Patent Publication Number: US-2023133431-A1

Title: Two-stroke internal combustion engine and engine working machine

Description:
RELATED APPLICATIONS 
     The present application is a National Phase of International Application No. PCT/JP2021/005491 filed Feb. 15, 2021, which claims priority to Japanese Patent Application No. 2020-034899, filed Mar. 2, 2020. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a two-stroke internal combustion engine and an engine working machine using the two-stroke internal combustion engine as a power source. 
     BACKGROUND ART 
     A two-stroke internal combustion engine is often used as a power source in portable working machines such as brush cutters, chain saws, and power blowers. In the two-stroke internal combustion engine, air and a fuel are mixed in a carburetor to generate an air-fuel mixture, and the air-fuel mixture is sucked into a crank chamber. The two-stroke internal combustion engine includes a scavenging passage that allows communication between the crank chamber and a combustion chamber. When a piston is actuated in a cylinder, the air-fuel mixture having been pre-compressed in the crank chamber is introduced into the combustion chamber through the scavenging passage, and scavenging is performed with the air-fuel mixture. 
     The two-stroke internal combustion engine has a well-known problem of “air-fuel mixture (fresh gas) blow-by”. Specifically, the air-fuel mixture for scavenging introduced into the combustion chamber is directly discharged from an exhaust port of the cylinder. The air-fuel mixture blow-by leads to a waste of fuel, and may cause air pollution. 
     To solve this problem, there has been known a so-called “stratified scavenging two-stroke engine”, which is configured to allow air and an air-fuel mixture to be introduced into a cylinder in the stated order as gases to be introduced thereinto (Republication of PCT International Publication No. 98/057053). The stratified scavenging two-stroke engine of this technology includes an air supply intake passage and an air-fuel mixture supply intake passage. Air is supplied to an upper part of a scavenging passage and an air-fuel mixture is supplied to a crankcase through a reciprocating operation of a piston inside the cylinder. As a result, the air stagnant in the upper part of the scavenging passage contributes to scavenging so that blow-by is reduced with use of so-called “leading air”. 
     With this configuration, supply amounts and timing of both of the leading air and the air-fuel mixture is determined dependently on a negative pressure generated in a combustion chamber and the amount of air stagnant in the upper part of the scavenging passage, and thus cannot be precisely controlled. As an idea, a countermeasure plan of, for example, appropriately setting a sectional area of each of intake passages for the leading air and the air-fuel mixture to adjust a ratio of air and the air-fuel mixture in intake air may be employed to define a supply ratio of the leading air and the air-fuel mixture. However, even the above-mentioned countermeasure does not enable such precise control as to achieve an optimal supply ratio over the entire operating range with a wide speed range. Further, to prevent damage to the engine, the air-fuel mixture supply amount is inevitably set larger (with a richer air-fuel mixture) to maintain stability of an operation over the entire speed range. 
     Further, the two-stroke internal combustion engine is widely used for portable working machines. For its features, the two-stroke internal combustion engine is not only used in a high-speed and high-load range but also often used for acceleration/deceleration between low and high speed, at an intermediate speed, or in a light-load range. In such a two-stroke internal combustion engine, it is also desired to supply the leading air and the air-fuel mixture that are most suitable for the operating state of the engine to the combustion chamber. More specifically, for example, accurate control of a leading air amount and an air-fuel mixture amount under various operating states as described above, setting a sufficient leading-air supply amount without depending on an air-fuel mixture supply amount required for each of the operating ranges of the engine, and maintaining compactness of the engine are demanded. 
     Solution to Problem 
     This disclosure has been made in view of the circumstances described above, and has an object to provide a two-stroke internal combustion engine that prevents air-fuel mixture blow-by at a time of scavenging and allows an air-fuel mixture suitable for an operating state of the engine to be supplied to a combustion chamber. 
     This disclosure has another object to provide an engine working machine including the two-stroke internal combustion engine as a power source. 
     In order to solve the problems described above, according to this disclosure, there is provided a two-stroke internal combustion engine, including: a cylinder, which defines a combustion chamber, and has an exhaust port; an ignition device configured to ignite an air-fuel mixture in the combustion chamber; a piston configured to reciprocate inside the cylinder through combustion and expansion that occur in the combustion chamber; a crank chamber communicating with an interior of the cylinder; a crankshaft, which is disposed in the crank chamber, and is operationally coupled to the piston; a fuel injection valve configured to inject a fuel into the crank chamber; an intake passage configured to allow only air to be sucked thereinto under a negative pressure generated when the piston is actuated; and a scavenging passage that allows communication between the crank chamber and the combustion chamber, wherein air passing through the intake passage is introduced into the scavenging passage, and air stagnant in the scavenging passage at end of air suction contributes to scavenging. 
     According to this disclosure, the fuel is injected into the crank chamber by the fuel injection valve. Further, the air passing through the intake passage is introduced into the scavenging passage. The air introduced into the scavenging passage is sucked into the crank chamber. At the end of air suction, the air stagnates in the scavenging passage. The air that has been introduced into the crank chamber through the scavenging passage is mixed with the fuel to generate an air-fuel mixture. The air-fuel mixture in the crank chamber is introduced into the combustion chamber through the scavenging passage by actuating the piston. The air-fuel mixture in the combustion chamber is compressed by the piston, and is ignited by the ignition device to cause combustion and expansion. Scavenging and discharge are achieved while the piston is being pushed back by the combustion and expansion. Specifically, the air stagnant in the scavenging passage is pressure-fed into the combustion chamber, and contributes to the scavenging. Through the scavenging, the combustion gas is discharged from the exhaust port. 
