Two-cycle internal combustion engine

Abstract: The invention provides a two-cycle internal combustion engine which can realize scavenging by air, prevent blow-by of a fuel mixture in a scavenging stroke, and prevent combustion of a lubricating oil in a combustion chamber to thereby purify an exhaust gas. To achieve this, a two-cycle internal combustion engine 1 includes a compressor section (20) for performing a compression stroke in concert with a scavenging stroke by a piston (7) reciprocating in a cylinder (8). A fuel mixture from a carburetor (17) is pressurized to be supplied to a combustion chamber (12) by the compressor section (20). On the other hand, a scavenging air is also pressurized to be supplied to the cylinder (8) from its side wall by the compressor section (20). An oil pump (60) driven by a crankshaft (3) is provided to pump up a lubricating oil stored in a bottom portion of a crankcase, thereby lubricating the crankshaft with the lubricating oil supplied from the oil pump.

Description: The present invention relates to a two-cycle internal combustion engine in which scavenging in a combustion chamber is carried out by an air flow induced from the outside of the engine. A two-cycle internal combustion engine is required to purify an exhaust gas, and various inventions for meeting this requirement have been proposed. For example, in the invention described in Japanese Patent Publication No. 50-25083, an auxiliary combustion chamber having a spark plug is provided additionally to a combustion chamber, wherein a rich fuel mixture is supplied to the auxiliary combustion chamber and a lean fuel mixture is supplied from a scavenging port of a cylinder into the combustion chamber by a compressing operation in a crankcase. The lean fuel mixture in the combustion chamber is burned by a strong flame generated in the auxiliary combustion chamber, thereby realizing purification of the exhaust gas from a two-cycle internal combustion engine. Further, in the invention described in Japanese Patent Laid-open No. 7-310554, a fuel mixture is supplied to a combustion chamber after the combustion chamber is scavenged by an air flow, thereby reducing blow-by of the fuel mixture to aim at purification of the exhaust gas from a two-cycle internal combustion engine. In this invention, a diaphragm opened poppet valve as a scavenging valve is provided at the top of a cylinder, and an air not containing a fuel is compressed in a crankcase to be supplied through a scavenging passage to the scavenging valve. A carburetor is provided in the scavenging passage extending from the crankcase to the scavenging valve and a diaphragm for operating the scavenging valve. In operation, when a scavenging pressure from the crankcase is applied to the diaphragm in a scavenging stroke to open the scavenging valve, the air from the crankcase flows through the scavenging passage into the combustion chamber to scavenge the combustion chamber. Subsequently, the fuel mixture is sucked from the carburetor by this air flow and supplied into the combustion chamber. As mentioned above, various inventions have conventionally been proposed to purify the exhaust gas from a two-cycle internal combustion engine, and further improvements are now desired also from the viewpoint of environmental protection. That is, while some degree of blow-by of a scavenging gas cannot be mechanically avoided in a two-cycle internal combustion engine, the invention described in Japanese Patent Publication No. 50-25083 has some limitation to the exhaust gas purification, because scavenging is carried out by using a fuel mixture although it is lean. Another challenge to achieve the exhaust gas purification in a two-cycle internal combustion engine is to prevent combustion of a lubricating oil. A conventional two-cycle internal combustion engine adopts a fuel mixture lubrication system such that a fuel mixture also containing a lubricating oil is supplied into a crankcase and a cylinder to lubricate movable portions such as a crankshaft and a piston. In this lubrication system, the lubricating oil contained in the fuel mixture is also burned in the combustion chamber, so that a white smoke or the like is generated by combustion of the lubricating oil to cause a large obstacle in purifying the exhaust gas. In the invention described in Japanese Patent Laid-open No. 7-310554, the scavenging in the initial stage is carried out by using the air not containing the fuel. However, this invention adopts a uniflow scavenging system such that the scavenging air and the fuel mixture are supplied from the scavenging valve located at the top of the cylinder toward the scavenging port formed at a lower portion of the cylinder in the same direction. Accordingly, there arises a problem that the fuel mixture supplied in the final stage of the scavenging may be attracted by the air flow to be exhausted together to some extent. Furthermore, when the scavenging valve is closed, the fuel mixture sucked from the carburetor stays on the upstream side of the scavenging valve, and this fuel mixture is urged to the diaphragm by the scavenging pressure from the crankcase. As a result, the diaphragm formed from an elastic film such as a rubber film is wetted with the fuel and may be deteriorated, causing a problem that the original operation characteristics of the diaphragm cannot be obtained to invite deviations of the timing of supplying the fuel mixture into the combustion chamber. It is accordingly an object of the present invention to provide a two-cycle internal combustion engine which can