US3752129A

US3752129A - Two-cycle internal combustion engines

Info

Publication number: US3752129A
Application number: US00232569A
Authority: US
Inventors: T Kobayashi; H Inaga; K Nakagawa
Original assignee: Kioritz Corp
Current assignee: Kioritz Corp
Priority date: 1971-12-17
Filing date: 1972-03-07
Publication date: 1973-08-14
Anticipated expiration: 1990-08-14
Also published as: JPS4867616A

Abstract

In a two-cycle internal combustion engine, a charging port is provided in the wall of a cylinder at the diametrically opposite position to an exhaust port; at least one main scavenging port at least one auxiliary scavenging port are provided on each side of a vertical plane which includes the straight line interconnecting the centers of said exhaust and charging ports and extends along the axis of the cylinder, in such a way that said auxiliary scavenging port is closer to said charging port than said main scavenging port; the directions of said main and auxiliary scavenging ports are so established that the mixture may be injected from said respective ports upwardly at an angle of about 15* to the transverse sectional plane of the cylinder and the jets of mixture may intersect each other in a predetermined region which is located on the diameter of the cylinder contained in said vertical plane and at a position more than the half of the distance between the center of the cylinder and said charging port distant from the center of the cylinder toward said charging port; the direction of said charging port is so established that the jet of mixture injected from said port may be directed toward a space slightly above said region of intersection of said scavenging flows and flow along the upper side of said scavenging flows without directly crossing them; and that reed valve means through which the mixture is sucked is disposed in that side of the crank case where the tangential component of rotational force of the crank shaft is directed toward the cylinder skirt and at a position adjacent said cylinder skirt, and the openings of said charging and auxiliary scavenging passage and, if necessary, the opening of said main scavenging passage are located circumferentially of said cylinder skirt and adjacent said reed valve means, whereby a kinetic energy is imparted to the mixture, sucked into the crank case, by making use of the tangential component of rotational force of the crank shaft and thereby the mixture is urged toward and directed into the openings of said respective passages and, therefore, the charging of the mixture into the cylinder is promoted, the scavenging and charging efficiency are enhanced and the engine output is increased.

Description

United States Patent Kobayashi et al.

[ TWO-CYCLE INTERNAL COMBUSTION ENGINES [73] Assignee: Kioritz Corporation, Tokyo, Japan [22] Filed: Mar. 7, 1972 21 App]. No.: 232,569

[30] Foreign Application Priority Data Dec. 17, 1971 Japan 46/102483 [52] US. Cl 123/73 A, 123/73 PP [51] Int. Cl. F02b 33/04 [58] Field of Search 123/73 A, 73 R, 65 R, 123/65 P, 73 V [56] References Cited UNITED STATES PATENTS 3,530,841 9/1970 Holt 123/73 R 1,780,175 11/1930 Curtis et a1... 123/65 R 3,382,857 5/1968 Foster et a1 123/73 R FOREIGN PATENTS OR APPLICATIONS 52,651 6/1942 Netherlands 123/65 R 674,658 4/1939 Germany 123/73 A Primary Examiner-Al Lawrence Smith Assistant Examiner-Dennis Toth Attorney-Karl W. Flocks [451 Aug. 14, 1973 port are provided on each side of a vertical plane which includes the straight line interconnecting the centers of said exhaust and charging ports and extends along the axis of the cylinder, in such a way that said auxiliary scavenging port is closer to said charging port than said main scavenging port; the directions of said main and auxiliary scavenging ports are so established that the mixture may be injected from said respective ports upwardly at an angle of about 15 to the transverse sectional plane of the cylinder and the jets of mixture may intersect each other in a predetermined region which is located on the diameter of the cylinder contained in said vertical plane and at a position more than the half of the distance between the center of the cylinder and said charging port distant from the center of the cylinder toward said charging port; the direction of said charging port is so established that the jet of mixture injected from said port may be directed toward a space slightly above said region of intersection of said scavenging flows and flow along the upper side of said scavenging flows without directly crossing them; and that reed valve means through which the mixture is sucked is disposed in that side of the crank case where the tangential component of rotational force of the crank shaft is directed toward the cylinder skirt and at a position adjacent said cylinder skirt, and the openings of said charging and auxiliary scavenging passage and, if necessary, the opening of said main scavenging passage are located circumferentially of said cylinder skirt and adjacent said reed valve means, whereby a kinetic energy is imparted to the mixture, sucked into the crank case, by making use of the tangential component of rotational force of the crank shaft and thereby the mixture is urged toward and directed into the openings of said respective passages and, therefore, the charging of the mixture into the cylinder is promoted, the scavenging and charging efficiency are enhanced and the engine output is increased.

