Abstract:
A dual crankshaft internal combustion engine is symmetrically constructed to form a perfectly balanced engine assembly. A first crankshaft, having a first end, a second end, and being formed of a shape and with a torsional flexibility, is housed within a cylinder block and connected to a first series of cooperating pistons and cylinders. A second crankshaft, having a first end and a second end, is formed of substantially the same shape as the first crankshaft and has substantially the same torsional flexibility as the first crankshaft. The second crankshaft is also housed within the cylinder block and connected to a second series of cooperating pistons and cylinders, while being positioned parallel to the first crankshaft, with the first end of the first crankshaft being positioned adjacent to the second end of the second crankshaft.

Description:
FIELD OF INVENTION 
   The present invention pertains to the art of internal combustion engines used in vehicles and, more specifically, to a balance and noise reduction system for a dual crankshaft engine. 
   BACKGROUND OF THE INVENTION 
   Conventional internal combustion engines employ piston and cylinder arrangements that tend to vibrate during operation. The vibration often creates a disturbance in a vehicle passenger compartment and is considered undesirable. 
   Most internal combustion engines develop power in impulses generated by the explosion of a combination of air and fuel in the engine&#39;s cylinders. The power is transferred to pistons that are located in the cylinders and are coupled to a rotating crankshaft with connecting rods. The power then flows to a flywheel that is connected to other downstream components of a powertrain. All conventional, single crankshaft, piston engines have a firing frequency vibration caused by uneven torque delivery to the flywheel. On a combustion or expansion event, the flywheel&#39;s rotational speed increases and, on a compression event, the rotational speed of the flywheel decreases. The torque that causes vibrational speed variations of the flywheel reacts against the cylinder block and causes torsional vibration of the cylinder block. 
   This fluctuating torque causes one source of vibration. Other disturbing engine vibrations are caused by unbalanced accelerations of internal engine components, especially linear accelerations of the piston masses within the cylinder bores. To address these problems, rubber engine mounts have been used to isolate the vehicle chassis from much of the cylinder block vibration. Still, some vibration is transmitted through the mounts and is sensed in the passenger compartment. 
   A partial solution is to have multi-cylinder engines generally configured so that the linear acceleration forces of the various pistons partially or completely cancel each other. Inline and opposed 6-cylinder engines, as well as inline and 90 degree V8 engines, usually have theoretically perfect balance of piston acceleration forces, but most other engines have residual unbalanced forces or moments. For example, all single crankshaft V6 engines with less than 180 degrees of bank angle have inherent unbalanced couples due to piston acceleration forces. Furthermore, all conventional single crankshaft engines have unbalanced torsional accelerations imposed upon the block structure due to flywheel rotational accelerations. 
   As an example, the Volkswagen 15 degree bank angle V6 engine is narrower than other 60 or 90 degree V6 engines and has a one-piece cylinder head that spans between two cylinder banks. However, there are numerous undesirable qualities with such a design. The intake manifold is on one side of the cylinder head and the exhaust manifold is on the other, causing three cylinders to have long intake and short exhaust passages while the other three cylinders have short intake and long exhaust passages. An asymmetry exists between the cylinder banks with regard to the location of the intake and exhaust valves. Further, one bank has mostly vertical intake valves and highly inclined exhaust valves, while the other bank has highly inclined intake valves and mostly vertical exhaust valves. Finally, the center planes of the cylinder bores intersect some distance below the crankshaft rotational axis, so that the cylinder bores on each bank are offset from the crankshaft axis in opposite directions. This arrangement causes the piston velocities in each of the two banks to be different. On one bank, the pistons are slower during upward motion than they are during downward motion. On the other bank, the pistons are faster during upward motion than they are during downward motion. 