realize scavenging by air, prevent blow-by of a fuel mixture in a scavenging stroke, and further prevent combustion of a lubricating oil in a combustion chamber to thereby remarkably purify an exhaust gas. It is another object of the present invention to provide a two-cycle internal combustion engine using a diaphragm operated poppet valve as a valve for controlling fuel mixture supply to a combustion chamber to realize exhaust gas purification, in which the poppet valve can be operated with its original operation characteristics to thereby ensure good combustion. Other objects of the present invention will become apparent from the following description. The two-cycle internal combustion engine of the present invention includes first and second compressors for performing a compression stroke in concert with a scavenging stroke by a piston reciprocating in a cylinder. The first compressor pressurizes a fuel mixture supplied from a fuel supplier such as a carburetor or a fuel injector to supply the fuel mixture under pressure into a combustion chamber defined in the cylinder from a top portion thereof. The second compressor pressurizes a scavenging air induced from the outside of the engine to supply the scavenging air under pressure into the cylinder from a side portion thereof, thus performing scavenging by means of an air flow not containing the fuel. A main passage is provided to connect the fuel supplier and the combustion chamber. A control valve such as an electromagnetic control valve or a diaphragm operated poppet valve for controlling supply of the fuel mixture to the combustion chamber is provided in the main passage so as to be adapted to open in the scavenging stroke. A fuel-mixture check valve for preventing reverse flow of the fuel mixture toward the fuel supplier, while allowing normal flow of the fuel mixture from the fuel supplier to the first compressor is provided in the main passage at a position between the control valve and the fuel supplier. A branch passage is provided to connect the first compressor and the main passage at a position between the fuel-mixture check valve and the control valve. A scavenging port is formed in a side wall of the cylinder so as to open into the cylinder. An auxiliary passage is provided to connect the scavenging port and the second compressor. An air check valve for preventing reverse flow of the scavenging air toward the outside of the engine, while allowing normal flow of the scavenging air from the outside of the engine to the second compressor is provided in the auxiliary passage. Accordingly, in the scavenging stroke where the piston is lowered in the cylinder, the scavenging air supplied under pressure by the second compressor enters the cylinder from its side portion to flow across the inside of the cylinder, thereby performing scavenging. On the other hand, the fuel mixture supplied under pressure by the first compressor enters the cylinder from its top portion to flow down in the cylinder. Thus, the scavenging air and the fuel mixture flow in different directions in the cylinder. That is, the flows of the scavenging air and the fuel mixture in the cylinder become two layers, so that the scavenging by the air flow can be performed without mixing with the fuel mixture. Furthermore, this two-cycle internal combustion engine is provided with an oil pump as in a four-cycle internal combustion engine, and a lubricating oil is supplied under pressure by the oil pump to movable portions of the engine such as a crankshaft. That is, this two-cycle internal combustion engine adopts a so-called direct lubrication system such that a lubricating oil reservoir is formed in a bottom portion of a crankcase of the engine and the lubricating oil is pumped up from the lubricating oil reservoir by the oil pump. In the case that the conventional two-cycle internal combustion engine merely adopts such a direct lubrication system, a lubricating oil mist generated in the crankcase is mixed with the fuel mixture or the scavenging air to cause irregular combustion and exhaust gas contamination. To the contrary, the two-cycle internal combustion engine of the present invention can adopt the direct lubrication system without such problems, because the crankcase is formed independently of the compression chambers for the fuel mixture and the scavenging air. The two-cycle internal combustion engine of the present invention is further provided with a stepped cylinder connected to the crankcase and located in 90° spaced relationship with the cylinder defining the combustion chamber, and a stepped piston reciprocating in the stepped cylinder. The stepped piston is connected to a crankpin for connecting the piston and the crankshaft. That is, the stepped piston and the piston are connected to the crankpin in coaxial relationship. The stepped cylinder and the stepped piston cooperate to configure the first and second compressors for performing the compression stroke in concert with the scavenging stroke by the piston. With this configuration, the cylinder defining the combustion chamber and the stepped cylinder forming the first and second compressors are arranged in a V shape 90° spaced relationship with each other, so that primary vibrations by the pistons reciprocating in the respective cylinders can be canceled to thereby suppress vibrations in operating the two-cycle internal combustion engine. In the case that a diaphragm operated poppet valve is used as the control valve mentioned above, a control passage is provided to connect the auxiliary passage and a diaphragm operation