Patent Publication Number: US-2015075476-A1

Title: Four-cycle engine and engine generator

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to four-cycle engines and engine generators. More specifically, the present invention relates to a four-cycle engine including a manual starter, and to a generator driven thereby. 
     2. Description of the Related Art 
     It is generally known that in order to improve engine thermal efficiency, increasing a compression ratio (expansion ratio) in the combustion chamber is effective. Increasing the compression ratio, however, will increase pressure in the compression step, resulting in an increased cranking torque required when starting the engine. Engines provided with manual starters are sometimes equipped with de-compressors as disclosed in JP-A S60-156976, for releasing the compression pressure to help human operators who would otherwise have to be more powerful to overcome the increased cranking torque when starting the engine. 
     However, opening the engine exhaust valve during the compression step when starting the engine to let the introduced air-fuel mixture escape to the exhaust pipe means that the unburned air-fuel mixture is supplied to the exhaust system, leading to occasional after fire. 
     SUMMARY OF THE INVENTION 
     Therefore, preferred embodiments of the present invention provide a four-cycle engine which has a high geometric compression ratio yet does not require as much cranking torque when starting the engine and is capable of reducing after fire, and also provide a generator driven by the engine. 
     According to an aspect of various preferred embodiments of the present invention, a four-cycle engine includes a cylinder; one inlet port configured to supply an air-fuel mixture into the cylinder; one inlet valve configured to open and close the inlet port; a piston reciprocable inside the cylinder; a crank shaft configured to convert reciprocating movement of the piston into rotating movement; a connecting rod configured to connect the piston and the crank shaft to each other; and a manual starter configured to rotate the crank shaft. The engine preferably has a geometric compression ratio not smaller than about 8.5, for example and the inlet valve has a valve closure timing selected from a range from after a bottom dead center to a top dead center, wherein the inlet valve closure timing is defined as a moment when the inlet valve in its closing stroke has a valve lift of about 1 mm, for example. 
     According to various preferred embodiments of the present invention, the inlet valve is open even after the bottom dead center and therefore, the air-fuel mixture which was once introduced into a combustion chamber of the cylinder is pushed back to an upstream of the inlet valve (toward the intake pipe). For this reason, even if the geometric compression ratio is high, being not smaller than about 8.5, an actual compression ratio is lower than the geometric compression ratio. Consequently, the actual compression pressure is not as high as a compression pressure under the geometric compression ratio. In other words, the engine provides a high expansion ratio but the actual compression pressure does not become high. This makes it possible to significantly reduce or prevent an increase in engine starting cranking torque necessary from the manual starter while improving thermal efficiency of the engine. Also, since the air-fuel mixture is not released to the exhaust pipe but to the intake pipe, the arrangement reduces after fire. As understood, the arrangement makes it possible, while keeping a high geometric compression ratio, to significantly reduce or prevent an increase in the engine starting cranking torque, and to significantly reduce or prevent after fire as well. 
     Preferably, the four-cycle engine is provided by a single-cylinder engine. Compared to a multi-cylinder engine, a single-cylinder engine has a smaller friction loss if the engine displacement is the same, because a single-cylinder engine has a smaller sliding surface area between the piston and the cylinder. Also, a single-cylinder engine has a smaller total surface area of the cylinder and the combustion chamber. Therefore, the single-cylinder engine has a smaller thermal loss and a higher thermal efficiency. In addition, it is often the case that a single-cylinder engine includes a manual starter. Since various preferred embodiments of the present invention make it possible to significantly reduce or prevent an increase in the engine starting cranking torque even if the geometric compression ratio is high, preferred embodiments of the present invention are suitable for a single-cylinder engine. 
     Further preferably, the valve closure timing of the inlet valve is selected from a range of not smaller than about 43 degrees to not greater than about 74 degrees, for example, in terms of a crank angle from the bottom dead center. In this case, the arrangement makes it possible to appropriately reduce the actual compression ratio, and therefore to significantly reduce or prevent an increase of engine starting cranking torque while improving thermal efficiency of the engine. 
