Patent Publication Number: US-2019170055-A1

Title: Two-stroke engine with improved performance

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/594,876, filed Dec. 5, 2017, the entire contents of which are hereby incorporated by reference. 
    
    
     FIELD OF INVENTION 
     The present disclosure relates to engines, and more particularly to two-stroke engines. 
     SUMMARY 
     In one embodiment, a two-stroke engine includes a crankcase. A power cylinder extends from the crankcase. The power cylinder selectively fluidly communicates with the crankcase to scavenge the power cylinder. A power piston is reciprocably disposed in the power cylinder. A compressor cylinder extends from the crankcase. The compressor cylinder selectively fluidly communicates with the power cylinder to introduce a pressurized air/fuel mixture into the power cylinder. A compressor piston is reciprocably disposed in the compressor cylinder. 
     In another embodiment, a two-stroke engine includes a crankcase. A first cylinder extends from the crankcase along a first cylinder axis. A first piston is reciprocably disposed in the first cylinder. A second cylinder extends from the crankcase along a second cylinder axis. The second cylinder axis is angled with respect to the first cylinder axis. A second piston is reciprocably disposed in the second cylinder. A conduit selectively fluidly connects the second cylinder with the first cylinder to introduce a pressurized air/fuel mixture into the first cylinder. 
     Other features and aspects of the disclosure will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an engine according to an embodiment of the invention. 
         FIG. 2  is a perspective view of an outdoor power tool in which the engine of  FIG. 1  may be incorporated. 
         FIG. 3  is a cross-sectional view of the engine of  FIG. 1 , with a power piston of the engine illustrated in a top-dead-center position. 
         FIG. 4  is a cross-sectional view of the engine of  FIG. 1 , with the power piston illustrated in a position about one hundred ten degrees past top-dead-center. 
         FIG. 5  is a cross-sectional view of the engine of  FIG. 1 , with the power piston illustrated in a bottom-dead-center position. 
         FIG. 6  is a cross-sectional view of the engine of  FIG. 1 , with the power piston illustrated in a position about two hundred thirty degrees past top-dead-center. 
         FIG. 7  is a cross-sectional view of the engine of  FIG. 1 , with the power piston illustrated in a position about two hundred seventy degrees past top-dead center. 
         FIG. 8  is a cross-sectional view of the engine of  FIG. 1 , with the power piston illustrated in a position about three hundred degrees past top-dead center. 
         FIG. 9  is a cross-sectional view of the engine of  FIG. 1 , with the power piston illustrated in a position about three hundred forty degrees past top-dead center. 
         FIG. 10  is a cross-sectional view of an engine according to another embodiment of the invention. 
     
    
    
     Before any embodiments of the disclosure are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION 
     Two-stroke engines (also known as two-cycle engines) are used in a wide variety of applications and are typically less costly and provide more power for a given displacement than four-stroke engines. However, typical two-stroke engines may produce greater hydrocarbon emissions due to unburned fuel escaping with the engine exhaust. Improving the performance of a two-stroke engine may allow for an engine that has lower cost and more power for a given displacement than an equivalent four-stroke engine while potentially reducing the hydrocarbon emissions relative to previous two-stroke engines. Additionally or alternatively, the two-stroke engine may provide more power than traditional two-stroke engines of the same displacement. 
       FIG. 1  illustrates an engine  10  according to one embodiment of the invention. The illustrated engine  10  is a compact, two-stroke engine particularly suitable for use with an outdoor power tool, such as a string trimmer  12  illustrated in  FIG. 2 . The engine  10  may also be used with other types of outdoor power tools (e.g., chainsaws, blowers, etc.), generators, or in any other small engine application. 
     With reference to  FIGS. 1 and 3 , the engine  10  includes first and second cylinders  14 ,  18 , each defining a respective cylinder axis  20 ,  21 . In the illustrated embodiment, the cylinders  14 ,  18  are arranged perpendicular to each other in a V-configuration, with the cylinder axes  20 ,  21  oriented perpendicularly. In other embodiments, the cylinders  14 ,  18  may have other orientations, including, for example, narrower or wider V-configurations, an in-line configuration, etc. The first cylinder  14  is configured as a power cylinder where an air-fuel mixture is ignited to power the engine  10  and drive an output shaft  19  ( FIG. 1 ). The second cylinder  18  is configured as a compressor cylinder where an air-fuel mixture is compressed before being injected into the power cylinder  14 . The compressor cylinder  18  thus acts as a supercharger, increasing the mass flow of air and fuel into the power cylinder  14  and producing a corresponding increase in engine power. 