     As described above, according to this disclosure, the fuel is supplied to the crank chamber by the fuel injection valve, thus fuel supply timing can easily be controlled, thereby enabling supply of the air-fuel mixture suitable for an operating state of the engine. Further, only air is sucked into the intake passage, and the air passing through the intake passage is introduced into the scavenging passage, thus the intake passage can be formed in a simple manner. The intake passage that allows only air to be sucked thereinto not only facilitates control of the intake air but also contributes to improvement of reliability of air control. At the end of air suction, the air stagnates in the scavenging passage, and since this air contributes to the scavenging, thus air-fuel mixture blow-by, which may occur at the time of scavenging, is prevented. As a result, an exhaust gas is improved in components thereof. 
     Further, according to this disclosure, since only an air passage is required to be arranged on an intake side, an intake passage and an intake port for air-fuel mixture supply, which have been provided and formed in a related-art stratified scavenging engine, are not required. Thus, the cylinder, a manifold, and other components can be formed in a simple manner. This configuration simplifies an air control valve to contribute to facilitation and improvement of reliability of control of an intake air amount. More specifically, a related-art cylinder is required to have a configuration in which an air port and an air-fuel mixture port are arranged vertically or in a circumferential direction. Meanwhile, according to this disclosure, since a port for air-fuel mixture supply is not required, a degree of freedom in design of a port opening in the cylinder is remarkably improved. 
     As one embodiment, the piston may have a piston groove, and an intake port and the scavenging passage may communicate with each other via the piston groove. In this case, since the intake port is opened and closed by the piston, it is not necessary to dispose a check valve. 
     As one embodiment, the piston groove may have a hole that allows communication with the crank chamber. In this case, air also flows from the intake port into the crank chamber through the hole and an air supply amount to the crank chamber can be increased. 
     As one embodiment, a communication portion, which is defined between a lower end of the piston and a lower end of the intake port, and allows communication between the intake passage and the crank chamber may be provided. In this case, air also flows from the intake port into the crank chamber through the communication portion and an air supply amount to the crank chamber can be increased. 
     As one embodiment, a cutout may be defined in a lower end of the piston, and the communication portion may be formed by the cutout. In this case, air flows from the intake port into the crank chamber through the cutout. 
     As one embodiment, an enlarged portion being enlarged in a downward direction may be provided at a lower end of the intake port, and the communication portion may be formed by the enlarged portion. In this case, air flows from the intake port into the crank chamber through the enlarged portion. 
     As one embodiment, the scavenging passage may have a branch passage communicating with the piston groove, and the branch passage may be closer to the crank chamber than a scavenging port is. In this case, since air flows from the intake port into the crank chamber through the branch passage, the air supply amount to the crank chamber can be increased. 
     As one embodiment, the fuel injection valve may be configured to inject the fuel in such a direction as to avoid hinderance of inflow of air into the crank chamber. 
     As one embodiment, the fuel injection valve may include a high-pressure fuel injection valve configured to receive a fuel pressure from at least one of an electrically-driven fuel pump or a pump configured to operate through rotation of the crankshaft. This high-pressure fuel injection valve enables the fuel injection to an aimed position. 
     As one embodiment, the fuel injection valve may be configured to inject the fuel at high pressure toward an area required to be cooled in the cylinder or the crank chamber. This configuration allows the area required to be cooled to be effectively cooled with the fuel injected from the fuel injection valve at high pressure. 
     As one embodiment, the fuel injection valve may include a high-pressure fuel injection valve capable of injecting the fuel when the crank chamber has a maximum internal pressure while the piston is being actuated. The use of the high-pressure fuel injection valve enables optimization of timing of fuel injection into the crank chamber. Thus, for example, fuel is prevented from being mixed with the stagnant air in the scavenging passage, and hence the exhaust gas is improved in components thereof. 
     As one embodiment, the intake passage may have an opening coupled to only one intake port. In this manner, in comparison to a related-art configuration having an air-fuel mixture port and two air ports, the intake passage can be formed in a simple manner, which significantly contributes to compactness of the working machine. 
     As one embodiment, a booster passage connecting the crank chamber and the combustion chamber is arranged separately from the scavenging passage, and fuel supply timing of the fuel injection valve may be controlled by a control device to achieve homogenous combustion during a full-load operation and to allow the fuel to flow into the combustion chamber through the booster passage to achieve stratified combustion during a light-load operation. In this case, the homogenous combustion is set to occur during the full-load operation, and the stratified combustion is set to occur during the light-load operation. In this manner, thermal efficiency is improved not only during the full-load operation but also during the light-load operation. 
     As one embodiment, a bottom dead center-side booster port that allows communication between the crank chamber and the booster passage may be provided, and a fuel supply configuration that allows the fuel supplied from the fuel injection valve to be introduced to a vicinity of the bottom dead center-side booster port during the light-load operation may be provided. In this case, since the fuel supplied from the fuel injection valve is introduced (flows) into the vicinity of the bottom dead center-side booster port during the light-load operation, the fuel easily flows into the combustion chamber through the booster passage. Meanwhile, air is introduced from the scavenging passage into the combustion chamber, and the air and the air-fuel mixture are introduced from different passages, which is suitable for the stratified combustion during the light-load operation. 
     As one embodiment, the fuel supply configuration may include both of a first configuration that allows the fuel injection valve to be arranged in the vicinity of the bottom dead center-side booster port and a second configuration that allows the fuel to be injected from the fuel injection valve at such an injection pressure that the fuel is allowed to reach and stay in the vicinity of the bottom dead center-side booster port. 
