EP0108706A2

EP0108706A2 - Cylinder liner for a uniflow type two cycle internal combustion engine

Info

Publication number: EP0108706A2
Application number: EP83730100A
Authority: EP
Inventors: Yoshinori C/O Mitsubishi Jukogyo K.K. Hirayama; Harutaka Nagasaki Technical Institute Tsujimura; Ichiro C/O Mitsubishi Jukogyo K.K. Hayashi; Horotoshi C/O Mitsubishi Jukogyo K.K. Kitagawa
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1982-11-04
Filing date: 1983-10-21
Publication date: 1984-05-16
Also published as: DK503283A; EP0108706B1; EP0108706A3; DK503283D0; DE3377804D1; KR840007136A; DK157701C; DK157701B

Abstract

An improved cylinder liner for an uniflow type two cycle internal combustion engine of the type including a number of scavenging holes formed thereon in an equally spaced relation in the peripheral direction is disclosed, wherein the improvement consists in that the centre line (Y, - Y_i) extending across the outer opening of the scavenging holes on the outer peripheral surface of the cylinder liner is inclined in the opposite direction to that of the swirl flow S and the upper swirl angle (0₂) is determined larger than the lower swirl angle (0'₂) so that an air passage extending through each of the scavenging holes is twisted away from the vertical plane. Further, another improved cylinder liner for an uniflow type two cycle internal combustion engine is disclosed, wherein the improvement consists in that each of the scavenging holes comprises a combination of plural circular holes arranged in an end-to-end relation in the substantially axial direction of the cylinder and the linear line (Y_i- Y₁) extending through the center (O₁) of the lowermost hole part and the centre (0₂) of the uppermost hole part on the outer opening of the scavenging holes on the outer peripheral surface of the cylinder liner is inclined in the opposite direction to that of the scavenging swirl flow S, wherein the upper swirl angle (θ₃) is determined larger than the lower swirl angle (θ₁) so that an air passage extending through each of the scavenging holes is twisted away from the vertical plane.

Description

The present invention relates to a cylinder liner for an uniflow type two cycle internal combustion engine and more particularly to improvement of or relating to a cylinder liner of the type including a number of scavenging holes formed thereon in an equally spaced relation in the peripheral direction so as to assure improved scavenging efficiency.
To facilitate understanding of the present invention it will be helpful that a conventional cylinder liner for an uniflow type two cycle internal combustion engine will be described with reference to Figs. 1 to 4.
Fig. 1 schematically illustrates a part of an uniflow type two cycle internal combustion engine, wherein reference numeral 1 designates a cylinder liner, referenve numeral 2 does a number of scavenging holes, reference 3 does a cylinder, reference numeral 6 does a piston, reference numeral 7 does a cylinder jacket, reference numeral 8 does a cylinder cover and reference numeral 9 does an exhaust valve.
As illustrated in Figs. 2 to 4, the conventional uniflow type two cycle internal combustion engine usually has a number of scavenging holes 2 arranged in an equally spaced relation along the cylindrical wall of the cylinder liner 1 at its lower part by way of which fresh air is caused to flow into the interior of the cylinder 3 thereby to expell combustion gas upwardly in the axial direction. Thus, combustion gas is replaced with fresh air required for next combustion. Specifically, each of the scavenging holes 2 is formed in such a manner that the center line 5 extending on the lower edge surface 2a is directed toward the center 4 of the cylinder and the upper edge surface 2_b is offset from the lower edge surface 2 a as illustrated in Fig. 3 while the direction of extension of both the upper and lower edge surfaces 2a and 2_bis maintained unchanged. As a result the whole air passage extending through the wall of the cylinder liner 1 is inclined in the same direction as that of swirl flow S. Accordingly, the upper edge surface 2_b of the scavenging hole extends inwardly in the radial direction with a certain offset from the center 4 of the cylinder equal to a radius R as illustrated in Fig. 4. Due'to the arrangement of the scavenging holes made in that way fresh air flows into the interior of the cylinder 3 toward the center 4 thereof at the lower part of the scavenging holes 2 but it flows thereinto at the upper part of the latter while it swirls in the horizontal direction.
Since the conventional cylinder liner has the scavenging holes 2 formed in the above-described manner,it has been found that scavenging flow in the cylinder 3 is achieved as illustrated in Fig. 5(a) and thereby the exhaust stroke is completed with a part of combustion gas staying in A, B, C and D sections in the drawing. As is apparent from the drawing, the section A occupies the largest space in the cylinder and acts as a significamt factor of adversely affecting scavenging efficiency. Further, the A section is an area where comparatively stable gas flow is achieved and the higher pressure difference between both the inside and outside of the cylinder 3 is, the bigger the space occupied by the section A becomes. As far as an internal combustion engine employing high scavenging pressure is concerned, it is an essential requirement that this section is reduced or minimized.