     The engine  10  further includes a first piston or power piston  22  received within the power cylinder  14  and a second piston or compressor piston  24  received within the compressor cylinder  18 . The pistons  22 ,  24  are reciprocable within the cylinders  14 ,  18  along the respective cylinder axes  20 ,  21 . In the illustrated embodiment, the power piston  22  and cylinder  14  have a displacement of about 10 cubic centimeters, and the compressor piston  24  and cylinder  18  also have a displacement of about 10 cubic centimeters. Accordingly, the power piston  22  and cylinder  14  have the same displacement as the compressor piston  24  and cylinder  18 . In other embodiments, the power piston  22  and cylinder  14  may have a different displacement than the compressor piston  24  and cylinder  18 . In addition, the engine  10  may have a larger or smaller total displacement. The power piston  22  and the compressor piston  24  preferably have the same mass. The V-configuration of the cylinders  14 ,  18  and the matched masses of the pistons  22 ,  24  advantageously reduce vibration of the engine  10  during operation. 
     With continued reference to  FIG. 3 , the pistons  22 ,  24  are eccentrically coupled to a crank  26  via respective crank arms  30 ,  32 . The crank  26  is housed within a crankcase  34 . The crankcase  34  is located at the base of the cylinders  14 ,  18  such that the cylinders  14 ,  18  generally extend from the crankcase  34 . The engine  10  further includes an intake port  38  in communication with the crankcase  34  and an exhaust port  42  in communication with the power cylinder  14 . The exhaust port  42  is coupled to an exhaust pipe  44  that discharges to the surrounding environment. The exhaust pipe  44  may include a muffler, one or more exhaust treatment elements, and the like. The intake port  38  is coupled to a feed passage  40  that receives a mixture of air and fuel from a carburetor (not shown) or other air/fuel mixing device. This air/fuel mix is supplied into the crankcase  34  through the intake port  38  as described in more detail below. 
     The power cylinder  14  includes a power cylinder head  48  at an end of the power cylinder  14  opposite the crankcase  34 , and the compressor cylinder  18  includes a compressor cylinder head  52  at an end of the compressor cylinder  18  opposite the crankcase  34 . An igniter  56  (e.g., a spark plug) extends through the power cylinder head  48  and provides a source of ignition for the air/fuel mix in the power cylinder  14 . In the illustrated embodiment, a compressor inlet passage  58  branches off the feed passage  40  and extends through the compressor cylinder head  52 . The compressor inlet passage  58  provides air/fuel mix from the feed passage  40  to the compressor cylinder  18  for subsequent compression. 
     With continued reference to  FIG. 3 , a connecting passage or conduit  60  extends from the compressor cylinder head  52  to the power cylinder  14 . The connecting passage  60  extends through a side wall of the power cylinder  14 , just below the head  48 . In other embodiments, the connecting passage  60  may connect to the power cylinder  14  through the power cylinder head  48 . A first valve  64  selectively permits fluid flow from the compressor cylinder  18  to the connecting passage  60 , and a second valve  68  selectively permits fluid flow from the connecting passage  60  to the power cylinder  14 . A third valve  70  is included in the compressor inlet passage  58  to selectively permit fluid flow from the feed passage  40  to the compressor cylinder  18 . In the illustrated embodiment, the first valve  64 , the second valve  68 , and the third valve  70  are one-way valves. The first valve  64  includes a first valve member  72  biased toward a closed position by a first spring  76 , and the second valve  68  includes a second valve member  80  biased toward a closed position by a second spring  84 . The third valve  70  may have a similar configuration. In other embodiments, other types of valves may be used. 
