US3641987A

US3641987A - Two-stroke engines

Info

Publication number: US3641987A
Application number: US15799A
Authority: US
Inventors: Bernard Hooper
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-03-07
Filing date: 1970-03-02
Publication date: 1972-02-15
Anticipated expiration: 1989-02-15
Also published as: GB1303251A

Abstract

The specification discloses a stepped-piston two-cycle engine in which the cylinders are arranged in one or more sets of two cylinders. The charge is pumped from a pumping part of each cylinder of the or each set to a working part of the other cylinder of the set through separate first and second separate transfer passages opening into the working part through first and second inlet ports respectively which are arranged symmetrically relative to the exhaust port of the cylinder. The first transfer passages of each set cross and the second transfer passages of the set cross. Each cylinder is arranged between parts of the transfer passages which feed its working part. The arrangement gives a compact engine with very good scavenging and capable of high performance.

Description

I Umted States Patent 1151 3,641,987

Hooper Feb.-l5, 1972 [54] TWO-STROKE ENGINES Primary ExaminerBenjamin W. Wyche [72] Inventor. Bernard Hooper, Maybank House, Hope Assistant Examiner Richard Rothman Street, Wordsley, Stourbndge, England Atmmey mm Kelman 221 Filed: Mar. 2, 1970 57 ABS CT 211 App]. No.: 15,799 1 The specification discloses a stepped-piston two-cycle engine in which the cylinders are arranged in one or more sets of two [30] Foreign Application I cylinders. The charge is pumped from a pumping part of each Mar. 7, 1969 Great Britain ..l2,077/69 cylinder "1 each 3 Part cylinder of the set through separate first and second separate transfer passages opening into the working part through first [1.8. CI. ..l23/71R F02, 33/14 and second inlet ports respectively which are arranged symmetrically relative to the exhaust port of the cylinder. The first FleldotSearch ..l23/7l R 71V 59 BS 65 S transfer passages of each set cross and the second transfer was passages of the set cross. Each cylinder is arranged between [561 Refer CIM parts of the transfer passages which feed its working part. The UNITED STA'IES PATENTS arrangement gives a compact engine with very good scaveng- 1 157 30s 10/1915 11 123/59 BS ing and capable ofhigh pefomance' I ogan I 3,301,237 1/1967 Wolf ..l23/7l l1 Claims,9Drawing Figures -s5- 3 so m 30 e so I m 4D Qfi I I82 4 31331655 38 75 N 0,8" 1o c -c2 26 GI 11 &

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sum 5 or e BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is concerned with stepped-piston two-stroke engines.

2. Description of the Prior Art This invention relates to multicylinder, two-stroke engines of the type comprising at least one set of two cylinders, each such cylinder having a working part of smaller diameter and a pumping part of larger diameter; a piston slidably reciprocable in each cylinder and having a working part which slides in the working part of the cylinder and a pumping part which slides in. the pumping part of the cylinder; and transfer passage means between the pumping part of each cylinder of the or each set and the working part of the other cylinder of the set so that, in the or each set, each charge of air, or fuel and air mixture, required by the working part of each cylinder of the set is pumped, during operation of the engine, to such working part along the transfer passage means from the pumping part of the other cylinder of the set.

Hereinafter, engines of the above type are referred to as being of the type specified and the respective cylinders of the or each set are referred to as the first and second cylinders of the set respectively.

It is an objectof the invention to provide an engine of the type specified having conveniently arranged transfer passage means.

SUMMARY OF THE INVENTION According to the invention there is provided an engine of the type specified wherein the working part of each cylinder has exhaust port means and first and second inlet port means arranged symmetrically about the longitudinal axis of the cylinder with respect to the exhaust port means, the pumping part of each cylinder delivers into transfer passage means which, for the or each set comprises four transfer passages which are separate for at least the major portions of their lengths and consist of two first transfer passages extending respectively between the first inlet port means of the first and second cylinders and the pumping parts of the second and first cylinders and two second transfer passages extending respectively between the second inlet port means of the first and second cylinders and the pumping parts of the second and first cylinders, each cylinder being interposed between parts of the first and second transfer passages which feed the working part of that cylinder, the first transfer passages of the set mutually crossing and the second transfer passages of the set mutually crossing and wherein portions of substantial length of the first and second transfer passages immediately adjacent to the first and second inlet port means of each working part are disposed so that the flow paths of the charge entering said working part through said first and second port means converge and meet adjacent a deflecting surface opposite to the exhaust port means and which directs the charge towards the cylinder head of said working part.

