Abstract:
An internal combustion engine has a multi-joint crank drive which includes a plurality of coupling members rotatably mounted on a crankshaft and a plurality of articulated connecting rods rotatably mounted on an eccentric shaft, wherein each of the coupling members is pivotably connected to a piston connecting rod of a piston of the internal combustion engine and to one of the articulated connecting rods. In order that the second-order mass forces can be better compensated without a considerable increase in the friction losses, the required packaging space, the weight of the multi-joint crank drive, or the bearing forces in the bearings of the crankshaft, the articulated connecting rods are provided with additional masses and have a center of mass that lies outside the longitudinal center planes of the articulated connecting rods.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2011/003489, filed Jul. 13, 2011, which designated the United States and has been published as International Publication No. WO 2012/013298 A2 and which claims the priority of German Patent Application, Serial No. 10 2010 032 441.8, filed Jul. 28, 2010, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to an internal combustion engine with multi-joint crank drive and additional masses on articulated connecting rods of the multi-joint crank drive for damping free inertia force. 
     Internal combustion engines of this type are known for example from DE-A-102005054761, DE-A-102005054760, EP-A-1126144, JP-A-2004124775 or WO-A-2007057149 and are often referred to as internal combustion engines with variable or changeable compression ratio, because the compression ratio can be changed by rotating the eccentric shaft. 
     In these internal combustion engines the multi-joint crank drive includes beside the eccentric shaft a number of coupling members which corresponds to the number of the cylinders, which coupling members are each rotatably supported on crank pins of the crankshaft and have two arms which protrude over the crankshaft toward opposite sides and are provided at their end with a pivot joint. One of the pivot joints serves for pivotally connecting a piston rod which connects a piston of the internal combustion engine with the crankshaft via the coupling member, while the other pivot joint serves for pivotally connecting a so called articulated connecting rod which is rotatably supported with its other end on a crank pin of the eccentric shaft. 
     Internal combustion engines of the above mentioned type are further known for example from EP-B-1359303, EP-A-1760290, EP-A-1760289 and U.S. Pat. No. 4,517,931. In these internal combustion engines which are often referred to as internal combustion engines with extended expansion, the eccentric shaft is driven with half the rotational speed of the crankshaft. 
     In all of these internal combustion engines free first and second-order inertia forces are caused by oscillating masses which inertia forces change with the crank angle. While the first order inertia forces are compensated by compensating weights on the crankshaft and the crank sequence of the crankshaft can be compensated, the free second-order inertia forces cannot be fully compensated in known internal combustion engines in spite of a broad spectrum of different measures. For this reason, internal combustion engines of the above mentioned type are inferior with regard to the running smoothness and running characteristics to conventional internal combustion engines without multi-joint crank drive in which the compensation of second inertia forces occurs often by means of two counter rotating compensation shafts which are driven with double the rotational speed as the crankshaft. However, these measures cannot readily be applied to internal combustion engines with multi-joint crank drive because in this case, on one hand the generated inertia forces do not have a purely oscillating but rather rotating course, and on the other hand, the friction losses of the multi-joint crank drive are already higher than the friction losses of conventional internal combustion engines and would be increased to an unacceptable degree by the additional friction losses of two compensating shafts. 
     In order to avoid this, the applicant has already proposed in the still unpublished German Patent application 10 2010 004 589 to provide only a single compensation shaft in internal combustion engines of the above mentioned type for damping second-order inertia forces with which these inertia forces can be almost completely compensated. 
     Taking the above into account, the invention is based on the object to improve an internal combustion engine of the above mentioned type in that the second-order inertia forces can be compensated even better without significant increase of the friction losses, the required construction space, the weight of the multi-joint crank drive or the support forces in the bearings of the crankshaft. 
     SUMMARY OF THE INVENTION 
     This object is solved according to the invention in that the articulated connecting rods are provided with additional masses and have a center of mass which lies outside their longitudinal center planes. Longitudinal center planes of the articulated connecting rods refers to the planes which are defined by two connecting-rod eyes of the articulated connecting rods. 