     The engine  10  further includes transfer ports  88  that are in communication with the crankcase  34  and that extend through a side wall of the power cylinder  14 . As described in more detail below, the transfer ports  88  provide crankcase scavenging (i.e. drawing fluid such as air or an air/fuel mixture from the crankcase  34  into the power cylinder  14  to force exhaust out of the cylinder  14  and prime the cylinder  14  for its compression stroke). 
     The relative positions of the intake port  38 , the exhaust port  42 , and the transfer ports  88  can be selected to provide a variety of advantages. For example, in the illustrated embodiment, the V-configuration of the engine  10  allows the intake port  38  to be positioned asymmetrically, resulting in asymmetric timing for air/fuel flow into the crankcase  34 . In addition, the exhaust port  42  and transfer ports  88  are preferably positioned to minimize the duration that these ports  42 ,  88  are open, thereby minimizing short-circuiting of non-combusted fuel through the exhaust port  42 . In some embodiments, the ports  38 ,  42 ,  88  may be positioned so as to provide in-cylinder exhaust gas recirculation (EGR), which may reduce NOx emissions from the engine  10 . 
     Operation of the engine  10  will now be described with reference to  FIGS. 3-9 . Combustion in the power cylinder  14  begins just before (e.g., about 10-30 degrees before) the power piston  22  reaches a top-dead-center (“TDC”) position illustrated in  FIG. 3 . With the first and second valves  64 ,  68  closed and the intake port  38  covered by the compressor piston  24 , the igniter  56  ignites an air/fuel mixture that has been compressed to a peak pressure between the top of the power piston  22  and the power cylinder head  48 . Once ignited, expanding combustion gases drive the power piston  22  downward toward the crank  26  in the direction of arrow  100 . This rotates the crank  26  in the direction of arrow  104 , which in turn draws the compressor piston  24  toward the crank  26  in the direction of arrow  108 . As the compressor piston  24  moves in the direction of arrow  108 , the third valve  70  opens and an air/fuel mixture enters the compressor cylinder  18  via the compressor inlet passage  58 . 
     As the pistons  22 ,  24  both move toward the crank  26 , the volume in the crankcase  34  is reduced, compressing an air/fuel mixture in the crankcase  34  to an elevated pressure. The pressure in the crankcase  34  is at or near its maximum when the power piston  22  drops below the exhaust port  42 , allowing combustion gases to begin escaping the power cylinder  14  through the exhaust port  42  ( FIG. 4 ). This occurs approximately 110 degrees past the TDC position of the power piston  22  in the illustrated embodiment. At this time, the compressor piston  24  also reverses direction and begins its compression stroke, moving in the direction of arrow  112 . The third valve  70  closes as the pressure of the air/fuel mixture in the compressor cylinder  18  begins to rise. 
     As the power piston  22  continues moving downward toward its bottom-dead-center (“BDC”) position illustrated in  FIG. 5 , it drops below the transfer ports  88 , allowing the pressurized air/fuel mixture from the crankcase  34  to flow through the transfer ports  88  and into the power cylinder  14 . The incoming air/fuel mix scavenges the power cylinder  14  by displacing any exhaust still remaining in the power cylinder  14  out through the exhaust port  42 . The compressor piston  24  continues moving in the direction of arrow  112 , compressing the air/fuel mix in the compressor cylinder  18  while the first and third valves  64 ,  70  are closed. 
     Next, with reference to  FIG. 6 , the power piston  22  begins moving upward in the direction of arrow  116 , completely sealing the transfer ports  88  and beginning to seal the exhaust port  42 . In the illustrated embodiment, this occurs at approximately 230 degrees past the TDC position (or approximately 50 degrees past the BDC position). As the compressor piston  24  approaches the end of its compression stroke, it unblocks the intake port  38 . An air/fuel mixture flows into the crankcase  34  through the intake port  38  for use in subsequent crankcase scavenging. 
