DECOMPRESSION DEVICE IN A TWO-CYCLE ENGINE

Abstract: For the purpose of preventing a decompression passageway in a two-cycle engine (1) from being blocked by carbon soot, facilitating manufacture of a two-cycle engine having decompression means and eliminating reduc­ tion of an output power of a two-cycle engine caused by decompression means, the known two-cycle engine having decompression means in the prior art has been improved. The improvements reside in that there are provided a scavenging passageway (20) formed along an inner wall of a cylinder as directed in the axial direction and communi­ cating with a crank case, and a decompression groove (21) scooped in the axial direction of the cylinder on the upstream side of a cylinder cooling airflow at the top end portion of the scavenging passageway, and the width and depth of the decompression groove are either varied along the axial direction of the groove so as to be narrowed towards its tip end on the side of a plug, or provided with a narrowed portion of venturi shape.

Description: BACKGROUND OF THE INVENTION: Field of the Invention: The present invention relates to a decompression device in a two-cycle engine as defined in the preamble of claim 1 which can reduce a starting torque upon start of the engine. Description of the Prior Art: One example of a two-cycle engine having decompression means in the prior art is illustrated in Fig. 7. The illustrated engine is a piston valve type spark-ignition two-cycle engine. In this figure, reference numeral 1 designates a cylinder, numeral 2 designates a cylinder liner forming an inner wall of the cylinder 1, numeral 3 designates a piston, numeral 4 designates a cylinder head, numeral 5 designates a combustion chamber, numeral 6 designates an exhaust port, and an opening 6a of the same exhaust port 6 on the side of the cylinder inner wall is opened and closed by slide movements of the piston 3. On the inner wall of the above-mentioned cylinder 1 is scooped out a compressed gas leak groove 7 within the range adapted to be opened and closed by slide movements of the piston 3 and extending from the top edge of the opening 6a of the exhaust port 6 on the side of the cylinder inner wall towards the upper dead point as shown in Fig. 8. This compressed gas leak groove 7 is formed in an inverse isosceles trapezoid shape in a plan configuration having its communicating portion 7a with the opening 6a of the exhaust port on the side of the cylinder inner wall side choked into a narrow width as compared to its top starting edge portion 7b, as shown in Fig. 9. It is to be noted that as examples of modification of the above-described compressed gas leak groove 7 of inverse isosceles trapezoid shape, sometimes a compressed gas leak groove 8 of oval shape in plan configuration as shown in Fig. 10, a compressed gas leak groove 9 of T-shape as shown in Fig. 11, a compressed gas leak groove 10 of inclined key hole shape as shown in Fig. 12, and a compressed gas leak groove 11 of X-mas tree shape as shown in Fig. 13 are provided, and their end portions 8a, 9a, 10a and 11a communicating with the opening 6a of the exhaust port 6 on the side of the inner wall of the cylinder are formed to be narrow in width. Also, in some cases, the above-mentioned compressed gas leak grooves 7 - 11 are provided along the opening 6a of the exhaust port 6 on the side of the inner wall of the cylinder in multiple as distributed rather than solely. Furthermore, the top starting end portion of the above-described compressed gas leak groove 7 - 11 is disposed preferably at a position of 40° or less in a crank shaft angle. Still further, preferably the above-mentioned compressed gas leak groove 7 (or 8 - 11) is formed in such manner that the depth of the groove at the top starting end portion 7b is relatively shallow but the depth at the communicating portion 7a is deep, resulting in an advantage that flow of the exhaust gas passing through the compressed gas leak groove 7 is made smooth. It is to be noted that reference numeral 13 designates an air feed port. In the above-described decompression device in a two-cycle engine in the prior art, the decompression passageway (compressed gas leak groove) 7 communicates with the exhaust port 6 and the combustion chamber 5. Accordingly, an incomplete combustion gas that is inherent to a two-cycle engine would pass through the decompression passageway 7 and would escape through the exhaust port 6. At this time, carbon soot is liable to block the decompression passageway 7, and so, the function of the passageway is deteriorated. Furthermore, regarding the process for manufacturing the engine, in order to provide the groove 7 in the prior art, an inner mold of a cylinder would be withdrawn in the opposite direction to the plug. Accordingly, at first a mold for forming the groove 7 must be moved to the central portion, and in the subsequent step of the process the inner mold must be withdrawn downwards, so that a man-hour for the manufacturing work is increased. In addition, if the groove is provided in the above-described manner, there is a disadvantage that reduction of an output power is resulted due to lowering of a compression pressure of the engine and due to leakage of gas through an escape groove upon an expansion stroke after ignition. GB-A-1 386 177 discloses a decompression device in a two-cycle engine having a decompression groove formed in a cylinder wall of said engine which communicates with an exhaust port. FR-A-2 525 687 discloses a decompression groove in a two-cycle engine which communicates with an end of a scavenging passageway directed toward the compression chamber. SUMMARY OF THE INVENTION: It is