Patent Publication Number: US-7210433-B2

Title: Stratified scavenged two-stroke engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of and is a divisional of U.S. patent application Ser. No. 11/026,209, filed Dec. 30, 2004, now U.S. Pat. No. 7,093,570, which claims priority to Provisional Patent Application Ser. No. 60/533,477 filed on Dec. 31, 2003, and entitled “STRATIFIED SCAVENGED TWO-STROKE ENGINE”, both of which are hereby incorporated by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The present invention relates to two stroke internal combustion engines and, particularly, to such engines with stratified scavenging. 
   A particular field of application of the invention is a two-stroke internal combustion engine. One application of the invention is to a small high speed two stroke engine, such as utilized in hand-held power equipment such as leaf blowers, string trimmers, hedge trimmers, also in wheeled vehicle applications such as mopeds, motorcycles, scooters, and in small outboard boat engines. The small two stroke engine has many desirable characteristics, including simplicity of construction, low cost of manufacturing, high power-to-weight ratios, high speed operational capability and, in many parts of the world, ease of maintenance. 
   Inherent drawbacks of two stroke engines are high emission levels and poor fuel economy due to short-circuit loss of fuel and air charge during the scavenging process. One drawback of the simple two-stroke engine is a loss of a portion of the fresh unburned fuel charge from the cylinder during the scavenging process. In the two-stroke engine, the homogeneous charge enters the cylinder through transfer ports during the scavenging process, when the exhaust port is also open. As such, some of the charge escapes through the exhaust port leading to high levels of hydrocarbons (HC) in the tailpipe. This leads to the poor fuel economy and high emission of unburned hydrocarbon, thus, rendering the simple two stroke engine difficult to comply with increasingly stringent governmental pollution restrictions. This drawback can be relieved by separating the scavenging of the cylinder, with fresh air, from the charging of the cylinder, with fuel. This separation can be achieved by having a buffer medium of air between the fresh charge and the burnt gas, during the scavenging process. 
   Several concepts and technologies have been proposed or tried to circumvent the short-circuit loss of fresh charge. Among these techniques are direct or indirect fuel injections, stratified scavenging, air-head, air assisted fuel injection, and compressed wave injection. Most of these technologies are either complex, expensive or need more parts. The fuel injection technology is not economical for small engines but air-head scavenging and stratified scavenging are promising. 
   An air-head scavenging system disclosed in U.S. Pat. No. 6,513,466 consists of an air channel leading into the storage space in the crankcase and has a reed valve. The filling time is very dependent on the pressure difference across the reed valve and is more likely dependent on engine speed and load. This may lead to an optimum performance only at a certain operating range of speed and load. The storage space may become a dead space when charge bypasses the storage space. U.S. Pat. Nos. 4,821,787, 6,112,708, and 6,367,432 describe reed valve controlled air passage in air-head scavenged two-stroke engines. The use of reed valves increases the cost and complexity and the performance is subject to quality of the reed valves. John Deere has used Reed valve controlled charge injection called compressed wave injection in the hand held application two-stroke engines. Again the use of reed in the engine can add cost and complexity to the engine. 
   It is desirable to have a simple two-stroke engine with fewer parts and that is easy to manufacture and assemble. It is also desirable to have an air volume high enough to improve the delivery ratio and scavenging and have asymmetric air inlet timing. 
   SUMMARY OF THE INVENTION 
   A two stroke internal combustion engine includes dual transfer passages in gaseous communication between a crankcase chamber and a combustion chamber of the engine, an air passage through the crankcase to the crankcase chamber and in gaseous communication with a carburetor of the engine, and a rotatable circular disk rotatably connected to a crankshaft of the engine. At least one first rotary shut-off valve is located in a radially outermost section of the circular disk bordered by a periphery of the circular disk and operatively disposed between the transfer passage and the crankcase chamber for opening and closing gaseous communication between the transfer passage and the crankcase chamber. At least one second rotary shut-off valve is located on the circular disk bordered by a periphery of the circular disk and operatively disposed between the air passage and the transfer passage for opening and closing gaseous communication between the air passage and the transfer passage. 
   In the exemplary embodiment of the two stroke internal combustion engine the first and second rotary shut-off valves are operably located on the on the circular disk to close the air passage to the transfer passage when the transfer passage is open between the combustion chamber and the crankcase chamber and to close off the transfer passage between the combustion chamber and the crankcase chamber when the air passage is opened to the transfer passage. In a more particular exemplary embodiment of the two stroke internal combustion engine the rotatable circular disk is a crank web, the first rotary shut-off valve is a conical cut out sector in a periphery of the crank web, and the second rotary shut-off valve is a notched cut out in the periphery of the crank web. An engine incudes a cylinder having at least one transfer passage that is a channel in a cylinder bore. A top end of the channel opens into a combustion chamber of the cylinder and the lower end opens into a crankcase chamber of the engine. The top end is opened and closed by a piston operably disposed in the cyliner bore, where as the lower end is alternatively opened and closed into the ambient air by a rotary valve, which in one embodiment of the engine is a crank web. When the rotary valve opens the air inlet to the lower end of transfer passage, as the piston is moving upward, a piston passage in a piston skirt of the piston opens a transfer port into the crankcase. The piston passage may be a window in the piston or a special passage with a fluid diode type that will be described later. The crank web also alternatively opens the lower end of the transfer passage into the crankcase. Connection of transfer passage to air and crankcase is alternative and is accomplished by a groove and cut out in the crank web. A main charge is injected into the crankcase in a usual manner either through a piston-controlled inlet, rotary valve, or a reed valve system. 