In addition to the first and second inlet ports each working part may have an auxiliary inlet port connected to one or both of the transfer passage or passages feeding the working part by an auxiliary transfer passage of lesser cross-sectional area than the or each transfer passage from which it is fed and opening into the working part through said auxiliary inlet port which is opposite to the exhaust port means. The first and second transfer passages of each working part may be mutually connected by a passage of less cross-sectional area than the transfer passages to balance any small pressure differences which may arise between the passages.

The transfer passages are substantially similar so that the amounts of charge introduced through the first and second inlet port means of a working part in a cycle are substantially equal and enter the working part at substantially equal speeds.

Preferably also the first transfer passages lie wholly on one side of a central plane (as hereinafter defined) and the second transfer passages lie on the opposite side of said plane.

By central plane" is meant, for the or each set, a plane containing the longitudinal axes of the cylinders of the set and which either (1) contains the axis of the engine crankshaft where the cylinders are arranged side by side or in opposition or (2) is perpendicular to the axis of the engine crankshaft when the cylinders of the set are arranged inopposition or in V-formation.

In stating that the transfer passages are separate" there is meant that the passages do not share any common boundary surfaces substantially throughout their lengths. The expression port means is used to include a single port or a plurality of ports which communicate with the same transfer passage or exhaust passage.

The advantage of this arrangement is that symmetrical and efficient scavenging of the working parts, which is essential for high performance, can be obtained in a compact engine. The arrangement also allows symmetrical exhaust outlets to be provided which is necessary on motorcycle engines. The provision of separate transfer passages allows these to be of adequate cross-sectional area and said portions thereof to be shaped to give the incoming charge the desired direction of flow.

The engine may include a unitary cylinder block and crankcase and all the transfer passages maybe formed in the unitary cylinder block and crankcase during casting thereof in a manner similar to the formation of water passages in cylinder blocks of water-cooled engines or the transfer passages may be formed partly in the crankcase and partly in separate cylinder blocks. If either of these constructions is used, the crankcase or unitary cylinder block and crankcase for the or each set may be made in two halves having mating faces arranged so that the passage parts mate when the two halves are secured together and form said four separate transfer passages.

For making two-stroke motorcycle engines it is preferred, where possible, to make the parts by diecasting and for such engines it'is preferred to use a construction wherein each set of cylinders comprises a crankcase having recesses formed therein, a cylinder block or cylinder blocks formed with recesses therein and a plate interposed between the crankcase and the cylinder block or blocks, the plate and the recesses cooperating to form at least parts of said four separate transfer passages. It is preferred to provide the swept volumes of the pumping parts of the cylinders in the crankcase and the working parts each in a separate cylinder block so as to facilitate alignment of the working and pumping parts of each cylinder, such cylinder block also containing the clearance volume of the associated pumping part. When using a plate, where the first and second passages cross they will be separated from each other by the plate.

In a still further arrangement, parts of the passages are formed in the crankcase and/or cylinder block or blocks; the passages opening to the external walls of the crankcase and/or cylinder block or blocks to which are secured crossover means which interconnect said parts.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in-detail by way of example with reference to the accompanying drawings in which;

FIG. 1 is an exploded perspective view of parts of the crankcase and cylinder blocks of a two-cylinder engine constituting a first embodiment of the invention and showing a perspective view from above of a plate between the crankcase and the cylinder blocks;

FIG. 2 is a perspective view from below of the plate of FIG.

FIG. 3 is a vertical section through one of the cylinders of the engine parts of which are shown in FIGS. 1 and 2;

FIG. 4 is a section on the line lV-IV of FIG. 3 through the working parts of both cylinders with the pistons omitted;

FIG. 5 is an exploded perspective view of the crankcase and cylinder blocks of a second embodiment of the invention;

FIG. 6 is an exploded perspective view, with the cylinder heads removed, of a third embodiment of the invention; and

FIGS. 7, 8 and 9 are diagrams illustrating modifications of the transfer passage connections between the pumping part of one cylinder of a set and the working part of the other cylinder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 to 4, the engine comprises a crankcase indicated generally at 10 and first and second cylinder blocks 11 and 12, each cylinder block having a cylinder head, that for the block 12 being indicated at 13. Each cylinder of the two-cylinder engine is formed of two parts, a working part of smaller diameter and a pumping part of larger diameter. Referring to the first cylinder this has a working part 15 and a pumping part 16. The second cylinder has a working part 17 and a pumping part 18. The construction of the two cylinders is identical and will be described in relation to the second cylinder and to FIGS. 1 and 3. Thus the working part 17 is formed within a liner 20 received within the cylinder block 12 and the pumping partlS of the cylinder is formed mainly within a liner 21 received within the crankcase l0 and projecting proud of the upper surface 22 of the crankcase'to have parts 23 with upper surfaces substantially level with the upper surface of a plate 24, which is interposed between the crankcase l0 and the cylinder blocks 11 and 12, and to have parts 25 which terminate within the thickness of the plate 24 and define the lower boundaries of

outlet ports

26 and 27 whose upper boundaries are formed by the radially inner ends 28a'and 29a of

recesses

28 and 29 formed in-the lower surface of the cylinder block 12. These recesses commucate at their radically inner ends with the annular clearance volume 180 ofthe pumping part 18 of the cylinder. The lower end of the cylinder block 12 is formed with spigot parts 30 which fit within the parts 23 and thus locate the cylinder block 12 relative to the liner 21 and thus locate the liner 20 relative to the liner 21 thus mutually locating the working and pumping parts of the cylinder.

