Abstract:
Method and arrangement for a turbocompound type internal combustion engine including an exhaust system for ducting the engine&#39;s exhaust gases. A supercharger turbine drives a compressor for the engine&#39;s combustion air, and an exhaust turbine is placed in the exhaust system downstream of the supercharger turbine for extracting residual energy from the exhaust flow via transmission to the combustion engine&#39;s crankshaft. The exhaust system also has an exhaust braking throttle placed downstream of the exhaust turbine. The exhaust braking throttle includes a pressure-controlled exhaust pressure regulator that makes possible variable regulation of an exhaust braking pressure in at least two steps.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation patent application of International Application No. PCT/SE02/01342 filed 4 Jul. 2002 which was published in English pursuant to Article 21(2) of the Patent Cooperation Treaty, and which claims priority to Swedish Application No. 0102769-7 filed 20 Aug. 2001. Both applications are expressly incorporated herein by reference in their entireties. 

   BACKGROUND OF INVENTION 
   1. Technical Field 
   The present invention relates to an arrangement for an internal combustion engine of the turbocompound type and that includes an exhaust system for ducting the engine&#39;s exhaust gases. A supercharger turbine drives a compressor for the engine&#39;s combustion air and an exhaust turbine is also included that is located in the exhaust system downstream of the supercharger turbine for extracting residual energy from the exhaust flow via transmission to the crankshaft of the internal combustion engine. The exhaust system also includes an exhaust braking throttle located downstream of the exhaust turbine. 
   2. Background Art 
   In a turbocompound engine (TC engine), power is transmitted from the power turbine of the TC unit, via a gear mechanism, down to the engine&#39;s crankshaft. This power is obtained by extracting the residual energy that remains in the engine&#39;s exhaust gases after having passed through the turbo compressor for compressing the engine&#39;s charging air. 
   For engine braking, it is normal for an exhaust brake to be used. For a TC engine, the exhaust brake consists of a suitable arrangement, normally a throttle valve that can throttle the exhaust flow, and it is placed downstream of the TC unit. When the valve is closed and the fuel injection ceases, power is transmitted instead from the crankshaft via a gear to the TC unit&#39;s power turbine. This power helps to increase the braking effect as it is an energy loss, but which is positive from a braking perspective. A problem that can arise, however, is that a valve that has been closed downstream of the TC unit will increase the density of the air in which the TC unit&#39;s power turbine is operating. This, of course, assists the braking, but it also gives rise to increased thermal and mechanical stresses. These increased stresses will depend upon the engine speed and will increase with increased engine speed. In order that the TC unit or other components do not break, they must be dimensioned for the increased stresses. This can lead to the construction being unnecessarily expensive, as it is made more complicated and as expensive heat-resistant material must be used. 
   With an exhaust brake, the braking effect increases for a given engine speed when the back pressure after the turbine increases. In order to obtain best braking function, as high as possible back pressure is desired. For a TC engine, this is particularly difficult, as the stresses that were mentioned above arise as a result of the back pressure increasing. For a given back pressure, the stresses also increase when the engine speed increases. In order that components do not break, they must be constructed so that they can withstand the stresses that arise at the maximal permitted engine brake speed. Alternatively, a lower back pressure can be selected. A lower back pressure can be presumed still to give acceptable braking performance at high engine speeds, but at low engine speeds the braking effect is commensurately low. Thus, in order to obtain a good braking effect at low engine speeds, high back pressure is required. This in turn leads to large forces at high engine speeds, or alternatively using low back pressures at a high engine speed and obtaining a poor braking effect at low engine speeds. 
   SUMMARY OF INVENTION 
   An object of the invention is therefore to achieve an arrangement that makes possible rapid and effective regulation of the exhaust back pressure during engine braking. 
   This object is achieved by a means for braking that is configured *so that the exhaust braking throttle comprises (includes, but is not limited to) a pressure-controlled exhaust pressure regulator that makes possible variable regulation of an exhaust braking pressure in at least one step in addition to “off” and “on” steps. Furthermore, the exhaust pressure regulator is provided with means for adapting the exhaust braking pressure to the engine speed. By means of this solution, it is possible to optimize the braking effect for all engine speeds without being forced to select expensive constructional solutions. At low engine speeds, a high back pressure is selected that gives acceptable braking effect and acceptable stresses. At high engine speeds, a lower back pressure is selected that gives acceptable stresses and acceptable braking effect. With this solution, the braking effect can thus be optimized for the selected constructional solution and the selected material over the entire range of engine speeds. This means in principle that for each engine speed there is a unique back pressure that gives maximal braking effect without leading to the inducement of unacceptable stresses. 
   According to an advantageous embodiment of the invention, the exhaust pressure regulator is provided with means for adapting the exhaust braking pressure to the engine speed. 
   This object is achieved by a means for braking that is configured so that the exhaust braking throttle comprises (includes, but is not limited to) a pressure-controlled exhaust pressure regulator that makes possible variable regulation of an exhaust braking pressure in at least one step in addition to “off” and “on” steps. Furthermore, the exhaust pressure regulator is provided with means for adapting the exhaust braking pressure to the engine speed. By means of this solution, it is possible to optimize the braking effect for all engine speeds without being forced to select expensive constructional solutions. At low engine speeds, a high back pressure is selected that gives acceptable braking effect and acceptable stresses. At high engine speeds, a lower back pressure is selected that gives acceptable stresses and acceptable braking effect. With this solution, the braking effect can thus be optimized for the selected constructional solution and the selected material over the entire range of engine speeds. This means in principle that for each engine speed there is a unique back pressure that gives maximal braking effect without leading to the inducement of unacceptable stresses. 
   The exhaust pressure regulator consists suitably of a piston valve that comprises a first piston surface that is acted upon by the exhaust pressure when the exhaust braking throttle is closed, and a second opposing piston surface permanently connected to the first piston surface, which second piston surface is acted upon by a control pressure. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be described in greater detail in the following, with reference to the embodiments that are shown in the attached drawings, in which: 
       FIG. 1  is a schematic diagram showing a first embodiment of an arrangement configured according to the teachings of the present invention, and 
       FIG. 2  schematically shows a second embodiment of an arrangement configured according to the teachings of the present invention. 
   

