JP2002332847A - Stratified scavenging two-cycle engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make sufficient amount of air sucked into a scavenging air passage during rising of a piston and to fully reduce THC in exhaust gas and a fuel consumption ratio. SOLUTION: The sufficient amount of air is sucked into the scavenging air passage 13, by preventing delay of timing of communicating an air inlet 16 with a scavenging port 20 by a recessed part 22, during rising of the piston 14 with respect timing is not delayed by more than communicating an air-fuel mixture inlet 17 with a crankcase 5 and preventing entering of an air-fuel mixture into the scavenging air passage via the crankcase. By scavenging combustion gas using sufficient amount of air sucked into the scavenging air passage 13, exhausting of a part of the air-fuel mixture along with the exhaust gas to an engine exterior from an exhaust passage 12 during scavenging is prevented and the THC in the exhaust gas and the fuel consumption ratio can be reduced fully.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a stratified scavenging two-cycle engine.

[0002]

2. Description of the Related Art Conventionally, stratified scavenging 2 for the purpose of reducing the THC (total amount of hydrocarbons) in the combustion gas (exhaust gas) discharged from the combustion chamber of an engine and reducing the fuel consumption rate.
Cycle engines are being developed and put into practical use. In such a stratified scavenging two-cycle engine, only air is sucked into the scavenging passage when the piston is lifted, and during scavenging, the air sucked in the scavenging passage is supplied to the combustion chamber prior to the air-fuel mixture. Thus, the scavenging of the combustion gas is performed only by air, and at the time of this scavenging, a part of the air-fuel mixture is prevented from being exhausted to the outside of the engine together with the combustion gas.
3, disclosed in JP-A-2001-27122.

[0003]

However, in the above-described stratified scavenging two-cycle engine, when the piston is lifted, the intake of the air-fuel mixture into the crank chamber is started, and thereafter, the intake of air into the scavenging passage is performed. Therefore, a situation occurs in which a part of the air-fuel mixture, which has started to be sucked in before the air, enters the scavenging passage through the crank chamber, and a sufficient amount of air cannot be sucked into the scavenging passage. For this reason, a part of the air-fuel mixture is still discharged together with the combustion gas during the scavenging, which causes a problem that the reduction of THC and the reduction of the fuel consumption rate cannot be sufficiently achieved.

Further, since a concave portion is formed in the outer peripheral portion of the piston to allow the air suction port and the scavenging port to communicate with each other, the edge of the concave portion slides greatly between the piston and the inner peripheral surface of the cylinder when the piston slides. It becomes a dynamic resistance and hinders the improvement of the engine output.

Further, by forming a concave portion for communicating the air suction port and the scavenging port on the outer periphery of the piston, a convex portion corresponding to the concave portion is formed inside the piston. And
When the piston slides, it is necessary to prevent the protrusion from interfering with the crank weight fixed to the crankshaft.To prevent the interference, the piston rod must be lengthened to separate the piston from the crankshaft. Therefore, there is a problem that the height of the engine is increased accordingly.

SUMMARY OF THE INVENTION An object of the present invention is to provide a stratified scavenging two-cycle engine capable of sufficiently reducing the THC and the fuel consumption rate by sucking a sufficient amount of air into a scavenging passage when the piston rises. That is.

An object of the present invention is to provide a stratified scavenging two-cycle engine in which a concave portion formed on the outer peripheral portion of a piston does not hinder an improvement in engine output.

[0008] It is an object of the present invention to provide a stratified scavenging two-cycle engine capable of suppressing an increase in the height of the engine.

An object of the present invention is to provide a stratified scavenging two-cycle engine capable of sufficiently reducing THC while maintaining the output and acceleration performance of the engine.

[0010]

According to the first aspect of the present invention,
An air inlet formed at an end of an air passage through which air is supplied and opened to a cylinder of the engine, and an exhaust outlet formed at an end of an exhaust passage through which combustion gas is discharged and opened to the cylinder. One end of which is formed at an end of a mixture passage in which the mixture is supplied and which is selectively communicated with a crank chamber of an engine by sliding of a piston sliding in the cylinder; A scavenging port formed at the other end of the scavenging passage communicated with the crank chamber and opened to the cylinder, wherein the air suction port and the air suction port are provided on an outer peripheral portion of the piston according to a sliding position of the piston. In the stratified scavenging two-cycle engine in which a recess for selectively communicating with the scavenging port is formed, the timing at which the air suction port and the scavenging port are communicated by the recess when the piston is lifted is determined by the mixed timing. Air inlet is configured so as not to be later than the timing to be communicated with the crank chamber.