   One embodiment of the engine includes quadruplet transfer passage having a lower end of a first transfer passages closest to an exhaust port is alternatively connected to the ambient air by the rotary valve. The top end of the first transfer passage is connected to an adjacent second transfer passage either through a cut out in the piston or directly through a connecting passage at the top between the first and second transfer passages. The quadruplet passage increases the total volume of air and air acts as a buffer medium in both the transfer passages. It also helps clear the fresh charge in the transfer passages from the previous cycle. 
   By controlling the lower of transfer passage during scavenging asymmetric timing may be accomplished by the use of rotary valve. Thus the lower end of the transfer passage closest to the exhaust port may be shut off early during the end of scavenging process and may also have delayed opening. 
   A total length of the transfer passage may be increased by having the transfer passage continue into the crankcase as a grove on the crankcase wall. By using the crank web as a rotary valve to open and close the air inlet to lower end of transfer passage and a window or passage in the piston to open and close the top end of transfer passage into the crankcase, asymmetric air inlet timing is achieved. Thus there is no need for reed valves in the engine disclosed herein. 
   In one embodiment of the engine, the crank web and passage in the piston has been used to effect three-way scavenging in which air enters the combustion chamber ahead of lean air-fuel charge followed by the rich air-fuel charge. In another embodiment of the engine the crank web and the passage in the piston control a rich charge, thus eliminating a reed valve used in John Deere&#39;s compressed wave injection engine and completely replacing it with the rotary valve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings where: 
       FIG. 1  is a longitudinal sectional view illustration of an exemplary embodiment of a two-stroke engine  10  with a rotary valve controlled air inlet system with air inlet open condition (connecting rod and piston pin not shown). 
       FIG. 2  is a section along the crankshaft of the engine  10  shown in  FIG. 1 . 
       FIG. 3  is a sectional view illustration of the engine  10  illustrated in  FIG. 1  when the air inlet is closed and crankcase open to transfer passage for scavenging. 
       FIG. 4  is a section along the crankshaft of the engine shown in  FIG. 3 . 
       FIG. 5  is a front view of the engine shown in  FIG. 1 . Carburetor not shown. 
       FIG. 6  is a top view of the crankcase of the engine shown in  FIG. 5 . 
       FIG. 7  is an enlarged view of crankcase ports with sealing inserts as viewed from top of crankcase. 
       FIG. 8  is a longitudinal sectional view illustration of an exemplary embodiment of a two-stroke engine  20  with a rotary valve controlled air inlet system with air inlet open condition, has air channel in the cylinder flange (connecting rod and piston not shown). 
       FIG. 8   a  is an enlarged view of crankcase inserts as viewed from the side. 
       FIG. 9  is a section along the crankshaft of the engine  20  shown in  FIG. 8 . 
       FIG. 10  is a bottom view of the cylinder of the engine  20  shown in  FIG. 8 . 
       FIG. 11  is a top view illustration of crankcase of an exemplary embodiment of a two-stroke engine  30  with air channel in the crankcase flange. 
       FIG. 12  is a longitudinal sectional view illustration of an exemplary embodiment of a two-stroke engine  40  with quadruplet transfer passages and rotary valve controlled air inlet system with air inlet open condition, has passage in the piston connecting each other at the top of two transfer passages. 
       FIG. 13  is a view illustration of  FIG. 12  with air inlet closed and lower end of both the transfer passages open to crankcase. 
       FIG. 14  is a longitudinal sectional view illustration of an exemplary embodiment of a two-stroke engine  50  with quadruplet transfer passages and rotary valve controlled air inlet system with air inlet open condition, has long transfer passages on the crankcase wall. 
       FIG. 15  is a view illustration of  FIG. 14  with air inlet closed and lower end of both the transfer passages open to crankcase. 
       FIG. 16  is a longitudinal sectional view illustration of an exemplary embodiment of a two-stroke engine  60  with quadruplet transfer passages and rotary valve controlled air inlet system with air inlet open condition, has a passage between the two transfer passages at the top. 
       FIG. 17(   a )– 17 ( f ) is an illustration of different piston configurations. 
       FIG. 18  is a longitudinal sectional view illustration of an exemplary embodiment of a two-stroke engine  70  with transfer passage opened and closed by the valve on the periphery of the crank web and the air inlet port by the cut out on the outside surface of the crank web, the air inlet port is shown open to crankcase through transfer passage and piston passage. 
       FIG. 19  is a view illustration of  FIG. 18  with air inlet port shut off and transfer passage open to crankcase. And transfer port open to combustion chamber. 
       FIG. 20  is a longitudinal sectional view illustration of an exemplary embodiment of a two-stroke engine  80  with transfer passage opened and closed by the valve on the periphery of the crank web and the air inlet port by the cut out on the outside surface of the crank web, and has piston with a closed passage for gaseous communication between the adjacent transfer passages (has quadruplet transfer passages). 
       FIG. 21  is a section along the crankshaft of the engine  80  shown in  FIG. 20 , with air inlet into the crankcase through a pair of transfer passages. 
       FIG. 22  is a section along the crankshaft of the engine  80  shown in  FIG. 20  with piston at BDC; the crank web shuts off air inlet. 
       FIG. 23  is a longitudinal sectional view illustration of an exemplary embodiment of a two-stroke engine  90  with transfer passage opened and closed by the valve on the periphery of the crank web and the air inlet port by the cut out on the outside surface of the crank web, and the adjacent transfer passages are in gaseous communication at the top and one of them has rotary valve controlled port at the lower end (has quadruplet transfer passages). 
       FIG. 24  is a cross sectional view illustration of the cylinder and port arrangement at the top of the engine  90  shown in  FIG. 23   
       FIG. 25  is a cross sectional view illustration of an exemplary embodiment of a two-stroke engine  100  with three-way scavenging, lower end of transfer passage opened and closed by the crank web for air inlet and piston skirt opens and closes a charge passage for charge injection. 