     As one embodiment, the fuel supply configuration may include both of a first configuration that allows the fuel injection valve to be arranged so that the fuel is injected toward the bottom dead center-side booster port and a second configuration that allows the fuel to be injected from the fuel injection valve at such an injection pressure that the fuel is allowed to reach the bottom dead center-side booster port. 
     As one embodiment, the fuel may be supplied from the fuel injection valve at timing at which the piston is positioned in a vicinity of a top dead center during the full-load operation, and the fuel may be supplied from the fuel injection valve at timing immediately before the piston reaches a vicinity of a bottom dead center during the light-load operation. In this manner, homogeneity of the air-fuel mixture in the crank chamber is improved during the full-load operation, which is suitable for the homogenous combustion. Meanwhile, during the light-load operation, the fuel supplied by the fuel injection valve flows into the combustion chamber through the booster passage before being mixed with the air supplied to the crank chamber. Since the air supplied to the crank chamber is introduced from the scavenging passage into the combustion chamber without being mixed with the fuel, it is suitable for stratified combustion. 
     As one embodiment, a pair of scavenging ports, which allow communication between the scavenging passage and the combustion chamber, may be arranged in an inner peripheral surface of the cylinder, and the exhaust port may be arranged on one arc-shaped surface side between the pair of scavenging ports, and a top dead center-side booster port that allows communication between the combustion chamber and the booster passage may be arranged on another arc-shaped surface side between the pair of scavenging ports. In this case, during the light-load operation, the fuel flows into the combustion chamber through the booster passage, and the air flows into the combustion chamber from the pair of scavenging ports. A rich air-fuel mixture flows from the booster port for air-fuel mixture supply, which is formed separately from the scavenging passage, mixes with the air in the combustion chamber. As a result, a rich air-fuel mixture is generated in the vicinity of a spark portion of the ignition device. In this manner, stability of the stratified combustion during the light-load operation is improved. 
     As described above, according to this disclosure, the leading air and the air-fuel mixture that are most suitable for the operating state of the engine can be supplied to the combustion chamber not only when the two-stroke internal combustion engine operates in a high-speed and high-load range but also when the two-stroke internal combustion engine accelerates or decelerates between low and high speed, operates at an intermediate speed, or operates in the light-load range. Further, a combination of the arrangement of the fuel injection device, timing control, and directivity allows accurate control of each of a leading air amount and an air-fuel mixture amount. Since the cylinder has only the air port for sucking air, a sufficient leading air supply amount can be set, and contributes to simplification and compactness of an intake-side (counter-exhaust side) structure of the engine. 
     As one embodiment, a check valve may be provided in the intake passage. When the check valve is provided, it is possible to prevent the blow-back from the combustion chamber in a previous cycle from being mixed into the intake passage, and the air supply amount and the fuel injection amount in a subsequent cycle can be more precisely managed. 
     As one embodiment, an engine working machine including the two-stroke internal combustion engine as a power source may be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic view of a two-stroke internal combustion engine according to one embodiment of this disclosure. 
         FIGS.  2 A to  2 D  are explanatory views for illustrating working strokes of an engine of  FIG.  1   . 
         FIG.  3    is a block diagram for illustrating flows of air and a fuel in the engine of  FIG.  1   . 
         FIGS.  4 A and  4 B  are schematic views of a two-stroke internal combustion engine according to modification embodiment of this disclosure. 
         FIG.  5    is a schematic view of a two-stroke internal combustion engine according to modification embodiment of this disclosure. 
         FIGS.  6 A and  6 B  are schematic views of a two-stroke internal combustion engine according to modification embodiment of this disclosure. 
         FIG.  7    is a schematic view of a two-stroke internal combustion engine according to modification embodiment of this disclosure. 
         FIGS.  8 A to  8 C  are explanatory views for illustrating a two-stroke internal combustion engine according to another embodiment of this disclosure during a full-load operation, in which  FIG.  8 A  is a schematic front view of the engine,  FIG.  8 B  is a diagram for illustrating fuel supply timing while the engine is performing the full-load operation, and  FIG.  8 C  is a schematic plan view of the engine. 
         FIGS.  9 A and  9 B  are explanatory views for illustrating the two-stroke internal combustion engine of  FIGS.  8 A to  8 C  during a light-load operation, in which  FIG.  9 A  is a schematic front view of the engine, and  FIG.  9 B  is a diagram for illustrating fuel supply timing while the engine is performing the light-load operation. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, with reference to the accompanying drawings, a description is given of the embodiments of this disclosure. 
     A two-stroke internal combustion engine (hereinafter simply referred to as “engine”) according to this disclosure is of air-cooled type to be mainly mounted in a portable engine working machine as a power source. Examples of a working machine in which the engine of this disclosure is used include a handheld, shoulder hanging, or backpack type portable working machine such as a chain saw, a brush cutter, a power cutter, a hedge-trimmer, and a power blower. 
     As illustrated in  FIG.  1   , an engine  1  according to one embodiment of this disclosure includes a cylinder block  2  and a piston  4 . The piston  4  slidably reciprocates inside a cylinder  3  forming the cylinder block  2 . A cylinder head  5 , which forms one end side of the cylinder block  2 , and the piston  4  define a combustion chamber  6 . A crankcase  7 , which forms another end side of the cylinder block  2 , and the piston  4  define a crank chamber  8 . A spark plug  9  forming to an ignition device is firmly fixed to the cylinder head  5 . The spark plug  9  projects into the combustion chamber  6 . A crankshaft  10  is pivotably supported in the crankcase  7 . The crankshaft  10  and a piston pin  4   a  of the piston  4  are coupled to each other with a connecting rod  11 . Combustion and expansion (explosion) in the combustion chamber  6  cause the piston  4  to slidably reciprocate inside the cylinder  3 . The reciprocation of the piston  4  rotationally drives the crankshaft  10  through the connecting rod  11 , and a rotational driving force is output to an output shaft (not shown) connected to the crankshaft  10 . 