To obviate the foregoing problem of expelling residual exhaust gas staying in the section A there was proposed a method of extending the upper edge surface 2_b of the scavenging holes inwardly in the radial direction with substantially increased offset from the center of the cylinder 3. However, when the proposed method is employed in accordance with the conventional arrangement of the scavenging holes 2, it results that ribs 2_c between the adjacent scavenging holes 2 are caused to incline at an increased inclination angle which leads to reduction of their mechanical strength. On the other hand, when the ribs 2_c has a wide width to compensate for reduction of mechanical strength, there is no fear of causing a problem relative to mechanical strength, but another problem is that there occurs shortage in total area of the scavenging holes 2.
Thus, the present invention has been made with the above-mentioned background in mind and its object resides in providing an improved cylinder liner for an uniflow type two cycle internal combustion engine which assures that improved scavenging efficiency is achieved while ribs between adjacent scavenging holes are configured properly.
Other object of the present invention is to provide an improved cylinder liner for an uniflow type two cycle internal combustion engine which assures that ribs have a sufficiently high mechanical strength and scavenging holes have sufficient area in total while the configuration of the ribs on the inner wall of the cylinder is designed so that good lubrication is achieved when a piston passes by the scavenging holes.
Another object of the present invention is to provide an improved cylinder liner for an uniflow type two cycle internal combustion engine which can be manufactured with reduced working hours.
To accomplish the above objects there is proposed in accordance with the present invention a cylinder liner for an uniflow type two cycle internal combustion engine of the type including a number of scavenging holes formed thereon in an equally spaced relation in the peripheral direction, characterized in that the center line (Y₁ - Y_l) extending across the outer opening of the scavenging holes on the outer peripheral surface of the cylinder liner is inclined in the opposite direction to that of swirl flow S and the upper swirl angle ( θ₂) is determined larger than the lower swirl angle ( θ'₂) so that an air passage extending through each of the scavenging holes is twisted away from the vertical plane.
Further, there is proposed in accordance with another aspect of the present invention a cylinder liner for an uniflow type two cycle internal combustion engine of the type including a number of scavenging holes formed thereon in an equally spaced relation in the peripheral direction, characterized in that each of the scavenging holes comprises a combination of plural circular holes arranged in an end-to-end relation in the substantially axial direction of the cylinder and the linear line (Y₁- Y_l) extending through the center O₁ of the lowermost hole part and the center O₂ of the uppermost hole part on the outer opening of the scavenging holes on the outer peripheral surface of the cylinder liner is inclined in the opposite direction to that of scavenging swirl flow S, wherein the upper swirl angle ( θ₃) is determined larger than the lower swirl angle ( 9₁) so that an air passage extending through each of the scavenging holes is twisted away from the vertical plane.
Other objects, features and advantages of the present invention will be apparent by reading of the following description which has been prepared in conjunction with the accompanying drawings.
The accompanying drawings will be briefly described below.

Fig. 1 is a vertical sectional view of a part of an uniflow type two cycle internal combustion engine, schematically illustrating how a cylinder liner is fitted into a cylinder head.
Fig. 2 is a fragmental vertical sectional view of the cylinder liner with a scavenging hole formed thereon, shown in an enlarged scale.
Fig. 3 is a front view of the cylinder liner as seen in the direction as identified with reference numeral III in Fig. 2.
Fig. 4 is a cross-sectional view of the cylinder liner taken in line IV - IV in Fig. 3.
Fig. 5(a) schematically illustrates how scavenging is effected with the conventional cylinder liner while a part of exhaust gas resides in several sections in the latter after completion of exhaust stroke.
Fig. 5(b) schematically illustrates how scavenging is effected with the cylinder liner of the invention while very few exhaust gas resides in several sections in the cylinder liner after completion of exhaust stroke.
Figs. 6(a) to (c) schematically illustrate a cylinder liner in accordance with the first embodiment of the invention, wherein Fig. 6(a) is a front view of the cylinder liner illustrating both the outer and inner openings of a scavenging hole formed on the cylinder liner, the outer opening being shown in real lines and the inner opening being shown in dotted lines, Fig. 6(b) is a fragmental cross-sectional view of the cylinder liner taken in line A - A in Fig. 6(a), and Fig. 6(c) is a fragmental cross-sectional view of the cylinder liner taken in line B - B in Fig. 6(a).
