Abstract:
Engines may be provided with one or more balance shafts to provide a balancing effect to the engine during operation. Normally, the balance shafts are located where they can be driven by the engine&#39;s front end drive assembly. However, driving balance shafts or any other engine accessory off the front end drive assembly has implications in respect of both increased engine noise and extended engine length. An engine accessory drive in accordance with the present invention reduces engine noise and engine length while providing a balancing effect. An engine accessory drive includes a primary drive ( 7 ) drivable by the crankshaft ( 9 ) of the engine and a rotary member ( 5 ) such as a drive pulley mounted on a drive shaft ( 27 ) which is drivable by the primary drive ( 8 ).

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to an accessory drive for driving a balance shaft, to an engine including the balance shaft, and to a method of driving a balance shaft.  
         BACKGROUND  
         [0002]    It is known to provide engines with one or more balance shafts to provide a balancing effect for the engine when in operation. Normally, such balance shafts are positioned on the engine where they can be driven by the engine&#39;s front end drive assembly. As a result, the engine designer is restricted as regards where the balance shafts can be located. Furthermore, the overall length of the engine is dictated by this conventional arrangement, with the result that a designer has limited scope for reducing engine dimensions and/or weight.  
           [0003]    The present invention sets out to overcome these and other disadvantages of the prior art.  
         SUMMARY OF THE INVENTION  
         [0004]    According to the invention there is provided an engine accessory drive comprising a primary drive drivable by a crankshaft of the engine, a rotational member mounted on a drive shaft drivable by the primary drive, a secondary drive external of the engine and drivable by the rotational member, and at least one engine accessory drivable by the secondary drive, the at least one engine accessory including a balance shaft.  
           [0005]    According to the invention there is also provided an engine comprising a crankshaft, a primary drive drivable by the crankshaft, a rotational member mounted on a drive shaft drivable by the primary drive, a secondary drive drivable by the rotational member, and at least one engine accessory drivable by the secondary drive, the at least one engine accessory including a balance shaft.  
           [0006]    According to the invention there is also provided a method of driving an engine accessory comprising the steps of providing an engine with a crankshaft, driving a primary drive from the crankshaft, driving a drive shaft from the primary drive, the drive shaft having a rotational member mounted thereon, driving a secondary drive from the rotational member, and driving at least one engine accessory from the secondary drive, the at least one engine accessory including a balance shaft. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which:  
         [0008]    [0008]FIG. 1 is an isometric view of an engine having an accessory drive for driving a balance shaft in accordance with the invention, and  
         [0009]    [0009]FIG. 2 is an isometric view of the accessory drive and accessories of the engine of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0010]    [0010]FIG. 1 shows an engine  1  having a front end  2  and a rear end  3 . The front end  2  is provided with an accessory or secondary drive  4  in accordance with the invention and shown in more detail in FIG. 2.  
         [0011]    In the illustrated embodiment the accessory drive  4  is made up of a drive pulley  5  and a belt  6  in communication with the drive pulley  5  so that the belt  6  is driven by the drive pulley  5 . It is to be understood that any flexible continuous drive member  6  may be used in place of a belt, such as a chain (not illustrated).  
         [0012]    The drive pulley  5  is mounted on a drive shaft  27  which in turn is driven by a primary drive in the form of a chain drive  7  within the engine  1 . Alternatively, chain drive  7  may be a belt drive, a gear drive, or any other conventional drive. The drive pulley  5  may be replaced by any suitable rotational member.  
         [0013]    The chain drive  7  is made up of a drive chain  8 , a crankshaft sprocket  10  mounted on a crankshaft  9  towards the front end  2  of the engine  1 , an idler sprocket  11  and a camshaft sprocket  12  mounted on a camshaft  13  at the front end  2  of the engine  1 . The drive chain  8  extends between the crankshaft sprocket  10 , the idler sprocket  11 , the camshaft sprocket  12  and a drive sprocket  14  to drive the drive pulley  5 .  
