Abstract:
An exhaust system for use with a four-stroke engine having first and second cylinders. The cylinders generate exhaust pulses at uneven time intervals with respect to each other. The exhaust system includes a muffler and first and second exhaust pipes that communicate between the mufflers and the first and second cylinders, respectively. The first and second pipes being sized and shaped to deliver the exhaust pulses to the muffler at even time intervals with respect to each other.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to motorcycles, and more specifically, to motorcycles having exhaust systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Known motorcycles include four-stroke internal combustion engines that include pistons reciprocating within cylinders defining combustion chambers. The pistons each execute four strokes for each cycle of the engine. The strokes are compression, expansion, exhaust, and intake. The piston moves in a first direction during the compression and exhaust strokes, and in a second opposite direction during the expansion and intake strokes.  
           [0003]    Exhaust systems are connected to the cylinders in order to direct the exhaust and reduce engine noise. Exhaust systems usually include a muffler and headers, or exhaust pipes, that are connected between the cylinders and the muffler. When an exhaust valve opens to begin the exhaust stroke of the piston, pressure pulses are created by the burst of high-pressure gas that suddenly escapes out of the combustion chamber and into the exhaust system. The pressure pulses, or sound waves, travel through the headers to the muffler where the amplitudes of the pressure pulses are reduced.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention is directed to an exhaust system that improves engine power by receiving pressure pulses from an engine at uneven time intervals and delivering the pressure pulses to a common muffler at even time intervals. By delivering the pressure pulse to the muffler at even time intervals, the torque and power of the engine is increased for a range of engine speeds. In addition, a smaller capacity muffler may be used, because when the pulses reach the muffler at evenly-spaced time intervals, the pressure pulses evenly share the muffler.  
           [0005]    One embodiment of the present invention is an exhaust system for a four-stroke engine. The engine includes first and second cylinders capable of producing pressure pulses. The engine includes a  360  degree cycle that begins with a first pressure pulse produced by the first cylinder, includes a second pressure pulse produced by the second cylinder, and ends with a next consecutive pressure pulse produced by the first cylinder. The phase difference between the production of the first pressure pulse and the production of the second pressure pulse is substantially less than  180  degrees. The exhaust system includes a muffler and first and second headers. The first header conducts the first pressure pulse from the first cylinder to the muffler, and the second header conducts the second pressure pulse from the second cylinder to the muffler. The first and second headers are configured such that, while the engine operates in a range of engine speeds, the first and second pressure pulses enter the muffler substantially  180  degrees out of phase with each other to improve the power of the engine within the range of normal engine operating speeds.  
           [0006]    Another embodiment of the invention includes a method for designing an exhaust system for a motorcycle including an engine. The method includes determining a first phase difference between the production of a first pressure pulse and the production of a second pressure pulse, and configuring first and second headers to maximize a phase difference between the first and second pressure pulses at the muffler for a range of engine speeds such that, while the engine operates in the range of engine speeds, the first and second pressure pulses enter the muffler at substantially  180  degrees out of phase with each other to increase the power of the engine within the range of engine cycle speeds.  
           [0007]    One exhaust system that embodies the invention is used with a four-stroke 45 degree V-twin engine having first and second cylinders that generate exhaust pulses at uneven time intervals with respect to each other. The exhaust system includes a muffler and first and second exhaust pipes that communicate between the mufflers and the first and second cylinders, respectively. The first and second pipes are sized and shaped to deliver the exhaust pulses to the muffler at even time intervals with respect to each other in a selected range of normal operating speeds.  
           [0008]    Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a side view of a motorcycle that includes an exhaust system embodying the invention.  
         [0010]    [0010]FIG. 2 is a perspective view of exhaust headers having substantially equal lengths.  
         [0011]    [0011]FIG. 3 is an enlarged perspective view of the exhaust headers shown in FIG. 1.  
         [0012]    [0012]FIG. 4 a  is a schematic illustration of the engine cycle of the motorcycle of FIG. 1.  
         [0013]    [0013]FIG. 4 b  is a schematic illustration of pressure pulses delivered to a muffler by the exhaust headers of FIG. 2.  
         [0014]    [0014]FIG. 4 c  is a schematic illustration of the pressure pulses delivered to the muffler by the exhaust headers of FIG. 3. 
     
    
       [0015]    Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The use of “consisting of” and variations thereof herein is meant to encompass only the items listed thereafter. The use of letters to identify elements of a method or process is simply for identification and is not meant to indicate that the elements should be performed in a particular order.  
       DETAILED DESCRIPTION  
       [0016]    [0016]FIG. 1 illustrates a motorcycle  10  including a frame  12 , front and rear wheels  14 ,  16 , a seat  18 , and an engine  20 . The front and rear wheels  14 ,  16  rotate with respect to the frame  12  and support the frame  12  above the ground. The engine  20  is mounted to the frame  12  and drives the rear wheel  16  through a transmission  22  and drive belt  24 . The seat  18  is mounted to the frame  12  and a fuel tank  26  is integrally formed with the frame  12 .  
         [0017]    Although the illustrated engine  20  is an air-cooled four stroke 45 degree V-twin engine having first and second cylinders  28 ,  30 , the invention may be embodied in other types of engines, such as multi-cylinder engines of either the water-cooled or air-cooled variety. Additionally, although the drawings illustrate the first and second cylinders  28 ,  30  as the front and rear cylinders, respectively, the invention may be embodied in an engine that has the cylinders positioned side-by-side rather than one behind the other. The invention may also be used in other uneven firing engines other than the  45  degree V-twin engine. The term “uneven firing,” as used herein, means that the cylinders fire at unevenly spaced intervals during rotations of the crankshaft (e.g., at 315 degrees of crankshaft rotation and again at  405  degrees of crankshaft rotation), as compared to even firing engines which fire at evenly spaced intervals (e.g., every 360 degrees of crankshaft rotation).  