     An exhaust port  12  and at least one scavenging port  14  are open on an inner wall of the cylinder  3 . These ports  12  and  14  are controlled to be opened and closed at predetermined timing through the reciprocation of the piston  4 . The exhaust port  12  communicates with a muffler  15   a  via an exhaust pipe  15 . The scavenging port  14  communicates with the crank chamber  8  via a scavenging passage  18 . 
     The engine  1  includes an air supply intake passage  17 . Only air is sucked into the intake passage  17  under a negative pressure generated when the piston  4  is actuated. A throttle valve  19  such as a butterfly throttle valve is disposed in the intake passage  17 , and an air cleaner  20  is disposed on an upstream side of the throttle valve  19 . When an output operating member (such as a throttle trigger or a throttle lever) of a working machine is operated by an operator, an opening degree of the throttle valve  19  is adjusted. 
     The intake passage  17  communicates with the scavenging passage  18 . A check valve  23  that prevents backflow of air from the scavenging passage  18  is disposed in the intake passage  17 . The intake passage  17  is coupled to an intake port  24  formed at an end of the scavenging passage  18  on the scavenging port  14  side. The intake passage  17  guides the air that has been purified in the air cleaner  20  to the scavenging passage  18 . The intake passage  17  has an opening coupled to only one intake port  24 . 
     The air that has been purified in the air cleaner  20  passes from the intake passage  17  through the check valve  23 , the intake port  24 , and the scavenging passage  18  in the stated order to be sucked into the crank chamber  8  during an intake stroke in which a pressure in the crank chamber  8  becomes negative. At the end of the intake stroke, the air stagnates in the scavenging passage  18 . During a scavenging stroke, the stagnant air flows from the scavenging port  14  into the cylinder  3  before the air-fuel mixture in the crank chamber  8  flows thereinto. As a result of the leading-air scavenging, a combustion gas in the combustion chamber  6  is discharged from the exhaust port  12 . 
     The engine  1  includes a fuel injection valve  25  for supplying a fuel into the crank chamber  8 . In the illustrated example, the fuel injection valve  25  is disposed at a lower part of the crank case  7 . 
     A fuel tank  26  and a fuel pump  27  are coupled to the fuel injection valve  25 . When the fuel pump  27  is actuated, the fuel in the fuel tank  26  is supplied to the fuel injection valve  25 , and the injection of the fuel is controlled by opening and closing the fuel injection valve  25 . 
     The fuel injection valve  25  is controlled to be opened and closed by a control device  28  including a microcomputer. A detection signal from at least one sensor  29  ( 29   a  to  29   g ) that detects an operating condition of the engine  1  is input to the control device  28 . The control device  28  controls the fuel supply from the fuel injection valve  25  to the crank chamber  8  based on the detection performed by the sensor  29 . 
     Examples of the at least one sensor  29  include, for example, an intake air temperature sensor  29   a , an intake air pressure sensor  29   b , a throttle valve opening-degree sensor  29   c , a crank chamber pressure sensor  29   d , a crank chamber temperature sensor  29   e , an engine speed sensor  29   f , and a crank angle sensor  29   g . The intake air temperature sensor  29   a  detects an intake air temperature, and sends out a signal. The intake air pressure sensor  29   b  detects an intake air pressure, and sends out a signal. The throttle valve opening-degree sensor  29   c  detects an opening degree of the throttle valve  19 , and sends out a signal. The crank chamber pressure sensor  29   d  detects a pressure in the crank chamber  8 , and sends out a signal. The crank chamber temperature sensor  29   e  detects a temperature in the crank chamber  8 , and sends out a signal. The engine speed sensor  29   f  detects a speed of the engine  1 , and sends out a signal. The crank angle sensor  29   g  detects a crank angle, and sends out a signal. The signals from these sensors  29   a  to  29   g  are input to the control device  28 . 
     The control device  28  sends out a fuel injection signal to the fuel injection valve  25  at appropriate timing and sends out an ignition signal to the spark plug  9  forming the ignition device at predetermined timing in accordance with a predetermined program based on various signals from the sensors  29   a  to  29   g . In this manner, the fuel supply to the crank chamber  8  and ignition of the air-fuel mixture in the combustion chamber  6  are achieved. 
     Next, an operation of the engine  1  of  FIG.  1    is described with reference to  FIGS.  2 A to  2 D . In  FIGS.  2 A to  2 D ,  FIG.  2 A  is an explanatory view for illustrating an operation at a time of scavenging,  FIG.  2 B  is an explanatory view for illustrating an operation at a time of intake and compression,  FIG.  2 C  is an explanatory view for illustrating an operation at a time of combustion and expansion (explosion), and  FIG.  2 D  is an explanatory view for illustrating an operation at a time of discharge. The fuel injection to the crank chamber  8  is only required to be performed at an appropriate time. As an example, the fuel injection is set to occur in a final phase of the intake stroke. 