In a similar manner the working part 15 is formed within a liner 31 and the pumping part 16 is formed mainly within a liner 32, the cylinder block 11 having spigot parts 33 which locate within parts 34 at the upper end of the liner 32, such parts having upper surfaces level with the upper surface of the plate 24, such upper end also having parts 35 which terminate within the thickness of the plate 24 and define the lower boundaries of

outlet ports

36 and 37 whose upper boundaries are defined by the radiallyinner ends 38a and 39a of recess 38 and 39 formed in the cylinder block 11 and communicating with the clearance volume 16a of the pumping part 16.

Rotatably mounted in the crankcase 10 is a crankshaft indicated generally at 40 having three journal bearings (not shown), the outer bearings being at the ends of the crankcase and the central bearing being in a central web 41 of the crankcase. The crankshaft has two throws each receiving the big end of a connecting rod the small end of which is connected to a piston reciprocable in one of the cylinders. Each piston has a working part and a pumping part. Thus referring to the second cylinder, the piston indicated generally at 42 has a working part 43 which is slidable in the working part 17 of the cylinder and the piston has a pumping part 44 which is slidable in the pumping part 18 of the cylinder. The working part 43 carries a gudgeon pin 45 and a connecting rod 46 extends between the gudgeon pin and the crankshaft. In a similar manner, the first cylinder has a piston (not shown) which has a working part slidable in the working part 15 and a pumping part slidable in the pumping part 16 and connected by a connecting rod to the crankshaft.

Transfer passage means, now to be described, are arranged so that during operation of the engine, charges are pumped from the pumping part 16 of the first cylinder to the working part 17 of the second cylinder and charges are pumped from the pumping part 18 of the second cylinder to the working part 15 of the first cylinder. Loop scavenging of the working parts of the cylinders is also obtained as will be described.

Referring now to FIG. 4, the working part 15 has a first inlet port 47, a second inlet port 48 and an exhaust port 49. Similarly, the second working part 17 has a first inlet port 50, a second inlet port 51 and an exhaust port 52. It will be seen that in each working part the inlet ports are arranged symmetrically about the longitudinal axis of the cylinder with respect to the exhaust port. Loop scavenging is obtained during operation of the engine the fresh charge entering through the inlet ports and converging and meeting adjacentthe inner surface of the working part opposite the exhaust port and being deflected by said surface towards the top of the cylinder to displace the burnt charge which flows downwardly and through the exhaust port. It will be noted that the

first inlet ports

47, and 50 lie to one side of a central plane indicated by the line 53. The central plane contains the longitudinal axes of the cylinders and the rotary axis of the crankshaft. The

exhaust ports

49 and 52 communicate with

exhaust passages

54 and 55 respectively which may be connected to exhaust pipes. It is to be noted that the

exhaust passages

54 and 55 are symmetrically arranged relative to a line perpendicular to the line 53 and midway between the axes of the two' cylinders.

Fresh charges are introduced into the engine through

inlet passages

56 and 57 leading respectively to the recesses 29 and 39 and thus to the

pumping parts

16 and 18 through the

ports

37 and 27. As shown in FIG. 3, for the inlet passage 57, each inlet passage may be fitted with an inlet fitting 58 containing a reed valve 59 and connected to the flange 60 of a carburettor, not shown.

Transfer passage means extend between the pumping part 16 of the first cylinder and the working part 17 of the second cylinder and between the pumping part 18 of the second cylinder and the working part 15 of the first cylinder. These transfer passage means comprise two separate transfer passages between each pumping part and its associated work ing part. The transfer passages include the

recesses

28, 29, 38 and 39 already described and are formed by passages in said blocks, apertures and recesses in the plate 24 and recesses in the crankcase 10. 4

The flow'path of a charge along each of the four transfer passages provided is shown in FIG. 1 by a series of arrows, the arrows being distinctively shown.