Therefore, the timing at which the suction of the air into the scavenging passage is started is not later than the timing at which the suction of the air-fuel mixture into the crank chamber is started. The air-fuel mixture sucked into the scavenging passage can be prevented from entering the scavenging passage via the crank chamber, whereby a sufficient amount of air can be sucked into the scavenging passage. By scavenging the combustion gas using the air sucked into the scavenging passage, it is possible to prevent a part of the air-fuel mixture from being exhausted from the exhaust passage together with the exhaust gas to the outside of the engine at the time of scavenging. It is possible to sufficiently reduce the THC in the gas and the fuel consumption rate.

According to a second aspect of the present invention, in the stratified scavenging two-stroke engine according to the first aspect of the present invention, the timing at which the air suction port and the scavenging port are communicated by the recess when the piston is lifted is controlled by the recess. The crank angle is set earlier in the range of 0 to 20 degrees than the timing at which the air-fuel mixture inlet port communicates with the crank chamber.

Therefore, a sufficient amount of air can be sucked into the scavenging passage, so that the THC in the exhaust gas scavenged and discharged outside the engine can be sufficiently reduced, and the fuel consumption rate can be sufficiently reduced. Can be. Furthermore, it is possible to prevent a large amount of air flowing through the scavenging passage from flowing into the crank chamber, thereby preventing the air-fuel ratio of the air-fuel mixture sucked into the crank chamber from deteriorating and lowering the output and acceleration performance of the engine. You.

According to a third aspect of the present invention, in the stratified scavenging two-cycle engine according to the first or second aspect of the present invention, the recess is formed in a shape that is long in an axial direction of the piston.

Therefore, it is possible to secure a sufficient time during which the air suction port and the scavenging port are in communication with each other, and a sufficient amount of air is sucked into the scavenging passage.

The invention described in claim 4 is the first to third aspects of the present invention.
In the stratified scavenging two-stroke engine according to any one of the above aspects, the air suction port and the scavenging port are close to each other along a circumferential direction of the piston.

Therefore, the length of the concave portion along the circumferential direction of the piston can be reduced. This allows
Since the length of the concave portion along the circumferential direction of the piston can be reduced, the resistance when the sucked air flows in the concave portion is reduced, so that the air can be smoothly sucked into the scavenging passage. .

Further, since the length of the concave portion along the circumferential direction of the piston can be reduced, the resistance between the outer peripheral surface of the piston and the inner peripheral surface of the cylinder when the piston slides can be reduced, and the output of the engine can be reduced. Can be up.

Further, since the concave portion is formed, the concave portion can be formed at a position where the convex portion protruding inside the piston does not interfere with the crank weight fixed to the crankshaft. Therefore, there is no need to separate the piston and the crankshaft, and the height of the engine can be kept small.

According to a fifth aspect of the present invention, in the stratified scavenging two-cycle engine according to the fourth aspect of the present invention, the air intake port and the scavenging port are within a range of 60 degrees along the circumferential direction of the piston. Is formed.

Therefore, the functions and effects described in the fourth aspect of the invention can be reliably achieved.

The invention according to claim 6 is the invention according to claims 1 to 5
In the stratified scavenging two-stroke engine according to any one of the above aspects, a chamfered portion is formed on an outer peripheral portion of a crank weight fixed to a crankshaft and disposed in the crank chamber.

Therefore, it is possible to prevent interference between the convex portion protruding inside the piston due to the concave portion formed on the outer periphery of the piston and the crank weight fixed to the crankshaft, and keeping the height of the engine small. Can be maintained. Further, even when the piston and the crankshaft are brought close to each other, the interference between the protrusion protruding inside the piston and the crank weight can be prevented, so that the height of the engine can be reduced.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to the drawings. The stratified scavenging two-cycle engine of the present embodiment is formed by connecting cylinder block 1 and crankcase 2. The connection between the cylinder block 1 and the crankcase 2 is performed by connecting a crankcase flange 3 formed at one end of the crankcase 2 (see FIG. 5) and a cylinder block flange 4 formed at one end of the cylinder block 1 (see FIG. 6). See) and interview
This is done by tightening with bolts.