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TWO-CYCLE INTERNAL COMBUSTION ENGINES BACKGROUND OF THE INVENTION This invention relates to two-cycle internal combustion engines of the type in which a mixture is sucked into a crank case through a reed valve, and more particularly to improvements in the scavenging system of such engines. The primary object of the invention is to improve the scavenging and charging effects of twocycle internal combustion engines and thereby to increase the engine output.

Numerous improvements have hitherto been made in the two-cycle internal combustion engines for the purpose of improving the scavenging and charging effects thereof. 1

A scavenging stroke is an operational phase of a cylinder in which the combustion gas remaining in the combustion chamber of the cylinder is scavenged and replaced by a fresh mixture, and in order to exhaust the residual combustion gas effectively, it has been a conventional practice to utilize the pulsation in the exhaust system including the exhaust pipe, the muffler, etc., to create scavenging flows within the combustion chamber by forced charging means, to utilize a jet of mixture injected from an additional charging port, or to use all of the above-mentioned methods in combination.

However, even with such methods, it has been inevitable that a certain amount of residual combustion gas remains, without being exhausted, in the bottom portion of the combustion chamber remote from the exhaust port, and the resulting disadvantages, such as lowering of the scavenging effect and temperature rise of a fresh fuel-air mixture, could not be completely eliminated. Such disadvantages have been the cause of low output of and unsatisfactory combustion in the two-cycle internal combustion engine.

The present invention contemplates the provision of a two-cycle internal combustion engine which is free of such disadvantages.

SUMMARY OF THE INVENTION.

In order to accomplish scavenging effectively, in the two-cycle internal combustion engine according to the present invention a main scavenging port (passage), an auxiliary scavenging port (passage) and a charging port (passage) are provided in the wall of a cylinder for the specific purposes to be attained thereby.

The charging port is located at the diametrically opposite position to the exhaust port. At least one main scavenging port and at least one auxiliary scavenging port are provided symmetrically on each side of a vertical plane which contains a straight line interconnecting the centers of said charging port and exhaust port, and extends along the axis of the cylinder, and the directions of said main and auxiliary scavenging ports are so established that the jets of mixture injected from said ports may intersect each other in a predetermined region in the combustion chamber of the cylinder, which is located at a position more than the half of the distance between the center of the cylinder and the charging port distant from said center of the cylinder toward said charging port. The direction of the charging port is so established that the jet of mixture injected from said port may be directed toward a space slightly above the region of intersection of the aforesaid scavenging flows and provide a uni-flow along the upper side of said scavenging flows, without directly crossing said scavenging flows. The positional relation of the auxiliary and main scavenging ports circumferentially of the cylinder wall is such that said auxiliary scavenging port is closer to the charging port than said main scavenging port.

Further, in the two-cycle internal combustion engine according to the present invention reed valve means is disposed in that side of the crank case where the tangential component of rotational force of the crankshaft is directed toward the cylinder skirt and at a position adjacent said cylinder skirt, and the other open ends of the charging and auxiliary passages are located circumferentially of said cylinder skirt and at positions in the proximity of said reed valve means.

The above-described arrangements which are the essential constitutional requirements of the present invention will bring about remarkable advantages which will be summarized as follows:

I. Since, in the scavenging stroke during operation of I the internal combustion engine, the jets of mixture injected from the main and auxiliary scavenging ports intersect each other in the predetermined region located in the deep portion of the combustion chamber and furthermore the jet of mixture injected from the charging port is directed to a portion slightly above said region of intersection and flows along the upper side of said main and auxiliary scavenging flows, the resultant flow sufficiently reaches the deep of the combustion chamber while swirling, so that the combustion gas remaining in the deep portion as well as other portion can be substantially completely scavenged. Therefore, the scavenging and charging effects of the two-cycle internal combustion engine can be improved.

2. Since the main and auxiliary scavenging flows acts on the underside of the flow of mixture injected from the charging port, urging the latter upwardly, the ignition plug on the cylinder head is also sufficiently exposed to the fresh mixture, so that said ignition plug is effectively cooled. Further, at the end of the compression stroke, only the fresh mixture resides around the ignition plug, so that the combustion state can be improved.