       FIG. 26  is a front view of the engine  100  shown in  FIG. 25  (carburetor not shown). 
       FIG. 27  is a sectional view illustration of the cylinder of the engine shown in  FIG. 25 . 
       FIG. 28  is a sectional view illustration of the cylinder of the engine shown in  FIG. 25 , showing alternative location of the charge port  549 . 
       FIG. 29  is a cross sectional view illustration of an exemplary embodiment of a two-stroke engine  110  with lower end of charge passage opened and closed by the crank web for rich charge inlet and piston passage opens and closes the charge passage into the crankcase. 
       FIG. 30  is a cross sectional view illustration of engine  110  shown in  FIG. 29  where piston is near BDC. 
       FIG. 31  is a front view of the engine  110  shown in  FIG. 29  (carburetor not shown). 
       FIG. 32  is a side view elevation of the piston for the engine shown in  FIG. 29 . 
       FIG. 33  is a longitudinal sectional view illustration of an exemplary embodiment of a two-stroke engine  120  with charge passage opened and closed by the valve on the periphery of the crank web and the charge inlet port by the cut out on the outside surface of the crank web, the charge inlet port is shown open to crankcase through charge injection port and piston passage. 
       FIG. 34  is an elevation of the cylinder flange for the engine  120  shown in  FIG. 31 . 
       FIG. 35  is a sectional view illustration of the crankcase for the engine  120  shown in  FIG. 31 . 
       FIG. 36  is an elevation of the cylinder flange without channel in the flange. 
       FIG. 37  is a sectional view illustration of the charge passage channel in the crankcase flange for the engine shown in  FIG. 36 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Air-head scavenged engines provide a buffer medium of air between the fresh charge and the burned gas during the scavenging process. When the transfer ports open, the air enters the combustion chamber first and is most likely to be short-circuited, in the sense a small fraction of air is lost into the exhaust. The air is inducted into the transfer passage during the intake process, when the piston is ascending. Typically, a reed valve is provided at the top of the transfer passages for inducting only air into top of the transfer passages that stays in the transfer passages to act as a buffer medium. In some instances, piston ports are also provided in place of reed valves. The disadvantage with the reed valves is that it adds parts and are speed sensitive and the performance is subject to quality of the assembly of reeds and reed themselves. 
   In the exemplary embodiment the rotary valve, which can be a crank web as described in this case, replaces the reed valves. The two-stroke engine described in this embodiment consists of air inlet ports, opened and closed by the crank web cut out in the crank web for gaseous communication between the air inlet ports and the crankcase port at the bottom end of the transfer passages and the transfer ports at the top end of the transfer passages, which are opened and closed by the top of the piston and also by either cut out in the piston or by the passages in the piston. The cut out in the crank web acts as a rotary valve that periodically establishes gaseous communication between the ambient air and the transfer passages. The second cut out provides gaseous communication between the crankcase and the transfer passage. Thus the crank web alternatively communicates bottom end of the transfer passage with the ambient air and crankcase. The two-stroke engine cycle processes determine which way the bottom of transfer passage opens into. 
   The air inlet port is in gaseous communication with lower end of the transfer passage at appropriate time only. The timing of the gaseous communication between the air inlet port and the transfer passage is controlled by the passage in the crank web (could be groove or counter sunk). The crank web during the scavenging and expansion process shuts off the air inlet port. The lower end of the transfer passage is open and closed to the crankcase at appropriate time by the cutout on the crank web. Thus the crank web acts as a rotary valve to time the flow air into transfer passage from ambient during intake process and opens the transfer passage to crank case during scavenging process. The air in the transfer passage acts as a buffer medium between the charge and the burnt gas to minimize the loss of charge into exhaust and hence lowers the exhaust emission. 
     FIGS. 1 through 11  illustrate a dual transfer passage two-stroke engine  10 , wherein there are two transfer passages  11  (and ports) one on each side of the exhaust port  50 . As the piston  16  moves upward after the exhaust port  50  is closed, the counter sunk passage  751  on the outer face  550  of the crank web  21  establishes a gaseous communication between the air inlet port  650  and the crankcase port  111  at the lower end of transfer passage  11 . Around the same time the transfer port  33  is open into the crankcase  26  by the passage  613  in the piston  16 . Thus the differential pressure between the crankcase and the ambient lets the air to flow into the transfer passage  11  through the carburetor  34 , air control valve  94 , passage  817  in the heat dam  134  and into the air passage  88  in the crankcase  28 . Air continues to flow into the transfer passage as long as there is pressure difference across ambient and crankcase  26  and until the air inlet port  650  is shut off by the crank web  21 . The gaseous communication between the crankcase port  111  and air inlet port  650  may be cut off either before the piston reaches TDC or slightly past TDC. The asymmetric timing of the air inlet port  650  is achievable by the location of trailing edge  687  and angular length B of the countersunk passage  751  on the crank web  21 . By closing the crankcase port  111  during the down ward stroke of the piston, the reverse flow of air into the countersunk passage in the crank web and hence back into ambient is prevented. By virtue of long passage  102  in the piston, the entry of live charge from crankcase  26  into the transfer passage  11  may be prevented. Also, the inertia of the air flowing into the crankcase through the passage past TDC helps prevent reverse flow of air and or charge into the transfer passage. 