therefore one object of the present invention to provide an improved decompression device in a two-cycle engine, in which blocking of a decompression passageway by carbon soot can be avoided, the process for manufacturing the engine is facilitated, and reduction of output power caused by decompression means can be eliminated. This object is solved by a decompression device as claimed in claim 1. More particularly, according to the present invention, a decompression groove for making a fuel-air mixture gas within a cylinder chamber escape into a crank case upon starting of an engine, is scooped at the top end portion of a scavenging passageway as directed in the axial direction. Furthermore, in order that the above-mentioned decompression groove may be hardly blocked by carbon soot, the decompression groove is provided in the scavenging passageway on the upstream side of a cylinder cooling airflow. On the other hand, the shape of the decompression groove is considered so as to be hardly blocked, and a discontinuity of section is provided between a first and a second portion of the decompression groove. In operation, since the decompression groove communicates with the crank case via the scavenging passageway, when a piston moves from the upper dead point to the lower dead point, a combustion gas passes through the decompression groove and enters the crank case. However, since a fresh air again enters the cylinder from the crank case through the scavenging passageway and the decompression groove, the decompression groove is hardly blocked by carbon soot. The decompression groove also can be formed through the process in the prior art, and so, rise of a cost due to construction of a mold can be prevented. Since the temperature and pressure of the gas in the expansion stroke upon operation are high, when the gas leaks through an escape groove, the gas flow velocity reaches the sound velocity. However, the gas flow is dammed by the discontinuity of section, hence leak of the gas is reduced, and reduction of output power can be prevented. The above-mentioned and other objects, features and advantages of the present invention will become more apparent by reference to the following description of preferred embodiments of the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS: In the accompanying drawings: Fig. 1(A) is a longitudinal cross-section view showing one preferred embodiment of the present invention, the decompression groove being shown only in a schematic manner; Fig. 1(B) is a general cross-section view of a cylinder in the structure shown in Fig. 1(A); Fig. 1(C) is a schematic transverse cross-section view taken along line A-A in Fig. 1(B) as viewed in the direction of arrows; Fig. 2 is a longitudinal cross-section view of a scavenging passageway and its neighborhood including a piston, the decompression groove being shown only in a schematic manner; Figs. 3(A) and 3(B) are a cross-section view and a side view, respectively of one form of decompression groove; Figs. 4(A) and 4(B) are a cross-section view and a side view, respectively, of a different form of decompression groove; Fig. 5(A) is a longitudinal cross-section view showing another preferred embodiment of the present invention; Fig. 5(B) is a bottom view of the structure shown in Fig. 5(A); Fig. 6 is another longitudinal cross-section view of the structure shown in Fig. 5(A) taken along line B-B in Fig. 5(A) as viewed in the direction of arrows; Fig. 7 is a longitudinal cross-section view of a two-cycle engine having decompression means in the prior art; Fig. 8 is an enlarged partial cross-section view of a compressed gas leak groove in Fig. 7; and Figs. 9 through 13 are enlarged partial cross-section views of different compressed gas leak grooves in the prior art taken along line C-C in Fig. 7 as viewed in the direction of arrows. DESCRIPTION OF THE PREFERRED EMBODIMENTS: Referring now to Fig. 1 and Fig. 2, a two-cycle engine having a decompression device according to the present invention is shown in longitudinal cross-section taken along a scavenging passageway in a cylinder 1. In this figure, reference numeral 19 designates a fan for cooling the cylinder 1, and a cooling airflow is made to flow by this fan 19 as show by bold white arrows. Reference numerals 20 and 20′ designate scavenging passageways of the cylinder 1, each of which communicates a crank case 23 with a combustion chamber 5. Reference numeral 6 designates an exhaust port, and reference numeral 13 designates an intake port. At the top end portion of the scavenging passageway 20 on the upstream side of the cooling airflow, is provided a decompression groove 21 for returning a fuel-gas mixture into the crank case 23, as directed in the axial direction of the cylinder 1. The top end of this decompression groove 21 is positioned at such location that before the suction port 13 is opened by the piston 14 it does not communicate with the combustion chamber 5 (Fig. 2). Figs. 3 and 4 are detailed illustrations for the configuration of the decompression groove. Fig. 3 shows a triangular shape sharpened towards the plug. Fig. 4 shows a flared shape broadened towards the scavenging passageway. In these figures, it is assumed that the dimensional relationships of h₂ ≦ h₁/2, B ≦ A/2 and b ≦ a/2 are fulfilled. In addition, Fig. 5 shows a cylinder of a two-cycle engine according to another preferred embodiment of the present invention, Fig. 6 shows the state where the same cylinder 1 is assembled in an engine, and in these figures reference numeral 21 designates an escape groove, and numeral 21a designates a venturi shaped narrowed portion. Owing to the existence of the escape groove 21, in the