Figs. 7(a) to (c) schematically illustrate a cylinder liner in accordance with the second embodiment of the invention, wherein Fig. 7(a) is a front view of the cylinder liner similar to Fig. 6(a), Fig. 7(b) is a fragmental cross-sectional view of the cylinder liner taken in line A' - A' in Fig. 7(a), and Fig. 7(c) is a fragmental cross-sectional view of the cylinder liner taken in line B' - B^* in Fig. 7(a).
Figs. 8(a) to (c) schematically illustrate a cylinder liner in accordance with the third embodiment of the invention, wherein Fig. 8(a) is a front view of the cylinder liner similar to Fig. 6(a), Fig. 8(b) is a fragmental cross-sectional view of the cylinder liner taken in line A" - A" in Fig. 8(a), and Fig. 8(c) is a fragmental cross-sectional view of the cylinder liner taken in line B" - B" in Fig. 8(a).
Figs. 9(a) to (c) illustrate results of experiments which were carried out under such a working condition that the lower swirl angle θ₁ is fixedly determined to 5 degrees and the upper swirl angle θ₂ varies in the range of 10 to 40 degrees, wherein Fig. 9(a) is a fragmental cross-sectional view of a cylinder liner schematically illustrating how the swirl angles θ₁ and θ₂ are determined, Fig. 9 (b) illustrates by way of diagrams a relation between air intake ratio f and scavenging efficiency η_S, wherein the angles θ₁ and θ₂ serve as a parameter, and Fig. 9(c) does a relation between swirl angle 0₂ and scavenging efficiency η_s, wherein swirl angle θ₁ is fixedly determined to 5 degrees and air intake ratio P is fixedly determined to 1.1.
Figs. 10(a) to (d) schematically illustrate a cylinder liner in accordance with the fourth embodiment of the invention, wherein Fig. 10(a) schematically illustrates an inner opening of a scavenging hole comprising three circular holes arranged in an end-to-end relation, shown by dotted lines, Fig. 10(b) does an outer opening of the same, shown by real lines, Fig. 10(c) is a fragmental cross-sectional view of the cylinder liner taken in line A - A in Fig. 10(a) and Fig. 10(d) is a fragmental cross-sectional view of the cylinder liner taken in line B - B in Fig. 10(a).
Figs. 11(a) to (d) schematically illustrate a cylinder liner in accordance with the fifth embodiment of the invention, wherein Fig. ll(a) schematically illustrates an inner opening of a scavenging hole comprising three circular holes arranged in an end-to-end relation, shown by dotted lines, Fig. 11(b) does an outer opening of the same, shown by real lines, Fig. 11(c) is a fragmental cross-sectional view of the cylinder liner taken in line A - A in Fig. 10(a) and Fig. 10(d) is a fragmental cross-sectional view of the cylinder liner taken in line B - B in Fig. ll(a).
Figs. 12(a) to (d) schematically illustrate a cylinder liner in accordance with the sixth embodiment of the invention, wherein Fig. 12(a) schematically illustrates an inner opening of a scavenging hole comprising three circular holes arranged in an end-to-end relation, shown by dotted lines, Fig. 12(b) does an outer opening of the same, shown by real lines, Fig. 12(c) is a fragmental cross-sectional view of the cylinder liner taken in line A - A in Fig. 12(a) and Fig. 12 (d) is a fragmental cross-sectional view of the cylinder liner taken in line B - B in Fig. 12(a).
Figs. 13(a) to (d) schematically illustrate a cylinder liner in accordance with the seventh embodiment of the invention, wherein Fig. 13(a) schematically illustrates an inner opening of a scavenging hole comprising three circular holes arranged in an end-to-end relation, shown by dotted lines, Fig. 13(b) does an outer opening of the same, shown by real lines, Fig. 13(c) is a fragmental cross-sectional view of the cylinder liner taken in line A - A in Fig. 13(a) and Fig. 13(d) is a fragmental cross-sectional view of the cylinder liner taken in line B - B in Fig. 13(a).