   As the piston descends, and before the top of the piston opens transfer port  33 , the crankcase port  111  at the lower end of the transfer passage  11  is opened by the cut out  244  on the periphery  43  of the crank web  21 . The location of leading edge  179  with respect to TDC position determines the start of scavenging process. The opening of the crankcase port  111  can be leading ahead or trailing behind the opening of the transfer port  33  by the piston. The angular length ‘A’ between the leading edge  179  and the trailing edge  178  determines the duration of the crankcase port  111  opened into the crankcase  26 . The intake of main air-fuel charge occurs though the inlet port  84  and through the carburetor control valve  585  in a normal way. The opening of the intake port  84  may be delayed with respect to the air inlet port  650 . A typical port timing for the exemplary air-head scavenged two-stroke engine is shown in Table 1. 
   As the piston descends down, it opens the exhaust port  50  first and then the transfer ports  33 . When the transfer ports  33  are opened, the air in the transfer passage  11  enters the combustion chamber  30  first ahead of the charge. Thus pure air acts as a buffer medium between the burnt gas and the fresh charge during the scavenging process. Since air enters the combustion chamber first and has the longest path to travel in the combustion chamber, it is the one that is most likely to be lost into the exhaust port  50 . Thus air-head scavenging minimizes the loss of fresh charge into the tail pipe and hence lowers the unburned hydrocarbon emission into the ambient. The scavenging duration by the charge may be delayed by delaying the opening of the crankcase port  111 . Thus the duration of time for which charge is likely to escape into the exhaust port may be shortened as determined by the angular length ‘A’ of the cut out  244  in the crank web  21 . Also, after discharging trapped air into the combustion chamber, the discharge of charge following the air may be momentarily interrupted by shutting off the crankcase port  111  by the crank web. In that case the cut out  244  is made of two segments; a first cut out  244   a  for the discharge of air through the port  33 . After momentarily shutting the crankcase port  111  the second cut out  751  opens the crankcase port  111  for discharge of charge. Descending of piston toward BDC helps build up crankcase pressure when the crankcase port  111  is momentarily shut off. Increased crankcase pressure around BDC position of the piston helps the delayed discharge of charge into the combustion chamber. 
   The proper functioning of the rotary valve depends on the good clearance between the port and the rotary valve. If the clearance between the two is excessive it may lead to poor sealing. In order to ensure proper seal between the face  550  of the crank web  21  and the crankcase wall, unique inserts  619  and  652  have been used.  FIGS. 7 and 8   a  show the air inlet port  650  and the crankcase port  111  with inserts  652  and  619  respectively in the corresponding ports. The insert is a small piece of tube inserted into the crankcase port  111  and the air inlet port  650 . The front face of the insert always keeps pressed against the face of the rotary valve, ensuring a proper seal between the insert and the rotary valve. At the back of the insert is a spring  614  that presses the insert away from the crankcase. The outer face of the insert pressed against the crank web always rests on the uncut face of the crank web and as such it does not get caught in the cut out. The insert  652  may be made of a non-metallic material and the spring  614  may either be a separate piece or an integral of the insert  652 . The inserts may be of soft material in comparison to the crank web. A high temperature plastic reinforced with glass fiber may be used. 
     FIGS. 8 and 9  show where the crank web  21  has a through passage  245  for uncovering the crankcase port  111  during the scavenging process. When the piston is ascending, the counter sunk passage  751  on the outer surface  550  of the crank web  21 , establishes gaseous communication between the air inlet port  650  and the crankcase port  111  for filling the transfer passage  11  with air during intake process. In  FIGS. 8 ,  8   a , and  9 , the crankcase port  111  is at a lower position and the transfer passage  11  is longer than it is illustrated in  FIGS. 1 through 4 . The air inlet passage  818  in the heat dam  638  is a single through passage. 
     FIGS. 8 through 10  show the air passage  861  splitting into left and right passages  950  on the cylinder flange  430  and then there is a air passage  851  in the crankcase  28  going down and opening into air inlet port  650 , through a passage  960  (shown in  FIGS. 6 and 7 ). The advantage is that the carburetor  34  containing control valves  585  for air-fuel and  94  for pure air is more compact. The adapter  638  between the carburetor  34  and the cylinder  12  is also small. 
     FIG. 11  shows where the air inlet passage  860  is in the crankcase splitting into left and right passages  850  in crankcase flange  428 . The air passage  850  opens into the passage  851  going down into the crankcase passage  960  (shown in  FIGS. 6 and 7 ) that runs along the crankshaft axis  19 , and into the air inlet port  650 . 
     FIGS. 12 through 16  illustrate quadruplet transfer passage system in a two-stroke engine. In the quadruplet transfer passages, there are four transfer passages one pair on each side of the exhaust port  50 . The air is inlet into the crankcase port  650  at lower end  100  of the transfer passage  11 , which is closest to the exhaust port  50 . However, the air instead of flowing out of transfer port  33  into the crankcase  26 , it flows into the adjacent transfer passage  211 . The transfer ports  33  and  233  are in gaseous communication with each other through passage  101  in the piston  16 .  FIG. 17(   e ) illustrates the passage in the piston. Where as in  FIG. 16 , the gaseous communication between the transfer passages  11  and  211  is through a direct passage  543  between the two passages. As the piston ascends the passage  101  in the piston  16  establishes at an appropriate time the communication between the adjacent transfer passages  11  and  211  through transfer ports  33  and  233 . Thus the air entering from port  619  at the bottom of the transfer passage  11  flows into the transfer passage  211  clearing the passage  11  of the fresh charge from the previous cycle. The charge and air in the transfer passage  211  flows into the crankcase  26  through the crankcase port  222  at the lower end of the transfer passage  211 . It may be observed that the location of the ports  619  and  222  at are a different heights, While  619  is opened closed by the crank web  21 , the port  222  may be either fully open all the time or may be closed by the piston as the piston descends toward BDC. Depending on the air inlet timing, the air may partially fill the transfer passage  211  after completely filling the transfer passage  11  or fill it completely. The intake of air-fuel mixture occurs in a normal way through the carburetor  34 , charge control valve  80 , inlet passage  107  and the inlet port  84 . The inlet port  84  opens later during the intake process after the start of induction of air into the transfer passage. The delay in charge inlet timing ensures filling of transfer passage  11  and at least partially the transfer passage  211  with pure air for an effective air-head scavenging. 