A crankcase 5 is formed in the crankcase 2, and a crankshaft 6 is formed in the crankcase 5.
Are rotatably supported. A pair of crank weights 7 are connected to the crankshaft 6, and both ends of the crankshaft 6 project outside the crankcase 2.

The crankcase 2, which is divided into two parts in the front and rear direction (axial direction of the crankshaft 6), is joined and fastened and fixed with bolts. A concave portion 8 communicating with a scavenging passage, which will be described later, is formed at the joint surface of the crankcase 2 which is divided and joined.
The recess 8 is formed as a pair at a position facing a portion between the two crank weights 7 (see FIG. 5).

In the cylinder block 1, a cylinder 9, an air passage 10, a mixture passage 11, an exhaust passage 12, and a scavenging passage 13 are formed.

A piston 14 is slidably provided in the cylinder 9, and the piston 14 and the crankshaft 6 are connected by a connecting rod 15.

The air passage 10 is a passage for taking in air for performing layered scavenging, and has one inlet side.
The exit side is branched into two. An air suction port 16 is formed at the end on the outlet side, and the air suction port 16 is open to the cylinder 9.

The mixture passage 11 is a passage for sucking a mixture as fuel, and a mixture intake port 17 is formed at an end on the outlet side. This mixture inlet 17
Is open to the cylinder 9, but is communicated with the crank chamber 5 at a predetermined timing when the piston 14 slides toward the top dead center, and is closed by the piston 14 at other times.

The exhaust passage 12 is a passage for discharging combustion gas generated by combustion in a combustion chamber 18 formed by being surrounded by the inner peripheral surface of the cylinder 9 and the upper end surface of the piston 14 to the outside of the engine. An exhaust port 19 opened to the cylinder 9 is formed at one end. Exhaust passage 12
Is connected to a muffler (not shown).

The scavenging passage 13 is a passage in which one end communicates with the crank chamber 5 and the other end has a scavenging port 20 opened to the cylinder 9. The air passing through the air passage 10 is sucked into the scavenging passage 13 when the piston 14 rises, and the air flows out into the combustion chamber 18 during scavenging to scavenge the combustion gas in the combustion chamber 18. Further, after the scavenging, the mixture in the crank chamber 5 is supplied into the combustion chamber 18 via the scavenging passage 13.

The scavenging passage 13 is formed in the cylinder block 1 and extends substantially parallel to the axial direction of the cylinder 9. The vertical passage 13 a is formed between the cylinder block flange 4 and the crankcase flange 3. It is formed by the horizontal passage portion 13b formed at the joint portion. The horizontal passage portion 13b has an arc-shaped groove portion 21 (see FIG. 6) surrounding the cylinder 9 in the cylinder block flange portion 4.
It is formed by covering with a crankcase flange 3. One end of the horizontal passage 13b communicates with the vertical passage 13a, and the other end of the horizontal passage 13b communicates with the recess 8. Thus, the vertical passage 13a
The volume of the scavenging passage 13 formed by the horizontal passage portion 13b and the horizontal passage portion 13b is formed to have a size of 40 to 60% of the displacement of the engine.

A recess 22 is formed in a part of the outer periphery of the piston 14.
Are formed. The recess 22 selectively connects the air suction port 16 and the scavenging port 20 according to the sliding position of the piston 14. In addition, the concave portion 2 is formed on the outer peripheral portion of the piston 14.
By forming 2, a protrusion 22a is formed at a position corresponding to the recess 22 on the inner peripheral portion of the piston 14 (see FIGS. 3, 8, and 12).

Here, the air suction port 16 and the scavenging port 20 are formed close to each other along the circumferential direction of the piston 14,
It is formed within a range of 60 degrees or less along the circumferential direction of the piston 14 (see FIG. 3).

The recess 22 is formed in a shape that is long in the axial direction of the piston 14 (see FIGS. 1 and 2).

Further, due to the positional relationship between the air inlet 16, the scavenging port 20, the recess 22, and the air-fuel mixture inlet 17, when the piston 14 rises, the air suction port 16 and the scavenging port 20 are connected by the recess 22. The timing is the mixture inlet 17
Is set not to be later than the timing of communication with the crank chamber 5. Specifically, when the crank angle exceeds 109.5 degrees, the air intake port 16 and the scavenging port 2
The communication between the mixture inlet 17 and the crank chamber 5 is started when the crank angle exceeds 127.7 degrees (see FIG. 2).