   The use of two crankshafts in one cylinder block is not unprecedented. One example can be found in the Ariel motorcycle. The Ariel motorcycle was manufactured for many years with a dual crankshaft engine. This Ariel “Square Four” engine included two inline, two-cylinder crankshafts operating in a common cylinder block structure, with the resulting four cylinder bores being oriented in a square fashion. Each of the two crankshafts operates two pistons, with a 180-degree phase angle between the crankpins on each crankshaft. One pair of straight cut spur gears is arranged to couple the crankshafts to each other to make the crankshafts rotate in opposite directions. This arrangement has some apparent drawbacks. First, the arrangement is very noisy in operation because the single gearset has backlash and rattles each time the direction of torque transfer reverses. Also, because each cylinder bank contains only two cylinders, each bank of cylinders has a second order vertical shaking force that is in phase with the vertical shake of the other bank. Thus, the whole engine assembly has a second order vertical shake equivalent to that of an inline four cylinder engine. Furthermore, the two counter-rotating crankshafts do not carry equal amounts of rotating inertia so the firing pulse accelerations of the crankshafts produce a reaction on the engine&#39;s cylinder block. 
   Based on the above, there is a need in the art for a dual crankshaft engine that is well balanced and produces much less vibration than conventional engines, while avoiding the disadvantages set forth above. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a dual crankshaft internal combustion engine that is symmetrically constructed to form a balanced engine assembly. The dual crankshaft internal combustion engine comprises first and second series of cooperating pistons and cylinders mounted in a cylinder block. A first crankshaft, formed of a distinct shape and with a certain torsional flexibility, is positioned within the cylinder block and connected to the first series of cooperating pistons and cylinders. A second crankshaft is formed of substantially the same distinct shape as the first crankshaft and has substantially the same torsional flexibility as the first crankshaft. The second crankshaft is also positioned within the cylinder block, while being connected to the second series of cooperating pistons and cylinders and positioned parallel to the first crankshaft, with a first end of the first crankshaft being positioned adjacent to a second end of the second crankshaft. 
   A first gearset is mounted in the internal combustion engine connecting the first end of the first crankshaft to the second end of the second crankshaft, while a second gearset is mounted in the internal combustion engine connecting the second end of the first crankshaft to the first end of the second crankshaft. With this interconnection, the first and second crankshafts are configured to rotate in opposite directions. A first mass, having an associated inertia, is connected to the first end of the first crankshaft and a second mass, having substantially the same rotational inertia as the first mass, is connected to the first end of the second crankshaft. Preferably, the first mass is a motor/generator or a starter, and the second mass constitutes a flywheel or a torque converter. The first and second crankshafts are preloaded with a rotational tension stored in the torsional flexibility of each shaft to eliminate gear rattle. An alternative embodiment is to have a single gearset between the two crankshafts with one of the gears including a spring loaded scissors gear. 
   In operation, the cylinders preferably have a firing order where the piston motion is diametrically symmetrical. That is to say that the rear piston of the left cylinder bank has a motion that is substantially identical to and in phase with the motion of the front piston of the right cylinder bank. Likewise, the second piston from the rear of the left bank is in phase with the second piston from the front of the right bank, etc. Also, the total rotational inertia of the left bank crankshaft, including flywheel, torque converter, and other rigidly attached rotating parts is substantially equal to that of the right bank crankshaft with its rigidly attached rotating parts. In any case, with this construction, the engine assembly will have substantially perfect internal balance of piston forces, while also providing substantially perfect internal balance of crankshaft rotational moments. 
   Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment when taken in conjunction with the drawings, wherein like reference numerals refer to corresponding parts in the several views. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic view of a vehicle incorporating a dual crankshaft engine embodying the invention; 
       FIG. 2  is a simplified isometric view of the dual crankshaft engine of  FIG. 1  shown in a simplified form with various parts of the engine, such as the valves and cylinders, being omitted so the moving parts of the engine attached to the crankshafts may be seen more easily; 
       FIG. 3  is an end view of the engine of  FIG. 2  showing one of the two gearsets that connect the crankshafts; 
       FIG. 4  is a downward looking cross-sectional view of the engine of  FIG. 3  taken along line  4 - 4  showing more details of the crankshafts, along with added inertial masses including a torque converter and a starter/generator; 
       FIG. 5  is a top view of the engine of  FIG. 3  showing the engine with six cylinders, along with associated intake ports and passages leading from an intake manifold; 
       FIG. 6  is a cross-sectional view of the engine of  FIG. 3  taken along line  6 - 6 , showing details of a gear that is incorporated into one of the counterweights on the crankshaft; and 
       FIG. 7  shows an embodiment where the gear of  FIG. 6  is a scissors gear. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   With initial reference to  FIG. 1 , there is shown an automotive vehicle  10  having a body  11  and a dual crankshaft engine  12 . The engine  12  is preferably attached to a first mass  14 , having an associated inertia such as motor/generator or a starter motor. The engine  12  is also attached to a second mass  16 , having an associated inertia, such as torque converter or a flywheel. The rotational inertia of the first mass  14  is preferably the same as the rotational inertia of the second mass  16 . Power from the engine  12  is transmitted through the flywheel or torque converter to a transmission  18 , then to the other portions of a powertrain  20  and eventually drives wheels  22 . 