   During the scavenging process, the transfer ports  33  and  233  open simultaneously or may have staggered timing, where port  233  farthest from exhaust port  50 , opens a few degrees ahead of port  33 . The air flowing from the transfer port  33  acts as a buffer medium between the charge and the burnt gas, thus minimizing the loss of charge into the exhaust. By virtue of crank web being able to provide asymmetric crankcase port timing, the opening of the crankcase port  619  may be delayed while opening the transfer port  33  ahead of  233  to have a blow down of exhaust gas into the transfer passage  11  without adversely effecting the crankcase pressure. When the air is discharged later during the scavenging process, it may trap a layer of burnt gas between the fresh charge and the air, which ensures better trapping of the charge. This minimizes the loss of charge into the exhaust, which lowers the engine out emission of unburned fuel. 
   It is also possible in a quadruplet transfer passage system for only the transfer passage  11  closest to the exhaust port to receive air while the transfer passage  211  is not in communication with passage  11 . In that case the piston may have a window for gaseous communication between transfer passage  11  and the crankcase  26  during intake of air into the transfer passage  11 . The piston with a window is shown in  FIG. 17(   f ). 
     FIGS. 17(   a ) through  17 ( f ) illustrate different piston configurations usable with the exemplary embodiment described above. In the case of a quadruplet transfer passages the piston  17 ( e ) provides communication between the transfer ports  33  and  233 . The height of the passage  103  determines the duration of the communication between the ports  33  and  233 . Similarly a window  104  illustrated in  FIG. 17(   f ) provides passage between the transfer port  33  and the crankcase  26  for filling the transfer passage  11  with pure air during air intake timing.  FIG. 17(   b ) and  FIG. 17(   c ) illustrates a long passage on the piston skirt  17 . The length of the piston passage  102  ( 612 ) may help prevent reverse flow of charge into the transfer passage when the piston is descending. 
     FIG. 17(   c ) illustrates a piston passage  612  with a fluid diode  615  which offers resistance for reverse flow of charge into the transfer passage  11  while offering no resistance or minimum resistance for the flow in one direction (toward crankcase). In a quadruplet transfer passage, any combination of the piston configurations may be used. In the sense that the piston may provide gaseous communication during early or late phase of air intake into transfer passages while providing a window or direct passage into crankcase during early or late intake phase of air into transfer passage. 
     FIG. 16  shows where there is no valve to regulate the inlet of pure air into transfer passages. The air inlet has just an air cleaner  95 . The inertia of air may keep most of air in the transfer passage  11  and  566  at high speeds, while expelling back some of the air into ambient at idle and low speeds. The air inlet timing may be such that the mass of air trapped in the passage may be proportional to engine speed and or load. Thus it may eliminate the need for expensive double barrel or butterfly valve type carburetor in an air-head scavenged engine. 
   The air and air-fuel control valves can either be a barrel valve type shown in  FIGS. 1 ,  8 , and  21  or a butter fly valve type shown in  FIGS. 12 through 15 . 
   In  FIG. 16 , the passage  543  between the transfer passage  11  and  211  is of unique shape. The top face  547  of the passage  543  and the lower face  551  are at an angle to the horizontal plane. The angles are such that when the transfer port  233  opens first it may provide a stratified charge discharge through the port  233  where some of the air in the transfer passage  11  is also discharged through the port  233  while maintaining a stratified layer of air and charge. Also, after the port  33  is open, the discharge in the ports  33  and  233  are such that the charge do not flow into the transfer port  33 , while flow of charge through  233  may draw some air from the passage  11 . Thus a layer of air may be provided between the charge flowing into chamber  30  and the burnt gas escaping into the exhaust port  50 . The same objective may also be achieved by the passage illustrated in  FIGS. 23 and 24 . 
   In  FIGS. 14 through 16 , the lower end of the transfer passage  11  has a crankcase port  41 . A passage around the crankshaft axis in the side walls of the crankcase  28  in the form of a channel  566  enclosed by the side face  550  of the crank web  21 . The intent of the long channel on the side walls of the crankcase  28  is to provide a compact but long transfer passage that holds a larger mass of pure air. One end of the channel  566  communicates with the crankcase port  41  and the other end has a ‘L’ shaped tip and an outlet  554  for gaseous communication with the air inlet port  650  through a cut out (recess)  751  on the outer face  550  of the crank web  21 . The functioning of the air intake and scavenging is identical to the description provided earlier for  FIGS. 1 through 11 . However, the crankcase port  41  remains closed all the time by the crank web. During the intake of air, the ambient air is in gaseous communication with the transfer passage  11  for induction of air through the air inlet port  650 , cut out  751  in the crank web, and the channel  566  at the midsection of the ‘L’ shaped tip, as shown in  FIGS. 14 and 16 . During the scavenging process, the cut out  244  opens the tip of ‘L’ section at the port  554 , as shown in  FIG. 15 . 
     FIGS. 18–23  illustrate an exemplary embodiment of a two-stroke engines with an alternative rotary valve design, where in the transfer passage port  620  is opened and closed to the crankcase by a conical cut out sector  755  in a periphery  753  of the crank web  21  while the air inlet port  650  is opened and closed by the outside surface and a notched cut out  680  on the crank web  21 . The crankcase port  619  is at an angle to the side wall of the crankcase. In the sense that the port  620  is directly at the lower end of the transfer passage  11 . Where as in  FIGS. 1 through 16  ports  111  and  619  are on the sidewall of the crankcase. 
