Abstract:
The invention relates to a DOHC sliding cam valve train of an internal combustion engine with a four-cylinder in-line arrangement and cylinder shutoff. One of the two camshafts has two shared cam pieces for the respective adjacent engine cylinders.

Description:
BACKGROUND 
     The invention relates to a valve train of an internal combustion engine with a four-cylinder in-line arrangement and variable-stroke actuation of the gas exchange valves. The valve train comprises a first camshaft having a carrier shaft and precisely two first cam pieces, which are arranged for conjoint rotation thereon and so as to be movable between at least two axial positions and which each actuate the intake or exhaust valves of two adjacent engine cylinders by at least two first groups of adjacent cams having different lobes. 
     A sliding cam valve train of this kind, in which the gas exchange valves of two adjacent engine cylinders are actuated by a common cam piece, is known from DE 10 2005 006 489 A1 and DE 10 2009 016 902 A1. A prerequisite for the reduction achieved here in the complexity of the camshaft is the adequate size of the common base circle angle of all the cams of a cam piece since only this angular range is available for the shifting thereof in operation. Consequently, in-line four-cylinder engines (R4) having two overhead camshafts (DOHC) and the standard ignition sequence 1-3-4-2, in particular, are suitable for this purpose. 
     As part of the continued tightening of the emission regulations, there is now also an R4 engine with a sliding cam valve train and cylinder shutdown in production—see Motortechnische Zeitschrift MTZ March 2012: “Der 1,4-L-TSI-Ottomotor mit Zylinderabschaltung” (“The 1.4-l TSI spark-ignition engine with cylinder shutdown”). 
     SUMMARY 
     It is the object of the invention to specify the variability of the stroke of a valve train of the type noted at the outset with a view to further reducing emissions from internal combustion engines in the various operating modes thereof. 
     This object is achieved by one or more features of the invention. According to this, the valve train should have a second camshaft, which comprises a carrier shaft and precisely two second cam pieces, which are arranged for conjoint rotation thereon and so as to be movable between at least two axial positions and which each actuate the respective other intake or exhaust valves of just one engine cylinder by at least one second group of adjacent cams having different lobes. In this case, the lobes of simultaneously actuating cams of the first cam groups are supposed to be different and to include a zero stroke and the second cam groups are each supposed to include a cam having a zero stroke in order to stop the intake and exhaust valves of the two engine cylinders, the exhaust and intake valves of which are actuated by the second cam pieces. 
     As a result, a DOHC sliding cam valve train for an R4 engine is obtained which has just four cam pieces and which, on the one hand, can be operated in the cylinder shutdown mode and the intake or exhaust valves of which, for the engine cylinders that cannot be shut down, are operated with two different cam lobes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features of the invention will become apparent from the following description and from the drawings, in which two illustrative embodiments of a valve train according to the invention are shown. Unless otherwise mentioned, features or components which are the same or functionally the same are provided with the same reference numbers here. In the drawings: 
         FIG. 1  shows a detail of a known sliding cam valve train in side view; 
         FIG. 2A  shows the intake and exhaust shafts of the first valve train according to the invention in schematic view; 
         FIG. 2B  shows the three operating modes of the first valve train in table form; 
         FIG. 3A  shows the intake and exhaust shafts of the second valve train according to the invention in schematic view; and 
         FIG. 3B  shows the three operating modes of the second valve train in table form. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be explained starting with  FIG. 1 , in which a known variable-stroke valve train of an internal combustion engine is shown. The basic operating principle of the valve train can be summarized in that a camshaft of conventionally rigid design is replaced by an externally toothed carrier shaft  1  and cam pieces  2  mounted for conjoint rotation and axial movement thereon by internal toothing. Each cam piece has two groups of axially adjacent cams having two lobes H and M of different sizes, the lift of which is transmitted selectively to the gas exchange valves  4  by finger levers  3 . The movement of the cam piece on the carrier shaft required to activate the respective cam in accordance with the operating point is accomplished by means of two axial slotted guides extending over the cam piece and having slotted guide tracks  5  and  6  in the form of grooves, which differ in orientation in accordance with the direction of movement and in which a respective actuating pin  7  of an actuator (not shown) engages, depending on the instantaneous position of the cam piece. 