The outer peripheral edge of the crank weight 7 has a convex portion 22a of the piston 14 when the crank shaft 6 rotates.
A chamfered portion 23 is formed to avoid interference with the head (see FIGS. 8 and 12).

In such a configuration, FIG. 1 shows a state in which the crank angle becomes 109.5 degrees from the bottom dead center position while the piston 14 is rising. This state is a state immediately before the air suction port 16 and the scavenging port 20 are communicated with each other by the concave portion 22, and the suction of air into the scavenging passage 13 has not been started. Further, the air-fuel mixture inlet 17 is closed by the outer peripheral surface of the piston 14 and the crank chamber 5 is closed.
The inhalation of the air-fuel mixture to has not started.

FIG. 2 shows that the crank angle is 1 from the bottom dead center position.
This is the state where the temperature reaches 27.7 degrees. In this state, the air suction port 16 and the scavenging port 20 are communicated with each other by the concave portion 22, and the outside air is communicated with the air passage 10 and the concave
, And is sucked into the scavenging passage 13 (more precisely, is sucked toward the crank chamber 5 through the scavenging passage 13). Note that the air-fuel mixture intake port 17 is still closed by the outer peripheral surface of the piston 14, and the intake of the air-fuel mixture into the crank chamber 5 has not been started. FIG. 3 is a sectional view taken along line AA in FIG.

FIG. 7 shows a state in which the piston has risen to the top dead center. In this state, the communication between the air suction port 16 and the scavenging port 20 by the concave portion 22 is continuously maintained, and external air enters the scavenging passage 13 through the air passage 10 and the concave portion 22 as shown by an arrow. Has been inhaled. Further, an air-fuel mixture intake port 17 is communicated with the crank chamber 5, and the air-fuel mixture is sucked into the crank chamber 5 via the air-fuel mixture passage 11 as indicated by an arrow. It is to be noted that the piston 14 prevents the air suction port 16 from directly communicating with the crank chamber 5.

FIG. 9 shows a state in which the piston 14 descends due to ignition and explosion in the combustion chamber 18, the exhaust port 19 is opened, and the combustion gas is discharged through the exhaust passage 12. At this time, the scavenging port 20 is closed by the piston 14, and the scavenging passage 13 is not communicated with the combustion chamber 18.

FIG. 10 shows a state where the piston 14 is slightly lowered from the state shown in FIG. 9, and the scavenging port 20 is in communication with the combustion chamber 18. Thereby, the air sucked into the scavenging passage 13 flows into the combustion chamber 18 from the scavenging port 20, and scavenging of the combustion gas in the combustion chamber 18 by the air is started.

FIG. 11 shows a state in which the piston 14 has descended to the bottom dead center position, and the air-fuel mixture sucked into the crank chamber 5 after the air sucked into the scavenging passage 13 passes through the scavenging passage 13. The gas flows into the combustion chamber 18 via
At the time of this scavenging, the air and the air-fuel mixture are stratified and flow into the combustion chamber 18, and the scavenging of the combustion gas is almost entirely performed by air, and the air-fuel mixture is discharged from the exhaust port 19 together with the combustion gas during the scavenging. Is prevented. Thus, the THC in the exhaust gas can be reduced, and the fuel consumption rate can be reduced.

FIG. 13 shows that the air intake port 16 and the scavenging port 20 correspond to the recessed portion 2 with respect to the timing at which the air-fuel mixture intake port 17 communicates with the crank chamber 5 (the air-fuel mixture intake start timing).
The difference between the timings of communication (air intake start timing) is represented by a crank angle.
5 is a graph showing a relationship between an HC emission rate, an output, and acceleration performance. From the graph of FIG. 13, the timing at which the air intake port 16 and the scavenging port 20 are communicated by the recess 22 is set to 0 ° to 20 ° in crank angle from the timing at which the mixture intake port 17 is communicated with the crank chamber 5. It can be seen that by setting the speed in the range, it is possible to maintain the output and maintain the acceleration performance while suppressing the THC emission rate.

The reason is that the air inlet 16 and the scavenging port 20
Is communicated by the concave portion 22 earlier than the timing at which the air-fuel mixture suction port 17 is communicated with the crank chamber 5 by a crank angle in the range of 0 to 20 degrees, so that the mixture is sucked into the crank chamber 5. Mixture is in scavenging passage 13
Before the air enters the scavenging passage 13, a sufficient amount of air can be sucked into the scavenging passage 13, and a large amount of air sucked from the scavenging passage 13 flows into the crank chamber 5 and
This is because it is possible to prevent deterioration of the air-fuel efficiency of the air-fuel mixture sucked into the inside.

