Abstract:
An engine is provided with features allowing a method of assembly that combines the ease of assembly of a net-assembled system with the positional accuracy of an index-assembled system in order to meet critical valve to piston clearance requirements, especially critical in a diesel engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/955,922, filed Aug. 15, 2007, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates to an engine, such as a diesel engine, having a camshaft drive system with one component that has an angular adjustment feature, allowing an improved method of assembly. 
     BACKGROUND OF THE INVENTION 
     Camshafts in a vehicle engine are often driven by timing chains and a drive sprocket or sprockets (or gears) off the front of the crankshaft, or off the front of an idler/balance shaft in some engine designs. Chain guides, tensioner arms and tensioning devices (which may be hydraulic or spring actuated) are used to maintain chain tension. A respective driven sprocket is attached for rotation with each camshaft and is driven rotationally by a timing chain. The camshafts control the opening and closing motion of engine valves that regulate airflow into and out of engine cylinders. The airflow is created by the upward and downward motion of pistons that is generated by the rotary motion of the crankshaft converted to linear motion by connecting rods. 
     The timing of the opening and closing of the engine valves in relation to the crankshaft is critical due to a typically low clearance of the pistons to the intake valves when opening and to the exhaust valves when closing. To accommodate this tight clearance, gasoline engines often have valve relief pockets cast or machined into the pistons to provide additional valve to piston clearance. Diesel engines have significantly higher compression ratios, with most of the volume of the combustion chamber in the crown of the piston. A machined or cast valve relief pocket puts a stress concentration in the crown area of the piston. Therefore, to maintain control of the combustion chamber volume and eliminate a piston stress concentration, diesel engines minimize the size of, or do not use valve relief pockets. This requires a lower running clearance between the pistons and valves. Diesel engines must therefore be designed and assembled to attain such a precise clearance. 
     SUMMARY OF THE INVENTION 
     A method of assembling an engine drive system, especially for a diesel engine, is provided that combines the ease of assembly of a net-assembled system with the positional accuracy of an index-assembled system in order to meet a critical valve to cylinder clearance. “Net assembly” of a camshaft drive system uses locating features (also referred to herein as positioning features, marking features, identifiers or alignment features) to angularly locate and fix members of the drive system to one another, without “locking” the positions of any of the components (i.e., without holding any of the components in a set angular position, without allowing rotation, until the assembly is completed). A net assembly method is relatively easy because of the locating features, but the accuracy of the relative angular positions of the crankshaft and the camshaft (i.e., the timing of the engine) is influenced by stack-up of the tolerances (i.e., variances in the positions) of the many components in the drive system, such as the crankshaft, camshaft, timing chains, etc. The accuracy of the net assembly method is suitable for a gasoline engine, with its typically larger minimum valve to piston clearance allowance. 
     “Index-assembly” of a camshaft drive system involves locking the crankshaft in a set angular position and also locking the camshaft in a set angular position. The accuracy of the relative angular positions of the crankshaft and the camshaft is generally higher than with net assembly, as only the tolerances of the locking features used to lock the crankshaft and the camshaft influence the accuracy, and the locking feature tolerances are greatly reduced in comparison to the many positional tolerances influencing accuracy in the net assembly method. However, an index assembly method is more difficult and time consuming, as locating features are not provided to aid in alignment of the components. 
     Thus, a method of assembling an engine, and specifically a method of assembling an engine drive system for an engine, are provided. 
     The method of assembling an engine includes locking a driveshaft in a first predetermined angular position and locking a camshaft in a second predetermined angular position. The method also includes installing a timing drive to operatively connect the driveshaft and the camshaft. Installing the timing drive involves many substeps, such as fixing a drive sprocket to the driveshaft to prevent rotation of the drive sprocket with respect to the driveshaft. Furthermore, installing the timing drive includes piloting a driven sprocket on the camshaft without preventing relative rotation thereof (i.e., such that the driven sprocket is free to rotate relative to the camshaft). The driven sprocket has an adjustment feature. Predetermined marked identifiers on a drive chain are then aligned with location identifiers on the sprockets to position the drive chain on the sprockets. The angular position of the driven sprocket relative to the camshaft is then adjusted to align the adjustment feature with a locking feature on the camshaft gear. The driven sprocket is then locked to the camshaft gear to prevent rotation of the driven sprocket relative to the camshaft gear. 