   In  FIG. 1 , the vehicle  10  is shown as a rear wheel drive vehicle but, as will become readily apparent from the discussion below, any type of powertrain arrangement, including a front wheel or all wheel drive, could be used. At this point, it should be readily recognized that a flywheel would be commonly used as second inertia mass  16  with a manual type countershaft transmission, while a torque converter would be used when an automatic type transmission is employed. Similarly, the first mass  14  is preferably a motor/generator in the case of a hybrid vehicle, otherwise the first mass  14  is a starter. As more fully discussed below, the first mass  14  may also include a combination of other accessories that, when taken together, have the same inertia as the second mass  16 . For simplicity in connection with describing the invention herein, the first mass  14  will generally be referred to as a motor/generator  24  and the second mass  16  will be generally be referred to as a flywheel  26  as shown in  FIG. 4 . 
   Referring now to  FIGS. 2-5 , the engine  12  is shown with a cylinder block  28  containing first and second counter-rotating crankshafts  30  and  32 . A first series  34  of cooperating pistons and cylinders are mounted in the cylinder block  28  and connected to first crankshaft  30 , while a second series  36  of cooperating pistons and cylinders are mounted in the cylinder block  28  and connected to second crank shaft  32 . More specifically, cylinders  41 - 46  slidably receive respective pistons  51 - 56  thus defining multiple combustion chambers, two of which are shown at  72  and  75  in  FIG. 3 . The pistons  51 - 53  connected to the first crankshaft  30 , along with their associated cylinders  41 - 43  as best seen in  FIG. 5 , collectively constitute the first series  34  of cooperating pistons and cylinders, while the pistons  54 - 56  connected to the second crankshaft  32 , along with their associated cylinders  44 - 46  as also best seen in  FIG. 5 , collectively constitute the second series  36  of cooperating pistons and cylinders. 
   Each of the first and second crankshafts  30  and  32  is rotatably mounted on respective main journals  81 - 84  and  85 - 88 . As one skilled in the art would understand, each main journal  81 - 88  is rotatably coupled to a main bearing of the engine  12 , thereby rotatably coupling the respective crankshafts  30  and  32  to the engine  12 . Each of the crankshafts  30 ,  32  also has a respective plurality of rod journals  90 ,  92  integrally formed therein. Each connecting rod  101 - 106  is mated to an associated journal  111 - 116  on one of the two crankshafts  30 ,  32 . Each connecting rod  101 - 106  is also connected to an associated reciprocating piston  51 - 56 , thus allowing the pistons  51 - 56  to drive the crankshafts  30 ,  32  when the engine  12  is in operation. As best seen in  FIG. 3 , the first plurality of rod journals  90  includes first, second and third connecting rod journals  111 - 113  that are evenly spaced around a rotational axis  120  of the first crank shaft  30  such that the journals  111 - 113  are spaced approximately 120 degrees apart. Similarly, the second plurality of journals, which includes the fourth, fifth and sixth journals  114 - 116  on the second crankshaft  32 , are set 120 degrees apart around a rotational axis  121 . 
