenger bus operation whereby accelerations with associated smoke emissions from the engine result in a large amount of particles accumulating in the filter. The nature of such

light-load operation results in obvious risk of insufficient frequency of combustion of the particles.

The invention provides a safe system based on the principle that the pressure drop across the particle filter and, analogously, the exhaust counterpressure upstream from the filter provides a description of how full of particles the filter is.

For the purposes of the invention, there is no need to lead the whole exhaust flow away from the filter, as it may be sufficient to divert past the filter one portion of the flow while a relatively smaller other portion of the exhaust gases continues to be allowed to pass through the filter. During such passing through there will of course also be a further, albeit smaller, accumulation of particles in the filter, typically resulting in an increasing proportion of the exhaust flow being led past the filter, given a certain volume of exhaust. Setting the means of bypassing the filter so that a suitable level of counterpressure/pressure drop causes bypassing can easily be achieved by simple experimentation.

Transitions from no-load or light-load running with associated low exhaust temperatures typically involve delays of the order one or a few minutes before the exhaust temperature brings the filter up to temperatures at which spontaneous regeneration takes place. A filter temperature of between 250 and 450°C is typically regarded as suitable for the occurrence of spontaneous ignition and combustion of particles and hence regeneration of the filter.

Leading exhaust gases past the particle filter through a valve which opens when the exhaust counterpressure is above said level results in a simple and reliable system which is enhanced if the valve opens in response to exhaust pressure against the action of a holding-back spring. The result is a reliable automatic mechanical system which is easy to set for achieving a desired opening level.

A possibility not excluded, however, is the counterpressure being detected by a pressure sensor which provides output signals used by control devices for controlling

the bypassing of the filter. In such cases a pressure sensor may be placed upstream from the filter. It is possible instead to place a pressure sensor upstream from the filter and a pressure sensor downstream from the filter so that the pressure drop across the filter can be detected. Bypassing can in practice be effected by a regulating device controlled by a computer unit so that all or part of the exhaust gases are switched.

It is preferable that the exhaust gases be led past the filter through a space within a silencer which encloses the filter. This means that the sound damping function of a silencer which comprises the filter can also be maintained during bypassing of the filter. It is in particular preferred that a catalyst, which is a filter connected in series, receives exhaust gases which are led past the filter. Such cases utilise the fact that N02 emitted from the catalyst takes part in the combustion process in the filter as indicated at the beginning of this description.

Corresponding advantages are achieved with a device, a silencer and a vehicle driven by combustion engine according to the invention.

Brief description of the drawings The invention will now be described in more detail on the basis of examples and with reference to the attached drawings, in which: Fig. 1 depicts in schematic section a silencer for an engine-driven vehicle with a particle filter through which exhaust gases from the engine flow, Fig. 2 depicts the silencer in Fig. 1 with exhaust gases bypassing the filter, Fig. 3a and 3b depict a valve arrangement according to a first embodiment in closed and open positions respectively, and Fig. 4a and 4b depict a valve arrangement according to a second embodiment in closed and open positions respectively.

Description of embodiments In Fig. 1, ref. 1 denotes a silencer for an exhaust system of a combustion engine, particularly a diesel engine for a heavy vehicle such as a bus or a truck. The silencer 1 encloses an integrated particle filter 3 fitted at a distance from a catalyst 2 such that in normal operation the catalyst 2 and the particle filter 3 are arranged in series. Exhaust gases flowing from the combustion engine through the silencer inlet 6 thus pass through both the catalyst 2 and the particle filter 3 before going through the outlet pipe 7 and on through the exhaust system to the environment.

The silencer 1 comprises two transverse walls 8 and 9 which support the catalyst 2 and are arranged sealingly with respect both to the inside of the silencer and the outer surface of the housing of the catalyst 2. A further transverse wall 10 is arranged for fastening the particle filter 3 on its upstream side and is sealingly arranged with respect to the inside of the silencer and the housing of the particle filter. In this transverse wall 10 is arranged a valve 4 which is closed in the position depicted in Fig. 1 so that exhaust gases from the catalyst 2 which fill the space 11 between the transverse walls 9 and 10 can only pass through the silencer by passing through the particle filter 3. A transverse wall 5 with at least one undepicted hole in it is so situated as to hold in place in the silencer 1 the downstream portion of the particle filter.

Fig. 2 depicts a situation in which the exhaust gases flowing through the inlet 6 and through the catalyst 2 meet such a powerful counterpressure in the particle filter 3 that the pressure in the space 11 between the transverse walls 9 and 10 increases. The result is that the valve 4, which is spring-loaded, lifts from its seat and allows exhaust gases to flow from the space 11 past the particle filter 3, through the perforated wall 5 and on through the outlet 7 to the environment. This flow path occurs, e. g. during light load, when so many particles have accumulated in the filter 3 that the counterpressure exceeds a certain level. This flow path will continue until greater load causes the exhaust gases to become so hot that the filter reaches a temperature at which spontaneous regeneration takes place. Thereafter the flow of exhaust gases will revert to the flow path depicted in Fig. 1.

Fig. 3a depicts an example of a version of the valve 4 in the form of a mechanically simple self-controlling element. In Fig. 3a, the valve 4 is in a closed position with a valve element 12 abutting sealingly against the intermediate wall 10 in order to seal a hole. On the upstream side of the intermediate wall 10 is arranged a snap spring 13 which holds back a gas pressure on the upper side (in the drawing) of the intermediate wall 10. At a gas pressure exceeding a certain level, this snap spring snaps to the valve open position as depicted in Fig. 3b, thereby opening a flow passage through the intermediate wall 10 past the valvepiece 12.

Fig. 4a depicts an alternative design of the valve 4 with a holding-back yoke 15 and a helical spring 16 which presses the valvepiece 4 against the intermediate wall 10 in order to seal a hole in said intermediate wall. A gas pressure exceeding a certain level on the upper side of the intermediate wall 10 in Figs. 4a and 4b will cause the valvepiece 14 to open a flow passage through said hole in the intermediate wall 10 by downward movement of the valvepiece 14 against the action of the force exerted by the helical spring 16.

The invention may be varied within the scopes of the ensuing claims. Thus the bypassing of the particle filter may be effected in a different manner, as indicated by broken lines in Fig. 1, e. g. by placing upstream from the particle filter a pressure sensor 17 whose output signals are led to a control unit (CDU) which causes bypassing of the particle filter by means of an undepicted regulating device.

The particle filter may also be arranged separately, i. e. not integrated in a silencer nor associated with a catalyst, although the version depicted in Figs. 1 and 2 is in principle preferred, particularly with a silencer in which both a catalyst and a particle filter are integrated.

It is preferred that exhaust gases be always allowed to pass through the catalyst even when the particle filter is bypassed, but this is not necessary for the invention.

If sensors are used for measuring counterpressure or pressure drop across the particle filter, a control system which reacts to signals from said sensors may also take into account signals from the engine which describe the exhaust flow, i. e. engine speed, accelerator position etc. , in order to achieve more exact values and levels at which the particle filter should be bypassed.