Patent Publication Number: US-6216650-B1

Title: Stratified scavenging two-cycle engine

Description:
TECHNICAL FIELD 
     The present invention relates to a stratified scavenging two-cycle engine, and more particularly to a stratified scavenging two-cycle engine, in which control of an air flow rate provides favorable acceleration performance and can prevent deterioration of exhaust gas. 
     BACKGROUND ART 
     As a conventional stratified scavenging two-cycle engine of this kind, a stratified scavenging two-cycle engine that includes a scavenging flow passage for connection between a cylinder chamber and a crank chamber and an air flow passage connected to the scavenging flow passage and that is structured in such a manner that pressure reduction in the crank chamber, with upward movement of a piston, permits a fuel mixture to be drawn into the crank chamber and permits air to be drawn into the crank chamber, through the scavenging flow passage from the air flow passage, is known. In the stratified scavenging two-cycle engine structured as described above, there is an advantage that combustion gas can be pushed out by air from the scavenging flow passage, thus making exhaust gas cleaner by greatly reducing an introduction of a fuel mixture during combustion gas expulsion. 
     In the aforesaid stratified scavenging two-cycle engine, however, there is a disadvantage that the fuel mixture is rarefied by air, whereby an air-fuel ratio (weight of air/weight of fuel) having a substantial ratio of air to fuel becomes thinner (increases), thus deteriorating acceleration performance. As a measure to improve acceleration performance, it is required that the air-fuel ratio is thickened (decreases) by increasing the supply amount of fuel also at a time of stationary engine speed in accordance with acceleration performance to draw an enriched fuel mixture into the crank chamber. In that case, however, an exhaust gas quality at the time of a stationary engine speed (i.e., other than a time of acceleration) deteriorates. 
     SUMMARY OF THE INVENTION 
     In view of the aforesaid disadvantages, an object of the present invention is to provide a stratified scavenging two-cycle engine, in which a fuel mixture and air are separately drawn and that controls a supplied flow rate of air to improve acceleration performance and to prevent deterioration of exhaust gas at a time of stationary engine speed and a time of acceleration. 
     To attain the aforesaid object, a stratified scavenging two-cycle engine according to the present invention is characterized by including a scavenging flow passage for connection between a cylinder chamber and a crank chamber, an air flow passage connected to the scavenging flow passage, an air flow rate controller for controlling a flow rate of air fed to the scavenging flow passage from the air flow passage, and a fuel mixture flow rate controller for controlling a flow rate of a fuel mixture drawn into the crank chamber from a fuel mixture flow passage, the aforesaid air flow rate controller throttling an air flow rate at the time of acceleration. 
     According to the aforesaid configuration, when a piston ascends, pressure in the crank chamber lowers so that a fuel mixture flows into the crank chamber, and air flows into the crank chamber through the scavenging flow passage from the air flow passage. Namely, the scavenging flow passage is filled with air, and inside the crank chamber, the fuel mixture is rarefied by air from the scavenging flow passage. Therefore, in the stratified scavenging two-cycle engine, an air-fuel ratio of a fuel mixture drawn from the fuel mixture flow passage is set in a higher range so as to make the air-fuel ratio optimum in combustion after the fuel mixture is rarefied by air. 
     Subsequently, when pressure in the cylinder chamber sharply rises by ignition of the fuel mixture in the cylinder chamber and the piston descends, pressure in the crank chamber rises. When the piston descends to a predetermined position, an exhaust port opens, for example, and combustion gas flows out of the exhaust port so that pressure in the cylinder chamber sharply drops, and a scavenging port which is an end portion on the side of the cylinder chamber of the scavenging flow passage opens. Then, air in the scavenging flow passage flows into the cylinder chamber, and subsequently the fuel mixture in the crank chamber flows into the cylinder chamber through the scavenging flow passage. 
     Specifically, combustion gas can be pushed out of the exhaust port by only air at a point in time when scavenge starts, thus preventing deterioration of exhaust gas due to an introduction of a fuel mixture. Moreover, a proper air-fuel ratio mixture fills the cylinder chamber, thereby also preventing deterioration of exhaust gas. Accordingly, exhaust gas can be cleaned at the time of stationary engine speeds. 