3. Since the tangential component of rotational force of the crank shaft imparts a kinetic energy to the mixture within the crank case and by which the mixture is urged toward and directed into the auxiliary scavenging passage and the charging passage and/or the main scavenging passage, the mixture can be effectively introduced into these passages and hence the scavenging and charging effects can be improved.

4. Since the transverse sectional areas of the auxiliary scavenging port and charging port are made smaller than that of the main scavenging port so as to increase the velocity of mixture injected therefrom, the scattering of the scavenging mixture can be prevented and a desired scavenging flow can be secured. Particularly, the scavenging during high speed operation of the engine can be achieved positively and effectively.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view of a two-cycle internal combustion engine according to the present invention;

FIG. 2 is a plan view of the cylinder mounting face of the crank case of the internal combustion engine shown in FIG. 1 exemplified as a two cylinders type;

FIG. 3 is a vertical sectional view of the cylinder of the two-cycle internal combustion engine;

FIG. 4 is a vertical sectional view of the reed valve means;

FIG. 5 is a transverse sectional view of the cylinder of a single cylinder, two-cycle internal combustion engine according to the invention;

FIG. 6 is a transverse sectional view of a multicylinder, two-cycle internal combustion engine according to the invention; and

FIG. 7 is a view similar to FIG. 6 but additionally showing the arrangement of the crank shaft within the crank case.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will be described by way of example with reference to the accompanying drawings.

Referring to FIG. 1 there is shown a two-cycle internal combustion engine according to the present invention, in which numeral 1 designates a cylinder, and 2 and 3 designate upper and lower parts of a crank case respectively. The upper part 2 of the crank case is formed with reed valve chambers 24 for mounting reed valve means 9 therein, and a mixture from a carbureter (not shown) is sucked into the crank case through said reed valve means. Numeral 8 designates a crank shaft which rotates in the direction of the arrow A or in a counterclockwise direction. Therefore, the reed valve means 9 are provided within the crank case on the side where the tangential component of rotational force of the crank shaft 8 is directed toward a cylinder skirt 19, and at locations adjacent said cylinder skirt respectively.

As seen in FIG. 2, in the cylinder mounting face 20 of the crank case is formed with cutouts 16" and 17" for communication with a main scavenging passage, an auxiliary scavenging passage and a charging passage respectively, which are formed in the wall of the cylinder 1. The crank case shown in FIG. 2 is of a two-cylinder, two-cycle internal combustion engine, and vertical planes X-X of symmetry of the port arrangements for the respective cylinders are inclined symmetrically with respect to a vertical plane S--S through the center of the crank case, for the purpose of reducing the distance between the axes of the two cylinders arranged side by side and thereby reducing the lateral width of the internal combustion engine (crank case). Numeral 26 designates cavities for mounting the cylinders 1 therein and 24 designates the profiles of the reed valve chamber formed in the crank case 2 for mounting the reed valve means therein. From the position of the chamber 24 in the circumferential direction of the cylinder, it will be understood that each reed valve means is arranged adjacent the auxiliary scavenging passage and charging passage.

The reed valve means 9 proper is shown in FIG. 4. As seen, the reed valve means includes a box body 18 having an isosceles-triangular cross section and projecting into the crank case, and plate valve elements 10 coven ing a plurality of openings formed in the chevronshaped wall of said box body. The plate valve elements 10 are biased against the chevron-shaped surface of the box body by means of limiting plates 11 and secured at one end to the box body 18 by means of set screws 21, respectively. The limiting plates 11 limit the amount of deformation of the plate valve elements 10 or the degree of opening of the valve means, and also prevent irregular operations of said plate valve elements caused by vibrations, etc. The plate valve elements 10 are opened toward downstream (the arrow B in FIG. 4 indicates the flow direction of mixture), but are tightly seated on the surface of the box body 18 when the pressure on the downstream side or interior of the crank case is relatively high, thereby interrupting the supply of mixture. Thus, the back flow of the mixture can be completely prevented.

FIGS. 5 and 45 are transverse sectional views of the cylinder of the two-cycle internal combustion engine of the invention, taken along the line Z-Z of FIG. I, respectively. FIG. 5 is of the engine having a single cylinder and FIG. 6 is of the engine having a multiplicity of cylinders. FIG. 7 is a transverse sectional view similar to FIG. 6 but with parts within the crank case, such as the crank shaft, added thereto as seen through the cylinder, to facilitate understanding of the positional relation between the crank shaft 8, and the auxiliary scavenging passages 16' and the charging passages 17' and further the main scavenging passages 15'.

