Patent Publication Number: US-2013247715-A1

Title: Crankshaft for an internal combustion engine

Description:
FIELD OF THE INVENTION 
     Exemplary embodiments of the invention relate to crankshafts for internal combustion engines and, more particularly, to a crankshaft for an internal combustion engine having a grouping of six crankpins, in which the crankpins are disposed asymmetrically about the crankshaft axis of rotation. 
     BACKGROUND 
     With the increased focus on vehicle emissions, exhaust gas recirculation (“EGR”) is utilized in many conventional internal combustion engines to assist in the reduction of throttling losses at low loads, to improve knock tolerance, and to reduce the level of oxides of nitrogen (“NO x ”) in the exhaust gas at high engine loads. EGR is especially important as an emissions reducer in internal combustion engines that run lean of stoichiometry and thereby are prone to emitting higher levels of NO x  emissions. 
     One proposition that has been considered in the construction of internal combustion engine systems is to utilize one or more of a plurality of cylinders as a dedicated source of EGR. For example, in an engine having two or more cylinders, the entire supply of exhaust gas produced in one of the cylinders is transferred to the intake ports of the other cylinders as EGR. In engines having greater numbers of cylinders (e.g., 4, 6, or 8 cylinders), timing considerations may cause it to be advantageous to dedicate up to half of the cylinders (i.e., 2, 3, or 4 cylinders) to the production of EGR. 
     A disadvantage to this type of internal combustion engine system is that an internal combustion engine that dedicates the use of one or more cylinders to production of EGR may not deliver EGR uniformly to the remaining cylinders. For example, the cylinder event following the dedicated EGR cylinder event may be prone to receive more EGR diluent than the subsequently firing cylinders. These variations in cylinder makeup (i.e. combustion air, fuel and EGR diluent) can result in uneven combustion performance that is difficult to control over a broad range of operating conditions. In addition, engines having displacements that are uniform among the cylinders, may be incapable of precisely delivering desired quantities of EGR. 
     To at least partially address these disadvantages, a number of configurations are being studied, including configurations wherein more than one in four cylinders operates as a dedicated EGR cylinder or where a dedicated EGR cylinder produces more than a single volume of exhaust gas for every four volumes of exhaust gas produced by other cylinders. To enable such configurations, it would be advantageous to have a crankshaft that can facilitate improved distribution of EGR among non-EGR cylinders. 
     SUMMARY 
     In an exemplary embodiment, a crankshaft for an internal combustion engine comprises at least four main journals aligned on a crankshaft axis of rotation and at least six crankpins, each being disposed about a respective crankpin axis and positioned between the at least four main journals. Each of the respective crankpin axes is oriented parallel to, and spaced radially from, the crankshaft axis of rotation. Each of the at least six crankpins is joined to a pair of crank arms for force transmission between each of the at least six crankpins and the respective pair of crank arms. Each crank arm is joined to a respective main journal for transmitting torque between the crank arm and the main journal. The at least six crankpins are disposed asymmetrically about the crankshaft axis of rotation. 
     The above features and advantages, and other features and advantages of the invention are readily apparent from the following detailed description of the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, advantages and details appear, by way of example only, in the following detailed description of the embodiments, the detailed description referring to the drawings in which: 
         FIG. 1  is a schematic plan view of portions of an internal combustion engine system embodying features of the invention; 
         FIG. 2  is a schematic end view of portions of an internal combustion engine system embodying features of another embodiment of the invention; 
         FIG. 3  is a schematic side view of portions of a crankshaft of an internal combustion engine system embodying features of another embodiment of the invention; 
         FIG. 4  is a schematic end view of portions of a crankshaft of an internal combustion engine system embodying features of another embodiment of the invention; 
         FIG. 5  is a schematic end view of portions of a crankshaft of an internal combustion engine system embodying features of another embodiment of the invention; 
         FIG. 6  is a schematic end view of portions of a crankshaft of an internal combustion engine system embodying features of another embodiment of the invention; and 
         FIG. 7  is a schematic end view of portions of a crankshaft of an internal combustion engine system embodying features of another embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     Referring now to  FIG. 1  and  FIG. 2 , an exemplary embodiment of the invention is directed to an internal combustion engine system  10  comprising a pair of dedicated EGR-producing cylinders  12 ,  14  arranged in a left bank  16 . Engine system  10  also comprises an EGR-consuming cylinder  18  that is also arranged in the left bank  16  and three additional EGR-consuming cylinders  20 ,  22 ,  24  arranged in a right bank  26 . Thus, in the embodiment illustrated, the internal combustion engine system  10  includes two EGR-producing cylinders  12 ,  14  and four EGR-consuming cylinders  18 ,  20 ,  22 ,  24 , however the configuration may also include any combination of number of EGR-producing cylinders and EGR-consuming cylinders (ex. 3, 4, 5, 6, 8, 10, 12, etc.) as well as configurations such as V-configured, horizontally opposed and the like, without affecting the application of the invention thereto. In an exemplary embodiment, both the EGR-producing cylinders  12 ,  14  are configured to operate on a two-stroke combustion cycle, and all four of the EGR-consuming cylinders  18 ,  20 ,  22 ,  24  are configured to operate on a four-stroke combustion cycle. In an exemplary embodiment, both of the EGR-producing cylinders  12 ,  14  are positioned adjacent to one another in the left bank  16  so as to facilitate a simplified arrangement of passages for carrying the EGR gases from the EGR-producing cylinders  12 ,  14  to the EGR-consuming cylinders  18 ,  20 ,  22 ,  24 . 