port of the poppet valve. The poppet valve is opened by an air pressure in the auxiliary passage to supply the fuel mixture into the combustion chamber in the scavenging stroke by the piston. Also in this case, the scavenging air and the fuel mixture form two layers in the cylinder to purify the exhaust gas. Accordingly, even in the case of using the diaphragm operated poppet valve for supply control of the fuel mixture, an air flow structurally isolated from the fuel mixture is applied to the diaphragm, thereby preventing the deterioration of the diaphragm due to wetting with the fuel. In the above two-cycle internal combustion engine, an operating pressure to the diaphragm operated poppet valve is determined by the air pressure from the second compressor driven by the crankshaft, so that the operating pressure to the diaphragm can be changed according to a rotating speed of the crankshaft. That is, the diaphragm operated poppet valve is opened according to the operation of the crankshaft, so that the timing of supplying the fuel mixture into the combustion chamber can be reliably set with a simple configuration, and the fuel mixture can be injected at a proper crank angle to realize good combustion. The two-cycle internal combustion engine using the diaphragm operated poppet valve mentioned above is further provided with a pressure control valve in the control passage for supplying a driving air pressure to the diaphragm. The pressure control valve is opened and closed in concert with throttle operation in such a manner that the opening angle of the pressure control valve is increased with an increase in throttle opening angle. Accordingly, the timing of supplying the fuel mixture from the diaphragm operated poppet valve can be changed according to throttle operation with a simple configuration, thus realizing fine timing control according to the operating condition of the two-cycle internal combustion engine. There will now be described a specific preferred embodiment of the two-cycle internal combustion engine according to the present invention with reference to the drawings: FIG. 1 is a vertical sectional view of a two-cycle internal combustion engine according to a preferred embodiment of the present invention. FIG. 2 is a vertical sectional view showing a control passage and its associated parts shown in FIG. 1. FIG. 3 is a vertical sectional view showing a diaphragm operated poppet valve and its associated parts shown in FIG. 1. FIG. 4 is a horizontal sectional view of the two-cycle internal combustion engine shown in FIG. 1. FIG. 5 is a schematic view for illustrating the operation of the two-cycle internal combustion engine shown in FIG. 1. FIG. 6 is a schematic view for illustrating the operation of the two-cycle internal combustion engine shown in FIG. 1. FIG. 7 is a schematic view for illustrating the operation of the two-cycle internal combustion engine shown in FIG. 1. FIG. 8 is a schematic view for illustrating the operation of the two-cycle internal combustion engine shown in FIG. 1. FIG. 9 is a schematic view for illustrating the operation of the two-cycle internal combustion engine shown in FIG. 1. FIG. 10 is a graph showing an operation characteristic of a poppet valve from the relation between a crank angle and a pressure applied to a diaphragm. FIG. 11 is a graph showing an operation characteristic of the poppet valve from the relation between a rotating speed of a crankshaft and a crank angle. FIG. 12 is a graph showing an operation characteristic of the poppet valve from the relation between a rotating speed of the crankshaft and a range of change in crank angle. While the two-cycle internal combustion engine shown is a single-cylinder two-cycle internal combustion engine, the configuration of the preferred embodiment may be applied similarly to a multi-cylinder two-cycle internal combustion engine. FIG. 1 is a vertical sectional view of a two-cycle internal combustion engine 1 according to a preferred embodiment of the present invention. The two-cycle internal combustion engine 1 (which will be hereinafter referred to simply as an engine 1) includes a crankcase 2, a crankshaft 3 rotatably supported in the crankcase 2, a connecting rod 5 rotatably mounted at one end thereof on a crankpin 4 of the crankshaft 3, a piston 7 rotatably mounted through a piston pin 6 at the other end of the connecting rod 5, a cylinder 8 in which the piston 7 is slidably accommodated, a cylinder block 9 mounted on the crankcase 2, and a cylinder head 10 mounted on the top end of the cylinder block 9. Reference numeral 10a is a spark plug. A lubricating oil OL is stored in a bottom portion of the crankcase 2. As in a four-cycle internal combustion engine, the lubricating oil OL is pumped up through a strainer 11 by the operation of an oil pump to be hereinafter described, and is supplied to a bearing portion of the crankshaft 3, a sliding portion of the piston 7, etc., thereby lubricating these mechanical movable portions. The engine 1 further includes a diaphragm operated poppet valve 13 as a control valve mounted on the cylinder head 10 for intermittently supplying a fuel mixture to a combustion chamber 12, a joint member 14 having three ports one of which is connected to the upstream side of the poppet valve 13, a reed valve 15 as a fuel-mixture check valve connected to another one of the three ports of the joint member 14, a fuel mixture pipe 16 connected to the upstream side of the reed valve 15, a carburetor 17 as a fuel supplier connected to the upstream side of the fuel mixture pipe 16, a branch pipe 