     The method of assembling an engine drive system includes locking the angular positions of the engine crankshaft and the camshaft (or camshafts) using locking features. Multiple rotatable members, such as sprockets and a timing chain, operatively connect the crankshaft for driving the camshafts. The relative orientation of all but one of the multiple rotatable members is fixed using locating features so that only that single member is adjustable to vary an angular position relative to the other members. The adjustable member is adjusted in angular position to align with one of the fixed members. The adjustable member is then connected to the fixed member it is aligned with to complete the drive system assembly. The locating features afford the ease of assembly of an “index assembly” method. The locking features ensure the positional accuracy of the “net assembly” method. The adjustment feature provides adjustability of one of the components relative to a locating feature on an adjacent component, preferably of the last component to be fixed in angular position, to ensure that the relative alignment of these last two components to be connected with one another may be realized. 
     An engine that may be assembled according to the above method includes a driveshaft operable for reciprocally driving pistons. Complementary locking features permit the driveshaft to be selectively locked in a fixed angular position with respect to an engine block that supports the driveshaft. Within the scope of the invention, the driveshaft may be a crankshaft or a balance transfer shaft driven by the crankshaft and rotating in a predetermined geared ratio with respect thereto. The engine further includes a camshaft operable for reciprocally moving engine valves to open and close compression chambers in which the pistons move. The camshaft has a camshaft gear connected for common rotation therewith. Other complementary locking features allow the camshaft to be selectively operatively locked to a cylinder head supporting the engine valves to thereby lock the angular position of the camshaft. 
     A drive sprocket is fixed to the end of the driveshaft and has an angular locating feature matable with a complementary locating feature on the driveshaft to prevent relative angular displacement therebetween. A driven sprocket is selectively lockable to the camshaft gear, using an adjustment feature discussed below, to prevent relative angular displacement therebetween. 
     The engine further includes a timing chain. The drive sprocket and the driven sprocket have respective marking features alignable with complementary marking features on the timing chain to position the timing chain on the sprockets in a predetermined relative position when the driveshaft and camshaft are locked. Finally, the adjustment feature of the driven sprocket, such as elongated slots in the sprocket, enables angular adjustment of the driven sprocket to properly align the adjustment feature with a locating feature on the camshaft gear, such as a series of apertures, prior to locking the driven sprocket to the camshaft gear. The driven sprocket may be locked to the camshaft gear by inserting a fastener through the aligned adjustment feature and locating feature when the driveshaft and camshaft are locked and the timing chain is positioned on the sprockets in the predetermined relative position. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective illustration of a diesel engine within the scope of the invention, assembled according to the method described herein; 
         FIG. 2  is a schematic perspective illustration in fragmentary partial cross-sectional view of a crankshaft counterweight lockable to the engine block of the engine of  FIG. 1  to lock respective angular positions of the crankshaft and balance transfer shaft; 
         FIG. 3A  is a schematic perspective illustration in fragmentary view of an intake camshaft gear locked to a cylinder head of the engine of  FIG. 1  to lock respective angular positions of the intake and exhaust camshafts; 
         FIG. 3B  is a schematic perspective illustration in fragmentary partial cross-sectional view of the locked intake camshaft gear intermeshing with and locking an exhaust camshaft gear; 
         FIG. 4A  is a schematic perspective illustration of a drive sprocket having a key slot serving as a positioning feature to fix the drive sprocket angular position relative to a key on the crankshaft, similar to the keyed balance transfer shaft of  FIG. 1  on which a drive sprocket with a key slot is fixed; 
         FIG. 4B  is schematic perspective illustration in fragmentary view of the crankshaft having a key to accept the drive sprocket of  FIG. 4A  for an embodiment of an engine drive system in which the drive sprocket is fixed to the crankshaft rather than the balance transfer shaft; 
         FIG. 5  is a schematic illustration in front fragmentary view of the engine of  FIG. 1 , showing marking features on the timing chains located with respect to complementary marking features on the drive sprocket and the driven sprockets; 
         FIG. 6  is a schematic illustration in front fragmentary view of one of the driven sprockets of  FIG. 1  having an adjustment feature aligned with a locating feature in the exhaust camshaft gear to allow the driven sprocket to be fixed to the exhaust camshaft gear and exhaust camshaft for rotation therewith; and 
         FIG. 7  is a schematic front illustration in fragmentary view of an intake and an exhaust valve of  FIG. 1  operatively positioned adjacent the head of a piston to define a clearance therebetween. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers refer to like components,  FIG. 1  shows a diesel engine  10  with a rotatable crankshaft  12 . Rotary motion of the crankshaft  12  is converted to linear motion of connecting rods  14 , as is known. Pistons  16  are connected at the ends of the connecting rods  14  and travel within cylinder bores in an engine block  18  (not shown in  FIG. 1 , but shown in fragmentary view in  FIG. 2 ). Air intake into each cylinder bore is controlled by the opening and closing of intake and exhaust valves  20 ,  22 , respectively, best shown in  FIG. 7 . As is apparent in  FIG. 7 , the clearance  24  between the respective valves  20 ,  22  and each piston  16  is relatively tight. Thus, the timing of the opening and closing of the valves  20 ,  22  with relation to the travel of the piston  16  is critical. This timing is a function of the angular orientation of intake camshafts  26  and exhaust camshafts  28 , which affect lowering and lifting of the valves  20 ,  22 , respectively, with respect to the angular orientation of the crankshaft  12 , which in turn affects the linear motion of the pistons  16 . 