     Meanwhile, when the flow rate of a fuel mixture fed to the crank chamber is increased by the fuel mixture flow rate controller, engine speed increases. At the time of such engine acceleration, an air flow rate is throttled by the air flow rate controller. Hence, the flow rate of air flowing into the crank chamber is relatively lower than the flow rate of a fuel mixture flowing into the same crank chamber, as compared with stationary engine speeds. 
     Namely, a thicker air-fuel ratio fuel mixture fills the cylinder chamber, thus improving acceleration performance of the engine. At this time, since the supply amount of fuel is not increased at the time of acceleration as in the prior art, the supply amount of fuel is small even at the time of acceleration, thus preventing deterioration of exhaust gas more than in the prior art. In addition, in the stratified two-cycle engine of the present invention, the supply amount of fuel is not increased at the time of acceleration, whereby deterioration of exhaust gas can be prevented more than in the prior art even at the time of a stationary engine speed. 
     A stratified scavenging two-cycle engine according to the present invention is characterized by including a scavenging flow passage for connection between a cylinder chamber and a crank chamber, an air flow passage connected to the scavenging flow passage, an air flow rate controller for controlling a flow rate of air fed to the scavenging flow passage from the air flow passage, and a mixture flow rate controller for controlling a flow rate of a fuel mixture drawn into the crank chamber from a fuel mixture flow passage, the aforesaid air flow rate controller being opened later than the mixture flow rate controller at the time of acceleration. 
     According to the aforesaid configuration, the same effect as that of the aforesaid embodiment can be obtained. In this embodiment, the same effect that is described above is obtained at the time of acceleration, and moreover an air-fuel ratio becomes the same as that at stationary engine speed by eliminating delay when predetermined acceleration is obtained, whereby accelerating performance can be improved and exhaust gas after acceleration can be made cleaner than in the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a stratified scavenging two-cycle engine according to one embodiment of the present invention, the engine being shown in a state of acceleration; 
     FIG. 2 is a sectional view of the stratified scavenging two-cycle engine of the one embodiment of the present invention, the engine being shown in a state of a stationary engine speed; 
     FIG. 3 is a schematic view of a first embodiment of an air supply delay device for the one embodiment of the present invention; 
     FIG. 4 is a diagram for explaining the relationship between points in time and valve openings in the first embodiment of the air supply delay device; 
     FIG. 5 is a block diagram of a second embodiment of the air supply delay device for the one embodiment of the present invention; 
     FIG. 6 is a flowchart of the second embodiment of the air supply delay device for the one embodiment of the present invention; 
     FIG. 7 is a diagram for explaining the relationship between points in time and valve openings in the second embodiment of the air supply delay device; 
     FIG. 8 is a block diagram of a third embodiment of the air supply delay device for the one embodiment of the present invention; 
     FIG. 9 is a flowchart of the third embodiment of the air supply delay device according to the present invention; and 
     FIG. 10 is a diagram for explaining the relationship between points in time and valve openings in the third embodiment of the air supply delay device. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     One embodiment of the present invention will be described below concerning the case of a crankcase reed valve-type engine with reference to FIG.  1  and FIG.  2 . Incidentally, the same effect as the above can be obtained in the case of a piston valve-type engine. In a stratified scavenging two-cycle engine shown in this embodiment, as shown in FIGS. 1 and 2, a fuel mixture flow passage  10  that provides a fuel mixture is connected to a crank chamber  1   a , and an air flow passage  2  that provides air is connected to a scavenging flow passage  3 . A check valve  20  is provided at the outlet of the air flow passage  2 . The check valve  20 , which is formed by a reed valve, allows a flow from the air flow passage  2  toward the scavenging flow passage  3 , and impedes a flow from the scavenging flow passage  3  toward the air flow passage  2 . A check valve  100  is provided in the fuel mixture flow passage  10 . The check valve  100  is also formed by a reed valve, allowing a flow from the fuel mixture flow passage  10  toward the crank chamber  1   a , and impeding flow from the crank chamber  1   a  toward the fuel mixture flow passage  10 . 
     Meanwhile, the scavenging flow passage  3  is provided in a crankcase  1  and a cylinder block  4  in order to lead from the crank chamber  1   a  into a cylinder chamber  4   a . In a cylinder inner face  4   b , scavenging ports  3   a  leading to the scavenging flow passage  3  are opened, and an exhaust port  4   c  for exhausting combustion gas is also opened. 