       FIG. 2A  shows the two camshafts of a first DOHC valve train according to the invention for an R4 engine having two intake valves and two exhaust valves per engine cylinder and the ignition sequence 1-3-4-2. The first camshaft, which is at the top in the figure, is the intake shaft  8 , and the second camshaft, at the bottom in the figure, is the exhaust shaft  9 . 
     The intake shaft  8  is made up of the externally toothed carrier shaft  1  and two first cam pieces  10  and  11 , which are mounted for conjoint rotation and so as to be axially movable between three positions on the carrier shaft by the internal toothing of said cam pieces. One first cam piece  10  actuates the intake valves of the two engine cylinders I and II, and the other first cam piece  11  actuates the intake valves of the two engine cylinders III and IV. Each of these cam pieces has four first groups, each comprising three adjacent inlet cams having different lobes. The Roman numeral in the designation I- 1  to IV- 2  of the intake- and exhaust-side cam groups refers to the respective engine cylinder, and the Arabic numeral in said designation refers to the first or second valve of said engine cylinder, each opening with the same lobe. The slotted guide tracks  5  and  6  of axially opposite orientation, into each of which one of two actuating pins engages in order to be able to move the first cam pieces into each of the three axial positions, extend between two adjacent cam groups of each engine cylinder. 
     The exhaust shaft  9  is made up of the externally toothed carrier shaft  1  and two second cam pieces  12  and  13 , which are mounted for conjoint rotation and axial movement between two positions on the carrier shaft by means of the internal toothing of said cam pieces. One cam piece  12  actuates the exhaust valves of engine cylinder I and the other second cam piece  13  actuates the exhaust valves of the engine cylinder IV. Each of these cam pieces has two second cam groups I- 1 , I- 2  and IV- 1 , IV- 2 , each having two adjacent exhaust cams with different lobes. Unlike the axially movable cam pieces, the exhaust cam pairs II- 1 , II- 2  and III- 1 , III- 2  of engine cylinders II and III, respectively, are joined firmly to the carrier shaft. The slotted guide tracks  5  and  6  arranged between the two cam groups intersect, and therefore only one actuating pin is required in each case to move these cam pieces into the two axial positions. 
     The qualitative size of the individual cam lobes and the three corresponding operating modes A to C of the internal combustion engine are listed in the form of a table in  FIG. 2B . The first cam groups I- 1  to IV- 2  of the intake shaft  8  have simultaneously actuating cams with different lobes. The cams, denoted by N, of cam groups I- 1  and I- 2 , and IV- 1  and IV- 2 , are namely cams with a zero stroke N for the purpose of stopping the intake valves of the engine cylinders I and IV while, at the same time, the intake valves of engine cylinders II and III are actuated by the cams with a large lobe H. In this cylinder shutdown mode C, the second cam pieces of the exhaust valves  9  are likewise in the left-hand axial position L, and therefore their cams with a zero stroke N also stop the exhaust valves of engine cylinders I and IV. In contrast, the exhaust valves of the engine cylinders II and III which are not shut down are actuated by the fixed cams having a large lobe H. 
     Mode B: starting from mode C, movement of the first and second cam pieces  10 ,  11  and  12 ,  13 , respectively, by one axial position (to the right into position M in  FIG. 2A ) has the effect that all the intake valves of cams with a medium-sized lobe M are actuated and that all the exhaust valves of cams having a large lobe H are actuated. 
     Mode A: starting from mode B, a further movement of the first cam pieces  10 ,  11  by one axial position (to the right into position R in  FIG. 2A ) has the effect that the intake valves are also actuated by cams having a large lobe H. 
     The second valve train according to the invention, as shown in  FIGS. 3A and 3B , differs from the valve train explained above only in the configuration of the intake shaft  8 ′: in this case, the intake valves of engine cylinders II and III are actuated by cams having a medium-sized lobe M in the cylinder shutdown mode C. 
     Alternative embodiments (not shown) of the inventions can be:
         stopping engine cylinders II and III instead of I and IV in the cylinder shutdown mode   in the figures, the first camshaft is the exhaust shaft, and the second camshaft is the intake shaft   cam pieces having other known slotted guide tracks   both camshafts have a stroke which is variable in either three stages or just two stages.       

     LIST OF REFERENCE SIGNS 
     
         
         
           
               1  carrier shaft 
               2  cam piece 
               3  end pivot follower 
               4  gas exchange valve 
               5  slotted guide track 
               6  slotted guide track 
               7  actuating pin 
               8  first camshaft, intake shaft 
               9  second camshaft, exhaust shaft 
               10  first cam piece 
               11  first cam piece 
               12  second cam piece 
               13  second cam piece