     The camshafts  26 ,  28  are driven by rotation of the crankshaft  12 . A transfer gear set includes a gear  30  connected for rotation with the crankshaft  12  and intermeshing with a gear  32  that is concentric for rotation with a balance transfer shaft  34  and is arranged generally parallel with an axis of rotation of the crankshaft  12 . A drive sprocket  36  is connected for common rotation with the balance transfer shaft  34 . The drive sprocket  36  transfers rotary motion of the balance transfer shaft  34  to respective driven sprockets  38 A,  38 B connected for rotation with the exhaust camshafts  28  via timing chains  39 A,  39 B, as further described below. Chain guides  40 , and tensioner arms  42  adjustable by tensioner devices  44  (which may be spring-actuated, hydraulically-actuated, or actuated by any other means known to those skilled in the art) are used to properly tension the timing chains  39 A,  39 B. The drive sprocket  36 , timing chains  39 A,  39 B, driven sprockets  38 A,  38 B, and camshaft gears  46 ,  48  are referred to herein as multiple rotary members constituting a timing drive or drive system to transfer rotary motion from the balance transfer shaft  34  to the camshafts  26 ,  28 . 
     Exhaust camshaft gears  46  (only one visible in  FIG. 1 ) are fastened to the driven sprockets  38 A,  38 B and fixed for rotation with the exhaust camshafts  28 , as further described below. The exhaust camshaft gears  46  intermesh with intake camshaft gears  48  fixed for rotation with the intake camshafts  26  to further transfer rotary motion to the intake camshafts  26 . 
     In order to reliably attain the appropriate timing and ensure the required valve clearance  24  (see  FIG. 7 ), the engine  10  is designed with and assembled using a combination of both “net build” and “index build” assembly features. First, as best shown in  FIG. 2 , the crankshaft  12  is locked in a predetermined angular position by aligning a locking feature  50  of the crankshaft  12 , with a complementary locking feature  54  in the engine block  18 . The complementary locking feature  50  is a key opening in a crankshaft counterweight  52 . The complementary locking feature  54  is a fastener opening in the engine block  18 . A locking pin  56 , shown in phantom, is inserted through the engine block  18  and the aligned locking features  50 ,  54  to lock the crankshaft  12  in the predetermined angular position associated with the locked position of the counterweight  52 . Because the balance transfer shaft  34  is driven by the crankshaft  12  through the transfer gear set  30 ,  32 , the locking features  50 ,  54  and pin  56  also lock the balance transfer shaft  34 . 
     Next, referring to  FIG. 1 , the drive sprocket  36  is fixed to the locked balance transfer shaft, also in a predetermined angular position, by aligning an angular positioning feature  57  of the drive sprocket  36 , which is a keyed opening, with a complementary positioning feature  59  of the balance transfer shaft  34 , which is a key. Similar features are illustrated in  FIGS. 4A and 4B , which show fixing of a drive sprocket  36 A to the crankshaft  12 , assuming an embodiment in which the crankshaft  12  rather than a balance transfer shaft  34  drives the timing chains  39 A,  39 B. In that embodiment, a keyed opening  57 A is aligned with a key  59 A of the crankshaft  12 . 