18 as a branch passage connected at one end thereof to the remaining one of the three ports of the joint member 14, and a compressor section 20 connected to the other end of the branch pipe 18. In this preferred embodiment, a main passage 19 for connecting the carburetor 17 and the top of the combustion chamber 12 is configured by the joint member 14 and the fuel mixture pipe 16. The upstream end of the carburetor 17 is connected to an air cleaner (not shown) for supplying an outside air to the carburetor 17. The reed valve 15 functions to prevent that the fuel mixture supplied from the carburetor 17 may reversely flow toward the carburetor 17. In this preferred embodiment, a direct lubrication system is adopted as mentioned above, so that no lubricating oil is contained in the fuel mixture supplied from the carburetor 17. The compressor section 20 is of a reciprocating type such that a compression stroke is carried out in the scavenging stroke of the piston 7 in concert with the piston 7. The compressor section 20 includes a connecting rod 21 rotatably mounted at one end thereof on the crankpin 4 of the crankshaft 3 in coaxial relationship with the piston 7, a stepped piston 23 rotatably mounted through a piston pin 22 at the other end of the connecting rod 21, and a stepped cylinder 24 in which the stepped piston 23 is slidably accommodated. The stepped piston 23 has a large-diameter portion 23a and a small-diameter portion 23b. The stepped cylinder 24 has a large-diameter portion 24a with which the large-diameter portion 23a of the piston 23 is kept in sliding contact and a small-diameter portion 24b with which the small-diameter portion 23b of the piston 23 is kept in sliding contact. A cylinder block 25 forming the stepped cylinder 24 therein is mounted on the crankcase 2, and a cylinder head 26 is mounted on the top end of the cylinder block 25. The compressor section 20 is composed generally of a first compressor and a second compressor. The first compressor is composed of the small-diameter portion 23b of the stepped piston 23 and the small-diameter portion 24b of the stepped cylinder 24. The second compressor is composed of the large-diameter portion 23a of the stepped piston 23 and the large-diameter portion 24a of the stepped cylinder 24. The first compressor includes a pressure chamber 27 to which the other end of the branch pipe 18 is connected. An air intake pipe 29 is mounted between the cylinder block 9 forming a combustion chamber and the cylinder block 25 forming a compressor. The air intake pipe 29 is formed with an air passage 30 (auxiliary passage for scavenging) having three open ends. One of the three open ends of the air passage 30 is connected to a pressure chamber 28 defined in the second compressor. Another one of the three open ends of the air passage 30 is connected to a scavenging port 31 formed in a lower portion of the side wall of the cylinder 8. Reference numerals 36 and 37 denote an exhaust port and an exhaust pipe, respectively. The remaining one of the three open ends of the air passage 30 is connected through an air supply pipe 32 to the air cleaner. A reed valve 33 as an air check valve is provided at a connecting portion between the air passage 30 and the air supply pipe 32. The reed valve 33 functions to prevent that the outside air supplied from the air cleaner may reversely flow toward the air cleaner. As shown in FIG. 2, a pipe 34 forming a control passage 34a is mounted between the air intake pipe 29 and the joint member 14. One end of the control passage 34a communicates with the air passage 30, and the other end of the control passage 34a communicates with a diaphragm operation port 35 of the poppet valve 13. As shown in FIG. 3, the diaphragm operated poppet valve 13 is provided at the top of the cylinder head 10. The poppet valve 13 has a rod-like valve element 40 passing through the main passage formed in the joint member 14. The valve element 40 has a conical valve head exposed to the combustion chamber 12 at the top of the cylinder 8. That is, the conical valve head of the valve element 40 of the poppet valve 13 openably closes a fuel-mixture inlet port formed at the top of the cylinder 8. In the open condition of the fuel-mixture inlet port, the fuel mixture from the main passage is supplied through a passage 41 formed around the stem of the valve element 40 into the combustion chamber 12. A coiled return spring 42 is provided around the valve element 40 to normally bias the valve element 40 in a valve closing direction (upward as viewed in FIG. 3). A diaphragm 43 is mounted at the tail end of the valve element 40 (the upper end as viewed in FIG. 3), and a pressure chamber 44 communicating with the diaphragm operation port 35 is defined on the upper side of the diaphragm 43. Accordingly, when compressed air is supplied from the air passage 30 through the control passage 34a to the diaphragm operation port 35, the pressure in the pressure chamber 44 rises to push the diaphragm 43 with the valve element 40, thereby opening the valve element 40 against the return spring 42. Reference numeral 45 denotes a spring provided on the upper side of the diaphragm 43 to suppress play of the diaphragm 43, and reference numeral 46 denotes an air vent passage formed on the lower side of the diaphragm 43. The diaphragm operation port 35 and the pressure chamber 44 are connected by a passage 47, and a shaft 48 is rotatably provided so as to traverse the passage 47. A pressure control valve 49 for opening and closing the passage 47 is mounted on the shaft 48 at its middle