     A crankshaft  5  is provided in the crankcase  1 , and a piston  7  is coupled to the crankshaft  5  via a connecting rod  6 . The piston  7  is put into the cylinder chamber  4   a  and movable along the axial direction of the cylinder chamber  4   a . In addition, a cylinder head  8  is provided on the cylinder block  4 , and an ignition plug  9  is provided on the cylinder head  8 . 
     A fuel mixture flow rate controller  11  for controlling a flow rate of a fuel mixture drawn into the crank chamber  1   a  is provided upstream of the fuel mixture flow passage  10 . Moreover, an air flow rate control means  12  for controlling a flow rate of air drawn into the scavenging flow passage  3  from the air flow passage  2  is provided upstream of the air flow passage  2 . 
     The fuel mixture flow rate controller  11  controls the flow rate of a fuel mixture with a throttle valve  11   a . Specifically, by opening the throttle valve  11   a , the flow rate of a fuel mixture drawn into the crank chamber  1   a  increases, whereby engine speed increases. In addition, in the fuel mixture flow rate controller  11 , a carburetor  11   b  is integrally provided upstream of the throttle valve  11   a.    
     The air flow rate controller  12  controls the flow rate of air with an on-off valve  12   a . The on-off valve  12   a  throttles an opening when the flow rate of a fuel mixture fed to the crank chamber  1   a  is increased by the throttle valve  11   a  and engine speed is increased, that is, at the time of engine acceleration. Specifically, the on-off valve  12   a  detects that the throttle valve  11   a  is changing in an opening direction and throttles an air flow rate. 
     In the stratified two-cycle engine structured as described above, as shown in FIG. 2, when the piston  7  ascends, pressure in the crank chamber  1   a  lowers so that a fuel mixture flows into the crank chamber  1   a  from the mixture flow passage  10 , and air flows into the crank chamber  1   a  through the scavenging flow passage  3  from the air flow passage  2 . Namely, the scavenging flow passage  3  is filled with air, and inside the crank chamber  1   a , the supplied mixture is rarefied by air. Therefore, an air-fuel ratio of a fuel mixture drawn from the fuel mixture flow passage  10  is set in a lower range so as to make the air-fuel ratio optimum in combustion after the fuel mixture is rarefied by air. 
     Subsequently, when pressure in the cylinder chamber  4   a  sharply rises by ignition of a fuel mixture in the cylinder chamber  4   a , the piston  7  descends, and pressure in the crank chamber  1   a  rises. When the piston  7  descends to a predetermined position, the exhaust port  4   c  opens, and combustion gas flows out of the exhaust port  4   c  so that pressure in the cylinder chamber  4   a  sharply drops and the scavenging ports  3   a  open. Then, air in the scavenging flow passage  3  flows into the cylinder chamber  4   a , and subsequently the fuel mixture in the crank chamber  1   a  flows into the cylinder chamber  4   a  through the scavenging flow passage  3 . 
     Specifically, combustion gas can be pushed out of the exhaust port  4   c  by only air at a point in time when scavenge starts, thus preventing deterioration of exhaust gas due to an introduction of uncombusted fuel mixture. Moreover, a proper air-fuel ratio mixture can fill the cylinder chamber  4   a , thereby also preventing deterioration of exhaust gas. Accordingly, exhaust gas can be cleaned at the time of stationary travel shown in FIG.  2 . 
     Meanwhile, when the flow rate of a fuel mixture fed to the crank chamber  1   a  increases by the mixture flow rate controller  11 , engine speed increases. At the time of such acceleration, an air flow rate is throttled by the air flow rate controller  12 , as shown in FIG.  1 . Hence, the flow rate of air flowing into the crank chamber  1   a  is relatively lower than the flow rate of a fuel mixture flowing into the same crank chamber  1   a  at stationary engine speeds, e.g., idle. Namely, a lower air-fuel ratio fuel mixture fills the cylinder chamber  4   a , thus improving acceleration performance of the engine. Since the total amount of fed fuel is smaller than in the prior art, with delay of a supplied quantity, exhaust gas at the time of acceleration can be made cleaner than in the prior art. Moreover, since the supply amount of fuel no longer needs to be determined in view of an air-fuel ratio at the time of acceleration, the supply amount of fuel can be set in a lower range at a stationary engine speed, and exhaust gas can be made cleaner than in the prior art. 