         [0014]    More particularly, the drive sprocket  14  is disposed on the drive shaft  27  which is in turn in direct communication with a common rail fuel pump  26  so that rotation of the drive sprocket  14  results in rotation of the drive shaft  27  to drive a common rail fuel pump  26 . Accordingly, the common rail fuel pump  26  is driven by the crankshaft  9  via the chain drive  7 . The drive pulley  5  is therefore driven simultaneously with the common rail fuel pump  26 . Typically, the common rail fuel pump  26  is driven at faster than engine speed e.g. up to one and a half times or twice engine speed. The drive shaft  27  has an axis of rotation which is offset from the axis of rotation of the crankshaft  9 .  
         [0015]    As will be appreciated by those skilled in the art, the common rail fuel pump  26  is optional and the drive shaft  27  could be in communication with an alternative accessory type or a stand-alone shaft. However, in general, the drive shaft  27  is rotatable at engine speed or greater.  
         [0016]    The common rail fuel pump  26  can be any other accessory e.g. an alternator, fan, power steering pump, air conditioning pump, oil pump or the like.  
         [0017]    A rotary seal is provided on the drive shaft  27  between the engine  1  and the drive pulley  5  at a location generally indicated by the reference numeral  31  to prevent engine oil leakage or dirt ingress.  
         [0018]    Drive from the crankshaft  9  is transmitted to the drive pulley  5  via the drive chain  8  to drive the belt  6  which can in turn drive engine accessories as shall be explained more fully below.  
         [0019]    Preferably, the primary chain drive  7  is located within the engine  1  while the secondary accessory drive  4  is located substantially externally of the engine  1 .  
         [0020]    In the present embodiment of the invention, the engine  1  is provided with engine accessories in the form of first and second tubular balance shafts  15 ,  16  externally mounted on the engine  1 . The balance shafts  15 ,  16  are offset vertically with respect to a centreline  17  of the crankshaft  9  i.e. the balance shaft  16  is set lower than the centreline  17  of the crankshaft  9  and the balance shaft  15  is set higher than the centreline  17 . A balance shaft design that may be used with this invention is illustrated in commonly-owned British Patent Application No 0106852.7 filed on Mar. 20, 2001.  
         [0021]    The crankshaft  9  is covered by a crankshaft cover plate  18  to enclose the crankshaft  9 .  
         [0022]    The drive pulley  5  is disposed adjacent the first balance shaft  15  above the centreline  17 .  
         [0023]    A further front end engine accessory in the form of an alternator  19  is disposed over the second balance shaft  16  on the engine  1  and is provided with an alternator pulley  20  for receiving the belt  6 . The first and second balance shafts  15 ,  16  are provided with first and second balance shaft pulleys  21 ,  22  respectively in communication with the belt  6  to drive the first and second balance shafts  15 ,  16 .  
         [0024]    The belt  6  is a synchronous belt having teeth  23  on a top face  24  and a bottom face  25  thereof. The belt  6  extends between the drive pulley  5 , the alternator idler pulley  20 , the first balance shaft pulley  21  and the second balance shaft pulley  22  so that the drive pulley  5  can effect contra-rotation of the first and second balance shafts  15 ,  16  via the belt  6 .  
         [0025]    At least the drive pulley  5  and the balance shaft pulleys  21 , 22  are synchronous drive pulleys to ensure correct rotational phasing of the balance shaft rotational elements (not shown) in relation to the phase angle of the crankshaft  9 .  
         [0026]    The effective diameters of the pulleys are configured such that the crankshaft  9 , via the internal chain drive  7  and drive pulley  5 , drives the first and second balance shafts  15 ,  16  at twice engine speed and the alternator  19  at an appropriate design speed.  
         [0027]    Selection of a drive shaft  27  rotatable at engine speed or greater reduces the speed step-up required to rotate the balance shafts  15 , 16  at twice engine speed (which speed would be required in an in-line four-cylinder engine for correct functioning of the balance shafts), therefore the driving load transmitted by belt  6  will be correspondingly lower than if shaft  27  is rotatable at less than engine speed. As an example, the camshaft  13  is rotatable at half engine speed in a four-stroke engine and is therefore unlikely to be suitable as the drive shaft for drive pulley  5  since a large speed step-up would be required.  