     Further, preferably, the four-cycle engine is provided by a gas-fuel engine which utilizes a gaseous fuel. Gaseous fuels such as a fuel gas typically have smaller Lower Heating Values, and therefore tend to produce lower output than liquid fuels such as gasoline if engine displacement is the same. For improved output, increasing the compression ratio is a key, but like in liquid-fuel engines, this often leads to a problem of abnormal combustion, like knocking. Since preferred embodiments of the present invention make it possible to maintain a high output while reducing abnormal combustion, preferred embodiments of the present invention are suitably applicable to gas-fuel engines. 
     Further, preferably, the four-cycle engine according to a preferred embodiment of the present invention is utilized in an engine generator. Typically, an engine generator uses its engine at 3000 rpm through 3600 rpm in normal power generation operation. If an actual compression ratio is high, then it becomes likely to see knocking in a medium-low rotation region of the engine before a normal, high rotation region is reached. A conventional attempt to this problem is, for example, to have two inlet valves in the engine, and control valve opening/closure timings of the two inlet valves independently from each other in accordance with the number of engine revolutions using a variable valve mechanism, thus adjusting the actual compression ratio to reduce knocking. In this case, however, complicated control must be provided and a number of engine parts must be increased, leading to increased cost and difficulty of manufacture. The engine generator may be equipped with a governor which controls the number of engine revolutions. Even in this case, however, the number of engine revolutions can drop into the medium-low rotation region and knocking can happen if the generator comes under a load fluctuation. 
     To solve these problems, the inventor of preferred embodiments of the present invention developed an arrangement involving a so called Atkinson cycle in which a valve closure timing of an inlet valve is after the bottom dead center, in an engine which includes a single inlet valve, paying special attention to a fact that the amount of air-fuel mixture which is pushed back to an upstream of the inlet valve varies when the number of revolutions of the engine varies. Specifically, as the number of revolutions of the engine becomes higher, fluid velocity of the air-fuel mixture becomes higher, and therefore there is a higher resistance to reversing of the flow of the air-fuel mixture to the upstream of the inlet valve. Simultaneously, the period from the time when the piston is at the bottom dead center to the time when the inlet valve is closed becomes shorter, so the air-fuel mixture becomes less prone to flow upstream of the inlet valve. This means that the actual compression ratio in a medium-low rotation region can be made smaller than in a high rotation region, and that the actual compression ratio in the high rotation region does not decrease as much. Hence, if the engine according to various preferred embodiments of the present invention, including a single inlet valve, is utilized in an engine generator, it is possible to reduce knocking in a medium-low rotation region while reducing cost of manufacture, and it is possible to obtain good thermal efficiency and output in a high rotation region (range of normal engine use) of 3000 rpm through 3600 rpm. Therefore, the four-cycle engine according to various preferred embodiments of the present invention can be suitably utilized for engine generators. 
     It should be noted here that the valve closure timing of the inlet valve is preferably defined as a moment when the inlet valve in its closing stroke has a valve lift of about 1 mm, for example. This is because it is difficult to accurately identify a crank angle when the valve lift is closer to 0 mm where the inlet valve is completely closed, since the amount of change in the valve lift becomes smaller as the valve lift becomes closer to 0 mm. 
     It should also be noted here that the geometric compression ratio refers to a ratio between a volume inside the cylinder when the piston is at the bottom dead center and a volume inside the cylinder when the piston is at the top dead center. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view from a front left view point, of an engine generator which includes a four-cycle engine according to a preferred embodiment of the present invention. 
         FIG. 2  is a perspective view from a rear right view point, of the engine generator which includes the four-cycle engine according to a preferred embodiment of the present invention. 
         FIG. 3  is an illustrative drawing, showing a longitudinal section of the four-cycle engine. 
         FIG. 4  is an illustrative drawing of a section taken in lines B-B in  FIG. 3 , showing an interior with cut surfaces of a cylinder head cover, a cylinder head, a cylinder body and a crank case. 
         FIG. 5  is an illustrative drawing, showing valve opening/closure timings, etc. of an inlet valve. 
         FIG. 6A  and  FIG. 6B  are illustrative drawings, showing air-fuel mixture flowing into/out of a combustion chamber. 
         FIG. 7  is a graph, showing a relationship between a crank angle and a valve lift in an inlet valve and an exhaust valve. 