Turning back to FIG. 5, the cylinder 1 is formed with an exhaust port 14 and a charging port 17 at the diametrically opposite position to said exhaust port. It is also provided with main scavenging ports 15 and auxiliary scavenging ports 16 on both sides of the vertical plane X-X which includes the straight line connecting the centers of the exhaust port 14 and charging port 17 and extending along the axis of the cylinder, and in symmetrical relation with respect to said vertical line. The positional relation in the circumferential direction of the cylinder of the auxiliary scavenging ports 16 and main scavenging ports 15 is such that said auxiliary scavenging ports are located closer to the charging port 17 than said main scavenging ports.

The directions of these main scavenging ports 15 and auxiliary scavenging ports 16 are so established that the mixture may be injected from these ports upwardly at an angle of about 15 to the transverse sectional plane of the cylinder 1 and further the jets of mixture from all of these ports may intersect each other in the region indicated by character 0 in FIG. 5. The region 0 when viewed from the upper side of the cylinder is located on the diameter of the cylinder contained in the aforesaid vertical plane X-X and at a position more than the half of the distance between the center of the cylinder and the charging port distant from said cylinder center toward said charging port. According to the experiment conducted by the present inventor, the best result was obtained when the region 0 was set in an area defined by l/D 0.2 wherein D is the diameter of the cylinder and l is the distance between said region and the charging port.

In the arrangement of the main and auxiliary scavenging ports shown in FIGS. 5-7, the positional relation of these ports is such that the jet of mixture injected from the main scavenging port on one side of the vertical plane X-X and the jet of mixture injected from the auxiliary scavenging port on the other side of said vertical plane are opposed by each other and collide against each other on a substantially straight line. According to the experiment conducted by the present inventor in this connection, the best result was obtained when the internal angle or with respect to the center of cylinder, defined by the jets of mixture injected from the main scavenging ports on both sides of the vertical plane X-X and crossing in the region 0, was set at about l25 and the external angle B with respect to the center of cylinder, defined by the jets of mixture injected from the auxiliary scavenging ports on both sides of said vertical plane and crossing in the region 0, was set at about 147". In the embodiment shown, the main and auxiliary scavenging ports are arranged as described above, namely the angles a and [3 will become 125 and l47 respectively and the jet of mixture injected from the main scavenging port on one side of. the vertical plane X-X and the jet mixture injectedfrom the auxiliary scavenging port on the other side of said vertical plane will be substantially opposed by each other.

The direction of the charging port is so established that the jet mixture injected from said charging port may be directed toward a space slightly above the region 0 and flow along the upper side of the aforesaid scavenging flows in the form of an independent flow without crossing said scavenging flows. According to the experiment conducted by the present inventor in this connection, a highly satisfactory result was obtained when the main and auxiliary scavenging flows were directed upwardly at an angle of with respect to the transverse sectional plane of the cylinder and the jet of mixture from the charging port was directed upwardly at an angle of about 60 with respect to said transverse sectional plane.

It was also found that a highly satisfactory result could be obtained when the transverse sectional areas of the auxiliary scavenging ports 16 and the charging port 17 were set within the range of 45-50 percent of that of the main scavenging ports 15.

The operation of the two-cycle internal combustion of the invention constructed as described above will be described hereunder:

When the engine is set in motion and the crank shaft 8 rotates in the direction of the arrow A in FlG. l, the tangential component of the rotational force of said crank shaft rotating incident to the rotation of crank pin and arm acts toward the cylinder skirt 19 along a line slightly spaced from the outlet end of the reed valve means 9. At the peripheral portion of the cylinder skirt (the cylinder mounting portion of the crank case, toward which the tangential component of the rotational force of the crank shaft are provided with the opening 17" of the charging passage and the openings 16" of the auxiliary scavenging passages (see FIG. 2). Therefore, a kinetic energy is imparted to the mixture within the crank case, which directs said mixture into the openings of these passages. By the same effect, the mixture is also directed into the openings 15" of the main scavenging passages.

When the engine goes into the exhaust stroke upon completion of the explosion stroke, the exhaust port 14 begins to open at first and the high temperature, high pressure combustion gas starts to be exhausted through said exhaust port. Just slightly thereafter, the scavenging ports l5, l6 begin to open and, after passage of the full-open state of the exhaust port and scavenging ports, both ports are closed again. The combustion gas in the cylinder is scavenged and replaced by fresh mixture during the period from the time when the scavenging ports and exhaust port start to be opened to the time when these ports are closed. In a conventional scavenging system, however, it has been difi'icult to scavenge substantially completely the combustion gas in the bottom of the combustion chamber remote from the exhaust port and it has been inevitable that a certain amount of combustion gas remains in said portion. Accordingly, the temperature of the fresh mixture has been elevated and the scavenging efficiency could not be increased more than a certain limit. Such phenomenon has been the cause of the low output of and unsatisfactory combustion in two-cycle internal combustion engines.