   DETAILED DESCRIPTION 
   The arrangement shown in the figures is configured for utilization with an essentially conventional internal combustion engine of the turbocompound type, preferably incorporated in the drive unit of a heavy truck or bus. The engine may advantageously be of the direct-injection diesel engine type in which a supercharger  10 , with exhaust gas driven turbine  11  and compressor  13  arranged on the turbine shaft  12 , are used for the compression and supply of combustion air. The inlet air is supplied to the compressor  13  for compression, after which the compressed air can be cooled during its passage through a charging air cooler before it enters the engine&#39;s inlet manifold. 
   The engine&#39;s exhaust gases are collected in the conventional way in an exhaust collector to be taken to the supercharger&#39;s  10  turbine  11  for driving the compressor  13 . The exhaust gases are then taken via a second exhaust turbine, which in the embodiment shown consists of an axial turbine  15  and an exhaust brake arrangement  16 , to a silencer unit with optional exhaust gas filter equipment. 
   The axial turbine  15  is used in turbocompound engines to extract residual energy from the exhaust gases after their passage through the supercharger&#39;s turbine. The exhaust gases drive the power turbine at very high speeds, up to approximately 90,000 rpm at a normal engine speed, which for a diesel engine for heavy trucks involves an engine speed of approximately 1,500–2,500 rpm. The torque that is obtained is transmitted to the crankshaft of the internal combustion engine via, among other things, a transmission  17  that gears down the speed, and a fluid coupling  18  that isolates the transmission  17  mechanically from the engine&#39;s crankshaft. 
   The exhaust brake arrangement  16  comprises a throttle  19  that can be moved between two end positions by means of a servo device  20 , with the throttle rapidly changing between a completely open and a completely closed position. The exhaust brake arrangement comprises, in addition, a bypass  21  past the throttle  19 , which bypass can be controlled by means of an exhaust brake regulator in the form of a piston valve  22  which is placed upstream of the throttle  19 . A first piston surface  23  is acted upon by the exhaust pressure when the exhaust braking throttle is closed, with the piston surface  23  being pressed against the action of a helical spring  24 , so that the bypass  21  is opened. A second piston surface  25  is permanently connected to the piston surface  23  via a rod  26  and is mounted in a cylinder  27  in such a way that it can be moved. 
   A regulating air pressure acts against the piston surface  25  via a compressed air pipe  28  which is connected to a compressed air system in the vehicle which is used to generate power for auxiliary units in the vehicle, for example the braking system and system for pneumatic operation of the vehicle&#39;s gearbox. This compressed air system comprises, among other things, a compressor  29 , an accumulator tank  30  and a valve housing  31 . 
   As the second piston surface  25  of the piston valve  22  has a slightly smaller diameter than the first piston surface  23 , the piston valve will be able to react during engine braking and open the bypass  21  past the exhaust braking throttle  16  in the event of an exhaust gas pressure acting against the first piston surface that is less than the pressure that is to be found in the compressed air pipe  29  and thus acts against the second piston surface  25 . For example, the piston surface  23  can have a diameter of ninety millimeters while the piston surface  25  has a diameter of eighty-four millimeters, whereby the piston valve  22  can react to an exhaust gas pressure which is approximately fifteen percent lower than the system pressure. 
   The valve unit  31  provides overpressure that can vary from the standby level of 0.5 bar overpressure to a higher level that can be regulated in relation to the required engine braking effect. For this purpose, the valve unit is connected to an engine control unit  32  as shown in FIG.  2  that is arranged to regulate the higher level of overpressure with regard to various parameters. For example, information about the brake pedal pressure and ABS system may be monitored so that the braking power is optimized in relation to the operation of the engine and to the state of the road. 
   As the second piston surface  25  of the piston valve  22  has a slightly smaller diameter than the first piston surface  23 , the piston valve will be able to react during engine braking and open the bypass  21  past the exhaust braking throttle  19  in the event of an exhaust gas pressure acting against the first piston surface that is less than the pressure that is to be found in the compressed air pipe  29  and thus acts against the second piston surface  25 . For example, the piston surface  23  can have a diameter of ninety millimeters while the piston surface  25  has a diameter of eighty-four millimeters, whereby the piston valve  22  can react to an exhaust gas pressure which is approximately fifteen percent lower than the system pressure. 
   A state of equilibrium gives: 
   