Dealing first with the flow from the pumping part 18 of the second cylinder to the working part 15 of the first cylinder the flow of a charge along the first transfer passage is shown by the ribbon arrows 61 and the flow of a charge along a second transfer passage is shown by the dashed arrows 62. The first transfer passage along which the flow is indicated by the arrows 61 is constituted by the recess 29, an aperture 63 in the plate 24, a recess 64 in the upper surface 22 of the crankcase, a further aperture 65 in the plate 24 and a passage 66 in the cylinder block 11 leading to the first inlet port 47 of the working part 15. The aperture 63 is aligned with one end of the recess 64 and the aperture 65 is aligned with the other end thereof, the plate 24 closing the recess between said apertures 63 and 65. The undersurface of the plate 24 is recessed at 67 to give a greater cross-sectional area for the flow of the charge along the recess 64.

The second transfer passage from the pumping part 18 to the working part 15 directs the charge along the path indicated by the arrows 62 and is constituted by the recess 28, an aperture 68 in the plate 24, a recess 69 in the upper surface 22 of the crankcase, a further aperture 70 in the plate 24 and a passage 71 in the cylinder block 11' leading to the second inlet port 48. The aperture 68 is aligned with one end of the recess 69 and the aperture 70 is aligned with'the other end of the recess, the plate closing off the recess 69 between the ends thereof but being recessed on its underside at 72 to increase the cross-sectional area of the recess 69 available for the fiow of charge therealong. The upper surface of the plate 24 is relieved at 73 adjacent the aperture 63 so as to be aligned with the upper surface of one of the parts 25 of the liner 2] and the upper surface of the plate 24 is also relieved at 74 adjacent the aperture 68 to be aligned with the upper surface of the other nnnn M- part 25. The relief at 74 has a smoothly curved surface leading to the aperture 68 to give good flow characteristics.

Dealing now with the flow from the pumping part 16 of the first cylinder to the working part 17 of the second cylinder there are again two separate transfer passages, the flow along the first passage being indicated by the full line arrows 75 and the flow along the second passage being indicated by the chain line arrows 76.

Dealing first with the transfer passage represented by the arrow 75, this passage is constituted by the recess 39, and is constituted by an aperture 77 in the plate .24, a recess 78 in the upper surface 22 of the crankcase, a further aperture 79 in the plate 24 and a passage 80 leading to the first inlet port 50 in the working part 17. The aperture 77 communicates with one end of the recess 78 and the aperture 79 with the other end of the recess, the recess being closed off by the plate between said ends and the plate being recessed on its underside at 81 to increase the cross-sectional area of the recess 78 available for flow. The upper surface of the plate 24 is relieved at 82 to bring it level with the upper surface of one of the parts 35 of the liner l6 and the upper surface of the relief is smoothly curved in a manner described in relation to the relief 74.

The second transfer passage represented by the chain line arrows 76 is constituted by the recess 38, an aperture 83 in the plate 24, a recess 84 in the upper surface 22 of the crankcase 10, a further aperture 85 in the plate 24 and a passage 86 leading to the second inlet port 51 of the working part 17 One end of the recess 84 is aligned with the aperture 83 and the other 'end of the recess is aligned with the aperture 85, the recess 84 being closed off by said plate between said ends but the underside of the plate being relieved at 87 for the purpose hereinbefore described.

The central plane referred to above is indicated by the line 53 in FIG. 3 in addition to the line 53 in FIG. 4. It will be seen that the first transfer passages represented by 'the

arrows

61 and 75 and which feed the

first inlet ports

47 and 50 lie wholly to one side of this plane and that the second transfer passages represented by the

arrows

62 and 76 and which feed the

second inlet ports

48 and 51 lie wholly to the opposite side of the plane represented by the line 53. By this arrangement, it is possible to locate the inlet ports in the working parts symmetrically as shown in FIG. 4 while still obtaining a compact engine and having the

exhaust passages

54 and 55 also symmetrical which is important for a motor cycle engine. It will also be seen that the first transfer passages whose flow is represented by the

arrows

61 and 75 cross over and are separated by the plate 24. Thus the crossover occurs as the charge from the pumping part 16 flows over the relieved portion 82 and into the aperture 77 thus crossing over the charge flowing in the recess 64. Similarly, the second transfer passages cross over as the charge represented by the arrow 62 flows over the relieved portion 74 and thus over the charge flowing in the recess 84. It

will also be seen that the

cylinder

15, 16 is interposed between.

parts of the transfer passages represented by the

arrows

61 and 62 and that the

cylinder

17, 18 is interposed between parts of the transfer passages represented by the

arrows

75 and 76.

It is believed that operation of the engine will be substantially comprehended from the foregoing description but one cycle will be described. It will be appreciated that the piston in the first cylinder is 180 out of phase with the piston in the second cylinder, the one being at bottom dead center while the other is at top dead center.