     As illustrated in  FIG.  2 A  and  FIG.  2 B , when the scavenging port  14  is closed by the piston  4  in a process in which the piston  4  is moving up from a bottom dead center, the pressure in the crank chamber  8  becomes negative due to the upward movement of the piston  4 . In this manner, air is sucked from the intake passage  17  through the intake port  24  and the scavenging passage  18  into the crank chamber  8 . As an example, the fuel is injected from the fuel injection valve  25  in the final phase of the intake stroke. The air-fuel mixture is compressed in the combustion chamber  6  until the piston  4  reaches a top dead center. When the piston  4  reaches the top dead center, the suction of air into the crank chamber  8  ends. At this time, the crank chamber  8  is filled with the air-fuel mixture corresponding to a mixture of a fuel and air, and air is stagnant in the scavenging passage  18 . 
     As illustrated in  FIG.  2 C , when the piston  4  reaches the top dead center, ignition is performed by the spark plug  9 . The ignition causes combustion and expansion (explosion) of the air-fuel mixture in the combustion chamber  6  to push down the piston  4  to the bottom dead center. As a result, the crankshaft  10  is rotated to generate power. With the piston  4  moving down, the air-fuel mixture in the crank chamber  8  is pre-compressed. At this time, the check valve  23  acts. Thus, air does not flow out from the intake port  24 . 
     As illustrated in  FIG.  2 D , when the exhaust port  12  is opened as a result of downward movement of the piston  4 , a combustion gas flows out to the exhaust pipe  15 . Subsequently, as illustrated in  FIG.  2 A , the scavenging port  14  is opened to allow the pre-compressed air-fuel mixture in the crank chamber  8  to be sent into the combustion chamber  6  through the scavenging passage  18 . At this time, the stagnant air in the scavenging passage  18  flows into the combustion chamber  6  as leading air before the air-fuel mixture in the crank chamber  8  flows thereinto, and pushes the combustion gas to the exhaust port  12 . Thus, blow-by of the air-fuel mixture at the time of scavenging is reduced. The piston  4  moves toward the top dead center again through the rotation of the crankshaft  10 . After that, the same actions are repeated. 
     Flows of the air and the fuel in the engine  1  of  FIG.  1    can be simplified as illustrated in a block diagram of  FIG.  3   . 
     In the engine  1  according to this embodiment, the fuel is supplied to the crank chamber  8  by the fuel injection valve  25 . Thus, timing of the fuel supply can easily be controlled, thereby enabling the supply of the air-fuel mixture suitable for the operating condition of the engine  1 . Further, air stagnates in the scavenging passage  18  at the end of the air suction, and the stagnant air serves as leading air to contribute to scavenging. Thus, air-fuel mixture air blow-by, which may occur at the time of scavenging, is prevented. As a result, an exhaust gas is improved in components thereof. The intake passage  17  is configured to allow only air to be sucked thereinto. This configuration not only facilitates control of intake air but also contributes to improvement of reliability of air control. Further, the intake passage  17  communicates with the scavenging passage  18  via the check valve  23 . Thus, this configuration contributes to simplification of the piston  4  and the scavenging passage  18 . 
     A mode in which the fuel injection valve  25  is disposed and a kind of the fuel injection valve  25  may be determined, for example, as follows. 
     As a suitable one embodiment, the fuel injection valve  25  may be configured to inject the fuel in a direction that avoids the scavenging passage  18 . In this manner, the fuel is less liable to be mixed with the stagnant air in the scavenging passage  18 . 
     As a suitable one embodiment, the fuel injection valve  25  may include a high-pressure fuel injection valve configured to receive a fuel pressure from at least one of an electrically-driven fuel pump or a pump configured to operate through rotation of the crankshaft  10 . This high-pressure fuel injection valve enables the fuel injection to an aimed position. For example, the fuel injection can be aimed at a position in the engine  1  where seizure is liable to occur. 
     As a suitable one embodiment, the fuel injection valve  25  may be configured to inject the fuel at high pressure toward an area required to be cooled in the cylinder  3  or the crank chamber  8 . This configuration allows the area required to be cooled to be effectively cooled with the fuel injected from the fuel injection valve  25  at high pressure. An example of the area required to be cooled includes an area where frictional heat is generated, such as a coupling portion between the connecting rod  11  and the piston pin  4   a  or a coupling portion between the connecting rod  11  and the crankshaft  10 . Further, the fuel may be injected at high pressure against an inner wall of the piston  4  so as to cool the piston  4 . 
     A mixed fuel containing gasoline and lubricating oil mixed therewith is used as a fuel for a two-stroke internal combustion engine. Thus, when the fuel (mixed fuel) is supplied to the crank chamber  8 , lubrication between the piston  4  and the cylinder  3  is readily and quickly achieved with the lubricating oil contained in the fuel. The lubrication also prevents the seizure of the engine  1 . In this case, the seizure of the engine  1  may be prevented by the fuel injection based on at least one of a temperature signal, a speed signal, an intake air pressure signal, or an opening-degree signal. For example, a fuel may be injected when a high temperature is detected during a high-speed operation. Also when a sudden stop is made during a high-speed operation, a fuel may be injected to prevent the seizure. 
     As a suitable one embodiment, the fuel injection valve  25  may include a high-pressure fuel injection valve capable of injecting the fuel when the crank chamber  8  has a maximum internal pressure while the piston  4  is being actuated. The use of the high-pressure fuel injection valve enables optimization of timing of fuel injection into the crank chamber. Thus, for example, gasoline is prevented from being mixed with the leading air, and hence the exhaust gas is improved in components thereof. 