   As can be seen from the above discussion, the two crankshafts  30  and  32  are fabricated to be essentially identical to each other and formed of the same distinct shape, but they are installed “end-for-end” so that crankshaft  30 , as viewed from the front of the engine  12 , has the same direction of rotation and rod journal positions as crankshaft  32 , as viewed from the rear of the engine  12 . This creates an arrangement that is symmetrical about a central axis  125  as shown in  FIG. 4 . In this manner, any pitching couples generated by piston linear acceleration forces of the first series  34  cooperating pistons and cylinders are equal in magnitude and phase angle, but opposite in sign to the couples generated by the second series  36  of cooperating pistons and cylinders. Note, for example, the sixth piston  56  closest to the flywheel  26  moves up and down in synchronism with the first piston  51  closest to the motor/generator  24 , as can best be seen in  FIG. 2  with respect to reference plane  126 . 
   Each crankshaft  30 ,  32  has a first end  131 ,  132  and a second end  133 ,  134  respectively. As best shown in  FIG. 4 , a first gearset  135  connects the first end  131  of the first crankshaft  30  to the second end  134  of the second crankshaft  32 , while a second gearset  136  connects the second end  133  of the first crankshaft  30  to the first end  132  of the second crankshaft  32 . With this construction, the two crankshafts  30  and  32  are forced to counter-rotate. In the case of the first crankshaft  30 , the first end  131  has a flange  140  integrally formed therein. As shown, the flange  140  is connected to the motor/generator  24 . In a single crankshaft engine, the second end of the crankshaft would normally be connected to a flywheel. However, the first crankshaft  30  as shown simply ends at the main journal  84  with or without a thrust surface  141 . In the case of the second crankshaft  32 , the first end  132  also has a flange  145  integrally formed therein but, in this case, the flange  145  connects to the flywheel  26 . 
   Firing frequency rotational accelerations of the flywheel  26  result in equal and opposite inertial vibration torque imposed upon cylinder block  28 . To cancel this torsional excitation of the cylinder block  26 , the motor/generator  24  and the flywheel  26  preferably have an equivalent amount of rotational inertia. Since the magnitude of both the clockwise and the counterclockwise rotating inertias are equal to each other and their rotational accelerations are equal but opposite, the reactions that they impose upon the cylinder block  28  mutually cancel, and the block  28  does not vibrate from internal inertial forces. This cancellation of the crankshaft&#39;s torsional reaction against the cylinder block structure  28  presents an opportunity. Since the cylinder block structure  28  has less vibration in response to crankshaft torsional vibrations than a conventional engine, the engine  12  may be operated in more fuel efficient modes where the crankshaft vibration increases. One example of a more fuel efficient mode of operation that increases crankshaft torsional vibration is cylinder deactivation, sometimes referred to as “variable displacement internal combustion” engine (VDIC). The present invention allows selective disabling or re-enabling of cylinders  41 - 46  from the first  34  and second  36  series of cooperating pistons and cylinders in accordance with power requirements of the vehicle  10 . Current production VDIC equipped vehicles are calibrated to avoid cylinder deactivation under many conditions where the engine  12  with deactivated cylinders could produce adequate power, but the resulting vehicle NVH (noise vibration and harshness) would be unacceptable. Furthermore, the increase of cylinder block width, to enclose two crankshafts  30 ,  32  and two banks of cylinders  41 - 43  and  44 - 46  in one integral structure, functions to stiffen the cylinder block  28  and reduce vibrations and radiated sound from the engine  12 . The motor/generator  24  serves to capture vehicle kinetic energy during deceleration as well as to add torque to enhance vehicle acceleration. 
   The motor generator  24  can also be used as an “active flywheel”. A control strategy for using a starter as an “active flywheel” is disclosed in U.S. Pat. No. 6,256,473, which is incorporated herein by reference. The additional rotating inertia of the motor/generator  24 , along with the ability of the motor/generator  24  to create or absorb torque, also allows the engine  12  to operate with a more efficient combustion process, such as HCCI (Homogeneous Charge Compression Ignition). The motor/generator  24 , with dynamic torque control, can remove torque from strong combustion events and add torque to weak combustion events. This torque compensation, along with the cancellation of the crankshaft internal inertial reaction torque on the cylinder block  28  allows reliable and smooth engine operation with a combustion process that may have reduced robustness in favor of more efficiency. 