Figs..14(a) to (d) schematically illustrate a cylinder liner in accordance with the eighth embodiment of the invention, wherein Fig. 14(a) schematically illustrates an inner opening of a scavenging hole comprising three circular holes arranged in an end-to-end relation, shovm by dotted lines, Fig. 14(b) does an outer opening of the same, shown by real lines, Fig. 14(c) is a fragmental cross-sectional view of the cylinder liner taken in line A - A in Fig. 14(a) and Fig. 14(d) is a fragmental cross-sectional view of the cylinder liner taken in line B - B in Fig. l4(a), and
Figs. 15(a) to (c) illustrate results of experiments which were carried out under such a working condition that the lower swirl angle θ₁ is fixedly determined to 5 degrees, the central swirl angle 6₂ is fixedly determined to 10 degrees and the upper swirl angle θ₃ varies as a parameter in the range of 20 to 40 degrees, wherein Fig. 15(a) is a fragmental cross-sectional view of a cylinder liner schematically illustrating how swirl angles θ₁, θ₂ and θ₃ are determined, Fig. 15(b) illustrates by way of diagrams a relation between air intake ratio P and scavenging efficiency wherein the upper swirl angle 0₃ varies in three steps of 20, 30 and 40 degrees, and Fig. 15(c) does a relation between swirl angle θ₃ and scavenging efficiency η_s, wherein swirl angle 6₁ is fixedly determined to 5 degrees, swirl angle θ₂ is fixedly determined to 10 degrees and air intake ratio ρ is fixedly determined to 1.1.

Now, the present invention will be described in a greater detail hereunder with reference to Figs. 6 to 15 which illustrate several preferred embodiments of the invention. In this connection it should be noted that the same or similar parts and components illustrated throughout the drawings as those in the conventional cylinder liner illustrated in Figs. 1 to 4 are identified with the same reference numerals.
First, Figs. 6(a) to (c) schematically illustrate the construction of a cylinder liner in accordance with the first embodiment of the invention. As is apparent from the drawings, the center line Y₁ - Y_l extending across the outer opening 2_d of the scavenging hole on the outer peripheral surface of the cylinder liner is inclined at an angle of α,relative to the axis line Z - Z of the same. The direction of inclination of the angle α, is oriented opposite to that of the swirl flow S. On the other hand, the center line Y₂- Y₂ extending across the inner opening 2_e of the scavenging hole on the inner peripheral surface of the cylinder liner is inclined at an angle of α₂relative to the axis line Z - Z of the same, wherein said inclination angle α₂is determined less than that of the conventional cylinder liner, as is readily seen from Fig. 6(a). It results that the center line Y₁- Y₁ on the outer opening formed on the outer peripheral surface of the cylinder liner is inclined at an angle of α, + α₂ ( > 0) relative to the other center line Y₂- Y₂ on the inner opening formed on the inner peripheral surface of the same.
Incidentally, Fig. 6(b) is a fragmental cross-sectional view of the cylinder liner taken in line A - A in Fig. 6 (a), whereas Fig 6(c) is a fragmental cross-sectional view similar to Fig. 6(b) taken in line B - B in Fig. 6(a). As is apparent from the drawings, the swirl angle 9₂ at the upper edge surface of the scavenging hole is designed substantially larger than the swirl angle θ₁ at the lower edge surface 2a of the same and the swirl angle θ'₂ at the central part of the scavenging hole is designed appreciably larger than the swirl angle θ₁ at the lower edge surface 2_.of the cylinder liner. Thus, the following inequality will be established.
θ₂> θ'₂ > 1 Thus, the scavenging hole extends through the cylinder liner while it is increasingly twisted away from the vertical plane in the above-described manner.
Next, operation of the cylinder liner as constructed in accordance with the first embodiment of the invention will be described below.
At the beggining time when the scavenging holes are opened as the piston is displaced downwardly during its downward stroke scavenging air is caused to flow through the upper part of each of the scavenging holes. Since the scavenging hole extends at a large swirl angle 9₂ across the aforesaid upper part thereof, intensive siwrling flow is produced whereby residual exhaust gas in the A section as illustrated in Fig. 5(b) is blown away. As the piston is displaced downward further, the swirl angle 9₂ is gradually reduced. When the upper surface of the piston becomes flush with the lower edge surface 2a of the scavenging hole, it assumes the minumum swirling angle θ₁ and thereby the swirling flow S is weakened with scaveniging effected toward the 'center of the cylinder. As a result residual exhaust gas staying at the cetral part of the cylinder is scavenged. Thus, remarkably improved scavenging is assured for residual exhaust gas in the interior of the cylinder, as schematically illustrated in Fig. 5(b). Further, since each of the scavenging hole ribs 2 _chas a reduced inclination angle α₂ on the inner surface of the cylinder liner, it is assured that excellent lubrication is achieved when piston rings pass by the scavenging holes 2.