         [0018]    The motorcycle  10  includes an exhaust system  32  that directs the exhaust and reduces the noise created by the engine  20 . The exhaust system  32  includes a muffler  34  and first and second exhaust pipes, or headers  36 ,  38 . The first header  36  is connected between the first cylinder  28  and the muffler  34 , and the second header  38  is connected between the second cylinder  30  and the muffler  34 . The headers  36 ,  38  receive pressure pulses A, B (FIGS. 4 a ,  4   b , and  4   c ) generated by the cylinders  28 ,  30  at uneven time intervals and deliver the pressure pulses A, B to the muffler  34  at even time intervals.  
         [0019]    [0019]FIG. 4 a  is a schematic representation of a 360 degree engine cycle. It should be noted that, in the illustrated embodiment, the engine cycle includes two full rotations of the crankshaft (not shown). Therefore, the  360  degree engine cycle includes 720 degrees of crankshaft rotation.  
         [0020]    The engine cycle begins with a first pressure pulse A created by the first cylinder  28  and ends with a next consecutive pressure pulse A′ generated by the first cylinder  28 . The engine cycle includes a second pressure pulse B generated by the second cylinder  30 . For the purpose of this discussion, each pressure pulse A, B is considered “generated” at the beginning of the exhaust stroke within each respective cylinder  28 ,  30 . In the illustrated embodiment, the second pressure pulse B is created 157.5 degrees out of phase with the first pressure pulse A as dictated by the uneven firing of the 45 degree V-twin engine  20  (i.e., the first pressure pulse A at  0  degrees and the second pressure pulse B at 157.5 degrees). Alternatively, an even firing engine would create the pressure pulses at a maximum 180 degree out of phase relationship. The difference between the maximum out of phase relationship (i.e., 180 degrees) and the out of phase relationship determined by the uneven firing engine  20  (i.e., 157.5 degrees) is shown schematically as phase gap δ.  
         [0021]    [0021]FIG. 4 b  schematically represents the phase relationship between the pressure pulses A, B reaching the muffler  34  after travelling through the headers  36 ,  38  illustrated in FIG. 2. The headers  36 ,  38  illustrated in FIG. 2 have equal cross-sectional areas and equal lengths, causing the pressure pulses A, B to travel at equal speeds and to maintain their phase relationship throughout the entire length of the headers  36 ,  38 . This phase difference remains constant for any engine speed.  
         [0022]    In contrast, FIG. 4 c  schematically represents the phase relationship between the pressure pulses A, B reaching the muffler  34  after travelling through the headers  36 ,  38  illustrated in FIGS. 1 and 3. The headers  36 ,  38  illustrated in FIGS. 1 and 3 have equal cross-sectional areas (e.g., 1.5 square inches in the illustrated embodiment) but have unequal lengths. Specifically, the illustrated first header  36  is  17  inches long and the illustrated second header  38  is 22.1 inches long such that the second pulse B travels through a greater distance than the first pressure pulse A. The greater distance of travel creates a lag for the second pressure pulse B, thereby increasing the phase relationship at the muffler  34 . The same would hold true if the headers  36 ,  38  had equal lengths, but the first header  36  had a smaller cross-sectional area than the second header  38 .  
         [0023]    It should be noted that because of the perspective from which FIGS. 2 and 3 are drawn, it is difficult to visually distinguish that the headers in FIG. 3 have equal lengths, and that the headers in FIG. 2 have unequal lengths. Irregardless, FIG. 3 illustrates headers having equal lengths and FIG. 2 illustrates headers having unequal lengths.  
         [0024]    Unlike the headers  36 ,  38  which have equal cross-sectional areas and lengths, the headers  36 ,  38  with unequal cross-sectional areas or lengths do not have a constant phase relationship at the muffler  34 . Instead, the phase relationship at the muffler  34  is dependent upon the engine speed. The difference in header lengths compensates for  6  so that the pressure pulses A, B reach the muffler 180 degrees out of phase for a specific engine speed.  
         [0025]    For example, when the engine  20  is running at 5000 rpm, or 2500 engine cycles per minute as illustrated in FIG. 4 c , δ defines a specific time interval T. The increased length of the second header  38  extends the travel distance for the second pressure pulse B to delay the second pressure pulse B for a period of time equal to T. As the engine  20  runs slower than 2500 engine cycles per minute, T increases and the phase relationship at the muffler  34  decreases from 180 degrees. As the engine speed increases from 2500 engine cycles per minute, T decreases and the phase relationship at the muffler  34  increases from 180 degrees.  
         [0026]    Therefore, by knowing the phase difference δ and the preferred operating speed of the engine  20  (e.g., 2500 engine cycles per minute), the power at the operating speed can be increased by configuring the headers  36 ,  38  so that second pressure pulse B is delayed by time T and the pressure pulses A, B reach the muffler  180  degrees out of phase when the engine operates at the preferred operating speed. The second pressure pulse B can be delayed by increasing the relative length of the second header  38  compared to the first header  36 , by increasing the cross-sectional area of the second header  38  relative to the first header  36 , or by modifying the cross-sectional area and the length such that the second pressure pulse B is delayed the required amount of time T.