   The lower end of the transfer passage  11  has a crankcase port  620  that is alternatively in gaseous communication with the ambient air through the cutout  680  on the outside face  550  of the crank web  21  and an air inlet port  650 . The crankcase port  620  is also alternatively in gaseous communication with the crankcase  26 . The crankcase port  620  is opened into the crankcase  26  by the cutout  753  on the periphery  43  of the crank web  21 . The lower end of the second transfer passage  211  is in gaseous communication with the crankcase  26  through a crankcase port  222  (shown in  FIGS. 12 through 16  and  FIGS. 21 and 22 ). Crankcase port  222  may or may not be controlled by the piston skirt, particularly as the piston approaches BDC. 
   As the piston  16  moves upward, the top edge of the piston skirt  17  closes the transfer port  33  first,  233  next and then the exhaust port  50 . Both the transfer ports  33  and  233  may be closed simultaneously if the transfer port timing is not staggered (in the sense one port opens earlier than the second). After the exhaust port  50  is closed the crank web shuts off the communication between crankcase port  620  and the crankcase  26 . As the piston continues to move upward the air inlet port  650  is opened by the cutout  680  and a little later the cutout  680  opens the crankcase port  620 , while the section of the crank web has shuts off direct flow of gas between crankcase port  620  and the crankcase  26 . However, the top of the transfer passage  11  can be in gaseous communication with the crankcase  26  either 1) directly through passage  102  in the piston (shown in  FIGS. 2 and 18 ), 2) through closed passage  103  in the piston into the adjacent transfer passage  211  (shown in  FIG. 20 ), 3) through a passage  542  between the transfer passages  11  and  211  (shown in  FIGS. 23 and 24 , or 4) a open passage  543  (shown in  FIG. 16  or a combination of any of the above. 
   As the piston continues to move upward, the sub-atmospheric pressure in the crankcase  26  draws air from ambient (outside the crankcase) into the transfer passage  11  through the air inlet passage  88 , air inlet port  650 , and into the crankcase port  620  shown in  FIGS. 21 through 23 . The air then passes through the transfer passage  11  and into the crankcase  26  either directly through piston passage  102  or into the adjacent transfer passage  211 . As the crankshaft continues to rotate and the piston moves past TDC, the air inlet port  650  is closed by the crank web outer face  550 . And a little later the crank web also closes the crankcase port  620  in  FIGS. 21 through 23 . The intake of air-fuel mixture called the charge occurs in a usual manner through the charge intake port  84 . The timing of the charge inlet may occur later than a conventional engine. Delayed intake opening for charge helps fill the transfer passage  11  with pure air. As the air is filled into the transfer passage, the passage  11  (and  211  in a quadruplet transfer passage system) is cleared of the charge from the previous cycle. 
   As the piston starts to move downward the charge in the crankcase  26  is pressurized. If the crankcase port  620  is not closed, then the fresh charge may enter the transfer passage  11 . However, since the crank web closes the crankcase port, the charge does not enter the transfer passage from the lower end. In a quadruplet type transfer passage and when the air is contained in both the transfer passages  11  and  211 , closing the crankcase port  620  prevents the reverse flow of air into the crankcase  26 . However, charge may enter the transfer passage  211  through the crankcase port  222 . The volume and length of the transfer passage  11  and  211  may be such that even when the charge enters the transfer passage  211 , it may not reach the transfer passage  11  as the crankcase port  620  is closed. 
   In order to completely eliminate the entry of charge into the transfer passage  211 , the crankcase port  222  may also be either closed by the crank web or by the piston port, where the piston skirt closes the port  222  until the transfer port  233  is open. The opening and closing of the transfer port in the crankcase (or in the cylinder) has been disclosed in patent application Ser. No. 10/446,393, filing date May 28, 2003 by the same Inventors. 
   As the piston descends the exhaust port  50  is open first. The transfer port is open next. Since it is the air that is entering the combustion chamber first and has the longest residential time, it is more likely that it is the air that gets short circuited into the exhaust port. Thus the air-head scavenging system minimizes the loss of charge into the exhaust and thus lowers the unburned hydrocarbons in the tail pipe exhaust. 
   When quadruplet transfer ports are used, most of the air is retained in the transfer passage  11 , which is closest to the exhaust port  50 . The transfer port  233  farthest from the exhaust port  50  may open first in the case of a staggered transfer ports. In that case, as the top of the transfer port  211  also has some air and it enters the combustion chamber first followed by the charge. The second transfer port  33  may open a few degrees later discharging pure air in front of charge and acts as a buffer medium between the fresh charge and the burnt exhaust gas. 
   It is possible to open the crankcase port  111  ( 620 ) later after the transfer port  33  is open, since the crankcase port is opened and closed by the crank web. Thus an asymmetric timing is possible with the crank web controlled crankcase port system. 
   In  FIGS. 23 and 24 , the cap  539  is a plug used after machining the transfer ports  33  and  233  and the connecting passage  542 . The included angles between faces  508  &amp;  512  and  511  &amp;  504  are important and they may converge close to the cylinder wall opposite the exhaust port. The included angle between the face  512  and the imaginary plane passing through cylinder axis  517  and the center of exhaust port  50  is such that the flow forces the charge flowing through transfer port  233  to be as close to the cylinder wall opposite the exhaust port as possible. The included angle between face  504  and the similar imaginary plane passing through  517  and center of exhaust port  50  is smaller than the angle formed by the face  512 . 
     FIG. 24  illustrates a cross sectional view of a quadruplet port type transfer passage arrangement. In that, there are pair of transfer passages  11  and  211  on each side of the exhaust port  50 . And there is a pair of transfer ports  33  and  233  associated with each pair of transfer passages respectively. In the exemplary embodiment the transfer passages  11  and  211  are interconnected at the top by a passage  542  and has a bridge  546  between the two ports  33  and  233  that separates the two transfer ports  33  and  233 . The interconnecting passage  542  has a diverging shape with a face  513  diverging toward the port  233  so as to prevent reverse low from passage  211  into  11  during scavenging. The passage  542  may be of different shape also so as to prevent or minimize the flow of media from passage  211  into  11 . The passage  542  may also be an insert with a fluid diode that allows a free flow of air from passage  11  to passage  211 , while resisting the reverse flow of charge from passage  211  into  11 . It may also have a one way valve between the passage  11  and  211 . 