Assuming that the first piston is at bottom dead center and the second piston is at top dead center, the pumping part of the first piston will have induced a charge into the pumping part 16 of the first cylinder through the inlet passage 56 and through a reed valve similar to that shown at 59. The charge in the second cylinder will have been ignited. As the piston in the first cylinder moves towards top dead center the charge in the pumping part 16 thereof will flow as indicated by the

arrows

75 and 76 towards the working part 17 of the second cylinder. The charge which has already been introduced into the workand the first inlet port 101. This transfer passage is constituted ing part 15 of the first cylinder will be compressed. As the piston in the second cylinder descends, when the working part of the piston uncovers the

ports

50,51 and 52, the charge which is being pumped from'the pumping part 16 will enter the working part 17, will flow upwardly towards the top of the cylinder and will displace the burnt charge which will flow out of the exhaust port 49. As the pumping part of the piston in the second cylinder descends it will induce a charge into the pumping part 18 through the'reed valve 59 and the recess 28. The charge in the first cylinder will now be exploded and as the piston in the second cylinder moves towards top dead center the charge in the pumping part 18 will be discharged through the

outlet ports

26 and 27 and will flow as indicated by the

arrows

61 and 62 towards the working part 15 of the first cylinder. When the working part of the piston in the first cylinder uncovers the

ports

47, 48 and 49 the charge will enter the cylinder through the

ports

47 and 48 and will displace the burnt charge through the exhaust port 49.

The portions of the transfer passages formed by the

passages

66, 71, and '86 are so shaped as to lead the charge into the working parts in the desired directions to obtain loop scavenging as described above. Moreover care is taken that the

passages

66 and 71 shall be similar in size and shape so that the

passages

80 and 86 shall be similar in size and shape so that a balanced flow of charge enters the working part.

Various modifications of the arrangement described in relation to FIGS. 1 to 4 are possible and one of these modifications is shown in FIG. 5 which illustrates diagrammatically an arrangement in which there is no plate such as 24 and the transfer passages are formed partly in two halves of a crankcase which mate along a plane perpendicular to the rotary axis of the crankshaft, and partly in the cylinder blocks. The cylinder block for the crankcase half 90 is indicated at 92 and the cylinder block for the crankcase half 91 is indicated at 9 3. A first cylinder is formed in the

parts

90 and 92 and has a pumping part 94 and a working part 95. The working part has an exhaust port 96 and first and second inlet ports of which only the first such port is shown at 97. The inlet ports are arranged symmetrically relative to the exhaust port in a manner similar to that described in relation to FIG. 4. The cylinder formed in the parts 91 and 93 has a pumping part 98 and a working part 99. The working part has an exhaust port 100 and first and second inlet ports arranged symmetrically relative to the exhaust port 100, only the first inlet port being shown and being indicated at 101.

Four transfer passages are provided as before thus there is a first transfer passage the flow along which is indicated by the full line arrows !02 and extends between the pumping part 94 by a recess 103 in the lower surface of the cylinder block 92 and a cooperating recess 104 in the upper surface of the crankcase half 90 (which recesses communicate with the clearance volume 94a of the pumping part 94) a passage 105 leading to a plane face 106 of the half 90, a passage 107 leading from a plane face 108 of the half 91 and a passage 109 in the cylinder block 93 leading to the first inlet port 101.

There is a first transfer passage leading from the pumping part 98 to the working part 95 and the flow along this passage is indicated by the ribbon arrows 110. The transfer passage is constituted by a recess 11 1 in the lower surface of the cylinder block 93 and a cooperating recess 112 in the upper surface of the crankcase half 91 and is constituted by a passage 113 extending to the face 108, a passage 114 in the crankcase half 90 extending from the face 106 and a passage 115 in the cylinder block 92 extending to the first inlet port 97 There is a second transfer passage indicated by the chaindotted arrows 116 extending from the pumping part 94 to the working part 99. This transfer passage is constituted by the

recesses

103 and 104 and a passage 117 in the crankcase half 90, a passage 118 in the crankcase half 91 and a passage 119 in the cylinder block 93 which leads to the second inlet port,

not shown, in the working part 99. There is a second transfer passage from the pumping part 98 to the working part 95 and innvm this is constituted by the recesses 111 and 112, a passage 120 in the crankcase part 91, a passage 121 in the crankcase part 90 and a passage 122 in the cylinder block 92 leading to the second inlet port, not shown, of the working part 95. The flow along this transfer passaged is represented by the dotted line arrows 123.

It will be seen that the first transfer passages, the flows along which are represented by the

arrows

102 and 110 lie to one side of a central plane containing the longitudinal axes of the cylinders and also the rotary axis of the crankshaft although the latter is not shown. The second transfer passages, the flows along which are represented by the

arrows

116 and 123 lie to the other side of said central plane.