     In an exemplary embodiment, combustion air  28  is compressed by a compressor  30 , which may comprise an engine driven supercharger, an exhaust driven turbocharger or a combination of both (i.e. super-turbocharger), before being delivered to each of the EGR-producing cylinders  12 ,  14  through intake runners  32 ,  34 . The intake runners  32 ,  34  deliver the compressed combustion air to the EGR-producing cylinders  12 ,  14  through intake ports  35 . The combustion air  28  is mixed with an EGR-producing flow of fuel  36  in the EGR-producing cylinders  12 ,  14  and is combusted therein. One or more ignition devices such as spark plugs  38  are located in communication with the EGR-producing cylinders  12 ,  14  and operate to ignite the fuel/air mixture therein at appropriate times. 
     In an exemplary embodiment, recirculation exhaust gas  40  from the combustion of the EGR-producing flow of fuel  36  and combustion air  28  in the EGR-producing cylinders  12 ,  14  is removed from each EGR-producing cylinder  12 ,  14  through one or more exhaust ports  42  in fluid communication with an EGR conduit  44 . EGR conduit  44  carries recirculation exhaust gas  40  from exhaust ports  42  and through a heat exchanger  46  to produce a cooled stream of EGR gas  48 . The heat exchanger  46  may be of an air cooled or liquid cooled configuration. In an exemplary embodiment, the cooled stream of EGR gas  48  is combined with a stream of fresh air  6  to form the stream of combustion air  28 , which is delivered to each of the EGR-consuming cylinders  18 ,  20 ,  22 ,  24  through intake runners  50 ,  52 ,  54 ,  56 . 
     The intake runners  50 ,  52 ,  54 ,  56  deliver the combustion air  28  to the EGR-consuming cylinders  18 ,  20 ,  22 ,  24  through intake ports  58 . The combustion air  28  is mixed with an EGR-consuming flow of fuel  60  in the EGR-consuming cylinders  18 ,  20 ,  22 ,  24  and is combusted therein. One or more ignition devices such as spark plugs  62  are located in communication with the EGR-consuming cylinders  18 ,  20 ,  22 ,  24  and operate to ignite the fuel/air mixture therein at appropriate times. 
     In an exemplary embodiment, discharge exhaust gas  64  from the combustion of the EGR-consuming flow of fuel  60  and combustion air  28  in the EGR-consuming cylinders  18 ,  20 ,  22 ,  24  is removed from each EGR-consuming cylinder  18 ,  20 ,  22 ,  24  through one or more exhaust ports  66  in fluid communication with a discharge exhaust system  68 . Discharge exhaust system  68  carries discharge exhaust gas  64  from exhaust ports  66  and through an exhaust treatment system  70  prior to being released to the atmosphere. The exhaust treatment system  70  may include various exhaust gas treatment devices such as a catalytic converter, a selective catalytic reduction device, a particulate trap or a combination thereof. 
     In an exemplary embodiment, the quantity of fuel mixed with the combustion air  28  in each of the EGR-producing cylinders  12 ,  14  is controlled such that each of the EGR-producing cylinders  12 ,  14  is operated at a customized level of air and fuel, as may be determined by an engine controller that is in signal communication with various engine, vehicle and exhaust system sensors. Since the exhaust gas discharged from the EGR-producing cylinders  12 ,  14  is to be ingested in one of the EGR-consuming cylinders  18 ,  20 ,  22 ,  24  before being released to the atmosphere, the customized air and fuel levels in each of the EGR-producing cylinders  12 ,  14  may be optimized to achieve selected goals such as engine efficiency, power, and operability. Accordingly, the EGR-producing cylinders are at least partially freed from encumbrances related to regulated constituents in gases they discharge. 