Further, in the present embodiment, the concave portion 22
Since the piston 14 is formed in a shape elongated in the axial direction, the air suction port 16 and the scavenging port 2
It is possible to ensure a sufficient time during which the communication with the zero is established, and to allow a sufficient amount of air to be sucked into the scavenging passage 13.

Further, in the present embodiment, the air inlet 1
6 and the scavenging port 20 are arranged along the circumferential direction of the piston 14 by 60
(Fig. 3)
), The length of the recess 22 communicating the air suction port 16 and the scavenging port 20 along the circumferential direction of the piston 14 is reduced. This reduces the resistance when the sucked air flows in the recess 22, so that the air can be smoothly sucked into the scavenging passage 13.

Further, since the length of the concave portion 22 along the circumferential direction of the piston 14 is reduced,
The resistance between the edge of the concave portion 22 and the inner peripheral surface of the cylinder 9 when the slidable member can slide can be reduced, and the output of the engine can be increased.

Further, by reducing the length dimension of the concave portion 22 along the circumferential direction of the piston 14,
Formed on the inside of the piston 14 due to the formation of
2a and the recess 2 at a position where the crank weight 7 does not interfere.
2 can be formed, and it is not necessary to separate the piston 14 and the crankshaft 6 to prevent such interference, and the height of the engine can be kept small. Furthermore, by forming the chamfered portion 23 on the outer peripheral edge of the crank weight 7, interference between the convex portion 22a and the crank weight 7 is more unlikely to occur, and the piston 14 and the crank shaft 6 are arranged closer to each other. It is possible to reduce the height of the engine.

[0051]

According to the stratified scavenging two-cycle engine according to the first aspect of the present invention, the timing at which the air suction port and the scavenging port are communicated by the concave portion when the piston is lifted, and the mixture suction port communicates with the crank chamber. It is set not to be later than the timing when the air is sucked into the scavenging passage, so that the air-fuel mixture is prevented from entering the scavenging passage via the crank chamber before entering the scavenging passage. A small amount of air can be sucked in, and by scavenging the combustion gas using the air sucked into the scavenging passage, part of the air-fuel mixture is discharged from the exhaust passage together with the exhaust gas to the outside of the engine during scavenging. And it is possible to sufficiently reduce the THC in the exhaust gas and the fuel consumption rate.

According to the second aspect of the present invention, in the stratified scavenging two-cycle engine according to the first aspect of the present invention, the timing at which the air intake port and the scavenging port communicate with each other by the concave portion when the piston is lifted is determined by the timing of the mixture intake. Since the mouth is set earlier in the range of 0 ° to 20 ° in crank angle than the timing of communication with the crank chamber, a sufficient amount of air can be sucked into the scavenging passage, thereby being scavenged. Reduction of THC in exhaust gas discharged outside the engine,
In addition, the fuel consumption rate can be sufficiently reduced, and furthermore, it is possible to prevent a large amount of air flowing through the scavenging passage from flowing into the crank chamber, thereby maintaining the output and acceleration performance of the engine.

According to the third aspect of the present invention, in the stratified scavenging two-cycle engine according to the first or second aspect of the present invention, since the recess is formed in a shape elongated in the axial direction of the piston, the air intake port is formed. It is possible to secure a sufficient time during which the air and the scavenging port are in communication with each other, and to suction a sufficient amount of air into the scavenging passage.

According to a fourth aspect of the present invention, in the stratified scavenging two-cycle engine according to any one of the first to third aspects, the air intake port and the scavenging port are brought close to each other along the circumferential direction of the piston. Since the length of the concave portion along the circumferential direction of the piston can be reduced, the convex portion protruding inside the piston due to the formation of the concave portion and the crank weight fixed to the crankshaft can be formed. The concave portion can be formed at a position where no interference occurs, and it is not necessary to separate the piston and the crankshaft in order to prevent such interference, and the height of the engine can be kept small. Furthermore, since the length of the concave portion along the circumferential direction of the piston can be reduced, the resistance when the air to be drawn flows in the concave portion can be reduced, and the air can be smoothly sucked into the scavenging passage. Can be made. Further, since the length of the concave portion along the circumferential direction of the piston can be reduced, the resistance between the outer peripheral surface of the piston and the inner peripheral surface of the cylinder when the piston slides can be reduced, thereby increasing the output of the engine. be able to.