     As a suitable one embodiment, as illustrated in  FIG.  1   , the fuel injection valve  25  may be installed on a side where a fuel tank  26  is arranged with respect to a plane containing an axis X of the cylinder  3  and an axis Y of the crankshaft  10 . This configuration allows the fuel tank  26  and the fuel injection valve  25  to be positioned closer to each other. Thus, a pipe provided between the fuel tank  26  and the fuel injection valve  25  requires only a short length, which further contributes to reduction in size and weight of the working machine. Further, when the fuel injection valve  25  is disposed so that a fuel inlet  25   c  of the fuel injection valve  25  faces the fuel tank  26 , a pipe extending from the fuel tank  26  to the fuel injection valve  25  can easily be provided. Further, in this case, a fuel can be rapidly fed to the fuel injection valve  25  because of a short distance between the fuel tank  26  and the fuel injection valve  25 . Thus, the engine  1  can be started in a desirable manner. In particular, in a case of a high-pressure fuel injection valve that is used together with an electrically-driven fuel pump installed in the fuel tank  26 , the distance between the electrically-driven fuel pump and the fuel injection valve  25  is short, the fuel can be quickly fed to thereby start the engine  1  in a desirable manner. 
     As a suitable one embodiment, the fuel injection valve  25   a  may have a fuel inlet  25   c  at a rear end and be configured to inject the fuel upward, and a fuel tank  26  may be arranged below the crank chamber  8 . Also in this case, the same actions and effects as those described above are obtained. 
     As the fuel injection valve  25 , not only an electrically controllable one but also a mechanically controllable one may be employed. When the latter one is used, for example, a mechanically openable and closable fuel injection valve may be operationally coupled to the crankshaft  10  so that the fuel injection valve is opened and closed at predetermined timing during working strokes of the piston  4 . 
     Next, a modification example of  FIG.  1    is described with reference to  FIGS.  4 A and  4 B . In the following description, components which are the same as or equivalent to those in the example of  FIG.  1    are denoted by the same reference symbols as those in  FIG.  1   , and overlapping description thereof is omitted.  FIG.  4 A  is an illustration of a state in which the piston is located at a top dead center, and  FIG.  4 B  is an illustration of a state in which the piston is located at a bottom dead center. 
     In an engine  40  of  FIGS.  4 A and  4 B , only one intake port  42  is formed in an inner wall of a cylinder  41 . An intake passage  17  communicates with the intake port  42 . The intake port  42  is opened and closed by a piston  43 . The piston  43  of  FIGS.  4 A and  4 B  have a piston groove  44  formed in a peripheral surface. The intake passage  17  and the scavenging port  14  communicate with each other via the piston groove  44  at predetermined timing. 
     In the engine  40  of  FIGS.  4 A and  4 B , when the piston  43  is actuated toward the top dead center, a pressure in a crank chamber  8  becomes negative. Then, when the piston groove  44  and the scavenging port  14  are aligned with each other while the piston  43  is moving upward, the crank chamber  8  communicates with the intake passage  17  via the scavenging passage  18 , the scavenging port  14 , the piston groove  44 , and the intake port  42 . Thus, air is sucked into the crank chamber  8  through the intake passage  17 , the intake port  42 , the piston groove  44 , the scavenging port  14 , and the scavenging passage  18  under the negative pressure of the crank chamber  8 . The sucked air comes into direct contact with the peripheral surface of the piston  43  to thereby improve cooling efficiency for the piston  43 . Further, the intake port  42  is opened and closed by the piston  43 . Thus, in contrast to the engine  1  of  FIG.  1   , a check valve is not required to be disposed in the intake passage  17 . 
     Next, a modification example of  FIGS.  4 A and  4 B  are described with reference to  FIG.  5   . In the following description, components which are the same as or equivalent to those in the example of  FIGS.  4 A and  4 B  are denoted by the same reference symbols as those in  FIGS.  4 A and  4 B , and overlapping description thereof is omitted. 
     In an engine  50  of  FIG.  5   , a piston groove  44  of a piston  51  has a hole  52  communicating with a crank chamber  8 . Air also flows into the crank chamber  8  from an intake port  42  through the hole  52 . Thus, an air supply amount to the crank chamber  8  can be increased. 
     Next, a modification example of  FIGS.  4 A and  4 B  are described with reference to  FIGS.  6 A and  6 B . In the following description, components which are the same as or equivalent to those in the example of  FIGS.  4 A and  4 B  are denoted by the same reference symbols as those in  FIGS.  4 A and  4 B , and overlapping description thereof is omitted. 
     An engine  60  of  FIGS.  6 A and  6 B  include a communication portion  61  defined between a lower end of a piston  43  and a lower end of an intake port  42 . The communication portion  61  allows communication between an intake passage  17  and a crank chamber  8 . In this case, when the piston  43  reaches a top dead center, air also flows from the intake port  42  into the crank chamber  8  through the communication portion  61 . Thus, an air supply amount to the crank chamber  8  can be increased. 
     As an example of a mode in which the communication portion  61  is defined, as illustrated in  FIG.  6 A , the communication portion  61  may be formed by an enlarged portion  61   a . The enlarged portion  61   a  is formed at the lower end of the intake port  42 , and is enlarged in a downward direction. In this case, air flows from the intake port  42  into the crank chamber  8  through the enlarged portion  61   a.    
     As another example of the communication portion  61 , as illustrated in  FIG.  6 B , the piston  62  may have a cutout  61   b  at its lower end so that the communication portion  61  is formed by the cutout  61   b . In this case, air flows from the intake port  42  into the crank chamber  8  through the cutout  61   b.    