     Next, a case will be explained where an air flow rate is throttled by the aforesaid air flow rate controller  12  and the air flow rate flows into the crank chamber  1   a  later than a fuel mixture flow rate. FIG. 3 shows a schematic view of a first embodiment of an air supply delay device  20 , which is controlled by a mechanism, to supply a later air flow rate. A fuel mixture link  21  is linked to the throttle valve  11   a  of the fuel mixture flow rate controller  11  (shown in FIG. 1) via a fuel mixture spring  22  and linked to a throttle lever  23  for accelerating or decelerating engine speed. A first air link  24  is linked to the on-off valve  12   a  of the air flow rate controller  12  (shown in FIG. 1) via a first air spring  25  and linked to the throttle lever  23  for accelerating or decelerating engine speed by a second air link  26  via a shock absorber  30 , together with the fuel mixture link  21 . In the shock absorber  30 , in an example shown, a second air spring  27  is inserted between the first air link  24  and the second air link  26 , and a spring constant Ka of the second air spring  27  is set in a lower range than a spring constant Kb of the first air spring  25 . Although a spring is used for the shock absorber  30  in the aforesaid embodiment, an assistant cylinder, an accumulator, or the like can be also used. 
     Next, operation will be described with reference to FIG.  3  and FIG.  4 . When an operator wants to accelerate the engine, the throttle lever  23  is manipulated in an accelerating direction. A movement of the throttle lever  23  in the accelerating direction is transmitted to the throttle valve  11   a  via the fuel mixture link  21  and the fuel mixture spring  22 , whereby the throttle valve  11   a  of the fuel mixture flow rate controller  11  is rotated to be opened further. Thus, the flow rate of a fuel mixture drawn into the crank chamber  1   a  is further increased and drawn in accordance with the amount of throttle lever  23  manipulation, as shown in a full line Zb in FIG.  4 . At the same time, the movement of the throttle lever  23  in the accelerating direction rotates the on-off valve  12   a  of the air flow rate controller  12  to be opened via the second air link  26 , the shock absorber  30 , and the first air link  24 , in sequence. At this time, in the shock absorber  30 , the second air spring  27  having the lower spring constant Ka is bent responsive to a movement of the second air link  26 , and the air first link  24  is moved after the second air spring  27  is bent by a predetermined amount. Accordingly, after receiving movement of the second air link  26 , the shock absorber  50  moves the first air link  24  with delay. Thus, in the opening amount of the on-off valve  12   a  of the air flow rate controller  12 , delay is brought about by the shock absorber  30  as shown in a dotted line Za in FIG. 4, and the on-off valve  12   a  is opened to a predetermined position which is set by the throttle lever  23  later than the throttle valve  11   a  at all times. By delay of the air quantity to be supplied, a lower air-fuel ratio fuel mixture fills the cylinder chamber  4   a , thus improving acceleration performance of the engine. At this time, with the delay of the air to be supplied, the total amount of fuel fed to the fuel mixture is smaller than in the prior art, whereby exhaust gas at the time of acceleration can be made cleaner than in the prior art. Moreover, since the supply amount of fuel no longer needs to be determined in view of an air-fuel ratio at the time of acceleration, the supply amount of fuel can be set in a lower range at a stationary engine speed, and exhaust gas can be made cleaner than in the prior art. 