         FIG. 8A  is an illustrative drawing, showing periods and valve opening/closure timings of an inlet valve shown in Table 1, whereas  FIG. 8B  is an illustrative drawing, showing periods and valve opening/closure timings of an exhaust valve shown in Table 1. 
         FIG. 9  is a graph, showing a relationship between the number of revolutions of the engine and an amount of air introduced. 
         FIG. 10  is a graph, showing a relationship between the number of revolutions of the engine and engine torque. 
         FIG. 11  is a graph, showing a relationship between the number of revolutions of the engine and fuel consumption rate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. 
       FIG. 1  and  FIG. 2  show an engine generator  10  which includes a four-cycle engine (hereinafter, will be called “engine”)  24  (to be described later) according to a preferred embodiment of the present invention. In the present specification, a “fore-aft direction” and a “left-right direction” in the engine generator  10  are defined as shown in  FIG. 1  and  FIG. 2  for the sake of descriptive convenience. Thus, a side on which the engine  24  is provided is a “front side”, a side on which a generator  26  (to be described later) is provided is a “rear side”, and a side on which an operation panel  48  (to be described later) is provided is a “left side”. 
     The engine generator  10  preferably is a portable generator, including a generator frame  12 . The generator frame  12  includes a front frame  14 , a rear frame  16 , an upper frame  18 , and a pair of lower frames  20 ,  22 . The front frame  14  is provided by a pipe-shaped member which is preferably formed into a general shape of inverted letter of U in a front view, whereas the rear frame  16  is provided by a pipe-shaped member which is preferably formed into a general shape of inverted letter of U in a rear view. The front frame  14  and the rear frame  16  are connected with each other at both of their end portions. The upper frame  18  is provided by a pipe-shaped member and extends in the fore-aft direction, connecting upper left end portions of the front frame  14  and the rear frame  16  respectively. The upper frame  18  defines and serves as a grip. The lower frame  20  is a platy member extending in the left-right direction, connecting left and right lower portions of the front frame  14  with each other. The lower frame  22  is a platy member extending in the left-right direction, connecting left and right lower portions of the rear frame  16  with each other. 
     The engine  24  is installed on the lower frame  20 , whereas the generator  26  is installed on the lower frame  22 . The engine  24  and the generator  26  are arranged in the fore-aft direction, with the engine  24  being on the front side and the generator  26  being on the rear side. The engine  24  includes a crank shaft  66  (to be described later), which is connected with a rotating shaft (not illustrated) of the generator  26 . 
     The engine  24  includes, on its front side, an air intake section  28  to introduce outside air. The air intake section  28  includes a cooling fan (not illustrated). An air cleaner  30  is provided on the right side of the air intake section  28 . As the cooling fan is driven, outside air introduced from the air intake section  28  cools the engine  24 . A recoil starter  32  defining and serving as a manual starter is provided near the air intake section  28 . 
     A muffler  34  is provided behind the engine  24 , on the right side of the generator  26 . Exhaust gas from the engine  24  is discharged to outside via the muffler  34 . A canister  36  is provided below the engine  24 . A fuel tank  38  is connected to the air cleaner  30  via the canister  36 . Gasoline vapor from the fuel tank  38  is adsorbed in the canister  36 . 
     The fuel tank  38  is arranged to cover the engine  24  and the generator  26  from above. The fuel tank  38  stores gasoline as a fuel to be supplied to the engine  24 . The fuel tank  38  has its right side portion attached to a support frame  40  which connects an upper right end portion of the front frame  14  and an upper right end portion of the rear frame  16  to each other. The fuel tank  38  has its left front portion and left rear portion connected to the front frame  14  and the rear frame  16  respectively via brackets  42  and  44 . 
     An operation box  46  is provided on the left side of the fuel tank  38 . The operation box  46  includes the operation panel  48 , and a case  50  which is provided on the right side of the operation panel  48  and incorporates an operation section (not illustrated), etc. A battery  52  is provided below the case  50 . 
     In the engine generator  10  described as above, the recoil starter  32  is pulled to rotate the crank shaft  66  and start the engine  24 . As the engine  24  starts, the generator  26  starts its power generating operation. The electric power from the generator  26  is taken out of the operation panel  48  or stored in the battery  52 . 
     Reference will now be made to  FIG. 3  to describe the engine  24 . 