The present invention provides an improved twocycle internal combustion engine which enables the above-described disadvantage to be substantially eliminated, the scavenging and charging efficiencies to be improved and the output to be increased accordingly.

The reasons therefor will be explained hereunder:

According to the present invention, the jets of mixture injected from the main scavenging ports of a relatively large capacity and the jets of mixture injected from the auxiliary scavenging ports are cross each other in the region 0, so that the combustion gas in the combustion chamber is urged toward the exhaust port and readily exhausted from said exhaust port.

Further, the mixture is injected from the charging port toward the portion slightly above the region 0 and flows as a uni-flow along the upper side of the main and auxiliary scavenging flows and said uni-flow of mixture is urged further upwardly due to the impact pressure of said main and auxiliary scavenging flows and the relatively large volumes (masses) of the same-Therefore, there can be obtained such advantage that the ignition plug and other portions of the cylinder head, which are high in temperature, are cooled directly by the fresh mixture which is relatively low in temperature, brought into contact therewith. Thereafter, this fresh mixture swirls in the upper portion of the combustion chamber, also urging the combustion gas, remaining in said portion, toward the exhausted therefrom, thus ensuring complete discharge of the combustion gas.

The jets of mixture injected from'the main scavenging ports are relatively large in amount and constitute the major portion of the scavenging flow, whereas the opening areas of the auxiliary scavenging ports and charging port are relatively small and hence the amount of mixture injected from these ports is relatively small but higher in velocity than the mixture injected from the main scavenging ports. Such high velocity auxiliary scavenging flows suppress the scattering tendency of the main scavenging flows imparted thereto by the collision against the auxiliary scavenging flow in the region 0 and a stable scavenging effect can be obtained, by which the combustion gas remaining in the end or deep portions of the combustion chamber can be smoothly exhausted.

Because of the smooth swirling flow of the scavenging mixture within the combustion chamber and the smooth swirling flow of the additionally charged mixture from the bottom to top of the combustion chamber, during the scavenging stroke, a fresh mixture can be smoothly admitted into the combustion chamber preparatory to the compression stroke, even around the ignition plug, so that propagation of combustion can be uniformalized and a satisfactory explosion can be achieved.

Owing to the synergistic effect of the abovedescribed facts, in the two-cycle internal combustion engine of this invention the scavenging of the cylinders can be achieved satisfactorily, and thereby the exhaust and charging efficiencies can be enhanced and the engine output can be increased, even if the cylinders or combustion chambers are of a shape which is quite undesirable for the smooth flow of scavenging and intake mixture and even when the period of the scavenging stroke is very short during rotation of the engine at a high speed.

Furthermore, in the two-cycle internal combustion engine of the invention is employed a mixture suction system comprising the reed valve means which operates automatically in response to the pressure of mixture, and, therefore, back-flow of the mixture can be completely prevented.

Still further, since the reed valve means is disposed in that side of the crank case where the tangential component of rotational force of the crank shaft is directed toward the cylinder skirt, and the openings of the auxiliary scavenging passages and charging passage are lo- "cated peripherally of said cylinder skirt and adjacent said reed valve means, the mixture within the crank case is urged toward and directed into said openings under the influence of said tangential component of rotational force, and thus an additional charging effect can be obtained.

As described herein, the two-cycle internal combustion engine according to the present invention is characterized by the facts that a charging port is provided in the wall of the cylinder at the diametrically opposite position to an exhaust port; that at least one main scavenging port and at least one auxiliary scavenging port are provided on each side of a vertical plane including the straight line connecting the centers of said exhaust port and charging port and extending along the axis, of the cylinder in such a way that said auxiliary scavenging port is closer to said charging port than said main scavenging port; that the directions of said main scavenging port and auxiliary scavenging port are so established that the mixture may be injected from these ports upwardly at an angle of about to the transverse sectional plane of the cylinder and further the jets of mixture from all of these ports may intersect each other in a predetermined region which is located on the diameter of the cylinder contained in said vertical plane and at a position distant from the center of the cylinder toward said charging port; that the direction of said charging port is so established that the jet of mixture injected from said charging port may be directed toward a space slightly above said region of intersection of said scavenging flows and flow along the upper side of said scavenging flows as a uni-flow without crossing them; and that reed valve means is disposed in that side of the crank case where the tangential component of rotational force of the crank shaft is directed toward the cylinder skirt and at a position adjacent said cylinder skirt, and further the openings of the charging passage and the auxiliary. scavenging passage and further the opening of the main scavenging passage are located circumferentially of said cylinder skirt and at positions adjacent said reed valve means, whereby the mixture sucked into the crank case is imparted with a kinetic energy by which said mixture is urged toward and led into said openings, and thus the charging of the mixture into the cylinder is promoted.