   In  FIG. 25  the function of the air inlet is similar to the description for the operation of engine shown in  FIG. 1 . However, in addition to the air, a rich charge system is added where a very rich air-fuel charge is inducted and injected into the combustion chamber  30  through a separate charge passage  39 . The engine consists of a three-way carburetor  547  and a three-way scavenging system. The charge passage  39  consists of segments  545 ,  552 ,  555  and  548 . Segment  545  has a charge injection port  40  at the top end open into the combustion chamber  30 . The port  40  is opened and closed by the piston. The segment  545  runs down in the cylinder  14  into the segment  552 , which is a channel on the cylinder flange  430 . The channel  552  runs around the cylinder  14  and opens into the lower end of the segment  555 . The charge passage  555  connects into the segment  548 , which has a port  549  in the cylinder  12  that opens into the crankcase. The port  549  is opened and closed by the piston  16 . The piston skirt  17  has a port  557  to time the start of injection when the piston is descending. 
   As the piston  16  ascends the piston skirt  17  opens the port  549  and thus establishing gaseous communication between the crankcase  26  and the ambient through the carburetor  547 . The rich charge now flows into the charge passage  39  through a one-way valve  36 . As the piston continues to ascend the air inlet into the transfer passage  11  and the lean air-fuel charge into the crankcase  26  occurs in a manner described earlier for the engine shown in  FIG. 1 . 
   The induction of rich charge into the charge passage  39  ends as the pistons begins to descend. The increase in crankcase pressure forces the one-way valve  36  to close. After the blow down of exhaust gas through the exhaust port  50 , the scavenging occurs first through the transfer port  33  where air enters the combustion chamber first followed by lean charge. As the piston continues to descend the crankcase port  111  may be closed and about the same time or before, the window  557  on the piston skirt  17  opens port  549  for injection of charge into the combustion chamber  30 . Thus the scavenging process occurs in three phases; first the air enters, followed by the lean charge through the transfer port  33  and then the rich charge is injected through the injection port  40 . The transfer passage system may be of quadruplet type described earlier and shown in  FIGS. 12 ,  15 , and  21 . Also, the air inlet and crank web design may be of any type described in this invention. 
     FIGS. 29 through 35  illustrate charge injection system where the lower end of the rich charge passage  39  is controlled by the crank web  21  and the top end by the piston  16  for start and end of charge induction into the charge passage. The start and end of charge injection into the combustion chamber may also be controlled by the crank web and have an asymmetric timing. 
   The carburetor  551  consists of two passages  300  for rich charge and  310  for either only air or very lean charge. The passage  310  opens into the passage  312  in the adapter plate, which communicates into the crankcase through the main inlet port  84 . The rich charge passage  300  opens into a charge inlet passage  302 , which has a charge inlet port  60  in the crankcase. 
   One end of the charge passage  39  has a charge injection port  40  opening into the combustion chamber where it is opened by the top of the piston  16  during scavenging and injection process. The charge passage  39  has a section  545  running down into the channel  552  in the cylinder flange  430  that runs around the cylinder  14  and opens into the passage  544  in the crankcase. The passage  544  in the crankcase opens into the crankcase  26  through a crankcase port  41  which is opened and closed by the cut outs in the crank web  21 . The rich charge passage  302  that is in communication with the carburetor  551  has a charge inlet port  60  in the crank case. The cut out  45  ( 556  in  FIG. 33 ) on the outside face  550  of the crank web  21  establishes gaseous communication between charge inlet port  60  and the crankcase port  41  when the piston is ascending. The rich charge flows into the charge passage  39  from the lower end of the charge passage and into the crankcase  26  through the charge injection port  40  and through the piston passage  603  (shown in  FIG. 32 ). Thus as the rich charge fills the charge passage  39  it clears the passage  39  of the residual lean charge from the previous cycle. Induction of rich charge ends when the crank web  21  closes the charge inlet port  60  as the piston reaches TDC or past TDC. In the case where the piston has a window similar to the one shown in  FIG. 17(   f ), then the height of the piston window determines the duration of induction. The induction of main lean charge or just air into the crankcase  26  occurs in a usual manner through the inlet port  84 . The main inlet  84  may be off set from the induction passage  39  as shown in  FIGS. 31 ,  33 ,  34 , and  36  or the inlet passage  84  may be split around the passage  39  as shown in  FIGS. 26 ,  27 , and  28 . 
   As the piston descends the piston opens the exhaust port  50  first and the scavenging occurs as the transfer ports  33  and  233  are opened. As the piston descends the crankcase port  41  is opened again by the cut out  44  ( 558  in  FIG. 33 ) in the crank web for injection. The lower ends  514  and  2514  of the transfer passages  11  and  211  shown in  FIGS. 29 and 30  may be shut off by the piston skirt  16  at the piston edge  520  thus forcing the charge and the crankcase content through the charge passage  39  through the charge injection port  40  into the combustion chamber. Thus the control of charge inlet by the crank web eliminates the need for one-way valve  39  (shown in  FIG. 25 ). Also, an asymmetric timing is achieved by the use of crank web for timing the charge induction and injection. 
   The segment  552  of the charge passage  39  may be on the cylinder flange  430  as shown in  FIG. 34  with the charge passage  544  in the crankcase  26  shown in  FIG. 35 . The segment  552  shown as  553  in  FIG. 37  may be on the crankcase flange  428  as shown in the Figure and the cylinder that matches this arrangement is shown in  FIG. 36 . 
   