     The engine  24  preferably is, for example, an air-cooled, single-cylinder, four-cycle, slanted-type OHV engine (Over Head Valve Engine) in which a cylinder center axis is slanted. The engine  24  is a so called Atkinson cycle engine, i.e., an engine in which a valve closure timing of an inlet valve  94  (to be described later) is after the bottom dead center. The engine  24  preferably has a geometric compression ratio not smaller than about 8.5, for example. The engine  24  includes a cylinder  54 . The cylinder  54  includes a cylinder body  56  and a cylinder head  58  which is attached to an upper end portion of the cylinder body  56 . A cylinder head cover  60  is attached to an upper end portion of the cylinder head  58 . A crank case  62  is provided in a lower portion of the cylinder body  56 . 
     The cylinder body  56  includes an inner circumferential surface provided with a cylinder liner  56   a . Inside the cylinder body  56 , a piston  64  is provided slidably with respect to the cylinder liner  56   a . The crank case  62  accommodates the crank shaft  66  and a cam shaft  68  which moves in association with the crank shaft  66 . The crank shaft  66  is disposed horizontally. The crank shaft  66  and the cam shaft  68  are parallel or substantially parallel to each other. The cam shaft  68  is disposed not to interfere (contact) with crank webs  70  of the crank shaft  66 . The piston  64  and the crank shaft  66  are connected to each other by a connecting rod  72 , such that reciprocating movement of the piston  64  is converted into rotating movement by the crank shaft  66 . The crank shaft  66  is provided with a drive gear  74 , whereas the cam shaft  68  is provided with a driven gear  76  which rotates in association with rotation of the drive gear  74 . The crank case  62  also accommodates a balancer  78 . The balancer  78  is in engagement with a gear  80  provided in the crank shaft  66 , to reduce vibration. As shown in  FIG. 3 , when the engine  24  is viewed from a position where the crank shaft  66  is located on the left side and the cam shaft  68  is located on the right side, rotation direction of the crank shaft  66  is counterclockwise as indicated by Arrow A. 
     Referring also to  FIG. 4 , from the cylinder body  56  to the cylinder head  58 , there is provided a communication path  84  configured to provide communication between inside of the crank case  62  and inside of a rocker arm chamber  82  in the cylinder head cover  60 . A pushrod  86 , and a tappet  88  provided on an end portion of the pushrod  86  are inserted through the communication path  84 . Inside the crank case  62 , the tappet  88  has its tip portion contacted to an inlet cam  90  of the cam shaft  68 . The push rod  86  includes another end portion contacted to a rocker arm  92  which is provided inside the rocker arm chamber  82 . The rocker arm  92  drives the inlet valve  94  which is under a constant upward urge from a valve spring  93 . The inlet valve  94  opens/closes an inlet port  96 . The inlet port  96  is connected to an unillustrated intake pipe. A push rod  98 , and a tappet  100  which is provided at an end portion of the push rod  98  are inserted through the communication path  84 . Inside the crank case  62 , the tappet  100  includes a tip portion contacted to an exhaust cam  102  of the cam shaft  68 . The push rod  98  includes another end portion contacted to a rocker arm  104  which is provided inside the rocker arm chamber  82 . The rocker arm  104  drives an exhaust valve  106  which is under a constant upward urge from a valve spring  105 . The exhaust valve  106  opens/closes an exhaust port  108 . The exhaust port  108  is connected to an unillustrated exhaust pipe. As described, the engine  24  includes one inlet valve  94 , one inlet port  96 , one exhaust valve  106  and one exhaust port  108 . 
     Valve opening/closure timings and valve lifts for the inlet valve  94  and the exhaust valve  106  are determined by respective profiles (sectional shapes) of the inlet cam  90  and the exhaust cam  102 . 
     The valve opening/closure timings for the inlet valve  94  are preferably set as shown in  FIG. 5 . The inlet valve  94  opens slightly before the top dead center, and closes after the bottom dead center. The valve closure timing of the inlet valve  94  preferably is defined as a time when the inlet valve  94  in its closing process has a valve lift of about 1 mm, for example, and this timing is selected to be within a range from after the bottom dead center to the top dead center. Specifically, the valve closure timing (when the valve lift is about 1 mm, for example) is preferably selected from a range of 0 degree&lt;θ&lt;180 degrees, where θ represents a crank angle from the bottom dead center. The valve closure timing is preferably selected from a range of about 43 degrees≦θ≦about 74 degrees, for example. 
     In  FIG. 5 , during the time when the inlet valve  94  is open, there is a period β, which is a period from the top dead center to the bottom dead center. In this period, as shown in  FIG. 6A , air-fuel mixture is introduced from the inlet port  96  to a combustion chamber  110 . As shown in  FIG. 6B , from the time when the inlet valve  94  is opened to the time when the top dead center is reached, i.e., in a period α, and from the time when the bottom dead center is reached to the time when the inlet valve  94  is closed (valve lift becomes 0 mm), i.e., in a period γ, the air-fuel mixture is pushed back, out of the combustion chamber  110 , to upstream of the inlet valve  94 . 