case of a low rotational speed such as upon starting, a compressed fuel-air mixture would escape, hence an operating load of a recoil starter or the like for use in starting is light, and the starter can be operated easily. On the other hand, upon regular operation since the gas within the cylinder becomes high-temperature and high-pressure, upon passing through the escape groove the gas velocity becomes equal to the sound velocity, in this case according to the generally well-known theory, the gas becomes hard to flow due to the venturi portion, hence leakage would be reduced, and accordingly, reduction of output power is little. As will be apparent from the detailed description above, according to the present invention, the following effects and advantages are obtained: 1) Owing to the fact that a decompression groove directed in the axial direction is provided at the top end portion of a scavenging passageway within a cylinder chamber communicating with a crank case, a fuel-air mixture within the cylinder chamber returns to the crank case through the decompression groove, and so, a starting torque upon starting of an engine can be reduced. 2) As a result of the fact that a decompression groove is provided on the side of the scavenging passageway on the upstream side of a cylinder cooling airflow, a temperature at the neighborhood of the decompression groove can be lowered, and this serves to prevent accumulation of carbon soot in the decompression groove. 3) Even if a combustion gas should pass through the decompression groove, since a fresh air would flow into the combustion chamber from the crank case through the scavenging passageway, the decompression groove would not be blocked by carbon soot. 4) Owing to the fact that the width and depth of the groove are varied along the axial direction of the groove so as to be narrowed at its tip end towards the plug, a flow velocity of the fuel-air mixture flowing through the decompression groove would vary, and carbon soot would be hardly deposited. In addition, control for lowering of an output power and reduction of a starting torque, would become possible. 5) Since the decompression groove is formed at the top end portion of the scavenging passageway, the inner mold within the cylinder chamber can be formed in one step of a process, and so, rise of a cost would not be resulted. 6) Upon normal operation, a high-velocity gas flow is limited by the narrowed portion of the venturi, hence leakage is reduced, and so reduction of an output power would be little.

Claims: A decompression device in a two-cycle engine including a cylinder (1) having an inner wall, a piston, a combustion chamber (5) formed by said piston and cylinder (1), a crank case (23), and means forming a cylinder cooling airflow, said decompression device comprising: a scavenging passageway (20) formed along said inner wall of said cylinder (1) in the axial direction thereof and communicating with said crank case (23), and a decompression groove (21) formed in said cylinder wall and extending in the axial direction thereof, and communicating with an end of said scavenging passageway (20) directed toward said combustion chamber (5) characterized in that, said decompression groove (21) is located on that side of the cylinder (1) being directed to the upstream side of said cylinder cooling airflow and said decompression groove has a first portion nearer to said scavenging passage (20), a second portion removed from said scavenging passage (20), said second portion having a section not greater than said first portion and each of said portions having an axial length less than the total length of said decompression groove, and a discontinuity of section located between said first portion and said second portion. A decompression device as claimed in claim 1, characterized in that said discontinuity of section is obtained by said first portion having a depth(A) in the radial direction of said cylinder (5) greater than the depth (B) in the radial direction of said cylinder (5) of said second portion and said end nearest said scavenging passageway (20) of said second portion having a width (b) in the circumferential direction of said cylinder (5) less than a width (a) of said end nearest said scavenging passageway (20) of said first portion. A decompression device as claimed in claim 1, characterized in that a narrowed portion of venturi shape is formed in the middle of said decompression groove (21). A decompression device as claimed in claims 1 or 2, characterized in that said decompression groove (21) is formed in a triangular shape. A decompression device as claimed in claims 1 or 2, characterized in that said decompression groove (21) is formed in a flared shape having the end nearest the scavenging passageway (20) broadened. A decompression device as claimed in any of the preceding claims, characterized in that said decompression groove (21) is provided at the top end portion of said scavenging passageway (20). A decompression device as claimed in any of the preceding claims, characterized in that said end of said first portion nearest said scavenging passageway (20) has a width (a) in the circumferential direction of said cylinder (5) less than the width of the end of said scavenging passageway (20) in the circumferential direction of said cylinder (5).

Inventors: NAKAHARA, TAKAFUMI NAGOYA TECHN.INST. MITSUBISHI, NAKATANI, YOSHIHIDE NAGOYA MACH.WORKS MITSUBISHI, OKADA, SHIGEICHI NAGOYA MACH.WORKS MITSUBISHI, TAKATA, MAKOTO NAGOYA MACH.WORKS MITSUBISHI, YAEZAWA, EIJI NAGOYA MACH.WORKS MITSUBISHI
Applicants: MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: F02B25
IPC codes: F02F1, F02B75, F02B25, F02B77