     
       
         
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
           
          
             
                 
               Typical port timings for a quadruplet ported engine for air-head 
             
             
                 
               scavenging are: 
             
             
                 
               EPO 50 opens at 100 to 125 aTDC 
             
             
                 
               TPO 233 opens at 110 to 135 aTDC 
             
             
                 
               TPO 33 opens at 105 to 140 aTDC 
             
             
                 
               Crankcase port 111 opens to crankcase at 100 to 130 aTDC 
             
             
                 
               Crankcase port 111 closes to crankcase at 20 to 35 aBDC 
             
             
                 
               Air inlet port 650 opens at 21 to 37 aBDC 
             
             
                 
               Air inlet port 650 closes at 20 bTDC to 30 aTDC 
             
             
                 
               Crankcase port 111 open to ambient for air induction at 106 to 
             
             
                 
               139 bTDC 
             
             
                 
               Crankcase port 111 closes to ambient at 10 bTDC to 35 aTDC 
             
             
                 
               Piston passage opens (connects transfer port to crankcase) at 106 
             
             
                 
               to 30 bTDC 
             
             
                 
               Piston passage closes at 106 to 30 aTDC 
             
             
                 
               Inlet 84 opens at 65 to 40 bTDC 
             
             
                 
               Inlet 84 closes at 65 to 40 aTDC 
             
             
                 
                 
             
          
         
       
     
   
   
     
       
         
             
             
           
             
                 
               TABLE 2 
             
             
                 
                 
             
           
          
             
                 
               Typical port timings for a three-way scavenged engine (example 
             
             
                 
               FIG. 25) are: 
             
             
                 
               EPO 50 opens at 100 to 125 aTDC 
             
             
                 
               TPO 33 opens at 105 to 140 aTDC 
             
             
                 
               Crankcase port 111 opens to crankcase at 100 to 130 aTDC 
             
             
                 
               Crankcase port 111 closes to crankcase at 40 bBDC to 35 aBDC 
             
             
                 
               Charge injection port 40 opens to combustion chamber at 115 to 
             
             
                 
               150 aTDC 
             
             
                 
               Charge injection port 40 closes at 115 bTDC to 150 bTDC 
             
             
                 
               Port 549 opens at 120 aTDC to 155 aTDC 
             
             
                 
               Port 549 closes at 120 bTDC to 155 bTDC 
             
             
                 
               Port 549 open for charge induction at 110 bTDC to 145 bTDC 
             
             
                 
               Port 549 closes for charge induction at 110 aTDC to 145 aTDC 
             
             
                 
               Air inlet port 650 opens at 21 to 37 aBDC 
             
             
                 
               Air inlet port 650 closes at 20 bTDC to 30 aTDC 
             
             
                 
               Crankcase port 111 open to ambient for air induction at 106 to 
             
             
                 
               139 bTDC 
             
             
                 
               Crankcase port 111 closes to ambient at 10 bTDC to 35 aTDC 
             
             
                 
               Piston passage opens (connects transfer port to crankcase) at 106 
             
             
                 
               to 30 bTDC 
             
             
                 
               Piston passage closes at 106 to 30 aTDC 
             
             
                 
               Inlet 84 opens at 65 to 40 bTDC 
             
             
                 
               Inlet 84 closes at 65 to 40 aTDC 
             
             
                 
                 
             
          
         
       
     
   
   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.