         [0028]    In the present embodiment of the invention the front end  2  of the engine is also provided with a bottom idler pulley  28  below the second balance shaft  16  to ensure a peripheral engagement of the belt  6  with the balance shaft pulley  22  which is sufficient to transfer the driving force. The alternator  19  is optional and the alternator pulley  20  can simply be an upper idler pulley  20 .  
         [0029]    As indicated above, the belt  6  is a double-sided synchronous (toothed) belt so that the belt  6  can engage with complementary pulley teeth  30  on at least the first balance shaft pulley  21 , second balance shaft pulley  22  and the drive pulley  5 .  
         [0030]    It will be appreciated by those skilled in the art that any number of front end accessories may be driven by the belt  6  via suitable pulleys.  
         [0031]    Industrial Applicability  
         [0032]    The invention finds application in machines where improved noise reduction and reduced vibration is required. The invention also provides the opportunity to improve engine oil sealing and reduce engine length.  
         [0033]    The primary drive can be substantially contained within the engine while the accessory drive  4  can be external to the engine  1 . Accordingly, as the drive for the engine accessories is taken from the drive pulley  5  via the belt  6 , no conventional external drive is required to be taken directly from the front end of the crankshaft  9  on the engine  1 . Accordingly, noise and vibration in the engine  1  of the invention is reduced. It is estimated that the reduction in noise can be as much as 25% or greater over engines having a conventional front end drive assembly. In addition, no rotary seal is required at the front end of the crankshaft  9 .  
         [0034]    An engine  1  having balance shafts  15 , 16  driven as previously described can also be made shorter in the longitudinal direction as compact gearing and sealing arrangements can be employed to drive the balance shafts  15 ,  16 . Such a reduction in length can provide significant cost advantages and advantages in designing and manufacture of the engine  1 .  
         [0035]    The reduction in length of the engine is further facilitated due to the absence of mechanical linkages at the front end or nose of the crankshaft  9  so that the crankshaft cover plate  18  can be provided with its own seal or accessories mounted on the crankshaft nose can be provided with their own seals which can be more compact than seals normally required between mechanical linkages at the nose of the crankshaft  9 .  
         [0036]    The rotary seal provided on the sprocket shaft  27  ensures sealing of the engine  1 .  
         [0037]    Moreover, as the nose of the crankshaft  9  is liberated, the nose is made available for driving other engine accessories if required e.g. AC/DC generators, hydraulic pumps, compressors, engine speed generators, transfer gear boxes and the like.  
         [0038]    In addition, where the engine  1  is provided with first and second balance shafts  15 ,  16  as hereinbefore described, the engine can have a lighter weight than conventional engines fitted with balance shafts driven by mechanical linkages at the nose of the crankshaft  9 .  
         [0039]    It is envisaged that the balance shafts  15 ,  16  could be mounted externally of the engine  1 , but suspended underneath an oil sump  29  to be below the crankshaft  9 . Such an arrangement allows the width of the engine  1  to be reduced (where the engine  1  is being used to power a machine, the width of the engine  1  can be critical as turning of the front wheels of the machine for steering can cause the wheels to engage the engine  1 ).  
         [0040]    Although the balance shafts  15 ,  16  are shown as being integral with the engine  1 , they need not be, and could be mounted on a flange cast onto the engine block. As the balance shafts  15 ,  16  are driven by the accessory drive  4 , an engine designer has a greater freedom regarding location of the balance shafts  15 ,  16  on the engine  1 . Thus, the designer can take into account the space that is available and select a position that provides optimum balancing effect as the balance shafts  15 ,  16  do not require to be placed where they must engage with the gearing of the front end drive assembly.  
         [0041]    The belt  6  can also be directed using idler pulleys to substantially any configuration.  
         [0042]    In an alternative embodiment of the invention, the balance shafts  15 ,  16  can be in communication with additional engine accessories at the rear end  3  of the engine  1  to facilitate transfer of the drive from the accessory drive to the additional engine accessories at the rear end.  
         [0043]    Modifications and improvements may be made to the foregoing without departing from the scope of the present invention.