     Next, a modification example of  FIGS.  4 A and  4 B  are described with reference to  FIG.  7   . In the following description, components which are the same as or equivalent to those in the example of  FIGS.  4 A and  4 B  are denoted by the same reference symbols as those in  FIGS.  4 A and  4 B , and overlapping description thereof is omitted. 
     In an engine  70  of  FIG.  7   , a scavenging passage  18  has a branch passage  71  that communicates with a piston groove  44 . The branch passage  71  is closer to a crank chamber  8  than a scavenging port  14  is. In this case, the branch passage  71  is located below the scavenging port  14 . Thus, time during which the piston groove  44  and the branch passage  71  communicate with each other through actuation of a piston  43  can be increased to thereby increase an air intake amount and improve an output. 
     Next, with reference to  FIGS.  8 A to  8 C  and  FIGS.  9 A and  9 B , another embodiment of this disclosure is described. An example of  FIGS.  8 A to  8 C  and  FIGS.  9 A and  9 B  are an embodiment based on a modification example of the engine  40  of  FIGS.  4 A and  4 B , which is a modification example of  FIG.  1   . Thus, components which are the same as or equivalent to those in the examples of  FIG.  1    and  FIGS.  4 A and  4 B  are denoted by the same reference symbols, and overlapping description thereof may be omitted. 
     An engine  80  of  FIGS.  8 A to  8 C  and  FIGS.  9 A and  9 B  include a cylinder  41 , an ignition device (spark plug  9 ), a piston  43 , a crank chamber  8 , a crankshaft  10 , a fuel injection valve  25 , an intake passage  17 , and scavenging passages  18 . The cylinder  41  defines a combustion chamber  6 , and has an exhaust port  12 . The ignition device ignites an air-fuel mixture in the combustion chamber  6 . The piston  43  reciprocates inside the cylinder  41  through combustion and expansion that occur in the combustion chamber  6 . The crank chamber  8  communicates with an interior of the cylinder  41 . The crankshaft  10  is disposed in the crank chamber  8 , and is operationally coupled to the piston  43 . The fuel injection valve  25  injects a fuel into the crank chamber  8 . The intake passage  17  supplies only air that is sucked under a negative pressure generated when the piston  43  is actuated. The scavenging passages  18  allow communication between the crank chamber  8  and the combustion chamber  6 . Air passing through the intake passage  17  is introduced into the scavenging passage  18 , and air stagnant in the scavenging passage  18  in a second half of an intake stroke contributes to scavenging. 
     As illustrated in  FIG.  8 C , the engine  80  has a pair of scavenging passages  18 . As a result, an inner peripheral surface of the cylinder  41  has a pair of scavenging ports  14  that allow communication between the scavenging passages  18 ,  18  and the combustion chamber  6 . The intake passage  17  branches into two branch paths  17   a  and  17   b . The branch paths  17   a  and  17   b  communicate with the pair of intake ports  42 ,  42  of the cylinder  41 , respectively. The intake ports  42 ,  42  are brought into communication with the scavenging ports  14 ,  14  at predetermined timing via two piston grooves  44 ,  44  formed in the peripheral surface of the piston  43 , respectively. 
     As illustrated in  FIG.  8 A  and  FIG.  9 A , the engine  80  includes a booster passage  81  that allows communication between the crank chamber  8  and the combustion chamber  6 . The booster passage  81  is separate from the pair of scavenging passages  18 ,  18 . The booster passage  81  communicates with the combustion chamber  6  via a top dead center-side booster port  81   a  of the cylinder  41 , and communicates with the crank chamber  8  via a bottom dead center-side booster port  81   b  of the crankcase  7 . The booster passage  81  allows a fuel in the crank chamber  8  to flow into the combustion chamber  6  while the engine  80  is performing a light-load operation to thereby achieve stratified combustion. Specifically, the fuel in the crank chamber  8  flows into the combustion chamber  6  through the booster passage  81  while the engine  80  is performing the light-load operation. At the same time, air in the crank chamber  8  flows into the combustion chamber  6  through the scavenging passages  18 ,  18 . In this manner, air and an air-fuel mixture are separately introduced into the combustion chamber  6  to achieve the stratified combustion. 
     In this embodiment, at least fuel supply timing of the fuel injection valve  25  is controlled by the control device  28  to achieve homogenous combustion during a full-load operation illustrated in  FIG.  8 A  and  FIG.  8 B . Meanwhile, the fuel flows into the combustion chamber  6  through the booster passage  81  to achieve the stratified combustion during the light-load operation illustrated in  FIG.  9 A  and  FIG.  9 B . In this manner, thermal efficiency is improved not only during the full-load operation but also during the light-load operation. 
     The fuel supply timing of the fuel injection valve  25  is controlled by the control device  28  so that fuel supply timing while the engine  80  is performing the full-load operation and fuel supply timing while the engine  80  is performing the light-load operation are set to be different from each other. Specifically, while the engine  80  is performing the full-load operation, the fuel is supplied from the fuel injection valve  25  at timing at which the piston  43  is located in the vicinity of the top dead center, as illustrated in  FIG.  8 B . This configuration allows the air-fuel mixture, which is generated as a result of mixture in the crank chamber  8 , to be introduced from lower end sides of the scavenging passages  18 ,  18  into the combustion chamber  6 . In this manner, during the full-load operation, homogeneity of the air-fuel mixture in the crank chamber  8  is improved, and hence such an air-fuel mixture is suitable for the homogeneous combustion. 