   As shown in  FIG. 1 , the flange  140  may also provide a mounting place for various ancillary equipment, generally indicated at  148  in  FIG. 1 , such as a camshaft drive mechanism, an engine driven coolant pump, a power steering pump, climate control system, fan belt pulleys, or the like. Power from the engine  12  could be taken from either end of either crankshaft  30 ,  32  in either hand of rotation as long as the two crankshafts  30 ,  32  have essentially symmetrical construction with equivalent rotating inertias and opposite directions of rotation. Preferably, the total inertia of the ancillary equipment  148  that is tightly coupled to the crankshaft  30  and the motor/generator  24  is the same as the inertia of the flywheel  26 . The motor/generator  24 , with inertia mass equivalent to the flywheel  26  or torque converter, has very large current generating capacity, even at engine idle, and could enable electrical powering of various ancillary equipment  148  that are normally driven mechanically by a belt from a crankshaft. Conventionally, an engine driven coolant pump and a power steering pump are sized for their most severe operating conditions, and during other engine operating conditions they are over-driven and waste much energy. Even with the conversion inefficiencies between electrical and mechanical energy, these machines would be more efficient if they were electrically driven at speeds in accordance with need. Technically, the vehicle  10  is not a full hybrid because the engine  12  will always be running when vehicle propulsion is needed. Regardless, the large electric motor/generator  24  could crank the engine  12  and bring it up to speed very quickly and quietly, so that if the vehicle  10  is equipped with an electrically powered climate control system or other ancillary equipment  148  powered from a rechargeable battery, the engine  12  may shut off whenever it is not needed for vehicle propulsion and restarted again, as needed, without adversely affecting the comfort of passengers or the drivability of the vehicle  10 . 
   Furthermore, each crankshaft  30 ,  32  includes two types of integral counterweights as shown in  FIG. 4 . Counterweights  152   a ,  152   b ,  155   a  and  155   b  for the second and fifth pistons  52 ,  55  are formed as two parallel lobed weights. The counterweights for the first, third, fourth and sixth pistons  51 ,  53 ,  54 ,  56  include a respective one lobed weight  151 ,  153 ,  154 ,  156  and a respective weight  161 ,  163 ,  164 ,  166  that is incorporated into respective gears  171 ,  173 ,  174 ,  176 . As can best be seen in  FIG. 6 , the crankshaft cheek  191  is provided with at least two annularly placed holes  181  and  182 . The gear  171  is also provided with holes  183  and  184  that align with the holes  181  and  182  in the cheek  191 . Threaded fasteners  185  and  186  pass through respective aligned ones of the holes  181 ,  182 ,  183 ,  184  to secure the gear  171  to the cheek  191  and thus to the crankshaft  30 . The counterweights  152   a ,  152   b ,  155   a ,  155   b ,  151 ,  153 ,  154 ,  156 ,  161 ,  163 ,  164 ,  166  have the appropriate masses and are located to generate forces on the crankshafts  30 ,  32  that cancel the lateral first order forces imposed on the crankshafts  30 ,  32  by the acceleration forces of the connecting rods  101 - 106 . First order unbalance refers to the forces and couples that vary as a sinusoidal function with one cycle of force occurring with each rotation of the crankshaft. Second order unbalance is caused by the changing vertical force components based on the varying vertical component of the connecting rod lengths caused by the cyclical inclinations of the rods  101 - 106  due to lateral movement of the associated journals  111 - 116 . 
   Preferably, pistons  51 - 56  and their associated connecting parts have the same reciprocating mass and stroke, and each crankshaft  30 ,  32  has equal angular and axial spacing between rod journals  111 - 116  so that the reciprocating pistons  51 - 56  and connecting rods  101 - 106  coupled to each crankshaft  30 ,  32  generate no unbalanced shaking forces or moments. Symmetry of construction achieves substantially perfect balance of the engine  12 . The crankshafts  30 ,  32  are far enough apart to prevent interference between the connecting rods  102 ,  105 , as illustrated in phantom lines in  FIG. 4 . The first series  34  of cooperating pistons and cylinders and the second series  36  of cooperating pistons and cylinders are preferably equidistant fore and aft, as illustrated, so that the engine  12  will have minimum length, and the block  28  will have maximum strength and stiffness. This overall arrangement will function to cancel those first order vibrations not cancelled by the counterweights  152   a ,  152   b ,  155   a ,  155   b ,  151 ,  153 ,  154 ,  156 ,  161 ,  163 ,  164 ,  166 , while also canceling any second order vibrations due to the symmetry of the engine  12  about axis  125 . 