     Next, referring again to  FIG. 1 , the camshafts  26 ,  28  are locked in desired angular positions relative to the locked angular position of the crankshaft  12 . This is accomplished by first fixing the intake camshaft gears  48  to the intake camshafts  26  using a key and keyed opening arrangement similar to that used to fix the drive sprocket  36  to the balance transfer shaft  34 . Then the exhaust camshaft gears  46  are piloted on the ends of the exhaust camshafts  28 , also using a key and keyed opening arrangement. As best shown in  FIG. 3B , a marking feature  60  on each exhaust camshaft gear  46  (e.g., gear teeth marked with a slight indentation, a color marking, or the like), is aligned with a complementary marking feature  62  (also marked teeth) on the adjacent, intermeshing intake camshaft gear  48 . Referring to  FIG. 3A , a locking feature  64 , which is an opening through the intake camshaft gear  48  is then aligned with a complementary locking feature  66 , which is an opening in the cylinder head  68 . A locking pin  70  is then inserted through the aligned locking features  64 ,  66  to temporarily lock the intake camshaft gears  48  and the intake camshafts  26  to the stationary cylinder head  68  in the predetermined angular position established by the marking features  60 ,  62  and the locking features  64 ,  66 . Because the exhaust camshaft gears  46  mesh with the intake camshaft gears  48  (as shown in  FIG. 1 ), the exhaust camshaft gears  46  and exhaust camshafts  28  are also locked in a predetermined angular position. 
     Referring now to  FIG. 1 , with the crankshaft  12 , balance transfer shaft  34  and camshafts  26 ,  28  all locked in set angular positions, the driven sprockets  38 A,  38 B are then piloted on to the ends of the exhaust camshafts  28 , but temporarily not fixed or locked in angular relation to the exhaust camshafts  28 . The timing chains  39 A,  39 B are then positioned over the drive sprocket  36  and the driven sprockets  38 A,  38 B between the chain guides  40  and the tensioner arms  42 . The drive sprocket  36  has two sets of circumferential teeth, spaced axially from one another, with a front set driving timing chain  39 A and a rear set driving timing chain  39 B (and obscured by the timing chain  39 B in  FIG. 1 ). Referring to  FIG. 5 , the drive sprocket  36  and driven sprockets  38 A,  38 B have location identifiers  72 A,  72 B,  72 C and  72 D, also referred to as marking features or positioning features, in the form of marked teeth, indicated by arrows in  FIG. 5 , that are similar to the marking features  60 ,  62  of the camshaft gears  46 ,  48 . The timing chains  39 A,  39 B have complementary marking features  74 A,  74 B,  74 C,  74 D, in the form of marked links, shown with circular markings in  FIG. 5 , which are aligned with the respective marking features  72 A,  72 B,  72 C and  72 D when the timing chains  39 A,  39 B are installed. The tensioner arms  42  are then set to ensure the marking features  72 A- 72 D and  74 A- 74 D remain in alignment with one another. 
     Finally, the angular orientation of the driven sprockets  38 A,  38 B is matched to the locked angular orientation of the exhaust camshaft gears  46  by rotating the driven sprockets  38 A,  38 B relative to the respective exhaust camshaft gears  46  as necessary to align the driven sprockets  38 A,  38 B with a locking feature  76  of the exhaust camshaft gears  46  (locking feature  76  of the camshaft gear  46  associated with drive sprocket  38 A is shown in  FIG. 6 ). The locking feature  76  is a series of spaced apertures at a predetermined radial distance R from the center of rotation of the exhaust camshaft gears  46 , as illustrated in  FIG. 6 . An adjustment feature  78  in the driven sprockets  38 A,  38 B, which is a series of elongated slots in the driven sprockets  38 A,  38 B also at the predetermined radial location R (shown in  FIG. 6  with respect to driven sprocket  38 A), serves as an indexing feature in that it allows the driven sprockets  38 A,  38 B to be adjusted prior to locking the driven sprockets  38 A,  38 B to the exhaust camshaft gears  46  by inserting fasteners  80  through the aligned locking features  76  (apertures) and adjustment features  78  (slots) so that the locked angular orientations of the crankshaft  12 , balance transfer shaft  34  and camshafts  26 ,  28 , as well as the aligned marking features  72 A- 72 D and  74 A- 74 D of the sprockets  36 ,  38 A,  38 B to timing chains  39 A,  39 B are maintained and tolerance stack-ups in the various components of the drive system are accommodated. In  FIG. 6 , one of the fasteners  80  is removed to show the aperture  76  and the slot  78 ; other respective aligned apertures and slots are hidden by the fasteners  80  that are shown. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.