portion exposed to the passage 47, and an operation member 50 is mounted on one end of the shaft 48. The operation member 50 is connected to a throttle adapted to be operated by an operator. Accordingly, an opening condition of the passage 47 between the control passage 34a and the pressure chamber 44 is controlled by the pressure control valve 49 in concert with the throttle operation by the operator, so that an opening timing of the valve element 40 of the poppet valve 13 is adjusted according to a vehicle running condition as hereinafter described. FIG. 4 is a cross section taken along the axis of the crankshaft 3 of the engine 1 and the axes of the two cylinders 8 and 24. As shown in FIG. 4, the engine 1 is provided with an AC generator 54 mounted on the crankshaft 3 at one end thereof, a crank angle detecting disc 55 mounted outside of the AC generator 54, a plurality of crank angle sensors 56 and 57 respectively corresponding to a plurality of projections 55a, 55b, and 55c formed on the crank angle detecting disc 55 for detecting crank angles or the like (the crank angle sensor corresponding to the projection 55a being not shown), and a water pump 58 connected to one end of the crankshaft 3. An oil pump 60 communicating with the strainer 11 is mounted in the crankcase 2. The oil pump 60 is connected to the crankshaft 3 by a known power transmitting mechanism 61 such as a chain or a gear mechanism, and is therefore driven by the rotation of the crankshaft 3. The crankcase 2 is formed with an oil passage 2a communicating with one of a pair of bearings 62 for rotatably supporting the crankshaft 3. The crankpin 4 is formed with an oil passage 4a connecting the two bearings 62 for the crankshaft 3 and communicating with bearings 63 for rotatably supporting the connecting rods 5 and 21. The crankshaft 3 is formed with an oil passage 3a connecting the oil passage 2a and the oil passage 4a. The oil passage 2a is connected at its one end to a discharge port of the oil pump 60. Reference numerals 64 denote a pair of oil seals. Accordingly, when the engine 1 is operated to rotate the crankshaft 3, the oil pump 60 is driven to pump up the lubricating oil OL stored in the bottom portion of the crankcase 2 through the strainer 11. The lubricating oil OL discharged from the oil pump 60 is supplied through the oil passage 2a, the oil passage 3a, and the oil passage 4a to the bearings 62 and 63 and the other movable portions of the engine 1. According to this preferred embodiment, the crankcase 2 is not used as a compression chamber for compressing a scavenging air and a fuel mixture, so that a direct lubrication system can be adopted by utilizing the bottom portion of the crankcase 2 as an oil reservoir. By adopting the direct lubrication system, the lubricating oil need not be mixed with the fuel mixture, so that contamination of the exhaust gas by combustion of the lubricating oil can be prevented. Further, the second compressor is a portion for compressing a scavenging air. Accordingly, even if a lubrication system by mixing of a lubricating oil with a fuel mixture is used, it is difficult to sufficiently lubricate the second compressor so as to endure intense driving associated with engine operation. In this respect, this preferred embodiment adopts a direct lubrication system capable of ensuring sufficient lubrication also to the second compressor, so that the second compressor for compressing a scavenging air can be achieved in a sufficiently lubricated condition. The operation of the engine 1 will now be described with reference to FIGS. 5 to 9. As shown in FIG. 5, the fuel mixture is burned in the combustion chamber 12, and the piston 7 is slightly lowered from the top dead center toward the bottom dead center. In concert therewith, the stepped piston 23 in the compressor section 20 is lowered to generate a negative pressure in the pressure chamber 27 of the first compressor. As a result, the outside air induced from the air cleaner is supplied to the carburetor 17 to obtain a fuel mixture, which is in turn supplied through the main passage 19 and the opened reed valve 15 to the branch pipe 18. At the same time, a negative pressure is generated also in the pressure chamber 28 of the second compressor. Accordingly, the outside air induced from the air cleaner is also supplied through the air supply pipe 32 and the opened reed valve 33 to the air passage 30. As shown in FIG. 6, the burned gas in the cylinder 8 is expanded to further lower the piston 7 to a position immediately before exhausting the burned gas. In this position, the exhaust port 36 and the scavenging port 31 are still closed by the piston 7, whereas the stepped piston 23 in the compressor section 20 changes its position from the bottom dead center to a rising stroke, so that the negative pressures in the first pressure chamber 27 and the second pressure chamber 28 are changed into positive pressures. Accordingly, the fuel mixture supplied midway into the branch pipe 18 in the condition shown in FIG. 5 is returned from the branch pipe 18 toward the main passage 19 as shown in FIG. 6, so that the reed valve 15 is closed to raise the pressure of the fuel mixture in the main passage 19. Similarly, the air supplied into the air passage 30 in the condition shown in FIG. 5 is raised in pressure to close the reed valve 33 as shown in FIG. 6, and is confined in the air passage 30. As shown in FIG. 7, the piston 7 is further lowered to open the exhaust port 36 and subsequently open the scavenging port 31. In this condition, the burned gas is exhausted from