     Referring now to FIG.  1  and FIG. 5, which show a schematic diagram of a second embodiment of an air supply delay device  20 A which supplies a later air flow rate. Incidentally, the second embodiment is electronically controlled, which shows an example in which the opening amount of the on-off valve  12   a  of the air flow rate controller  12  is throttled more than that of the throttle valve  11   a  of the mixture flow rate controller  11 . A fuel mixture servo-motor  31  is attached to the throttle valve  11   a  of the fuel mixture flow rate controller  11 . The fuel mixture servo-motor  31  is connected to a control element  34 , such as a digit controller, via a fuel mixture position control servo amplifier  32  and a fuel mixture D/A converter  33  and operates in accordance with commands from the control element  34 . An air servo-motor  35  is attached to the on-off valve  12   a  of the air flow rate controller  12 , the air servo-motor  35  being connected to the control element  34 , such as a digital controller, via an air position control servo amplifier  36  and an air D/A converter  37  and operates in accordance with commands from the control element  34 . Provided in the throttle lever  23  is a movement sensor  38  for detecting the amount of movement (or the amount of rotation) of the throttle lever  23 . A signal from the movement sensor  38  is inputted to the control element  34  via an A/D converter  39 . A CPU  43   a , a ROM  43   b , a RAM  43   c , and a timer  43   d  are provided in the control element  34 . Although an example in which the servo-motors  31 , 35  are used for opening and closing the throttle valve  11   a  and the on-off valve  12   a  is shown above, an electromagnetic proportional control valve which controls a flow rate with a solenoid, a step motor, or the like may be used. 
     Next, operation will be described, based on a flowchart shown in FIG. 6 with reference to FIGS. 1 and 5. 
     At START in step  1 , when the engine starts, the control element  34  executes control operations at regular intervals, for example, at 10 msec intervals by interrupt of a timer  43   d.    
     In step  2 , input processing of throttle openings is executed. A voltage value according to the amount of movement from the movement sensor  38  is converted to a digital value through the A/D converter  39  to be inputted to the CPU  43   a . In the control element  34 , address data corresponding to a throttle opening, which is already stored in the RAM  43   c , are moved to data stored in an address corresponding to the preceding throttle opening, and data corresponding to a throttle opening which is inputted to the CPU  43   a  from the A/D converter  39  this time is stored in an address corresponding to a throttle opening which is already stored. In addition, the control element  34  converts a voltage value according to the amount of movement from the movement sensor  38  to a digital value through the A/D converter  39  and receives it in the CPU  43   a , and subsequently outputs an opening command to the mixture servo-motor  31  so that the flow rate of a fuel mixture is in accord with the amount of movement stored in the ROM  43   b  flows. 
     In step  3 , data of an address corresponding to an air flow rate map stored in the ROM  43   c  are read out from the present throttle opening, which is obtained in step  2 . 
     In step  4 , data of a throttle opening obtained last time and data of a throttle opening obtained this time are compared, and whether the engine is in acceleration or not is determined from whether the throttle opening obtained this time is increased more than the throttle opening obtained last time or not. 
     When the throttle opening obtained this time is the same as or is smaller than the throttle opening obtained last time in step  4 , the procedure advances to step  5 . 
     In step  5 , when the throttle opening obtained this time is the same as the throttle opening obtained last time, the same command value as that of the throttle opening obtained last time is outputted to the on-off valve  12   a  of the air flow rate controller  12  as an opening command, and when the throttle opening obtained this time is smaller than the throttle opening obtained last time, a command value for letting the flow rate of air according to the amount of movement of the throttle lever  23 , which is stored in the ROM  43   c  flow, is outputted to the on-off valve  12   a  of the air flow rate controller  12  as an opening command, respectively. The control element  34  outputs an opening command to the fuel mixture servo-motor  31  so that a flow rate of a fuel mixture is in accord with an amount of movement of the throttle lever  23  stored in the ROM  43   c . Further in the above, the mixture flow rate controller  11  may be a mechanical control means, which uses the mixture link  21  shown in FIG. 3, without being electronically controlled. 
     When the throttle opening obtained this time is larger than the throttle opening obtained last time in step  4 , the procedure advances to step  6  after the amount of acceleration is obtained. 
     In step  6 , predetermined throttle amount data X, according to the amount of acceleration stored in the ROM  43   c  are subtracted from air quantity data D, found from the air flow rate map obtained in step  3 , to find throttle air flow rate data Dx. 
     In step  7 , whether the throttle air flow rate data Dx obtained in step  6  are larger than minimum air flow rate data Do of the engine or not is determined. 
     When the throttle air flow rate data Dx are smaller than the minimum air flow rata data Do, the procedure advances to step  8 . 