The recesses 103 and 111 in the cylinder blocks 92 and 93 respectively communicate with

inlet passages

124 and 125 respectively which in the complete engine will have reed-valve containing fittings secured thereto.

The transfer passages are separate throughout the major parts of their lengths, being common only at the

recesses

103, 104, and 111, 112. It will be seen that the first transfer passages represented by the

arrows

102 and 110 cross and the second transfer passages represented by the

arrows

116 and 123 cross. The cylinder 94, 95 is interposed between parts of the passages 1 10 and 123 and the cylinder 98, 99 is interposed between parts of the

passages

102 and 116.

The operation of the embodiment of FIG. is similar to the operation of the embodiment of FIGS. 1 to 4 and will therefore not be described.

FIG. 6 shows a further modification. In this modification, the engine comprises a cylinder block 130 in which are formed a first cylinder 131 and a second cylinder 132. The first cylinder 131 has a working part133 and a pumping part 134 and the second cylinder has a working part 135 and a pumping part not visible in the drawing. A piston 136 is reciprocable in the cylinder 131 and has a working part 137 and a pumping part 138. The piston is connected to a crankshaft 139 by means of a connecting rod 140. The working part 133 has a first inlet port 141', a second inlet port 142 and a exhaust port 143, the

ports

141, 142 and 143 being arranged for loop scavenging as described above; the pumping part 134 has port means 144. The first inlet port 141 is on one side of a plane indicated by the line 145 through the longitudinal axes of the cylinders and the axis of the crankshaft 139. The working part 135 similarly has first and second inlet ports on the same sides respectively of the plane 145 as the inlet ports 141 and 142 of the working part 131. The working part 135 also has an exhaust port arranged in a manner similar to the exhaust port 143.

The inlet ports and the outlet ports communicate with the external walls of the cylinder block and the crossing over of the four transfer passages which are provided and which are arranged as described below is effected by first and

second transfer members

146 and 147 operating in conjunction with passages and recesses formed in the external walls of the cylinder block and cover members 148 and 149.

There is a first transfer passage, the flow along which is indicated by the full line arrow 150 between the pumping part 134 and the working part 135. This transfer passage extends from the clearance volume 151 of the pumping part 134 and comprises a Z-shaped recess 152 in a transfer member 147 and a passage, not shown, in the cylinder block leading from the recess 152 to the first inlet port of the working part 135.

There is a first transfer passage from the pumping part of the cylinder 132 to the first inlet port 141 in the working part 133, the flow along this transfer passage is indicated by the ribbon arrow 153. The transfer passage starts from the recess in communication with the pumping part of the cylinder 132, not shown, and comprises an aperture 154 in the transfer member 147, a Z-shaped recess 155 in the cover member 149, another aperture 156 in the transfer member 147 and a passage 157 in the cylinder block leading to the first inlet port 141.

There is a second transfer passage leading from the pumping part 134 to the working part 135 and the flow path of a charge along this passage is indicated by the chain-dotted arrow 158. The passage is formed by a recess 158a in the cylinder block which is closed by a wall 159 in the transfer member 146, the recess 158a leading to the second inlet port of the working part 135. The recess 158a communicates with an aperture 160 in the wall 159 which in turn communicates with an aperture 161 in the cover member 148. The aperture 161 is adapted to be connected to a carburettor to allow a charge to be induced into the pumping part 134 through the apertures 161, 160 and through the port means 144.

There is a second transfer passage from the pumping part of the cylinder 132 to the second inlet port 142 of the working part 133. The flow path of a charge in this transfer path is indicated by the dotted arrow 162. The transfer path is formed by a passage 163 which communicates with the pumping part of the cylinder 132, an aperture 164 in the wall 159, a recess 165 in the transfer member 146 and which is closed by the cover plate 148, a further aperture 166 in the transfer member 146 and a passage 167 leading to. the second inlet port 142.

In operation the pumping part 134 pumps charges along the first and second transfer passages represented by the

arrows

150 and 158 to the working part 135 and the pumping part of the cylinder 132 pumps charges along first and second transfer passages represented by the arrows 153 and 162 to the working part 133. The first transfer passages 150, 153 are on one side of the plane indicated by the line 145 and the second transfer passages 158, 162 are on the other side of said plane. The transfer passages represented'by the arrows 150 and 153 cross and the transfer passages represented by the arrows 158 and 162 cross. The cylinder 131 is interposed between parts of the

passages

150 and 158 and the cylinder 132 is interposed between parts of the passages 153 and 162.