the exhaust port 36. In concert therewith, the stepped piston 23 is further raised to further raise both the pressure of the fuel mixture in the main passage 19 and the pressure of the air in the air passage 30. At the time the scavenging port 31 is opened, the highpressure air confined in the air passage 30 is allowed to flow from the scavenging port 31 into the cylinder 8. As a result, a lateral air flow leading from the scavenging port 31 to the exhaust port 36 is generated in the cylinder 8 to thereby scavenge the burned gas. As shown in FIG. 8, the piston 7 changes its position from the bottom dead center to a rising stroke to just close the scavenging port 31 and subsequently close the exhaust port 36. At this time, the stepped piston 23 in the compressor section 20 takes a position near the top dead center to obtain the most compressed condition of the air in the first pressure chamber 27 and the second pressure chamber 28. Accordingly, the pressure of the fuel mixture confined in the main passage 19 and the pressure of the air confined again in the air passage 30 are maximized. In this condition, the valve element 40 of the poppet valve 13 is pushed down against the return spring 42 by the air pressure led from the air passage 30 through the control passage 34a to the pressure chamber 44 formed on the upper side of the diaphragm 43. Accordingly, the fuel-mixture supply port formed at the top of the cylinder 8 is opened to thereby supply the pressurized fuel mixture from the main passage 19 into the combustion chamber 12. In the case of using an electromagnetic control valve in place of the diaphragm operated poppet valve 13 as a control valve for controlling the supply of the fuel mixture, the electromagnetic control valve is excited in the above-mentioned condition to be opened for a given period of time. As shown in FIG. 9, the piston 7 is further raised to reach a position immediately before the top dead center. In this condition, the stepped piston 23 changes its position from the top dead center to a lowering stroke. As a result, the air pressure in the air passage 30, that is, in the pressure chamber 44 is lowered, so that the valve element 40 of the poppet valve 13 is raised by the return spring 42 to close the fuel-mixture supply port, thereby obtaining a most compressed condition of the fuel mixture in the combustion chamber 12. In this condition, the spark plug 10a is excited to ignite the fuel mixture. The operation by the pressure control valve 49 to be opened and closed in concert with the throttle operation will now be described with reference to FIGS. 10 to 12. FIG. 10 is a graph showing an operation characteristic of the diaphragm operated poppet valve 13 controlled by the pressure control valve 49. In FIG. 10, the horizontal axis represents an angle  (deg.) of the crankshaft 3, and the vertical axis represents a pressure Pd (Pa) in the pressure chamber 44 of the diaphragm operated poppet valve 13. When the piston 7 is in the position of top dead center TDC, the crank angle  is 0 (deg.), and when the piston 7 is in the position of bottom dead center BDC, the crank angle  is 180 (deg.). In FIG. 10, the line A denotes a pressure line showing a set load of the return spring 42 in the diaphragm operated poppet valve 13 converted into a pressure in the pressure chamber 44. This pressure is set to Pd1. The compressor section 20 is so set as to start compression when the crank angle is 1. The pressure Pd in the pressure chamber 44 changes as shown by a curve B or C every time the crankshaft 3 is rotated. The curve B is a pressure curve in the pressure chamber 44 in the case that the pressure control valve 49 is fully opened, and the curve C is a pressure curve in the pressure chamber 44 in the case that the pressure control valve 49 is opened by a given angle. The pressure Pd in the pressure chamber 44 changes with the opening angle of the pressure control valve 49, and the larger the opening angle of the pressure control valve 49, the larger the waveform of the pressure curve in the pressure chamber 44. Accordingly, as apparent from the graph shown in FIG. 10, the waveform of the curve B is larger than that of the curve C. In the case that the pressure control valve 44 is fully opened, the curve B intersects the line A at a point B1 where the pressure Pd in the pressure chamber 44 coincides with the pressure Pd1 corresponding to the set load of the return spring 42. When the pressure Pd exceeds the pressure Pd1, the diaphragm operated poppet valve 13 is opened. On the other hand, in the case that the pressure control valve 49 is opened by a given angle, the curve C intersects the line A at a point C1 where the pressure Pd in the pressure chamber 44 coincides with the pressure Pd1 corresponding to the set load of the return spring 42. When the pressure Pd exceeds the pressure Pd1, the diaphragm operated poppet valve 13 is opened. By changing the opening angle of the pressure control valve 49 so that it increases with an increase in throttle opening angle, the opening timing of the diaphragm operated poppet valve 13 can be changed according to the crank angle , thus effecting combustion according to the operating condition of the engine 1. FIG. 11 is a graph showing an opening timing control characteristic of the diaphragm operated poppet valve 13. In FIG. 11, the horizontal axis represents a rotating speed Ne (rpm) of the crankshaft 3, and the vertical axis represents a crank angle  (deg.) of the crankshaft 3. In FIG. 11, the terms of "exhaust start" and "exhaust end" mean the start timing of