     In step  8 , the CPU  43   a  outputs the minimum air flow rate data Do to the air D/A converter  37 , and the air D/A converter  37  converts the data to a predetermined voltage value to be outputted to the air position control servo amplifier  36 . The air position control servo amplifier  36  rotates the air servo-motor  35  to a position proportional to the voltage value. The control element  34  outputs an opening command to the mixture servo-motor  31  so that the flow rate of a fuel mixture is in accord with the amount of movement of the throttle lever  23  stored in the ROM  43   c . Further in the above, the fuel mixture flow rate controller  11  may be a mechanical control means which uses the mixture link  21  shown in FIG. 3 without being electronically controlled. 
     When the throttle air flow rate data Dx is larger than the minimum air flow rate data Do in step  7 , the procedure advances to step  9 . 
     In step  9 , the CPU  43   a  outputs the throttle air flow rate data Dx to the air D/A converter  37 , and the air D/A converter  37  converts the data to a predetermined voltage value to be outputted to the air position control servo amplifier  36 . The air position control servo amplifier  36  rotates the air servo-motor  35  to a position proportional to the voltage value so that the on-off valve  12   a  of the air flow rate controller  12  is throttled. The control element  34  outputs an opening command to the fuel mixture servo-motor  31  so that the flow rate of a fuel mixture is in accord with the amount of movement of the throttle lever  23  stored in the ROM  43   c . Further in the above, the mixture flow rate controller  11  may be a mechanical control means which uses the fuel mixture link  21  shown in FIG. 3 without being electronically controlled. 
     As shown with a dotted line Va in FIG. 7 with reference to FIGS. 1 and 5, the on-off valve  12   a  of the air flow rate controller  12  is throttled more than the throttle valve  11   a  of the fuel mixture flow rate controller  11  by the throttle amount data X, and the air servo-motor  35  operates while being throttled more than the fuel mixture servo-motor  31 . Therefore, a supplied air quantity is decreased, and a fuel mixture having a lower air-fuel ratio fills the cylinder chamber  4   a , thus improving acceleration performance of the engine. In FIG. 7, the horizontal axis represents time, the vertical axis represents the opening amount of a valve, the dotted line Va shows the case of the on-off valve  12   a  of the air flow rate controller  12 , and a full line Vb shows the case of the throttle valve  11   a  of the mixture flow rate controller  11 . When a valve opening amount Qa is changed to an acceleration valve opening amount Qb in the drawing, the opening amount of the throttle valve  11   a  of the fuel mixture flow rate controller  11  increases as shown with the full line Vb, and the opening amount of the on-off valve  12   a  of the air flow rate controller  12  remains in a position where it is for a predetermined period of time as shown with a dotted line Va. As a result, the opening amount of the on-off valve  12   a  of the air flow rate controller  12  increases later than the opening amount of the throttle valve  11   a  of the fuel mixture flow rate controller  11  while being throttled more than the opening amount of the throttle valve  11   a  of the fuel mixture flow rate controller  11 . Thus, similar to the above, with delay in an air quantity to be supplied, the total amount of fuel fed to the fuel mixture is smaller than in the prior art, whereby exhaust gas at the time of acceleration can be made cleaner than in the prior art. Moreover, since the supply amount of fuel no longer needs to be determined in view of an air-fuel ratio at the time of acceleration, the supply amount of fuel can be set in a lower range at a stationary engine speed, and exhaust gas can be made cleaner than in the prior art. 
     Referring now to FIG. 8, a third embodiment of an air supply delay device  20 B is described, with reference also to FIGS. 3 and 5. The configuration of parts of the third embodiment is different from that of the second embodiment shown in FIG. 5 in that: two timers  41  and  42  are provided in a control element  34 A; the mixture D/A converter  33 , the mixture position control servo amplifier  32 , and the mixture servo-motor  31  are omitted; and the throttle valve  11   a  in the fuel mixture flow rate controller  11  is connected to the throttle lever  23  via the fuel mixture link  21 . A controlling method of the third embodiment is an example in which the opening of the on-off valve  12   a  of the air flow rate controller  12  is made later than the throttle valve  11   a  of the fuel mixture flow rate controller  11 . Incidentally, the same parts as those in FIG. 5 are denoted by the same numerals and symbols and the explanation thereof is omitted. 
     The controlling method by the control element  34 A will be described, based on a flowchart shown in FIG. 9 with reference to FIGS. 1 and 8. 
     At START in step  21 , when the engine starts, the control element  34 A executes control operations at regular intervals, for example, at 10 msec intervals by interrupt of a timer  41 . 