     Since exhaust gas produced by the EGR-consuming cylinders  18 ,  20 ,  22 ,  24  is to be released to the atmosphere, either directly or following treatment in an exhaust gas treatment system, the air and fuel mixtures of these EGR-consuming cylinders  18 ,  20 ,  22 ,  24  may be operated so as to meet a number of goals, such as engine efficiency, power, and operability, in addition to emission standards. The EGR-consuming cylinders  18 ,  20 ,  22 ,  24  enjoy benefits associated with ingestion of EGR from the EGR-producing cylinders  12 . These benefits include reduced combustion temperatures and associated levels of NO x , allowing richer levels of EGR in the remaining cylinders with increased levels of hydrogen, thereby improving knock resistance, fuel consumption and combustion stability, while still allowing stoichiometric gas to be maintained in the exhaust gas treatment system for compatibility with the catalytic treatment devices. Accordingly, the re-ingestion of exhaust gas discharged from the EGR-producing cylinders  12 ,  14  assists in the reduction of throttling losses at low loads and improves knock tolerance while reducing levels of oxides of nitrogen (“NO x ”) in the discharge exhaust gas  64 . 
     In an exemplary embodiment, the EGR-producing cylinders  12 ,  14  and the EGR-consuming cylinders  18 ,  20 ,  22 ,  24  interact with a rotating group that comprises pistons (not shown) that are each associated with a respective cylinder and connected through a respective connecting rod (not shown) to a respective crankpin, the crankpins being disposed on a single crankshaft. In an exemplary embodiment, a central axis defined by each EGR-producing cylinder  12 ,  14  and the EGR-consuming cylinder  18  arranged in left bank  16  is parallel to, and coplanar with, each other central axis defined by those three cylinders  12 ,  14 ,  18  arranged in left bank  16 . Thus, the central axes of the cylinders  12 ,  14 ,  16  of the left bank define a left bank plane  15 . In an exemplary embodiment, a central axis defined by each of the EGR-consuming cylinders  20 ,  22 ,  24  arranged in right bank  26  is parallel to, and coplanar with, each other central axis defined by each other cylinder  16  arranged in right bank  26 . Thus, the central axes of the EGR-consuming cylinders  20 ,  22 ,  24  define a right bank plane  19 . In a V-configured embodiment, the left bank plane  15  and the right bank plane  19  intersect approximately at a crankshaft axis of rotation and form an engine block angle  21  between the left bank plane  15  and the right bank plane  19 . In an exemplary embodiment, the engine block angle  21  is approximately 90 degrees. In another exemplary embodiment, the engine block angle  21  is approximately 60 degrees. 
     In an exemplary embodiment, as shown in  FIG. 3 , a crankshaft  300  for an internal combustion engine comprises a plurality of main journals  302 ,  304 ,  306 ,  308  aligned sequentially on a crankshaft axis of rotation  310 . A first crankpin  312  is disposed about a first crankpin axis  314  and positioned between the first main journal  302  and the second main journal  304 . A second crankpin  316  is disposed about a second crankpin axis  318  and is also positioned between the first main journal  302  and the second main journal  304 . A third crankpin  320  is disposed about a third crankpin axis  322  and positioned between the second main journal  304  and the third main journal  306 . A fourth crankpin  324  is disposed about a fourth crankpin axis  326  and is also positioned between the second main journal  304  and the third main journal  306 . A fifth crankpin  328  is disposed about a fifth crankpin axis  330  and positioned between the third main journal  306  and the fourth main journal  308 . A sixth crankpin  332  is disposed about a sixth crankpin axis  334  and is also positioned between the third main journal  306  and the fourth main journal  308 . Thus, each of the six crankpins  312 ,  316 ,  320 ,  324 ,  328 ,  332  is disposed about a respective crankpin axis  314 ,  318 ,  322 ,  326 ,  330 ,  334  and positioned between two of the four main journals  302 ,  304 ,  306 ,  308 . In an exemplary embodiment, each crankpin axis  314 ,  318 ,  322 ,  326 ,  330 ,  334  is spaced radially a semi-stroke distance  34  from the crankshaft axis of rotation  310 . 
