Abstract:
A method and a device for exhaust gas recycling (EGR) in a combustion engine ( 1 ), wherein EGR gases are led into a section of the intake channel where a venturi ( 9 ) is arranged for modifying the flow of the intake air, and where a reduced pressure prevails because of modified flow, wherein at least a portion of the wall ( 10 ) of the venturi ( 9 ) is adjustable for varying the cross section of the venture and thereby for desired adjustment to prevailing operational conditions.

Description:
FIELD OF THE INVENTION 
     This invention concerns a method and a device for adjusting the flow of exhaust gas recycling gasses and intake air in a venturi section of an intake channel of an engine. It also concerns a supercharged engine including such a device. 
     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 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. 
     It is previously known to use a separate supercharging device in order to obtain the desired increase of exhaust gas pressure, for example through WO96/18030 and WO96/18031. A drawback with these solutions is the need of the extra super-charging device or any other pressure increasing means, which makes these solutions both costly and bulky. 
     Also U.S. Pat. Nos. 5,611,203 and 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. 
     A major problem, which the devices according to these documents suffer from, is that flow resistance losses occur in the intake channel because of the configuration of the devices. As an example, the use of a valve blade in a by-pass channel brings along a pressure drop with resulting power waste for the engine. 
     SUMMARY 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. A main aim is thus to provide a simpler and more effective transfer of EGR gases in a combustion engine. 
     According to the invention this is achieved in a method and a device as above through providing a venturi in the intake channel of the engine and adjusting the cross section of the venturi by adjusting the position in the channel of a wall of the venturi, for modifying the flow of intake air and exhaust gas recycling gasses for the engine. 
     The invention makes it possible to minimise flow resistance losses since valve blades and similar control devices become unnecessary. Further, all intake air may be used for controlled EGR pumping in the venturi since no by-pass channel needs to be arranged. This gives great advantages since the pressure drop over the venturi device may be held at a minimum. Altogether the result will be enhanced engine performance. 
     By making it possible to adjust flow actuation by adjusting at least a portion of the wall, so that the amount of air and recycled gasses as well as time for recycling may be optimized. This adjustment is simple to obtain and results in effective variation, whereby EGR recycling simply can be optimized for the prevailing operation. The adjustment is preferably controlled by a suitable sequence being programmed in the usual engine control computer, said sequence preferably using data being supplied by the ordinary parameter sensors of the engine. 
     By the adjustment being obtained through a pivoting movement, a simply controlled movement pattern is achieved. 
     The channel for supplying EGR gases into the venturi device may be placed in a fixed or movable wall depending on what is suitable in a specific installation. 
     It is preferred that the venturi wall has a sideward recess wherein the adjustable wall portion is variably insertable. This way achieves a simple construction, making it possible for the venturi to be in principle out of function with a resulting minimal pressure drop when the adjustable wall portion is entirely inserted into the recess. 
     It is further preferred that the upstream portion of the adjustable wall portion is formed such that together with the adjoining wall of the recess, in inserted positions of the insertable wall portion, it forms a space where a natural vortex may be formed and stay in operation. This may be obtained for example by having these wall portions forming a space with essentially equilateral triangular section, that is that the surfaces includes an angle of about 60°. 
     As an alternative, a deformable membrane may be provided, which in a bridging manner connects the-upstream portion of the adjustable wall portion with the adjoining wall of the recess. Hereby an essentially smooth wall configuration is obtained also in case of inserted positions of the adjustable wall portion. 
     If the opening of the EGR channel in the venturi is directed essentially in the flow direction of the intake air, it is achieved that pressure pulses in the EGR channel will not reduce the pumping effect. 
     It is preferred that the section of the venturi is essentially rectangular with adjustable height. This geometry gives simple construction and adjustability. 
     Further features and advantages of the invention become clear from the following description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments exemplifying the invention will now be described in more detail with reference to the drawings, wherein: 
         FIG. 1  diagrammatically shows an embodiment of the invention in connection with a four-stroke 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 manner provided with at least one intake valve for supply of combustion air and at least one exhaust valve for discharge 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. According to the invention the EGR gases are 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 and length in the flow direction, it may be assured that an adequate negative pressure may be obtained so that a suitable amount of EGR gases may be transferred. S indicates a control computer for controlling the venturi as a response to parameter signals from the engine. 
     The venturi  9  is shown in more detail in  FIG. 2 , where an adjustable wall portion  10  is shown in a position  10 ′, where it is brought into the channel  2  as much as possible and a position  10 ″, where it is brought out from the channel  2  as much as possible with respect to a recess  12 . The EGR transfer channel  4  debouches just about opposite that portion of the venturi  9  where the greatest negative pressure prevails. It is preferred that the channel  4  is arranged in a fixed wall  20  in the venturi but it may also debouch in the upper surface of the adjustable wall portion  10  (not shown). It is also preferred that the opening of the channel  4  in the venturi is directed in the flow direction in order to enhance transfer conditions. 
       11  indicates a pivoting shaft whereby the wall portion  9  may be adjusted for regulating the amount/portion of supplied EGR gases. In principle, the most inserted position  10 ′ results in a greater negative pressure and a greater content of supplied EGR gases, whereas the brought out position  10 ″ results in smaller or possibly no amount of supplied EGR gases at all. 
     The section through A–A of the shown venturi is rectangular which means that the surface of the wall portion  10 , which is directed against the channel, is single-curved. The shape of this surface and its extension in the length direction and height direction is to be tested and dimensioned outgoing from engine data and desired EGR mixing conditions. 
     The recess  12  thus serves for receiving-the wall portion  10  in different retracted positions. It is preferred that the upstream portion  14  of the wall portion and the adjoining surface  13  of the recess in different retracted positions form a space having a section wherein a natural vortex V may be formed and be retained in operation. This gives an undisturbed operation without unwanted pressure fluctuations, which would otherwise occur if unstable vortices would be formed in this position. 
     Control of the amount of exhaust gases to be recycled is accomplished by the control system S ( FIG. 1 ), which in a per see known manner communicates with sensors for sensing suitable engine parameters. The cooler  7  is arranged for cooling the recycled EGR gases.  FIGS. 2  (and  3 ) also shows a setting means  17 , which is controlled by the control system S ( FIG. 1 ) and may be provided with an electrically hydraulically or pneumatically operated motor. 
       FIG. 3  shows an embodiment wherein a deformable membrane  15  is attached which provides a curve adjoining between the wall portion  10  and the fixed wall  16 . The membrane may for example be made of spring steel. 
     The shape of the venturi  9  may thus vary depending on the present application. This also concerns the shape of the channel, but with the preferred embodiment, having a channel with a rectangular section, similar advantageous flow conditions are created over the width of the wall portion  10 . The movement pattern of the wall portion  10  may be different if insertion into the channel  2  is accomplished in any other way, for example by displacement. Larger portions of the wall may also be deformable besides only the membrane  15 . 
     It is within the scope of the invention that 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 four-stroke combustion engine, it is however, applicable in other kinds of combustion engines wherein similar problems or conditions prevail.