FIGS. 7, 8 and 9 indicate diagrammatically modifications of the arrangement described in detail with'reference to FIGS. 1 and 6 and each diagram shows the transfer path connections between the working part of one cylinder and the pumping part of the other. It is to be understood that the arrangement will be symmetrical and that the pumping part of the other I cylinder will communicate with the working part of the one cylinder through a similar arrangement of transfer passage means.

Referring to FIG. 7 the pumping part of the first cylinder is indicated at 170 and the working part of the other cylinder at 171. First and

second transfer passages

172 and 173 extend from the pumping part 170 to the working part 171 and open into the latter through first and

second inlet ports

174 and 175 respectively. The inlet ports are arranged for loop scavenging with reference to an exhaust port 176. Thus far the arrangement is as described in relation to FIGS. 1 to 4. The modification is that a balance passage 177 of lesser cross-sectional area than the

transfer passages

172 and 173 is connected between the passages. The purpose of the balance passage is to even out any small differences in pressure there may be between the charges flowing along the

transfer passages

172 and 173. The central plane is indicated by the line 178. I

Referring now to FIG. 8, the pumping part of one cylinder is indicated at 179 and the working part of the other at 180. First and second transfer passages are indicated at 181 and 182 and open into first and

second inlet ports

183 and 184 in the working part 180. The first transfer passage 181 and the first inlet port 183 are to one side of the central plane indicated by the line 185 and the transfer passage 182 and the second inlet 184 are arranged on the other side of the plane 185. The working part has an auxiliary inlet port 186 which is intersected by the plane and which is fed from the first transfer passage 181 by an auxiliary transfer passage 187. If desired the passage 187 may be fed also from the passage 182 by a further auxiliary passage 187a shown dotted. It will be seen that in this arrangement the main inlet ports are arranged symmetrically with respect to an exhaust port 188.

The auxiliary inlet port 186 gives cross scavenging towards the exhaust port 188.

FIG. 9 shows an arrangement which is similar to PK]. 8 except that the axis of symmetry of the parts of the working part is inclined to the central plane. Thus referring to PK]. 9, the pumping part of one cylinder is indicated at 189 and the working part of the other at 190. First and second transfer passages are indicated at 191 and 192 and are on opposite sides of the central plane indicated by the line 193. The first transfer passage 191 feeds the first inlet port 194 and the second transfer passage 192 feeds the second inlet port 195. The exhaust port of the'working part is indicated at 196 and the axis of symmetry of the workingpart by the line 197. An auxiliary inlet port 198 is provided on the axis of symmetry opposite to the exhaust port 196 and is fed from the transfer passage 191 by an auxiliary transfer passage 199.

It will be seen that the invention provides a number of different arrangements of a stepped piston engine in which the transfer passages are arranged symmetrically and enable a compact engine to be obtained with efficient scavenging and a symmetrical arrangement of the exhaust ports and therefore of the exhaust pipes which is important for a motor cycle engine. It is possible also to make the transfer passages feeding each. cylinder of substantially equal length which gives balanced flow and efficient scavenging and charging of the cylinders.

I claim:

1. A multicylinder, two-stroke engine comprising at least one set of two cylinders, each cylinder having a working part of smaller diameter, and a pumping part of larger diameter; a piston slidably reciprocable in each cylinder and having a working part which slides in the working part of the cylinder and a pumping part which slides in the pumping part of the cylinder; transfer passage means between the pumping part of each cylinder of the or each set and the working part of the other cylinder of the set to conduct charges from the pumping parts to the working parts during operation of the engine; port means in each working part and comprising exhaust port means and first and second inlet port means arranged symmetrically about the longitudinal axis of the cylinder with respect to the exhaust port means and a deflecting surface formed by the cylinder wall opposite the exhaust port means; the transfer passage means for the or each set comprising four transfer passages which are separate for at least the major portions of their lengths and consist of two first transfer passages extending respectively between the first inlet port means of the first and second cylinders and the pumping parts of the second and first cylinders and two second transfer passages extending respectively between the second inlet port means of the first and second cylinders and the pumping parts of the second and first cylinders, each cylinder being interposed between parts of the first and second transfer passages which feed the working part of-that cylinder, the first transfer passages of the set mutually crossing and the second transfer passages of the set mutually crossing, all the four transfer passages for a set of cylinders'being substantially similar so that the amounts of charge introduced through the first and second inlet port means of a working part in a cycle are substantially equal and enter the working part at substantially equal speeds, and portions of the first and second transfer passages immediately adjacent to the first and second inlet port means of each working part being disposed so that the flow paths of the charge entering said working part through said first and second inlet port means coverage and meet adjacent said deflecting surface and so that said surface directs the charge toward the cylinder head of said working part.