According to the fifth aspect of the present invention, in the stratified scavenging two-cycle engine according to the fourth aspect of the present invention, the air inlet and the scavenging port are formed within a range of 60 degrees or less along the circumferential direction of the piston. Therefore, the operation and effect described in the invention of claim 4 can be reliably achieved.

According to a sixth aspect of the present invention, in the stratified scavenging two-cycle engine according to any one of the first to fifth aspects, the outer peripheral edge of the crank weight fixed to the crankshaft and disposed in the crank chamber. Since the chamfered portion is formed in the portion, it is possible to prevent interference between the convex portion protruding inside the piston due to the formation of the concave portion on the outer periphery of the piston and the crank weight fixed to the crankshaft, and The height dimension can be kept small. Further, even when the piston and the crankshaft are brought close to each other, the interference between the protrusion protruding inside the piston and the crankweight can be prevented, so that the height of the engine can be reduced.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional front view showing a stratified scavenging two-cycle engine according to an embodiment of the present invention.

FIG. 2 is a vertical sectional front view showing a state in which air is sucked from an air inlet.

FIG. 3 is a sectional view taken along line AA in FIG. 2;

FIG. 4 is a side view showing a cylinder block.

FIG. 5 is a plan view showing a crankcase.

FIG. 6 is a bottom view showing the cylinder block.

FIG. 7 is a vertical sectional front view showing a state where a piston is located at a top dead center and air is sucked from an air inlet and air-fuel mixture is sucked from an air-fuel mixture inlet;

FIG. 8 is a vertical sectional side view thereof.

FIG. 9 is a vertical sectional front view showing a state where exhaust of combustion gas is being performed.

FIG. 10 is a longitudinal sectional front view showing a state immediately after scavenging is started.

FIG. 11 is a vertical sectional front view showing a state where a piston has descended to a bottom dead center.

FIG. 12 is a vertical sectional side view thereof.

FIG. 13 is a graph showing the relationship between the timing at which the intake of air and the intake of air-fuel mixture is started, and the relationship between the THC emission rate, output, and acceleration performance.

[Explanation of symbols]

 Reference Signs List 5 crank chamber 6 crankshaft 7 crank weight 9 cylinder 10 air passage 11 mixture passage 12 exhaust passage 13 scavenging passage 14 piston 16 air intake 17 mixture intake 19 exhaust outlet 20 scavenging opening 22 recess 23 chamfer

Claims (6)

[Claims] 1. An air intake opening formed at an end of an air passage through which air is supplied and opened to a cylinder of an engine, and an air intake formed at an end of an exhaust passage through which combustion gas is discharged. A mixture intake selectively communicated with a crank chamber of an engine by sliding of a piston which is formed at an end of an mixture passage where the mixture is supplied and an exhaust port which is opened and slides in the cylinder. A scavenging port formed at the other end of the scavenging passage, one end of which is communicated with the crank chamber and opened to the cylinder, and an outer peripheral portion of the piston according to a sliding position of the piston. In a stratified scavenging two-stroke engine having a concave portion for selectively communicating an air intake port and the scavenging port, a timing in which the air intake port and the scavenging port are communicated by the concave portion when the piston is lifted. Grayed is stratified scavenging two-cycle engine, wherein the air-fuel mixture intake port is set so as not to be later than the timing to be communicated with the crank chamber. 2. The timing at which the air suction port and the scavenging port are communicated by the recess when the piston is lifted is 0 degrees or more in crank angle from the timing at which the mixture suction port is communicated with the crank chamber. 2. The stratified scavenging two-stroke engine according to claim 1, wherein the engine is set earlier in a range of 20 degrees. 3. The stratified scavenging two-stroke engine according to claim 1, wherein the recess is formed in a shape elongated in the axial direction of the piston. 4. The stratified scavenging two-stroke engine according to claim 1, wherein the air suction port and the scavenging port are close to each other along a circumferential direction of the piston. 5. The stratified scavenging two-stroke engine according to claim 4, wherein the air suction port and the scavenging port are formed within a range of 60 degrees or less along a circumferential direction of the piston. 6. A chamfered portion is formed on an outer peripheral portion of a crank weight fixed to a crankshaft and disposed in the crank chamber.
A stratified scavenging two-cycle engine according to any one of the preceding claims.