     In step  22 , input processing of throttle openings is executed. A voltage value according to the amount of movement from the movement sensor  38  is converted to a digital value through the A/D converter  39  to be inputted to the CPU. In the control element  34 A, data of an address corresponding to a throttle opening, which is already stored in the RAM  43   c , are moved to data stored in an address corresponding to the preceding throttle opening, and data corresponding to a throttle opening, which is inputted to the CPU  43   a  from the A/D converter  39  at this time, are stored in an address corresponding to a throttle opening which is already stored. 
     In step  23 , data of an address corresponding to an air flow rate map stored in the ROM  43   c  are read out from the present throttle opening, which is obtained in step  22 . 
     In step  24 , data of an address corresponding to the air flow rate map stored in the ROM  43   b  from the present throttle opening which is obtained in step  23  is outputted to the air D/A converter  37 , and the air D/A converter  37  converts the data to a predetermined voltage value to be outputted to the air position control servo amplifier  36 . The air position control servo amplifier  36  rotates the air servo-motor  35  to a position proportional to the voltage value. 
     In step  25 , data of the throttle opening obtained last time and data of a throttle opening obtained this time are compared, and whether the engine is in acceleration or not is determined from whether the throttle opening obtained this time is increased more than the throttle opening obtained last time or not. 
     When the throttle opening obtained this time is the same as or is smaller than the throttle opening obtained last time in step  25 , the air servo-motor  35  is rotated to a position at which output is conducted to the air D/A converter  37  in step  24 . 
     When the throttle opening obtained this time is larger than the throttle opening obtained last time in step  25 , the procedure advances to step  26 . 
     In step  26 , a delay time t o  is counted by a timer  42 , during which interrupt for executing control operations by the timer  41  is prevented. After the delay time t o  is counted by the timer  42  interrupt is resumed. Thus, the air servo-motor  35  starts to operate later than the throttle valve  11   a  in the fuel mixture flow rate controller  11 . Consequently, as shown with a dotted line Ya in FIG. 10, the on-off valve  12   a  of the air flow rate controller  12  starts to operate later than the throttle valve  11   a  of the fuel mixture flow rate controller  11  by the delay time t o , whereby delay in an air quantity to be supplied occurs and a thicker air-fuel ratio fuel mixture fills the cylinder chamber  4   a , thus improving acceleration performance of the engine. In FIG. 10, the horizontal axis represents time, the vertical axis represents the opening amount of a valve, a dotted line Ya shows the case of the on-off valve  12   a  of the air flow rate controller  12 , and a full line Yb shows the case of the throttle valve  11   a  of the fuel mixture flow rate controller  11 . When a valve opening amount Qa is changed to an acceleration valve opening amount Qb (in the drawing), the opening amount of the throttle valve  11   a  of the fuel mixture flow rate controller  11  increases, as shown with the full line Yb, and the opening amount of the on-off valve  12   a  of the air flow rate control means  12  increases after the delay time t o  as shown with the dotted line Ya, and subsequently increases similarly to that of the throttle valve  11   a  of the fuel mixture flow rate controller  11 . As a result, the same effect that is described above can be obtained at the time of acceleration, and moreover since an air quantity increases when predetermined acceleration is obtained, the air-fuel ratio becomes the same as that at a stationary engine speed, whereby acceleration performance can be improved, and exhaust gas after acceleration can be made cleaner than in the prior art. 
     In the aforesaid embodiment, the on-off valve  12   a  is structured to be throttled by detecting that the throttle valve  11   a  is changing in an opening direction. Specifically, when the throttle valve  11   a  is changing in an opening direction, the engine is regarded as being then subject to acceleration, whereby the on-off valve  12   a  is throttled. However, the engine may be also regarded as being subject to acceleration by an increase in engine speed, and thereby the on-off valve  12   a  is structured to be throttled. Namely, the on-off valve  12   a  may be structured to throttle an opening by detecting that the rotational frequency of the crankshaft  5  is changing in an increasing direction, for example. 
     INDUSTRIAL AVAILABILITY 
     The present invention is useful as a stratified scavenging two-cycle engine, in which control of an air flow rate provides favorable accelerating performance and can prevent deterioration of exhaust gas.