2. An engine according to claim 1 having a first part with one of said first and second parts. I I

4. An engine as claimed 1n claim 1 comprising a first part constituted by a crankcase and cylinder block, first and second recesses formed in said first part and respectively communicating with the inlet port'means and the pumping parts of the cylinders and opening to the external walls of thefirst part and crossover means releasably connected to the first part and interconnecting the first and second recesses to form therewith the transfer passages.

5. An engine as claimed in claim 4 wherein said crossover means comprises a recessed second part and an apertured third part interposed between'the first and second parts, the apertures in the third part being disposed to place selected recesses of the first and second parts in communication such that the transfer passages are formed by the recesses and apertures.

6. An engine according to claim 1 wherein each working part has an auxiliary transfer port connected to one or both of the transfer passage or passages feeding the working part by an auxiliary transfer passage of lesser cross-sectional area than the or each transfer passage from which it is fed and opening into the working part through said auxiliary inlet port which is opposite to the exhaust port means.

7. 'An engine according to claim 1 wherein the first and second transfer passages of each working part are mutually connected by a passage of lesser cross-sectional area than the transfer passages to equalize the pressure in the transfer passages.

8. An engine according to claim 1 including a crankcase for the or each set made in two halves having mating faces, at least parts of the transfer passages being formed in said halves and being arranged so that the passage parts mate when the two halves are secured together.

9. An engine according to claim 1 wherein each set of cylinders comprise a crankcase having recesses formed therein, a cylinder block means formed with recesses therein and a plate interposed between the crankcase and the cylinder block means, the plate and the recesses cooperating to form at least parts of said four separate transfer passages.

10. An engine according to claim 9 wherein the swept volumes of the pumping parts of the cylinders are provided in the crankcase and the working parts each in a a separate cylinder block.

11. An engine according to claim 9 wherein where the passages cross they are separated by the plate.

nun,

Claims

1. A multicylinder, two-stroke engine comprising at least one set of two cylinders, each cylinder having a working part of smaller diameter, and a pumping part of larger diameter; a piston slidably reciprocable in each cylinder and having a working part which slides in the working part of the cylinder and a pumping part which slides in the pumping part of the cylinder; transfer passage means between the pumping part of each cylinder of the or each set and the working part of the other cylinder of the set to conduct charges from the pumping parts to the working parts during operation of the engine; port means in each working part and comprising exhaust port means and first and second inlet port means arranged symmetrically about the longitudinal axis of the cylinder with respect to the exhaust port means and a deflecting surface formed by the cylinder wall opposite the exhaust port means; the transfer passage means for the or each set comprising four transfer passages which are separate for at least the major portions of their lengths and consist of two first transfer passages extending respectively between the first inlet port means of the first and second cylinders and the pumping parts of the second and first cylinders and two second transfer passages extending respectively between the second inlet port means of the first and second cylinders and the pumping parts of the second and first cylinders, each cylinder being interposed between parts of the first and second transfer passages which feed the working part of that cylinder, the first transfer passages of the set mutually crossing and the second transfer passages of the set mutually crossing, all the four transfer passages for a set of cylinders being substantially similar so that the amounts of charge introduced through the first and second inlet port means of a working part in a cycle are substantially equal and enter the working part at substantially equal speeds, and portions of the first and second transfer passages immediately adjacent to the first and second inlet port means of each working part being disposed so that the flow paths of the charge entering said working part through said first and second inlet port means coverage and meet adjacent said deflecting surface and so that said surface directs the charge toward the cylinder head of said working part.

2. An engine according to claim 1 having a first part with recesses formed therein communicating with the inlet port means, at least one second part having recesses formed therein and an apertured third part interposed between the first part and the second part, the apertures in such third part being disposed to provide communication between selected recesses in said first and second parts such that the transfer passages are formed by said recesses and apertures.

3. An engine as claimed in claim 2 wherein each third part is also provided with recesses which are aligned with recesses in one of said first and second parts.

4. An engine as claimed in claim 1 comprising a first part constituted by a crankcase and cylinder block, first and second recesses formed in said first part and respectively communicating with the inlet port means and the pumping parts of the cylinders and opening to the external walls of the first part and crossover means releasably connected to the first part and inteRconnecting the first and second recesses to form therewith the transfer passages.

5. An engine as claimed in claim 4 wherein said crossover means comprises a recessed second part and an apertured third part interposed between the first and second parts, the apertures in the third part being disposed to place selected recesses of the first and second parts in communication such that the transfer passages are formed by the recesses and apertures.

7. An engine according to claim 1 wherein the first and second transfer passages of each working part are mutually connected by a passage of lesser cross-sectional area than the transfer passages to equalize the pressure in the transfer passages.