     A first plurality of crank arms  336  is joined to first crankpin  312  and second crankpin  316  for force transmission among first crankpin  312 , second crankpin  316 , and the first plurality of crank arms  336 . In an exemplary embodiment, each of the crank arms  336  is also joined to a respective main journal  302 ,  304  for transmitting torque among the first plurality of crank arms  336  and the main journals  302 ,  304 . A second plurality of crank arms  338  is joined to third crankpin  320  and fourth crankpin  324  for force transmission among third crankpin  320 , fourth crankpin  324 , and the second plurality of crank arms  338 . In an exemplary embodiment, each of the crank arms  338  is also joined to a respective main journal  304 ,  306  for transmitting torque among the second plurality of crank arms  338  and the main journals  304 ,  306 . A third plurality of crank arms  340  is joined to fifth crankpin  328  and sixth crankpin  332  for force transmission among fifth crankpin  328 , sixth crankpin  332 , and the third plurality of crank arms  340 . In an exemplary embodiment, each of the crank arms  340  is also joined to a respective main journal  306 ,  308  for transmitting torque among the third plurality of crank arms  340  and the main journals  306 ,  308 . 
     In an exemplary embodiment, the right bank  26  ( FIGS. 1 and 2 ) includes three EGR-consuming cylinders  20 ,  22 ,  24 . The first crankpin  312  ( FIG. 3 ) is mechanically coupled to a piston (not shown) that interacts with a first EGR-consuming cylinder  20 . Similarly, the third crankpin  320  is mechanically coupled to a piston that interacts with a second EGR-consuming cylinder  22 , and the fifth crankpin  328  is mechanically coupled to a piston that interacts with a third EGR-consuming cylinder  24 . One of the remaining three crankpins  316 ,  324 ,  332  is mechanically coupled to a piston that interacts with a fourth EGR-consuming cylinder  18 . The remaining two crankpins (either  316  and  324  or  324  and  332 ) are mechanically coupled to a piston that interacts with first and second EGR-producing cylinders  12 ,  14 . 
     As discussed above, the EGR-producing cylinders  12 ,  14  are to be operated differently from the EGR-consuming cylinders  18 ,  20 ,  22 ,  24 . For example, the EGR-producing cylinders  12 ,  14  are to be operated at different ratios of fuel to air than the EGR-consuming cylinders  18 ,  20 ,  22 ,  24 . In addition, each of the EGR-producing cylinders  12 ,  14  is to be operated on a two stroke cycle. Thus, each of the EGR-producing cylinders  12 ,  14  will undergo two combustion events for each single combustion event associated with the EGR-consuming cylinders  18 ,  20 ,  22 ,  24 . As a result, there will be equivalent numbers of combustion events among the EGR-producing cylinders  12 ,  14  and the EGR-consuming cylinders  18 ,  20 ,  22 ,  24 . In order to provide adequate quantities of EGR gas  48  to the EGR-consuming cylinders  18 ,  20 ,  22 ,  24 , it is desirable to schedule the combustion events among the cylinders such that each combustion event of an EGR-consuming cylinder is immediately preceded by a combustion event in an EGR-producing cylinder  12 ,  14 . 
     In addition, it is desirable that the crankpins  312 ,  316 ,  320 ,  324 ,  328 , and  332  be arranged to enable, with respect to their associated cylinders, a “near-even fire” combustion sequence. Thus, in an exemplary embodiment with two EGR-producing cylinders  12 ,  14  and four EGR consuming cylinders  18 ,  20 ,  22 ,  24 , eight nearly evenly spaced firing events are produced in about 720 degrees of rotation of the crankshaft. In one embodiment, the firing sequence is such that a firing event occurs at approximately even intervals associated with each 90 degrees of rotation of the crankshaft. According to this fairly precise even-firing embodiment, firing events occur at 0 degrees, 90 degrees, 180 degrees, 270 degrees, 360 degrees, 450 degrees, 540 degrees, 630 degrees, and again at 720 degrees. In another embodiment, firing events associated with one of the EGR-producing cylinders is delayed approximately 30 degrees from the standard interval while firing events associated with the other cylinders occurs on the 90 degree interval. According to this less precise even-firing embodiment, firing events occur at 0 degrees, 120 degrees, 180 degrees, 270 degrees, 360 degrees, 480 degrees, 540 degrees, 630 degrees, and again at 720 degrees. 