     Meanwhile, while the engine  80  is performing the light-load operation, the fuel is supplied, as illustrated in  FIG.  9 B , from the fuel injection valve  25  at timing at which the piston  43  is located on the bottom dead center side, more preferably, immediately before the piston  43  reaches the vicinity of the bottom dead center. As a result, during the light-load operation, the fuel supplied from the fuel injection valve  25  easily flows into the combustion chamber  6  through the booster passage  81  before being mixed with air in the crank chamber  8 . The air, which has been supplied to the crank chamber  8 , is introduced from the scavenging passages  18 ,  18  independently of the above-mentioned flow of the air-fuel mixture. As a result, an air layer and an air-fuel mixture layer are formed in the combustion chamber  6  to thereby achieve stratified combustion. 
     Preferably, the fuel injection of the fuel injection valve  25  is controlled by the control device  28  so that fuel injection while the engine  80  is performing the full-load operation and fuel injection while the engine  80  is performing the light-load operation are set to be different from each other. Specifically, while the engine  8  is performing the full-load operation, it is preferred that the fuel be powerfully and radially injected from the fuel injection valve  25 , as illustrated in  FIG.  8 A . As a result, the fuel is easily scattered throughout the whole crank chamber  8 . Thus, the fuel and the air are easily homogeneously mixed with each other in the entire space in the crank chamber  8 . 
     Meanwhile, while the engine  80  is performing the light-load operation, it is preferred that the fuel be supplied from the fuel injection valve  25  so as to be injected to the vicinity of the bottom dead center-side booster port  81   b . To achieve such fuel supply, the engine  80  includes a fuel supply configuration  82 . The fuel supply configuration  82  positions the fuel supplied from the fuel injection valve  25  in the vicinity of the bottom dead center booster port  81   b  while the engine  80  is performing the light-load operation. The fuel supply configuration  82  cause the fuel to easily flow into the combustion chamber  6  through the booster passage  81  while the engine  80  is performing the light-load operation. Thus, this configuration is suitable for the stratified combustion during the light-load operation. 
     As a specific example of the fuel supply configuration  82 , as illustrated in  FIG.  9 A , the fuel supply configuration  82  includes both a first configuration  82   a  and a second configuration  82   b . The first configuration  82   a  allows the fuel injection valve  25  to be arranged in the vicinity of the bottom dead center-side booster port  81   b . The second configuration  82   b  allows the fuel to be injected from the fuel injection valve at such an injection pressure that the fuel can reach and stay in the vicinity of the bottom dead center-side booster port  81   b . As described above, the injection pressure of the fuel injection valve  25  is controlled by the control device  28 . 
     As indicated by imaginary lines in  FIG.  9 A , another specific example of the fuel supply configuration  82  may include both a first configuration  82   c  and a second configuration  82   d . The first configuration  82   c  allows the fuel injection valve  25  to be arranged so that the fuel is injected toward the bottom dead center-side booster port  81   b . The second configuration  82   d  allows the fuel to be injected from the fuel injection valve  25  at such an injection pressure that the fuel can reach the bottom dead center-side booster port  81   b . Also in this case, the injection pressure of the fuel injection valve  25  is controlled by the control device  28 . 
     Next, a preferred positional relationship among the exhaust port, the two scavenging ports, and the top dead center-side booster port is described. As illustrated in  FIG.  8 C , the inner peripheral surface of the cylinder has the two scavenging ports. The exhaust port  12  is arranged on one arc-shaped surface side of the inner peripheral surface of the cylinder, which is located between the two scavenging ports  14 , 14 . The top dead center-side booster port  81   a  is arranged on another arc-shaped surface side of the inner peripheral surface of the cylinder, which is located between the two scavenging ports. An opening position of the top dead center-side booster port in an axial direction of the cylinder is only required to be lower than the exhaust port (bottom dead center side), and may be substantially the same height position as height positions of the scavenging ports. 
     The above-mentioned configuration allows the fuel to flow into the combustion chamber through the booster passage and the air to flow from the two scavenging ports into the combustion chamber during the light-load operation. The fuel flowing into the combustion chamber through the booster passage is a rich fuel containing little air. Further, the air flowing from the two scavenging ports into the combustion chamber contains little fuel. As illustrated in  FIG.  9 A , the fuel flowing from the top dead center-side booster port into the combustion chamber is carried away with the air flowing from the two scavenging ports into the combustion chamber along the another arc-shaped surface of the inner peripheral surface of the cylinder in a direction toward a cylinder head. As described above, the separate introduction of the air and the air-fuel mixture into the combustion chamber  6  further improves stability of the stratified combustion during the light-load operation. 
     It is apparent that the configuration that includes the booster passage and controls the fuel supply timing from the fuel injection valve to achieve the stratified combustion during the light-load operation is applicable not only to the embodiment illustrated in  FIGS.  8 A to  8 C  and  FIGS.  9 A and  9 B  but also to the embodiments of  FIG.  1    to  FIG.  7   . 
     It is important for the above-mentioned engine to more precisely control a ratio of the air supplied from the intake passage and the air-fuel mixture containing fuel components supplied by the fuel injection device. For this purpose, it is desired that a well-known check valve (see the check valve  23  of  FIG.  1   ) be provided in the intake passage, more preferably, in the vicinity of the intake port. The check valve prevents blow-back from the combustion chamber in a previous cycle from being mixed into the intake passage to thereby enable more precise management of the air supply amount and the fuel injection amount in a subsequent cycle. 
     The embodiments of this disclosure have been described in detail with reference to the drawings. However, a specific configuration is not limited to those of the embodiments described above. For example, changes in design without departing from the scope of this disclosure are encompassed in this disclosure. Further, technologies in the above-mentioned embodiments described above may be used in combination as long as there is no particular contradiction or problem in, for example, purpose and configuration.