     With reference to  FIG. 7 , the compressor piston  24  reaches the end of its compression stroke at approximately 270 degrees past the power piston&#39;s TDC position. By this point, the power piston  22  has sealed the exhaust port  42 , and the piston  22  continues moving upward in the direction of arrow  116  to compress the air/fuel mix contained within the power cylinder  14 . In the illustrated embodiment, the compressor piston  24  contacts the first valve member  72 , displacing it against the force of the first spring  76  and allowing the high pressure air/fuel mixture to flow into the connecting passage  60 . This causes the second valve  68  to open, and the high pressure air/fuel mix that was compressed by the compressor piston  24  is injected into the power cylinder  14  during the compression stroke of the power piston  22 . Additional air/fuel mixture continues to flow into the crankcase  34  through the intake port  38 . In other embodiments, the first valve  64  may open in response to the pressure within the compressor cylinder  18  exceeding a cracking pressure of the first valve  64  such that the piston  24  need not engage the first valve member  72 . Because the exhaust port  42  has been closed by the power piston  22 , the injected air/fuel mixture cannot escape through the exhaust port  42 . This advantageously reduces hydrocarbon emissions. In addition, injecting high pressure air/fuel into the power cylinder  14  before combustion supercharges the engine  10  to produce more power. 
     Referring to  FIG. 8 , the compressor piston  24  begins moving back toward the crank  26  in the direction of arrow  108  when the power piston  22  is at approximately 300 degrees past the TDC position. This allows the first valve  64  to close. The power piston  22  continues moving upward in the direction of arrow  116 , further compressing the air/fuel mixture in the power cylinder  14 . The piston  22  seals the connecting passage  60 , and the second valve  68  also closes in preparation for combustion. Additional air/fuel mixture can continue to enter the crankcase  34  through the intake port  38 . 
     With reference to  FIG. 9 , just prior to combustion (e.g., at approximately 20 degrees before the TDC position of the power piston  22 ), the compressor piston  24  is moving in the direction of arrow  108  to draw air/fuel mix into the compressor cylinder  18  through the compressor inlet passage  58 . The compressor piston  24  also seals the intake port  38  so that compression of the air/fuel mix in the crankcase  34  may begin. Combustion begins, and the power piston  22  moves to the TDC position described above with respect to  FIG. 3 . The operating sequence then repeats. 
       FIG. 10  illustrates an engine  210  according to another embodiment of the invention. The engine  210  is similar to the engine  10  described above with reference to  FIGS. 1-9 . Accordingly, the following description focuses only on differences between the engine  210  and the engine  10 , and features and elements of the engine  210  corresponding with features and element of the engine  10  are given like reference numbers plus “200.” 
     The engine  210  includes a feed passage  240  provided with a dividing wall  320  that separates the feed passage  240  to provide a first feed path  324  and a second feed path  328 . The first feed path  324  communicates with the compressor inlet passage  258 , and the second feed path  328  communicates with the crankcase intake port  238 . A rich air/fuel mixture is supplied to the compressor cylinder  218  via the first feed path  324 , and substantially fuel-free air is supplied to the crankcase  234  via the second feed path  328 . Alternatively, substantially fuel-free air may be supplied via the feed passage  240  into both the compressor cylinder  218  and the crankcase  234 , or a relatively leaner air/fuel mixture may be supplied via the first feed path  324 . In such embodiments, all or additional fuel may be introduced downstream of the compressor cylinder  218  (i.e. between the compressor cylinder  218  and the power cylinder  214 ). For example, fuel may be injected into the connecting passage  260  to mix with compressed air exiting the compressor cylinder  218 . A fuel pump may be provided for this purpose. In some embodiments, an oil pump (not shown) may be provided to introduce oil into the fuel-free air that flows into the crankcase  234  and/or the compressor cylinder  218  for lubrication. 
     Because there is substantially no fuel in the air drawn into the crankcase  234  of the engine  210  in at least one embodiment, substantially all of the fuel for combustion is supplied to the power cylinder  214  via the compressor cylinder  218 . The rich air/fuel mixture from the compressor cylinder  218  combines with additional air in the power cylinder  214  (supplied to the power cylinder  214  from the crankcase  234  during crankcase scavenging) to achieve a proper stoichiometric ratio for combustion. Because the air in the crankcase  234  used for crankcase scavenging is substantially fuel-free, no fuel short-circuits combustion by prematurely exiting through the exhaust port  242 . As such, the engine  210  advantageously has reduced hydrocarbon emissions. 
     Although the disclosure has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of one or more independent aspects of the disclosure as described. Various features and advantages of the disclosure are set forth in the following claims.