     As discussed above, it is desirable to have EGR-producing cylinders  12 ,  14  positioned adjacent to one another, and this is facilitated by coupling pistons that interact with the first and second EGR-producing cylinders  12 ,  14  to either: (1) the second crankpin  316  and the fourth crankpin  324 ; or (2) the fourth crankpin  324  and the sixth crankpin  332 . The first crankpin  312  is coupled to a first EGR-consuming cylinder  20  in the right bank  26 . The third crankpin  320  is coupled to a second EGR-consuming cylinder  22  in the right bank  26 , and the fifth crankpin  328  is coupled to a third EGR-consuming cylinder  24  in the right bank  26 . Either the second crankpin  316  or the sixth crankpin  332  is coupled to the EGR-consuming cylinder  18  in the left bank  16 . With respect to each cylinder, a respective crankpin is coupled, through a connecting rod (not shown), to a piston (not shown) that is disposed in either the respective cylinder. Thus, as crankshaft  300  rotates about the crankshaft axis of rotation  310 , each crankpin associated with a piston in an EGR-producing cylinder interacts with working fluid (i.e., fuel, air) in the respective EGR-producing cylinder and encounters a combustion event once for every 360 degrees of crankshaft rotation. Similarly, as crankshaft  300  rotates about the crankshaft axis of rotation  310 , each crankpin associated with a piston in an EGR-consuming cylinder interacts with working fluid (i.e., fuel, air and EGR mixture) in the respective EGR-consuming cylinder and encounters a combustion event once for every 720 degrees of crankshaft rotation. 
     Where the second crankpin  316  and the fourth crankpin  324  are coupled to pistons that interact with the first and second EGR-producing cylinders  12 ,  14 , respectively, an exemplary firing order facilitating either a fairly precise even-firing embodiment or a less precise even-firing embodiment includes: (1) a firing event associated with the first EGR-consuming cylinder  20  located in right bank  26  occurring at a crankshaft rotational position of approximately 0 degrees; (2) a firing event associated with the first EGR-producing cylinder  12  occurring at a crankshaft rotational position of approximately 90 or 120 degrees; (3) a firing event associated with the second EGR-consuming cylinder  22  located in right bank  26  occurring at a crankshaft rotational position of approximately 180 degrees; (4) a firing event associated with the second EGR-producing cylinder  14  occurring at a crankshaft rotational position of approximately 270 degrees; (5) a firing event associated with the third EGR-consuming cylinder  24  located in right bank  26  occurring at a crankshaft rotational position of approximately 360 degrees; (6) a firing event associated with the first EGR-producing cylinder  12  occurring at a crankshaft rotational position of approximately 450 or 480 degrees; (7) a firing event associated with the EGR-consuming cylinder  18  located in left bank  16  occurring at a crankshaft rotational position of approximately 540 degrees; and (8) a firing event associated with the second EGR-producing cylinder  14  occurring at a crankshaft rotational position of approximately 630 degrees. 
     To facilitate such a firing sequence, with an engine block angle  21  of 90 degrees, as shown in  FIG. 4 , the crankpins  312 ,  316 ,  320 ,  324 ,  328 , and  332  are arranged asymmetrically about the crankshaft axis of rotation as follows: (1) the first crankpin  312  is arranged at a rotational position of approximately 0 degrees; (2) the second crankpin  316  is arranged at a rotational position of approximately 180 degrees (i.e., the second crankpin  316  is disposed approximately 180 degrees from the position of the first crankpin  312 ) for a fairly precise even-firing embodiment or approximately 210 degrees for a less precise even-firing embodiment; (3) the third crankpin  320  is arranged at a rotational position of approximately 180 degrees; (4) the fourth crankpin  324  is arranged at a rotational position of approximately 0 degrees; (5) the fifth crankpin  328  is arranged at a rotational position of approximately 0 degrees; and (6) the sixth crankpin  332  is arranged at a rotational position of approximately 270 degrees. 
     To facilitate such a firing sequence, with an engine block angle  21  of 60 degrees, as shown in  FIG. 5 , the crankpins  316 ,  324 , and  332  interacting with the cylinders of the left bank  16  are shifted 30 degrees about the crankshaft axis of rotation. Accordingly, the crankpins  312 ,  316 ,  320 ,  324 ,  328 , and  332  are arranged asymmetrically about the crankshaft axis of rotation as follows: (1) the first crankpin  312  is arranged at a rotational position of approximately 0 degrees; (2) the second crankpin  316  is arranged at a rotational position of approximately 150 degrees (for a fairly precise even-firing embodiment) or approximately 180 degrees (for a less precise even-firing embodiment); (3) the third crankpin  320  is arranged at a rotational position of approximately 180 degrees; (4) the fourth crankpin  324  is arranged at a rotational position of approximately 330 degrees; (5) the fifth crankpin  328  is arranged at a rotational position of approximately 0 degrees; and (6) the sixth crankpin  332  is arranged at a rotational position of approximately 240 degrees. 