exhaust from the engine 1 and the end timing of exhaust from the engine 1, respectively, and the terms of "scavenging start" and "scavenging end" mean the start timing of scavenging to the engine 1 and the end timing of scavenging to the engine 1, respectively. In the graph shown in FIG. 11, the curve D is an opening timing characteristic curve of a control valve for controlling the supply of an optimum fuel mixture required by the engine 1, and the curve E is an opening timing characteristic curve inherent to the diaphragm operated poppet valve 13. As mentioned above, the pressure control valve 49 controls the operation characteristic of the diaphragm operated poppet valve 13 in concert with a throttle opening angle (i.e., substantially in concert with a rotating speed Ne of the crankshaft 3). By advancing the opening timing of the poppet valve 13 and increasing the open duration of the poppet valve 13 with an increase in throttle opening angle, the curve E showing the opening timing characteristic of the diaphragm operated poppet valve 13 can be approximated to the curve D showing an optimum characteristic. FIG. 12 is a graph showing an open duration characteristic of the diaphragm operated poppet valve 13. In FIG. 12, the horizontal axis represents a rotating speed Ne (rpm) of the crankshaft 3, and the vertical axis represents a range α (deg.) of change in crank angle. The graph shown in FIG. 12 shows the duration of an open condition of the diaphragm operated poppet valve 13 with respect to a change of a certain crank angle according to a rotating speed Ne of the crankshaft 3. This relation will be hereinafter referred to as "open duration characteristic", and the curve showing the open duration characteristic will be hereinafter referred to as "open duration characteristic curve". For example, in an open duration characteristic curve F shown in FIG. 12, the diaphragm operated poppet valve 13 continues to open for a period of time required for a change of about 210 deg. in the crank angle in the case that the rotating speed Ne of the crankshaft 3 is 7,000 rpm. The curve F is an open duration characteristic curve of an optimum fuel mixture injection valve required by the engine 1, and the curve G is an open duration characteristic curve inherent to the diaphragm operated poppet valve 13. As mentioned above, the operation characteristic of the diaphragm operated poppet valve 13 can be changed by the pressure control valve 49. Accordingly, the open duration characteristic of the diaphragm operated poppet valve 13 can be approximated to the characteristic shown by the curve F irrespective of the rotating speed Ne of the crankshaft 3. As described above, the operation characteristic of the diaphragm operated poppet valve 13 can be changed by the pressure control valve 49 according to a throttle opening angle, thereby maintaining the supply of a fuel mixture in an optimum condition over the substantially entire rotating speed region of the crankshaft 3. In the above preferred embodiment, the compressor section 20 is of a reciprocating type configured by a cylinder and a piston, and is so located as to form a V shape in 90° spaced relationship with the cylinder 8 forming the combustion chamber 12, thereby canceling primary vibrations due to the operation of the engine 1 to allow silent operation. However, the type and location of the compressor section are not especially limited from the viewpoint of the exhaust gas purification as one of the objects of the present invention. Further, while the pressure control valve 49 is provided to effect better engine operation according to throttle operation in the above preferred embodiment, the pressure control valve 49 may be omitted from the viewpoint of the exhaust gas purification. According to the present invention as described above, by adopting a direct lubrication system using an oil pump in a two-cycle internal combustion engine, the combustion of a lubricating oil and the blow-by of a fuel mixture can be prevented to thereby purify an exhaust gas. Furthermore, the combustion cylinder and the compressor section are so arranged as to form a V shape in 90° spaced relationship, thereby allowing silent engine operation. Although a diaphragm operated poppet valve is used as a fuel mixture supply control valve, a diaphragm for operating the poppet valve is operated by a mechanism isolated from the fuel mixture, so that the deterioration proper condition. Additionally, the opening characteristic of the poppet valve can be changed according to throttle operation, thereby effecting optimum fuel mixture supply according to engine operating condition. The invention provides a two-cycle internal combustion engine which can realize scavenging by air, prevent blow-by of a fuel mixture in a scavenging stroke, and prevent combustion of a lubricating oil in a combustion chamber to thereby purify an exhaust gas. To achieve this, a two-cycle internal combustion engine 1 includes a compressor section 20 for performing a compression stroke in concert with a scavenging stroke by a piston 7 reciprocating in a cylinder 8. A fuel mixture from a carburetor 17 is pressurized to be supplied to a combustion chamber 12 by the compressor section 20. On the other hand, a scavenging air is also pressurized to be supplied to the cylinder 8 from its side wall by the compressor section 20. An oil pump 60 driven by a crankshaft 3 is provided to pump up a lubricating oil stored in a bottom portion of a crankcase, thereby lubricating the crankshaft with the lubricating oil supplied from the oil pump.