     On one hand, the invention is based on the recognition that in internal combustion engines of the afore mentioned type, beside the classical oscillating masses such as the pistons, the piston rods and the coupling members, the articulated connecting rods which are moving back and forth also have a significant influence on the second-order inertia forces. Further, the invention is based on the recognition that the free second-order inertia forces can be partially or entirely eliminated when the vector sum of the force vectors of the inertia forces of all oscillating masses is brought to zero or is moved close to zero. 
     By changing the moved masses of the articulated connecting rods at selected sites of the articulated connecting rods, the positions of the centers of mass of the articulated connecting rods and with this the force vectors of the second-order inertia forces of the articulated connecting rods, can be influenced to make the vector sum of the force vectors of the inertia forces of all oscillating masses zero or move them closer to zero, without causing other disadvantageous effects such as for example a strong increase of the bearing forces in the crankshaft bearings. 
     Simulations have shown that in internal combustion engines of the aforementioned type, the mass compensation of the second-order inertia forces can be significantly improved with the solution according to the invention with a relatively small increase in construction space and weight. 
     The solution according to the invention is suited for internal combustion engines with multi-joint crank drive and variable compression as well as for internal combustion engines with multi-joint crank drive and extended expansion. 
     In a preferred embodiment of the invention, the additional masses and/or the centers of mass of the articulated connecting rods are each located on the side of the longitudinal center planes of the articulated connecting rods which faces away from the crankshaft, wherein the additional masses preferably protrude over the articulated connecting rods which faces away from the crankshaft in the region of a great connecting-rod eye asymmetrically to the longitudinal center planes. 
     In order for the dimensions of the additional masses to be the same in each pivot position of the articulated connecting rods, the additional masses advantageously have a circular-arc-shaped border on their side which faces away from the articulated connecting rod, which border is generally coaxial to a longitudinal center axis of the great connecting-rod eye and has expediently a radius R between 40 and 80 mm. 
     In order to be able to minimize the construction space required for the additional masses as far as possible, the at least portions of the additional masses can advantageously be made from a material with a higher specific weight than the ones of the articulated connecting rods themselves. 
     The solution according to the invention cannot only be used for one piece articulated connecting rods but also for two piece articulated connecting rods in which the additional masses are expediently arranged in the region of the great connecting rod eye on a connecting rod foot of the articulated connecting rods. 
     The additional masses are expediently molded two-piece, however they can alternatively also be bolted to the articulated connecting rods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the following, the invention is explained in more detail by way of an exemplary embodiment shown in the drawing. It is shown in 
         FIG. 1  a perspective view of parts of an internal combustion engine with variable compression ratio and a multi-joint-crank drive; 
         FIG. 2  a front side view of the parts shown in  FIG. 1 ; 
         FIG. 3  a perspective view of an articulated connecting rod of the multi-joint crank drive; 
         FIG. 4  a front side view of parts of an internal combustion engine with extended expansion and a multi-joint crank drive; 
         FIG. 5  a front side view of parts of another internal combustion engine with extended expansion and a multi-joint crank drive; 
         FIG. 6  a bar diagram with different bar pairs which show second-order and fourth order free inertia forces in the direction of a vertical and a transverse axis of the internal combustion engine; 
         FIG. 7  two vector representations of the free second-order inertia forces in the direction of a vertical axis of the internal combustion engine. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As best shown in the  FIGS. 1 and 2  as well as  4  and  5 , the four tact-4-cylinder internal combustion engines  1  which are only partially shown in the drawing, include a crankshaft  2  and four pistons  3  arranged in series, of which only one is respectively shown in the drawing. The pistons  3  are movable up and down in a cylinder (not shown) of the internal combustion engine  1  and are connected with the crankshaft  2  by a connecting rod  4 . The crankshaft  2  is supported in a cylinder crank case (not shown) of the internal combustion engine  1  for rotation about a rotation axis  5 . 
     The internal combustion engines  1  further include an eccentric shaft  6 , which has a rotation axis  7  which is parallel to the rotation axis  5  of the crankshaft  2 , is rotatably supported adjacent the crankshaft  2  and slightly below the crankshaft  2  in the cylinder crank case and is coupled with the crankshaft  2  via a multi-joint crank drive  8 . 