     Where the fourth crankpin  324  and the sixth crankpin  332  are coupled to pistons that interact with the first and second EGR-producing cylinders  12 ,  14 , respectively, an exemplary firing order facilitating either a fairly precise even-firing embodiment or a less precise even-firing embodiment includes: (1) a firing event associated with the first EGR-consuming cylinder  20  located in right bank  26  occurring at a crankshaft rotational position of approximately 0 degrees; (2) a firing event associated with the second EGR-producing cylinder  14  occurring at a crankshaft rotational position of approximately 90 or 120 degrees; (3) a firing event associated with the third EGR-consuming cylinder  24  located in right bank  26  occurring at a crankshaft rotational position of approximately 180 degrees; (4) a firing event associated with the first EGR-producing cylinder  12  occurring at a crankshaft rotational position of approximately 270 degrees; (5) a firing event associated with the second EGR-consuming cylinder  22  located in right bank  26  occurring at a crankshaft rotational position of approximately 360 degrees; (6) a firing event associated with the second EGR-producing cylinder  14  occurring at a crankshaft rotational position of approximately 450 or 480 degrees; (7) a firing event associated with the EGR-consuming cylinder  18  located in left bank  16  occurring at a crankshaft rotational position of approximately 540 degrees; and (8) a firing event associated with the first EGR-producing cylinder  12  occurring at a crankshaft rotational position of approximately 630 degrees. 
     To facilitate such a firing sequence, with an engine block angle  21  of 90 degrees, as shown in  FIG. 6 , the crankpins  312 ,  316 ,  320 ,  324 ,  328 , and  332  are arranged asymmetrically about the crankshaft axis of rotation as follows: (1) the first crankpin  312  is arranged at a rotational position of approximately 0 degrees; (2) the second crankpin  316  is arranged at a rotational position of approximately 270 degrees; (3) the third crankpin  320  is arranged at a rotational position of approximately 0 degrees; (4) the fourth crankpin  324  is arranged at a rotational position of approximately 0 degrees; (5) the fifth crankpin  328  is arranged at a rotational position of approximately 180 degrees; and (6) the sixth crankpin  332  is arranged at a rotational position of approximately 180 degrees (for a fairly precise even-firing embodiment) or approximately 210 degrees (for a less precise even-firing embodiment). 
     To facilitate such a firing sequence, with an engine block angle  21  of 60 degrees, as shown in  FIG. 7 , the crankpins  316 ,  324 , and  332  interacting with the cylinders of the left bank  16  are shifted 30 degrees about the crankshaft axis of rotation. Accordingly, the crankpins the crankpins  312 ,  316 ,  320 ,  324 ,  328 , and  332  are arranged asymmetrically about the crankshaft axis of rotation as follows: (1) the first crankpin  312  is arranged at a rotational position of approximately 0 degrees; (2) the second crankpin  316  is arranged at a rotational position of approximately 240 degrees; (3) the third crankpin  320  is arranged at a rotational position of approximately 0 degrees; (4) the fourth crankpin  324  is arranged at a rotational position of approximately 330 degrees; (5) the fifth crankpin  328  is arranged at a rotational position of approximately 180 degrees; and (6) the sixth crankpin  332  is arranged at a rotational position of approximately 150 degrees (for a fairly precise even-firing embodiment) or approximately 180 degrees (for a less precise even-firing embodiment). 
     Thus, a “near-even fire” combustion sequence is facilitated, whereby, in the case of a 6-cylinder internal combustion engine, with two cylinders operating on a 2-stroke cycle and four cylinders operating on a four-stroke cycle, eight nearly evenly spaced firing events occur in about 720 degrees of rotation of the crankshaft. The invention has been described above primarily with reference to its application in a 6-cylinder engine. It should be clear to one skilled in the art of internal combustion engines that engines of other cylinder numbers, and varied configurations, can easily be envisaged and that the invention should not, and cannot be limited to those examples provided herein. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the present application.