     FIGS. 3 and 5  illustrate how an intake manifold  200  is preferably oriented above the engine  10  in a fashion that advantageously provides the same length passages  205  between the manifold plenum  207  and the intake valves  210  of every cylinder  41 - 46 . 
   Since gear sets  135  and  136  are driven directly from the crankshafts  30  and  32 , the gear sets  135 ,  136  have the potential to be very noisy due to the crankshafts&#39; torsional vibrations causing rattle between the gears  171 ,  173 ,  174 ,  176 . A properly designed scissors gear set  270 , shown in  FIG. 7 , is therefore provided to eliminate rattle between the gears  171 ,  173 ,  174 ,  176 . The scissors gear set  270  includes a one-piece gear  174  on the crankshaft  32  meshing with a split gear  271  on the crankshaft  30 . The split gear  271  has a first rigid portion  273  that is attached to the crankshaft  30 , while a second spring-loaded portion  274  is rotationally biased relative to the first portion  273  by a spring  275 . In operation, the first rigid portion  273  transfers torque to the gear  174  in one direction of torque, while the second spring-loaded portion  274 , through the spring  275 , transfers torque to the mating gear  174  in the opposite direction of torque. If the spring pre-load is greater than the maximum reverse torque that the second spring-loaded portion  274  of the gear set  270  must carry, the gear set  270  will not rattle. Since the crankshaft  32  receives very large input torques in both the forward and backward directions, the scissors gear set  270  is designed so that the spring  275  is very strong. 
   In another preferred embodiment of the invention, the torsional flexibility of the crankshafts  30 ,  32  is used to reduce vibration instead of using the scissors gear set  270 . This is done when mounting the gear sets  135 ,  136  between the crankshafts  30 ,  32  at both ends of the engine  12 . More specifically, the first three gears  171 ,  173 ,  174  are clamped to the crankshafts  30 ,  32  as they are installed into the engine  12 . Then the fourth gear  176  is placed, but not clamped, so that it is free to rotate slightly relative to crankshaft  32 . The ends  132 ,  133  of the crankshafts  30 ,  32  near the unclamped gear  176  can be twisted relative to each other and held in the twisted position while the last gear  176  is clamped in place to provide a torsional preload. The twisted crankshafts  30 ,  32  serve as preload springs for the gear sets  135 ,  136 , thereby eliminating gear rattle by preloading the first and second crankshafts  30 ,  32  with a rotational tension stored in the torsional flexibility of each shaft  30 ,  32 . Preferably, one crankshaft  30  carries two gears  171 ,  173  with a handed helix, while the other crankshaft  32  carries two gears  174 ,  176  with an opposite handed helix. In this manner, the thrust loads generated by the torsional preload will create both fore and aft thrust on each crankshaft  30 ,  32 , allowing the thrusts from the preload to cancel within each crankshaft  30 ,  32  without imposing extra loads on the crankshaft support bearings. 
   The engine configuration described in this invention disclosure provides near perfect internal balance of piston acceleration forces, unlike conventional single crankshaft V6 engines, and it will provide near perfect internal balance of rotational accelerations of the flywheel, unlike all conventional single crankshaft engines. Furthermore, the upper portion of this dual crankshaft engine is narrower than that of conventional V6 and V8 engines. Although described with reference to a preferred embodiment of the invention, it should be readily understood that various changes and/or modifications could be made to the invention without departing from the spirit thereof. For instance, both crankshafts could be offset from the plane of their corresponding cylinders to reduce piston side loading during the power stroke, as long as symmetry is maintained with both crankshafts having the same magnitude of offset. The shown engine has three cylinders in each bank, however each crankshaft may have more cylinders so long as the rows of cylinders each have the same number of cylinders. If the engine assembly has fewer than six cylinders, such as was done in the construction of the Ariel Square Four motorcycle engine, balance shafts or another balancing mechanism would be required to achieve the desired engine balance. In general, the invention is only intended to be limited by the scope of the following claims.