     
       
         
           
             P 
             m 
           
           = 
           
             
               
                 P 
                 s 
               
               · 
               
                 
                   A 
                   25 
                 
                 
                   A 
                   23 
                 
               
             
             = 
             
               
                 P 
                 control 
               
               · 
               
                 constant 
                 . 
               
             
           
         
       
     
   
   The control pressure, P control , is adjustable and is adjusted by the valve unit  31 . Since the back pressure is directly proportional to the control pressure, the back pressure will be changed when the control pressure is changed. 
     FIG. 2  shows a variation of the invention in which the exhaust brake regulator  16  is designed in a different way to that shown in  FIG. 1 . Thus the exhaust pipe is Land the piston valve  22  is set in the angle between the two parts of the pipe. In this case, the throttle  19  and the bypass  21  are not needed, as the changeover from normal operation to exhaust braking is carried out by the piston valve  22  being moved from an inner inactive position to an outer active position. In this position, the piston surface  23  blocks off the exhaust pipe with a pressure that is determined by the valve housing  31  and the engine control unit  40 , so that excess pressure can leak past the piston surface  23 . The variant of the invention shown in  FIG. 2  is less expensive to implement than the solution shown in  FIG. 1 , but does result in a greater pressure loss in the exhaust pipe. 
   An advantage of the arrangement according to the invention is that the braking effect of the engine brake can be regulated. This means that it is possible to obtain different braking effects at different engine speeds. This adjustable braking effect can, for example, be used to reduce the fuel consumption and to increase driving comfort. These side effects are, of course, also obtained if the exhaust back pressure is regulated in an ordinary turbo engine or aspirating engine. 
   With the modern control units that are on current engines, this is possible to achieve, and mechanical components are available that are sufficiently quick to be able to achieve the required back pressure. In the example of the regulation system disclosed herein, an indirect setting of the back pressure is carried out by varying the feed pressure to the EPG. This feed pressure gives rise to a predetermined back pressure. An alternative way of controlling the back pressure is to mount a pressure sensor in the collector housing and to measure the pressure and regulate the feed pressure to the air throttle (the EPG) by means of the control unit in such a way that the required back pressure is obtained. This method is, however, more complicated and more expensive than the indirect method. Practical tests have shown that for a normal engine size for heavy trucks, it is almost possible to achieve a constant braking moment over a very large range of engine speeds using the disclosed arrangement. 
   The invention is not to be regarded as being limited to the embodiments described above, a number of further variants and modifications being possible within the framework of the following patent claims.