Abstract:
A transfer passage design for a two stroke internal combustion engine that allows for regulation of the air/fuel flow as the piston nears the bottom of its stroke. Additionally, a cut out in the skirt of the piston may allow for additional gaseous communication between the crankcase chamber and the transfer passage.

Description:
[0001]    This patent application claims the benefit of, priority of, and incorporates by reference U.S. Provisional Patent Application Ser. No. 61/362,294, entitled “Piston for a Two-Stroke Engine” by Nagesh Mavinahally, Bernardo Herzer, and David Kostka filed on Jul. 7, 2010. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a two stroke engine, specifically an improved two stroke engine piston and cylinder port design. 
         [0004]    2. Description of the Related Art 
         [0005]    Conventional low cost two-stroke engines, particularly the ones used in lawn and garden applications typically have higher emissions compared to higher cost stratified engines. The emissions in a conventional two-stroke engine are higher due to scavenging loss of fuel-air mixture, particularly in a simpler port and piston design. 
         [0006]    The design described here lowers the emission in a conventional two-stroke engine without adding cost or complexity to the engine. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Accordingly, the design for a new and improved method of lowering the emissions and lowering the fuel consumption in a two-stroke engine is disclosed. This design may be applied to the hand-held two-stroke engines, and lawn and garden engines in general. 
         [0008]    The low cost two-stroke engine is especially suited for hand-held, lawn and garden equipments such as trimmers, blowers, chainsaws, generator engines, and mopeds. 
         [0009]    This low cost two-stroke engine reduces the scavenging loss of unburned fuel and therefore reduces the pollutants and improves the efficiency of the engine. In the improved design, the piston controls the scavenging process by restricting the flow of air-fuel mixture when the piston is closer to and at bottom dead center (BDC). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    A more complete appreciation of the invention and many of the advantages thereof will be readily obtained as the same becomes better understood by reference to the detailed description when considered in connection with the accompanying drawings, wherein: 
           [0011]      FIG. 1  shows a sectional side view of an embodiment of the engine when the piston is close to BDC. 
           [0012]      FIG. 2  shows a sectional front view of an embodiment of the engine shown in  FIG. 1 . 
           [0013]      FIG. 3  shows a sectional side view of an embodiment of the engine when the piston is at BDC. 
           [0014]      FIG. 4  shows the a sectional front view of an embodiment of the engine shown in  FIG. 3 . 
           [0015]      FIG. 5  shows the a cross sectional view (section A-A of  FIG. 3 ) of the transfer ports of an embodiment of the engine. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A two-stroke engine constructed in accordance with the preferred embodiment, is shown in  FIG. 1  through  FIG. 5 .  FIG. 1  through  FIG. 5  depict an embodiment of a two-stroke engine having two pairs of transfer passages  111  and  112 ; one pair on each side of the exhaust port  50 . The engine has a cylinder  12  and a crankcase  28  which are firmly fastened with a gasket  25  in between the cylinder and the crankcase  28 . As seen in  FIG. 5 , the transfer passages  111   112  may have outer side walls formed from the casting of the cylinder  12 . The crankcase  28  has a crankcase chamber  26  having a crankshaft  22  for reciprocatively moving a piston  16  inside the cylinder bore  13  through a connecting rod  18 . The crankshaft  22  is supported by the crankcase  28  through a pair of main bearings  14  ( FIG. 4 ) on either side of the crank pin  24 . The piston  16  has openings (cut outs)  216  on the skirt that align with the front transfer passages  111 , which is farthest from the exhaust port  50 . The opening  216  may be aligned with both the front passage  111  and rear passage  112 , just the front passage  111 , or just the rear passage  112 . The pair of rear passages  112  may be deleted and just one pair of passages  111  may be sized and shaped to perform the same function. The transfer passage inlet ports  211  and  311  at the lower ends of the transfer passages  111  and  112 , unlike with conventional engines (also unlike U.S. Pat. No. 6,513,465), do not open into the crankcase chamber  26  via openings at the bottom of the passages  111  and  112 . The inlet ports  111  and  112  are blocked at the bottom by the gasket  25  and or a wall on the crankcase  28 . The transfer passages  111  and  112  are, however, open on the side that contacts the piston  16 . Thus the piston  16  acts as a movable gate that alternately opens and closes the transfer passages  111  and  112  at the top which is outlet ports  133   134  and at the bottom which is inlet ports  211  and  311 . When the piston is close to BDC, as seen in  FIG. 2 , the transfer passages  111  provide a gaseous communication between the combustion chamber  30  above the piston, at the transfer passage outlet port  133 , and the crankcase chamber  26  below the piston through transfer passage inlet ports  211  and  311  and also through the opening  216  on the piston skirt. 
         [0017]    In the operation of the engine, as the piston  16  moves upward to compress the air-fuel mixture in the combustion chamber  30 , it first closes the transfer passages  111  and then closes the exhaust port  50 . At the same time, pressure in the crankcase chamber  26  reduces as the piston continues to ascend. As the piston  16  continues to ascend, it closes the exhaust port  50  and shortly later the intake port  84  is opened by the lower edge  116  of the piston  16 . Thus fresh air-fuel mixture is drawn into the crankcase chamber  26 . The air-fuel mixture in the combustion chamber  30  is compressed by the ascending piston  16  and is ignited by the spark from the spark plug  44  causing high combustion pressure to push the piston  16  down generating the power. As the piston  16  descends, the piston  16  first closes the intake port  84  and as the piston  16  continues to descend, the pressure increases in the crankcase chamber  26 . As the piston  16  continues to descend, the exhaust port  50  is opened first. After the piston  16  descends a little further, the outlet ports  133  of the transfer passages  111  are opened and the inlet ports  211  and  311  are already open. At this time, the air-fuel mixture in the crankcase chamber  26  is pushed toward the combustion chamber  30  through the fully open inlet ports  211  and  311 . As the piston continues to descend, the lower edge  116  of the piston  16  begins to close the transfer passage inlet ports  211  and  311  as shown in  FIGS. 3 and 4 . However, since the cut outs  216  are aligned with the transfer passages  111 , the air-fuel mixture continues to flow into the combustion chamber  30 , but at a rate determined by the shape and size of the cut outs  216  on the piston skirt. Thus the discharge rate of the air-fuel mixture is now regulated by the shape of the cut outs  216 . The flow rate is reduced as the piston  16  approaches BDC and it is possible to completely cut off the flow. Reduction in flow of air-fuel mixture thus minimizes the loss of fresh air-fuel mixture through the exhaust port  50 . Thus a significant reduction in emission may be seen with the engine built according to this embodiment. 
         [0018]    The present invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. While there have been described herein, what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.