Next, Figs. 7(a) to (c) schematically illustrate the construction of a cylinder liner in accordance with the second embodiment of the invention. In practice, this embodiment corresponds to the case where the inclination angle α₂ in the first embodiment as illustrated in Fig. 6 liner becomes zero and therefore the center line Y₁- Y₁ extending aross the outer opening of the scavenging hole 2 on the outer peripheral surface of the cylinder liner is inclined at an angle of α₁ - α₂ > 0) relative to the center line Y₂- Y₂ extending across the inner opening on the inner peripheral surface of the same. As is apparent from Fig. 7(a), the center line Y₂- Y₂ is superimposed on the axis line Z - Z of the cylinder.
Next, Figs. 8(a) to (c) schematically illustrate the construction of a cylinder liner in accordance with the third embodiment of the invention. In this embodiment the center line Y₂- Y₂ extending across the inner opening of the scavenging hole 2 on the inner peripheral surface of the cylinder liner is inclined at an angle α₂ relative to the center axis Z - Z of the cylinder in the same direction as the center line Y_l- Y₁ extending across the outer opening on the outer peripheral surface of the same, the latter being inclined at an angle α₁ correlative to the center axis Z-Z of the cylinder. In this case the center line Y₁- Y₁ is inclined at an angle of α - α₂( > 0) relative to the center line Y₂- Y₂but the cylinder liner in accordance with this embodiment functions in the substantially same manner as in the first embodiment.
As described above, the cylinder liner in accordance with any one of the first to third embodiments is constructed in such a manner that the center line Y_l- Y₁ extending across the outer opening of the scavenging hole is inclined in the opposite direction to that of the swirl S.
Figs..9(a) to (c) illustrates improvement in scavenging efficiency which can be achieved by the construction of the scavenging holes on the cylinder liner in accordance with the first to third embodiments of the invention. Figs. 9(b) and (.c) show results of experiments which were conducted under the operating conditions where swirl angle θ₁ at the lower edge surface of the scavenging hole is fixedly determined to 5 degrees and swirl angle θ₂ on the upper edge surface of the same varies in the range of 10 to 40 degrees, as illustrated in Fig.9(a). Specifically, Fig. 9(b) illustrates by way of diagrams a relation between air intake ratio ρ and scavenging efficiency η_s, wherein the angles θ₁ and θ₂ serve as a parameter, and Fig. 9(c) does a relation between swirl angle θ₂and scavenging efficiency η_s, wherein swirl angle θ₁ is fixedly determined to 5 degrees and air intake ratio ρ is fixedly determined to 1.1.
As is apparent from Fig. 9(c) where air intake ratio ρ is determined to 1.1 and swirl angle θ1 is determined to 5 degrees, scavenging efficiency tends to increase when swirl angle 6₂ at the upper part of the scavenging hole is determined to 10 degrees or more or less and it reaches the highest level when swirl angle e₂ is determined to 30 degrees or more or less. In this case of experiments inclination angles α₁, and α₂of openings of a scavenging hole on the cylinder line are determined as a function relative to height of scavenging hole, number of scavenging holes and swirl angle θ'₂.
As is readily understood from the above description, a cylinder liner for an uniflow type two cycle inner combustion engine in accordance with the first to third embodiments of the invention is constructed such that the center line Y_l- Y₁ extending across the outer opening of each of the scavenging holes on the outer peripheral surface of the cylinder liner is inclined in the opposite direction to that of the swirl flow S and the upper swirl angle θ₂is determined larger than the lower swirl angle θ'₂ so that air passage extending through the scavenging holes is twisted away from the vertical plane. Thus, it is assured that scavenging efficiency is substantially improved while the configuration of ribs between a series of scavenging holes is determined properly.
Next, Figs.lO(a) to (d) schematically illustrate a cylinder liner in accordance with the fourth embodiment of the invention, wherein Figs. 10(a) and (b) are a fragmental view of a scavenging hole as seen from the position located outwardly of the peripheral surface of the cylinder liner. As is apparent from the drawings, the axis line Y₁ - Y₁ extending across the outer opening 2_f of the scavenging hole on the outer peripheral surface of the cylinder liner ( linear line extending through both the center O₁ of the lower hole and the center 0₃ of the upper hole) is inclined at an angle of α, relative to the axis line Z - Z of the cylinder. It should be noted that the axis line Y1 - Y₁ is inclined in the opposite direction to that of the swirl S as illustrated in Figs. 10(c) and (d). Further, the axis line Y₂- Y₂ extending across the inner opening 2_e of the scavenging hole on the inner peripheral surface of the cylinder liner is superimposed on the axis line Z - Z of the cylinder as is apparent from Fig. 10(a). Accordingly, the axis line Y₁- Y₁ extending across the outer opening on the outer peripheral surface of the cylinder liner is inclined at an angle of α, relative to the axis line Y2 - Y₂ extending across the inner opening on the inner peripheral surface of the same.