The two-cycle internal combustion engine of the invention is also characterized by the fact that the transverse sectional areas of the charging port and auxiliary scavenging port are made smaller than that of the main scavenging port and, in a preferred embodiment, are within 45-50 percent of the latter, so that the mixture may be injected from said charging and auxiliary scavenging ports at higher velocities, whereby the scattering tendency of the scavenging flows resulting from collision against each other in said region of intersection can be suppressed and the desired scavenging flows can be maintained.

By the combination of the above-described features of the present invention, it has become possible to substantially increase the output and enhance the scavenging efficiency'of the twocycle internal combustion engine as compared with those of the conventional one.

We claim:

1. A two-cycle internal combustion engine with crank case containing a crank shaft in a cylinder, comprising a reed valve means for introducing the combustible mixture therethrough disposed in that side of the crank case where the tangential component of rotational force of the crank shaft is directed toward a skirt of the cylinder; a charging port provided in the wall of the cylinder in a diametrically opposite position to an exhaust port; at least one main scavenging port and at least one auxiliary scavenging port provided on each side of a vertical plane containing the straight line connecting the centers of said exhaust port and said charging port and extending along the axis of the cylinder, the relative position of said'main scavenging port and said auxiliary scavenging port being such that said auxiliary scavenging port is closer to said charging port than said main scavenging'port; the directions of said main and auxiliary scavenging ports so established that the mixture may be injected from said ports upwardly at an angle of approximately l5 to the transverse sectional plane of the cylinder and jets of mixture injected from all of said ports may intersect each other in a region located between the center of the cylinder and a generating line of the cylinder inclusive of said charging port on a diameter of the cylinder contained in said vertical plane; the direction of said charging port so established that the mixture injected from said port is directed upwardly at an angle of approximately 60 to the transverse sectional plane of the cylinder toward a space slightly above said region of intersection and provides a uni-flow not directly crossing the main and auxiliary scavenging flows; other open ends of passages communicating with said auxiliary scavenging port and said charging port being located in the cylinder mounting face of the crank case and at positions adjacent said reed valve means to utilize the tangential component of rotational force of the crank shaft to urge the mixture within the crank case toward and direct it into said auxiliary scavenging ports and said charging port to be injected into the cylinder.

Claims

1. A two-cycle internal combustion engine with crank case containing a crank shaft in a cylinder, comprising a reed valve means for introducing the combustible mixture therethrough disposed in that side of the crank case where the tangeNtial component of rotational force of the crank shaft is directed toward a skirt of the cylinder; a charging port provided in the wall of the cylinder in a diametrically opposite position to an exhaust port; at least one main scavenging port and at least one auxiliary scavenging port provided on each side of a vertical plane containing the straight line connecting the centers of said exhaust port and said charging port and extending along the axis of the cylinder, the relative position of said main scavenging port and said auxiliary scavenging port being such that said auxiliary scavenging port is closer to said charging port than said main scavenging port; the directions of said main and auxiliary scavenging ports so established that the mixture may be injected from said ports upwardly at an angle of approximately 15* to the transverse sectional plane of the cylinder and jets of mixture injected from all of said ports may intersect each other in a region located between the center of the cylinder and a generating line of the cylinder inclusive of said charging port on a diameter of the cylinder contained in said vertical plane; the direction of said charging port so established that the mixture injected from said port is directed upwardly at an angle of approximately 60* to the transverse sectional plane of the cylinder toward a space slightly above said region of intersection and provides a uni-flow not directly crossing the main and auxiliary scavenging flows; other open ends of passages communicating with said auxiliary scavenging port and said charging port being located in the cylinder mounting face of the crank case and at positions adjacent said reed valve means to utilize the tangential component of rotational force of the crank shaft to urge the mixture within the crank case toward and direct it into said auxiliary scavenging ports and said charging port to be injected into the cylinder.