     An oil dipper  112  is attached to a big end portion  72   a  of the connecting rod  72 , and oil (not illustrated) is stored inside the crank case  62 . The oil is splashed by the oil dipper  112  to the cylinder body  56 , the cylinder head  58 , the cylinder head cover  60  and so on, directly or indirectly after spattering on the crank shaft  66 , the cam shaft  68 , etc., such that lubrication of the crank shaft  66 , the cam shaft  68 , the cylinder body  56 , the rocker arms  92 ,  104  etc. is achieved. 
     According to the engine  24  as has been described, the inlet valve  94  is open even after the bottom dead center and therefore, the air-fuel mixture which was once introduced into the combustion chamber  110  of the cylinder  54  is pushed back to the upstream of the inlet valve  94  (toward the intake pipe). For this reason, even if a geometric compression ratio is high, preferably being not smaller than about 8.5, for example, an actual compression ratio is lower than the geometric compression ratio. Consequently, the actual compression pressure is not as high as a compression pressure under the geometric compression ratio. In other words, the engine provides a high expansion ratio but the actual compression pressure does not become high. Therefore, the arrangement reduces increase in cranking torque necessary to start the engine  24  with the recoil starter  32 , while improving thermal efficiency of the engine  24 . Also, the air-fuel mixture is not released toward the exhaust pipe, but toward the intake pipe. Therefore, the arrangement reduces after fire. As understood, the arrangement makes it possible, while keeping a high geometric compression ratio, to significantly reduce or prevent an increase in the engine starting cranking torque, and to significantly reduce or prevent after fire as well. 
     Compared to a multi-cylinders engine, a single-cylinder engine has a smaller friction loss if the engine displacement is the same, because a single-cylinder engine has a smaller sliding surface area between the piston and the cylinder. Also, a single-cylinder engine has a smaller total surface area of the cylinder and the combustion chamber. Therefore, the single-cylinder engine has a smaller thermal loss and a higher thermal efficiency. In addition, it is often the case that a single-cylinder engine includes a manual starter. According to the engine  24 , it is possible to reduce increase in the engine starting cranking torque even if the geometric compression ratio is high. Hence, the engine  24  is suitable for single-cylinder engine configurations. 
     According to the engine  24 , a valve closure timing of the inlet valve  94  represented by a crank angle from the bottom dead center, preferably is selected from a range of not smaller than about 43 degrees to not greater than about 74 degrees, for example. This makes it possible to appropriately reduce the actual compression ratio, and therefore to significantly reduce or prevent an increase of engine starting cranking torque while improving thermal efficiency of the engine  24 . 
     The inventor of the present invention discovered and took special notice of a fact that in the engine  24 , which utilizes a so called Atkinson cycle where the valve closure timing of the inlet valve  94  is after the bottom dead center, and which includes only one inlet valve  94 , the amount of air-fuel mixture which is pushed back to the upstream of the inlet valve  94  during the period γ in  FIG. 5  varies when the number of revolutions of the engine varies. Specifically, as the number of revolutions of the engine becomes higher, fluid velocity of the air-fuel mixture becomes higher, and therefore there is a higher resistance to reversing of the flow of the air-fuel mixture to the upstream of the inlet valve  94 . Simultaneously, the period from the time when the piston  64  is at the bottom dead center to the time when the inlet valve  94  is closed becomes shorter, so the air-fuel mixture becomes less prone to flow upstream of the inlet valve  94 . This means that the actual compression ratio in a medium-low rotation region can be made smaller than in a high rotation region, and that the actual compression ratio in the high rotation region does not decrease as much. Hence, in cases where the engine  24  which has a single inlet valve  94  is utilized in the engine generator  10 , it is possible to reduce or prevent knocking in a medium-low rotation region, such that it is possible to obtain good thermal efficiency and output in a high rotation region (range of normal engine use) of 3000 rpm through 3600 rpm, so the engine  24  can be suitably utilized for the engine generator  10 . 
     Further, reference will be made to  FIG. 7  through  FIG. 11 , to describe an experiment in which the engine  24  according to a preferred embodiment according to the present invention was compared to a comparable engine (hereinafter called “comparative example”). 
     Table 1 and  FIG. 7  show various settings of the engine  24  and the comparative example.  FIG. 8A  shows graphical representation of the periods and valve opening/closure timings for the inlet valves given in Table 1, whereas  FIG. 8B  shows graphical representation of the periods and valve opening/closure timings for the exhaust valves given in Table 1. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Compar- 
                   