Incidentally, Fig. 10(c) is a fragmental cross-sectional view of the cylinder liner taken in line A - A in Fig. 10(a) and Fig. 10(d) is a fragmental cross-sectional view of the same taken in line B - B in Fig. 10(a). As illustrated in Figs. 10(c) and (d), the swirl angle θ₃ at the upper hole part of the scavenging hole is determined substantially larger than the swirl angle θ₁ at the lower hole part of the same. Further, as illustrated in Fig.lO(c), the swirl angle θ₂ of the central part of the scavenging hole is determined appreciably larger than the swirl angle θ₁of the lower part of the same. Accordingly, the following inequality will be established
Thus, the scavenging hole extends through the cylinder liner while it is increasingly twisted away from the vertical plane in the above-described manner.
As is apparent from Figs. 10(c) and (d), the centers (01, O'₁), (0₂, O'₂) and (03, O'₃) of both the outer and inner openings of the scavenging hole are located on a plane extending at a right angle relative to the axis line Z - Z of the cylinder respectively.
It should be noted that as is apparent from Figs. 10(c) and (d), the inner opening on the inner peripheral surface of the cylinder liner at each of the upper, central and lower parts of the scavenging hole has the same width B as measured in the peripheral direction.
Next, Figs. 11(a) to (d) schematically illustrate a cylinder liner in accordance with the fifth embodiment of the invention. In this embodiment the center 0₂ of the central part of the scavenging hole is located in alignment with the axis line Y₁- Y₁ which extends through both the center 0₁ of the lower part and the center 0₃ of the upper part as is the case with the fourth embodiment illustrated in Fig. 10(b).
Next, Figs. 12(a) to (d) schematically illustrate a cylinder liner in accordance with the sixth embodiment of the invention. This embodiment is substantially same to the foregoing ones as illustrated in Figs. 10 and 11 with exception that each of hole parts constituting the scavenging hole has the same inner diameter. Assuming that each of the hole parts on the inner peripheral surface of the cylinder liner has the same width as measured in the peripheral direction as in case of the foregoing embodiments illustrated in Figs. 10 and 11, it results that the inner diameter of each of the hole parts is caused to decrease as each of the swirl angles θ₁, 9₂ and 9₃ increases. On the other hand, when each of the hole parts has the same inner diameter as in case of the sixth embodiment in illustrated in Fig. 12, the rib between the adjacent scavenging holes has a width which varies in dependence on the position on the latter. Therefore, in view of the above-described embodiments an intermediate case may be proposed where the width of the rib between the adjacent scavenging holes varies appreciably in dependence on the position on the latter and moreover the inner diameter of each of the hole parts varies appreciably in dependence on the position on the scavenging hole.
Next, Figs. 13(a) to (d) schematically illustrate a cylinder liner in accordance with the seventh embodiment of the invention. In this embodiment the cylinder liner is constructed in the different manner from the fourth embodiment as illustrated in Fig. 10 such that the axis line Y₂- Y₂ extending across the inner opening 2_e on the inner peripheral surface of the cylinder liner is inclined at an angle α₂ relative to the axis line Z - Z of the cylinder in the leftward direction while the axis line Y₁- Y₁ extending across the outer opening 2_f on the outer peripheral surface of the cylinder liner is inclined at an angle α, relative to the axis line Z - Z of the cylinder in the righthand direction as seen in the drawing. Accordingly, the axis line Y₁- Y₁ on the outer opening of the scavenging hole is inclined by an angle α₁ + α₂ ( > 0) relative to the axis line Y₂- Y₂ on the inner opening of the same.
Finally, Figs. 14(a) to (d) schematically illustrate a cylinder liner in accordance with the eighth embodiment of the invention. In this embodiment the axis line Y₂- Y₂ extending across the inner opening 2_e on the inner peripheral surface of the cylinder liner is inclined at an angle of α₂ relative to the axis line Z - Z of the cylinder in the rightward direction and the axis line Y₁- Y₁ extending across the outer opening 2_f on the outer peripheral surface of the same is inclined at an angle of a₁ relative to the axis line Z - Z of the cylinder in the rightward direction as seen in the drawing, that is, in the same direction as that of the axis line Y₂- Y₂. In this case the axis line Y₁- Y₁ is inclined at an angle α₁ - α₂ >0) relative to the axis line Y₂- ^Y ₂.
