DESCRIPTION OF PRIOR ART In turbo-supercharged diesel engines it is previously known to recycle exhaust gases to the engine inlet in order to reduce contents of nitrous oxides in the exhaust gases. Hereby the recycled exhaust gases function so as to lower the combustion temperature resulting in that a smaller amount of the nitrogen in the inlet air can be converted into nitrogen oxides. This process, usually called EGR (exhaust gas recirculation) has often been used in Otto engines as a relatively simple way of reducing the contents of harmful exhaust gas emissions.

In diesel engines, on the other hand, this technique has not been employed to such a great extent depending i. a. on the fact that there are particular problems associated with these engines, making Otto engine solution not directly applicable in diesel engines.

One of these particular problems is that the combustion in diesel engines normally occurs with excess air. This indirectly results in need of transferring relatively large amounts of exhaust gases during a relatively large operating range of the engine in order to achieve the desired function.

This problem is accentuated in case of an engine of the super- charged type, because in that case the pressure in the intake

system of the engine is greater than the pressure in the exhaust gas system during a great part of the operating range.

Among known solutions to be used in supercharged engines, two main principle solutions can be distinguished, usually named "short route EGR"and"long route EGR". In the first mentioned case exhaust gases are taken from a position before an exhaust turbine in the exhaust system and is recycled to a position after an intake air compressor which is arranged in the intake system. In the latter case exhaust gases are taken from a position. after the exhaust. turbine and-are recycled to a position before the intake air compressor. Both of these principle solutions have advantages and disadvantages.

Also US-A-5 611 203 and US-A-5 611 204 could be mentioned as previously known art with respect to this invention. These documents describe how exhaust gases are recycled to the intake in turbo-supercharged diesel engines through a venturi device or-any-other kind of ejector being placed in the intake channel. The system according to these documents uses the low static pressure prevailing in a certain section of the ejector device for pumping-in an EGR flow into the charged air.

When venturi devices are used as pump means for transferring EGR gases in supercharged four-stroke combustion engines, in some operational cases such pressure conditions prevail in the engine that the negative pressure in the venturi is not sufficient. In practice the maximum available negative pressure in a venturi is limited. by the sonic speed of the medium.

Conventional venturi devices are constructed with a chamber surrounding the injection area. This chamber thus functions as

a pressure source. This construction however results in transition losses to and from the chamber as well as loss of efficiency when the EGR gas is sucked into the intake air.

AIM OF THE INVENTION It is an aim of this invention to provide a solution to or a reduction of the problems of the prior art. The main purpose of the invention is thus to provide an effective and cost- saving solution to the task of transferring EGR gases.

According to the invention this is achieved through a method and a device as above through the features of the characterising portion of claims 1 and 5 respectively.

According to the invention, the chamber is thus replaced with a channel having a contraction for accelerating EGR gases.

Hereby the flow losses of the EGR gases are reduced up to the venturi, further the impulse loss in the venturi itself is reduced, since the EGR gases already from the start has got a higher speed. The pulse energy in the EGR gases is also utilised more effectively.

By, according to a preferred embodiment, the contraction being chosen such that at the inlet in the venturi device of the EGR gases, these have been accelerated to a speed which essentially, corresponds to the speed of the intake air in this area, optimised feed with minimised losses is achieved.

Further features and advantages of the invention will come clear from the following description of an embodiment.

BRIEF DESCRIPTION OF DRAWINGS Embodiments exemplifying the invention will now be described in more detail with reference to the annexed drawings, wherein: Fig. 1 diagrammatically shows an embodiment of the invention in connection with a turbo-supercharged diesel engine, Fig. 2 shows in more detail a device according to a first embodiment of the invention, and Fig. 3'shows a device according to a-second embodiment of the invention.

DESCRIPTION OF EMBODIMENTS Fig. 1 shows diagrammatically a combustion engine 1 of the piston engine type with cylinders arranged in a straight inline cylinder block. The engine is a four-stroke diesel engine adapted for a heavy vehicle such as a truck or a bus.

Each cylinder is in its respective cylinder head in a conventional not in detail shown manner provided with at least one intake valve for supply of combustion air and at least one exhaust valve for discharging of exhaust gases from the combustion. An intake channel 2 leads the intake air to the cylinders whereas an exhaust collector 3 leads the exhaust gases from the cylinders to the turbine T and subsequently to the exhaust pipe.

Further, a transfer channel 4 is arranged for recycling EGR gases from the exhaust side of the cylinders to their intake side. The transfer channel 4 debouches in the intake channel 2 after a charging air cooler 5 and before a manifold to the cylinders. An EGR control valve 6 is positioned in the

transfer channel whereby the transfer may be disconnected and possibly controlled to a certain extent.

The transfer channel 4 debouches in a section of the intake channel wherein venturi 9 is arranged in such a way that the flow of intake gases, which are charged by the compressor C, is modified so as to create a negative pressure at the passage of the venturi. This is because the air at the passage of a convex curved portion will be given an increased speed. The EGR gases are thus led into said section in a portion in connection with-the venturi where thus a negative pressure prevails. By shaping the venturi 9, in particular, i. a. with respect to curvature, length in the flow direction and cross- section, width and height, it may be assured that an adequate negative pressure may be obtained so that a suitable amount of EGR gases may be transferred.

With 7 an EGR gas cooler is indicated.

In Fig. 2 the venturi 9 is shown in more detail from where diagrammatically the construction is shown with a contraction portion 10, a mixing section 11 and a diffuser portion 12. The intake air flows, as seen in the Figure, from the left to the right. In the region of the mixing section 11 an EGR channel 13 debouches, which includes a contraction 14 for accelerating the EGR gases before intake into the venturi 9. The port 15 of the channel 13 is directed essentially in the flow direction R of the main flow. The venturi in this Figure is constructed with rectangular sections across the plane of the paper. I. e. in planes parallel with the plane of. the drawing venturi sections are essentially identical.

Fig. 3 shows an embodiment, wherein the venturi is rotary- symmetrical around the axis A-A. Further, the elements

corresponding to the ones in Fig. 2 are shown. The contraction portion is denoted 10', the mixing section 11', the diffuser portion 12', the EGR channel 13', the contraction 14'and the port 15'. The negative pressure in the port 15'is hereby essentially proportional to the radius R14, where R1 is the narrowest section of the venturi.

The invention may be modified further within the scope of the patent claims.. The venturi may thus be given another design than the one shown. It should however be stressed that the region of the port 15'should be constructed with respect to i. a. the relation between the radiuses Rl and R2 with the intention to have the least possible loss. R2 is the radius of the mixing section after the port of the EGR channel. The diffuser portion is to be carefully tested in order to avoid that disturbing vortices are formed through diversion of the gas flow along the walls. The construction of the port is likely to be denoted as a slot 15 and 15'respectively, which may be straight (for example Fig. 2) or curved (for example Fig. 3) It is not excluded that the device is adjustable, for example by the walls V1 and V2 in Fig. 2 being mutually displaceable for variation of the contraction 14.

Within the scope of the invention is that all or only some of the cylinders of an engine contribute to EGR recycling, for example through one of the blocks of a V engine.

The invention has been described at the background of a super- charged diesel engine but it is applicable also for other types of combustion engines, wherein similar problems and conditions prevail.