     Beside the crankshaft  2  and the eccentric shaft  6 , the multi-joint crank drive  8  includes overall four coupling members  9  (only one is shown) which are each rotatably supported on a crank pin  10  of the crankshaft  2 . Each coupling member  9  has a shorter lift arm  11  which is pivotally connected with the lower end of one of the connecting rods  4  via a pivot joint  12 , the end of which connecting rod  4  is articulated on the associated piston via a further pivot joint  13 . 
     The multi-joint crank drive  8  includes further articulated connecting rods  14  which whose number corresponds to the number of piston connecting rods  4  and the coupling members  9 , which articulated connecting rods  14  are oriented substantially parallel to the piston connecting rods  4  and arranged in axial direction of the crankshaft  2  and the eccentric shaft  6  in approximately the same plane as the associated piston connecting rod  4 , however on the opposite side of the cranks shaft  2 . 
     As best shown in  FIG. 3 , each articulated connecting rod  14  includes a connecting rod  15  and two connecting rod eyes  16 ,  17  which are arranged at opposing ends of the connecting rod  15 , which eyes have different inner diameters. The large connecting-rod eye  17  of each articulated connecting rod  14  on the lower end of the connecting rod  15  serves for receiving a crank pin of the eccentric shaft  6 , which crank pin is eccentric in relation to the rotation axis  7 , on which crank pin the articulated connecting rod  14  is rotatably supported by means of a sliding bearing bushing  20 . The smaller connecting-rod eye  16  on the upper end of the connecting rod  15  of each articulated connecting rod  14  serves for receiving a pivot bolt  18  of a pivot joint between the articulated connecting rod  14  and a longer coupling arm  19  of the neighboring coupling member  9 , which protrudes over the crankshaft  2  on the side of the crankshaft  2  which is opposite to the lift arm  11 . 
     In the internal combustion engine  1  with variable compression ratio shown in  FIGS. 1 and 2 , the eccentric shaft  6  can be rotated about its rotation axis  7 , in order to change the compression ratio of the internal combustion engine  1 . 
     In the internal combustion engines  1  with extended expansion shown in  FIGS. 4 and 5  on the other hand, the eccentric shaft  6  is driven with halve the rotational speed of the crankshaft  2 . 
     The oscillating masses of the pistons  3 , the piston connecting rods  4 , the coupling members  9  and the articulated connecting rods  14 , cause free inertia forces which should be compensated as much as possible to improve the smooth running and the acoustic of the internal combustion engine  1 . These free inertia forces include predominantly first order inertia forces F — 1.0, second-order inertia forces F — 2.0 as well as further order inertia forces, of which the comparatively great second-order inertia forces F — 2.0 and the comparatively small fourth-order inertia forces F — 4.0 are shown as bar diagrams in  FIG. 6 . 
     As shown in  FIG. 6 , the fourth-order inertia forces F — 4.0 and second-order inertia forces F — 2.0 can be divided into a force component Fy — 2.0 or and Fy — 4.0 which is oriented parallel to the Y-or transverse axis of the internal combustion engine  1 , and a force component Fz — 2.0 or Fz — 4.0 which is oriented parallel to the Z-or vertical axis of the internal combustion engine  1  (cylinder axle direction). 
     As also shown in  FIG. 6 , the force components Fy — 2.0 and Fy — 4.0 oriented parallel to the Y-or transverse axis of the internal combustion engine  1 , are each composed of a mass force component F KP  of the oscillating masses of the piston connecting rods  4 , a mass force component F KG  of the coupling members  9  and a mass force component F AP  of the oscillating masses of the articulated connecting rods  14 , while the force components Fz — 2.0 and Fz — 4.0 oriented parallel to the Z-or vertical axis of the internal combustion engine  1 , i.e. in cylinder axle direction, are each composed of a mass force component F KO  of the oscillating masses of the pistons  3 , a mass force component F KP  of the oscillating masses of the piston connecting rods  4 , a mass force component F KG  of the oscillating masses of the coupling members  9  and a mass force component F AP  of the oscillating masses of the articulated connecting rods  14 . 