As is readily understood from the above description, the cylinder liner in accordance with any one of the fourth to eight embodiments is constructed such that the axis line Y₁ - Y₁ extending across the outer opening of the scavenging hole is inclined in the opposite direction to that of the swirl flow S as in case of the first to third embodiments.
Next, operation of the cylinder liner as constructed in accordance with each of the fourth to eighth embodiments of the invention will be described below.
At the beginning time when the scavenging holes are opened as the piston is displaced downwardly during its downward stroke scavenging air is caused to flow through the upper part of each of the scavenging holes. Since the upper part of the scavenging hole extends through the wall of the cylinder liner at a large swirl angle 9₃, intensive swirling flow S is produced whereby residual exhaust gas staying in the A section as illustrated in Fig. 5(b) is blown away.
When the piston is displaced downward further until all the scavenging holes are completely opened, swirl angle is reduced to an angle of θ₁ whereby scavenging air is caused to flow toward the center of the cylinder, resulting in residual exhaust gas in the central part of the cylinder being scavenged completely.
On the other hand, with respect to ribs between the adjacent scavenging holes the cylinder liner in accordance with the fifth embodiment as illustrated in Fig. 11 has no inclination relative to the axis line Z - Z of the cylinder and the cylinder liner in accordance with the fourth and sixth embodiments as illustrated in Figs. 10 and 12 has a certain inclination less than that of the ribs on the conventional cylinder liner. Owing to the arrangement made in that way it is assured that excellent lubrication is achieved when piston rings pass by the scavenging holes.
Figs. 15(a) to (c) illustrates improvement in scavenging efficiency which can be achieved by the construction of the scavenging holes on the cylinder liner in accordance with the fourth to eighth embodiments of the invention, wherein Fig. 15(a) is a fragmental cross-sectional view of the cylinder liner particularly illustrating how swirl angles θ₁, e₂ and θ₃ are determined and Pigs. 15 (b) and (c) show results of experiments which were conducted under the operationg conditions where swirl angle θ₁ at the lower hole part is fixedly determined to 5 degrees, swirl angle θ₂ at the central hole part is fixedly determined to 10 degrees and swirl angle θ₃ at the upper hole part varies at three stages of 20, 30 and 40 degrees. Specifically, Fig. 15(b) illustrates by way of diagrams a relation between air intake ratio f and scavenging efficiency η_s with swirl angle θ₃ fixedly determined as a parameter and Fig. 15(c) does a relation between swirl angle 8₃ and scavenging efficiency η_s when swirl angle θ₁ is fixedly determined to 5 degrees, swirl angle θ₂ is fixedly determined to 10 degrees and air intake ratio ρ is fixedly determined to 1.1. As is apparent from Fig. 15(c), scavenging efficiency η_s increases as swirl angle θ₃ increases in the range of 20 to 30 degrees and it reaches the highest level when swirl angle θ₃ is determined to 30 degrees or more or less.
Inclination angles α₁ and α₂ of the scavenging holes are determined as function relative to height of scavenging holes, number of the latter and swirl angles. Since each of the scavenging holes on the cylinder liner in accordance with the fourth to eighth embodiments of the invention is constructed by a combination of plural circular holes, it is assured that it has a large swirl angle θ₃ at its upper hole part and a small swirl angle θ₁, at its lower hole part while each of the ribs between the adjacent scavenging holes has a proper configuration with working hours required for forming them being reduced to the minimum, resulting in improved scavenging efficiency guaranteed.
As described above, a cylinder liner for an uniflow type two cycle internal combustion engine in accordance with the fourth to eighth embodiments is constructed so that each of scavenging holes comprises a combination of plural circular holes which are arranged in an end-to-end relation in the substantially axial direction of the cylinder liner in such a manner that the axis line Y_i-Y_l extending through the center of the lowermost hole part and the center of the uppermost hole part on the outer opening of the scavenging hole on the outer peripheral surface of the cylinder liner is inclined in the opposite direction to that of the scavenging swirl flow S, wherein the upper swirl angle θ₃ is determined larger than the lower swirl angle θ₁ so that the center line extending through the center of each of the hole parts constituting the scavenging hole is twisted away from the vertical plane. Owing to the arrangement made in that way it is assured that each of the scavenging holes is formed with the minimized working hours required while ribs between the adjacent scavenging holes have proper configuration and sufficiently high strength and moreover remarkably improved scavenging efficiency is achieved with excellent lubricating function being maintained when the piston passes by the scavenging holes.