               
               
                   
                   
                 ative 
                 Engine 
               
               
                   
                   
                 Example 
                 24 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Displacement (Single Cylinder): cc 
                 357 
                 357 
               
               
                   
                 Geometric Compression Ratio (Expansion Ratio) 
                 8.1 
                 8.8 
               
               
                 C 
                 Inlet Cam Opening Degree: Period in which Inlet 
                 240 
                 272 
               
               
                   
                 valve is lifted (Angle): Degrees 
                   
                   
               
               
                 D 
                 Inlet Valve Opening Timing (0 mm Valve Lift)  
                 −20 
                 −20 
               
               
                   
                 (Crank Angle) Degrees 
                   
                   
               
               
                 E 
                 Inlet Valve Closure Timing (1 mm Valve Lift)  
                 194 
                 230 
               
               
                   
                 (Crank Angle) Degrees 
                   
                   
               
               
                 F 
                 Inlet Valve Closure Timing (0 mm Valve Lift)  
                 220 
                 252 
               
               
                   
                 (Crank Angle) Degrees 
                   
                   
               
               
                 θ 
                 Inlet Valve Closure Timing at 1 mm Valve Lift  
                 14 
                 50 
               
               
                   
                 from Bottom Dead Center) (Crank Angle)  
                   
                   
               
               
                   
                 Degrees 
                   
                   
               
               
                 G 
                 Exhaust Cam Opening Degree: Period in which 
                 256 
                 256 
               
               
                   
                 Exhaust valve is lifted (Angle): Degrees 
                   
                   
               
               
                 H 
                 Exhaust Valve Opening Timing (0 mm Valve  
                 −236 
                 −236 
               
               
                   
                 Lift) (Crank Angle) Degrees 
                   
                   
               
               
                 I 
                 Exhaust Valve Closure Timing (1 mm Valve  
                 −2 
                 −2 
               