Claims: Two-cycle internal combustion engine having first and second compressors (23b, 24b; 23a, 24a) for performing a compression stroke in concert with a scavenging stroke by a piston (7) reciprocating in a cylinder (8), said first compressor (23b, 24b) pressurizing a fuel mixture supplied from a fuel supplier (17) to supply said fuel mixture under pressure into a combustion chamber (12) defined in said cylinder (8), said second compressor (23a, 24a) pressurizing a scavenging air induced from the outside of said engine (1) to supply said scavenging air under pressure into said cylinder (8); the engine further comprising: a main passage (19) for connecting said fuel supplier (17) and said combustion chamber (12); a control valve (13) provided in said main passage (19) and adapted to open in said scavenging stroke for controlling supply of said fuel mixture to said combustion chamber (12); a fuel-mixture check valve (15) provided in said main passage (19) at a position between said control valve (13) and said fuel supplier (17) for preventing reverse flow of said fuel mixture toward said fuel supplier (17), while allowing normal flow of said fuel mixture from said fuel supplier (17) to said first compressor (23b, 24b); a branch passage (18) for connecting said first compressor (23b,24b) and said main passage (19) at a position between said fuel-mixture check valve (15) and said control valve (13); a scavenging port (31) formed in a side wall of said cylinder (8) so as to open into said cylinder (8); an auxiliary passage (30) for connecting said scavenging port (31) and said second compressor (23a, 24a); an air check valve (33) provided in said auxiliary passage (30) for preventing reverse flow of said scavenging air toward the outside of said engine (1), while allowing normal flow of said scavenging air from the outside of said engine (1) to said second compressor (23a, 24a); a lubricating oil(OL) reservoir formed in a bottom portion of a crankcase (2) of said engine (1); and an oil pump (60) for supplying a lubricating oil (OL) from said lubricating oil (OL) reservoir to movable portions of said engine (1). Two-cycle internal combustion engine according to claim 1, further comprising: a stepped cylinder (24) connected to said crankcase (2) and located in 90° spaced relationship with said cylinder (8) defining said combustion chamber (12); and a stepped piston (23) reciprocating in said stepped cylinder (24), said stepped piston (23) being connected to a crankpin (4) for connecting said piston (7) and a crankshaft (3); said stepped cylinder (24) and said stepped piston (23) cooperating to define a first pressure chamber (27) of said first compressor (23b, 24b) and a second pressure chamber (28) of said second compressor (23a, 24a). Two-cycle internal combustion engine according to claim 1 or 2, characterized in that said control valve (13) is a diaphragm operated poppet valve (13) provided in said main passage (19) for controlling supply of said fuel mixture to said combustion chamber (12); wherein said engine further comprises: a control passage (34a) for connecting said auxiliary passage (30) and a diaphragm operation port (35) of said poppet valve (13); said poppet valve (13) being opened by an air pressure in said auxiliary passage (30) to supply said fuel mixture into said combustion chamber (12) in said scavenging stroke by said piston (7). Two-cycle internal combustion engine according to daim 3, further comprising a pressure control valve (49) provided in said control passage (34a) and adapted to be opened and closed in concert with throttle operation.

Inventors: OOKI KENJI, TSUSHIMA YUJI, UEDA MINORU, YAGASAKI AKIO, OOKI, KENJI, TSUSHIMA, YUJI, UEDA, MINORU, YAGASAKI, AKIO, Ooki, Kenji, c/o K.K. Honda Gijutsu Kenkyusho, Tsushima, Yuji, c/o K.K. Honda Gijutsu Kenkyusho, Ueda, Minoru, c/o K.K. Honda Gijutsu Kenkyusho, Yagasaki, Akio, c/o K.K. Honda Gijutsu Kenkyusho
Applicants: HONDA MOTOR CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 2000
Main classification: F02B75
IPC codes: F02F1, F02B33, F02B75, F02B1, F01M3