While the present invention has been described above with respect to several preferred embodiments, it should of cource be understood that it should not be limited only to them but various changes or modifications may be made in a suitable manner without any departure from the spirit and scope of the invention as defined in appended claims.

Claims

(1) A cylinder liner for an uniflow type two cycle internal combustion engine of the type including a number of scavenging holes formed in an equally spaced relation in the peripheral direction, characterized in that the center line (Y_i- Y_l) extending across the outer opening on the outer peripheral surface of the cylinder liner is inclined in the opposite direction to that of swirl flow S and the upper swirl angle ( 8₂) is determined larger than the lower swirl angle ( θ'₂) so that an air passage extending through each of the scavenging holes is twisted away from the vertical plane.
(2) A cylinder liner for an uniflow type two cycle internal combustion engine as defined in claim 1, characterized in that each of the scavenging holes has a substantially rectangular cross-sectional configuration.
(3) A cylinder liner for an uniflow type two cycle internal combustion engine as defined in claim 2, characterized in that the center line (Y₂- Y₂) extending across the inner opening of the scavenging holes on the inner peripheral surface of the cylinder liner is inclined in the same direction as that of the swirl flow S relative to the axis line (Z - Z) of the cylinder.
(4) A cylinder liner for an uniflow type two cycle internal combustion engine as defined in claim 2, characterized in that the center line (Y₂- Y₂) extending across the inner opening of the scavenging holes on the inner peripheral surface of the cylinder liner extends in parallel to the axis line (Z - Z) of the cylinder.
(5) A cylinder liner for an uniflow type two cycle internal combustion engine as defined in claim 2, characterized in that the center line (Y₂- Y₂) extending across the inner opening of the scavenging holes on the inner peripheral surface of the cylinder liner is inclined in the opposite direction to that of the swirl flow S relative to the axis line (Z - Z) of the cylinder and its inclination angle 0(₂is determined smaller than an inclination angle α₁ of the outer opening of the scavenging holes on the outer peripheral surface of the cylinder liner.
(6) A cylinder liner for an uniflow type two cycle internal combustion engine of the type including a number of scavenging holes formed in an equally spaced relation in the peripheral direction, characterized in that each of the scavenging holes comprises a combination of plural circular holes arranged in an end-to-end relation in the substantially axial direction of the cylinder and the linear line (Y_l- Y_l) extending through the center ( O₁) of the lowermost hole part and the center (O₃) of the uppermost hole part on the outer opening of the scavenging holes on the outer peripheral surface of the cylinder liner is inclined in the opposite direction to that of scavenging swirl flow S, wherein the upper swirl angle ( 9₃) is determined larger than the lower swirl angle ( 9₁) so that an air passage extending through each of the scavenging holes is twisted away from the vertical plane.
(7) A cylinder liner for an uniflow type two cycle internal combustion engine as defined in claim 6, characterized in that the linear line (Y₂- Y₂) extending across the inner opening of the scavenging holes on the inner peripheral surface of the cylinder liner extends in parallel to the axis line(Z - Z) of the cylinder.
(8) A cylinder liner for an uniflow type two cycle internal combustion engine as defined in claim 6, characterized in that the linear line (Y₂- Y₂) extending across the inner opening of the scavenging holes on the inner peripheral surface of the cylinder liner is inclined in the same direction as that of the scavenging swirl flow S relative to the axis line (Z - Z) of the cylinder.
(9) A cylinder liner for an uniflow type two cycle internal combustion engine as defined in claim 6, characterized in that the linear line (Y₂- Y₂) extending across the inner opening of the scavenging holes on the inner peripheral surface of the cylinder liner is inclined in the opposite direction to that of the scavenging swirl flow S relative to the axis line (Z - Z) of the cylinder and its inclination angle α₂is determined smaller than an inclination angle α₁ of the linear line (_Yl- Y₁) extending across the outer opening of the scavenging holes on the outer peripheral surface of the cylinder liner.
(10) A cylinder liner for an uniflow type two cycle internal combustion engine as defined in claim 6, characterized in that the inner opening of each of the scavenging holes on the inner peripheral surface of the cylinder liner has the same width as measured in the peripheral direction.
(11) A cylinder liner for an uniflow type two cycle internal combustion engine as defined in claim 6, characterized in that each of the scavenging holes has the same inner diameter.