       FIG. 7  shows the mass force proportions F KO , F KP , F KG , and F AP  of the parallel to the Z-or vertical axis of the internal combustion engine  1  oriented force component F — 2.0 not only as in  FIG. 6  with its value, but in addition also with its respective phase position as force vectors, whose lengths corresponds to the respective value of the mass force proportion, while the orientation indicates the phase position. The vector sum VS corresponds to the free second-order inertia forces in a direction parallel to the Z-or vertical axis of the internal combustion engine  1  and in the left image in  FIG. 7  has a significant value so that significant free second-order inertia forces act in this direction. As can be seen in the right image of  FIG. 7 , the scaling of individual mass force proportions F KO , F KP , F KG  and F AP , i.e., an increase or decrease of the masses of individual components  4 ,  6 ,  9 ,  14 , allows decreasing the vector sum VS in this direction to zero, which in the right image of the exemplary embodiment in  FIG. 7  occurs by an increase of the masses of the coupling members  9  and the articulated connecting rods  14 . This allows completely damping the second-order inertia forces oriented parallel to the Z-or vertical axis of the internal combustion engine  1  and thereby improving the smooth running of the internal combustion engine. As an alternative, the free second-order inertia forces oriented parallel to the Y-or transverse axis of the internal combustion engine can be damped in a corresponding manner or the geometric sum of the free inertia forces in the direction of the Y-or transverse axis and the Z-or vertical axis of the internal combustion engine  1  can be decreased. 
     The increase of the mass of the articulated connecting rod  14  for increasing the force vector of the mass force proportions F AP  of the free second-order inertia forces in a direction parallel to the Z-or vertical axis of the internal combustion engine  1 , as shown in the right image of  FIG. 7 , occurs by an additional mass arranged on each articulated connecting rod  14 , which mass is molded in such a manner to the articulated connecting rod  14  in the region of the greater connecting-rod eye  17  and asymmetrical to a longitudinal center plane  22  defined by the center axes of the two connecting-rod eyes  16 ,  17 , that the center of mass of the articulated connecting rod  14  is displaced by the additional mass  21  out of the longitudinal center plane  22 . 
     As best shown in  FIGS. 1 ,  2 ,  4  and  5 , the additional masses  21  are each molded in one piece on the outside of the connecting-rod eyes  17  on the side of the greater connecting-rod eyes of the articulated connecting rods  14  which faces away from the crankshaft  2 . As best shown in  FIG. 3 , the additional masses  21  have the shape of a sector of a circular ring, which extends around a portion of the greater connecting-rod eye  17  and on its side which faces away from the connecting-rod eye  17  has a circular arc-shaped border  23  which is coaxial to a longitudinal center axis of the great connecting-rod eye  17 . In circumferential direction of the connecting-rod eye  17 , the additional masses  21  have two plane borders  24 ,  25  which are oriented approximately tangential to a cylindrical opening which is surrounded by the connecting-rod eye  17  and extend over a circumferential angle of 60 to 80 degrees. 
     As shown in  FIGS. 2 ,  4  and  5 , the radius R of the circular-arc-shaped border  23  can vary depending on the configuration of the internal combustion engine  1 , wherein the radius in the internal combustion engine  1  with variable compression ratio shown in  FIGS. 1 and 2  with 45 mm is the smallest and in the internal combustion engines  1  with extended expansion lift shown in  FIGS. 4 and 5  is 72.7 mm or 55 mm. 
     When producing the additional masses  21  from a material with a higher specific weight compared to material of the articulated connecting rods  14 , such as for example a heavy metal, the dimensions of the additional masses  21  can still be decreased. 
     While the articulated connecting rods  14  shown in  FIGS. 1 to 3  and  5  are configured one-piece, the articulated connecting rod  14  in  FIG. 4  is configured two-piece, wherein it has a separated bearing eye  17  with a connecting-rod foot  28  and a connecting-rod cover  30  which is fastened on the connecting-rod foot  28  with bolts  29 . In this case, the additional mass  21  is molded on the connecting rod foot  28 , wherein it protrudes somewhat in the direction of the connecting rod over  30 .