               
                   
                 Lift) (Crank Angle) Degrees 
                   
                   
               
               
                 J 
                 Exhaust Valve Closure Timing (0 mm Valve  
                 20 
                 20 
               
               
                   
                 Lift) (Crank Angle) Degrees 
               
               
                   
               
            
           
         
       
     
     As understood from Table 1 and  FIG. 7 , in this experiment, the engine  24  and the comparative example are set to the same inlet valve opening timing, but to different inlet valve closure timings, namely, the engine  24  is set to a later closure timing. Therefore, the engine  24  has a greater inlet cam opening degree. Also, the engine  24  has a greater valve lift of the inlet valve. In Table 1, the valve opening/closure timings for the inlet valve and the exhaust valve are indicated by crank angles from the top dead center. For example, the valve closure timing (about 1 mm valve lift) of the inlet valve  94  in the engine  24  is about 230 degrees, which is a crank angle from the top dead center, which namely is a crank angle of about 50 degrees from the bottom dead center. All the other settings of the engine  24  and the comparative example, including the valve opening/closure timings for the respective exhaust valves, are identical between the two. 
     By using air instead of air-fuel mixture, an experiment was performed to determine how much volume of air introduced is inside the cylinder  54  when the inlet valve  94  is closed (0 mm valve lift). Results are shown in  FIG. 9 . Referring to  FIG. 9 , as the number of revolutions of the engine increases, the volume of air introduced increases because air becomes less prone to flow back to the upstream of the inlet valve  94 . In comparison of the two engines in terms of the volume of air introduced, the engine  24 , in which the inlet valve  94  closes later than in the comparative example, shows smaller volumes in the medium-low rotation region of the number of engine revolutions. However, the situation is reversed at approximately 3200 rpm, and in the higher rotation region, the engine  24  which has a higher geometric compression ratio shows greater volumes of air than the comparative example. The amount of air-fuel mixture introduced behaves similarly to the amount of air introduced. Therefore, the engine  24  takes smaller volumes of air-fuel mixture than the comparative example in the medium-low rotation region of the number of engine revolutions, but the situation is reversed at approximately 3200 rpm, and in the higher rotation region, the engine  24  takes greater volumes than the comparative example. 
     Referring to  FIG. 10  which shows engine torque comparison, like  FIG. 9 , the engine  24  generates lower torques than the comparative example in the medium-low rotation region of the number of engine revolutions, but the situation is reversed at approximately 3000 rpm, and in the higher rotation region, the engine  24  generates higher torques than the comparative example. 
     With reference to  FIG. 11 , fuel consumption rate will be compared. Throughout the entire range from the medium-low rotation region to the high rotation region of the number of engine revolutions, the engine  24  has a lower fuel consumption rate than the comparative example, with a larger difference between the two as the engine rotation region goes higher. As understood, the engine  24  has a better fuel economy than the comparative example. 
     As is clear from these results of experiments, according to the engine  24 , air-fuel mixture becomes less prone to flow back to the upstream of the inlet valve  94  at a higher number of revolutions of the engine, so it is possible to make an actual compression ratio smaller in the medium-low rotation region than in the high rotation region, without much decrease in the actual compression ratio in the high rotation region. Therefore, it is possible to reduce knocking in the medium-low rotation region while obtaining good thermal efficiency (fuel economy) and output (torque) in the high rotation region. 
     In the preferred embodiment described above, description was made for a case where the fuel preferably is gasoline, for example. However, the fuel may be gaseous fuel such as a fuel gas. Gaseous fuels such as a fuel gas typically have smaller Lower Heating Values, and therefore tend to produce lower output than liquid fuels such as gasoline if engine displacement is the same. For improved output, increasing the compression ratio is a key, but like in liquid-fuel engines, this often leads to a problem of abnormal combustion, like knocking. Since the engine  24  is capable of maintaining a high output while reducing abnormal combustion, the engine  24  is suitably applicable to gas-fuel engines. 
     The manual starter is not limited to a recoil starter, and may be provided by a kick starter. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.