A two-stroke cycle internal combustion engine and a method of charging same

Abstract: A two-stroke cycle internal combustion engine (11) comprises a cylinder block (12) defining at least one cylinder bore (13). Further, a piston (16) is reciprocatably received within the cylinder bore (13) and an exhaust port (32) is formed in one side of the cylinder bore (13) and being operable by said piston. This engine is provided with at least one scavenging passage (28) opening into said cylinder bore at a first scavenging port (29) for inducing a first scavenging flow. This first scavenging flow starts at said first scavenging port, (29) crosses said cylinder bore and terminates at said exhaust port. Further, there is provided a means (39,35,34) for inducing a tumbling flow within said cylinder bore.

Description: The invention relates to a two-stroke cycle internal combustion engine comprising the features of the preamble portion of Claim 1. Further, the invention relates to a method defined by the features of the preamble portion of claim 14. The advantages of two-stroke cycle engines are well known. The primary advantages of a two-cycle engine relative to a four-stroke cycle engine are its simplicity and its potentially higher specific output. Because these engines are primarily ported and because they fire every revolution rather than every other revolution, these results are obtained. However, the necessity for ensuring complete combustion and total scavenging of the engine presents significant problems. In addition, there is always the risk that some of the fresh fuel-air charge will pass out of the exhaust port at the end of the scavenging cycle. Thus, emission control is a problem that is greater with a two-stroke cycle engine generally than with a four-stroke cycle engine. One highly successful type of scavenging system employed with two-stroke cycle engines is the so-called Schnurle-type scavenging. With this type of scavenging there is provided a scavenging port that is generally opposed to the exhaust port. The scavenging air flow enters the combustion chamber from the scavenging port and flows generally axially toward the cylinder head. The charge then crosses the combustion chamber and flows downwardly toward the exhaust port. This type of scavenging is quite effective in that it takes a relatively long time for the fresh air charge to reach the exhaust port. In addition, the flow pattern tends to ensure that the chamber will be substantially purged of the combustion product by the end of the exhaust cycle. Under some running conditions, however, it is desirable to obtain a high degree of turbulence in the combustion chamber. This is particularly true when operating at low speeds and low loads. It has been found that turbulence can improve the flame propagation and ensure complete combustion under these difficult running conditions. An internal combustion engine showing features of the preamble portion of claim 1 is known from GB-A-345 156. Further, this reference also discloses a method according to the preamble of claim 14. The auxiliary scavenging ports cause additional scavenging air entering through them to stream over the concave piston between the cylinder axis and the main outlet ports. The course of these auxiliary currents from two opposed auxiliary scavenging inlet ports converges at the center of the cylinder to follow the main scavenging current. Accordingly, it is an objective of the present invention to provide an improved two-stroke cycle internal combustion engine as well as a method of charging a cylinder bore of a two-stroke cycle internal combustion engine which will not only effectively scavenge the cylinder bore but which will also introduce adequate turbulence to ensure complete combustion. This objective is solved by a two-stroke cycle internal combustion engine according to claim 1. This objective is further solved by a method according to claim 14. Preferably, the so-called Schnürle-type scavenging is used so that a first main scavenging port is positioned diametrically opposite to the exhaust port. However, with the Schnurle-type scavenging it is also somewhat difficult to achieve stratification in the combustion chamber. Stratification is, however, desirable at lower loads and speeds to insure good fuel economy and exhaust emission control. Stratification permits a total cylinder charging that is less than stoichiometric. A stoichiometric charge can be maintained in proximity to the spark plug at the time of firing to insure ignition. In order to further enhance the mixing rate between air and fuel, it is advantageous when a fuel injection device is adapted to inject fuel into the tumble flow by spraying with the tumble flow direction or traverse to the tumble flow direction. Other preferred embodiments of the present invention are laid down in further dependent claims. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein: Figure 1 is a cross-sectional view, taken primarily along the line I-I of Figure 2, through a single cylinder of an internal combustion engine constructed and operated in accordance with an embodiment of the invention, with various fuel injector locations being shown both in solid and phantom lines; Figure 2 is a transverse cross-sectional view taken along the line II-II of Figure 1; Figure 3 is a timing diagram showing the events of the engine depicted in Figures 1 and 2; Figure 4 is a perspective view showing the normal scavenge flow and the flow generated by the added tumble scavange port; Figure 5 is a cross-sectional view, in part similar to Figure 2, and shows another embodiment of the invention; Figure 6 is a cross-sectional view, in part similar to Figures 2 and 5, and shows a further embodiment of the invention; Figure 7 is a cross-sectional view, in part similar to Figures 2, 5, and 6, and shows a still further embodiment of the invention; Figure 8 is a cross-sectional view, in part similar to Figures 2 and 5-7, and shows yet another embodiment of the invention; Figure 9 is a cross-sectional view, in part similar to Figure 1, and shows a further embodiment of the invention; and Figure 10 is a graphical view showing the relation of scavenge flow in relation to load with the embodiment of Figure 9. Referring now in detail to the drawings, and initially to Figures 1-4, a two-cycle internal combustion engine constructed in accordance with a first embodiment of the invention is shown in these figures and is identified generally by the reference numeral 11. In the figures showing the engine 11, many of the components are shown in only schematic form. Where any component is so shown, this is because those components are primarily conventional and any prior art type of construction may be employed for them. As should be apparent from the foregoing description, the invention relates primarily to the scavenging system for the engine 11. Therefore, other components of the engine 11, which are not primarily related to the scavenging system, have been either shown schematically, only partially, or not at all. Where that is the case, those skilled in the art can readily resort to any known construction. The invention also is depicted in conjunction with only a single cylinder of a multiple-cylinder internal combustion engine. Again, those skilled in the art will readily understand how the invention may be practiced with multiple-cylinder engines of any configuration. The engine 11 is comprised of a cylinder block 12 that is formed with at least one cylinder bore 13. The lower end of the cylinder bore 13 is closed by the skirt of the cylinder block 12 and a crankcase member 14. The opposite end is closed by a cylinder head assembly 15 that is affixed to the cylinder block in any known manner. This may also include engines wherein the cylinder block 12 and the cylinder head 15 are integrally formed. A piston 16 reciprocates in the cylinder bore 13 and is connected by means of a piston pin 17 to the small or upper end of a connecting rod 18. The lower or big end 19 of the connecting rod 18 is journaled on a throw 21 of a crankshaft 22. The crankshaft 22 is rotatably journaled in a crankcase chamber 23 that is formed by the cylinder block and crankcase member 14. If a multiple-cylinder engine is employed, the crankcase chamber 23 associated with each cylinder bore 13 is preferably sealed from the others. An induction system supplies an air charge to the crankcase chamber 23 through an intake port 24 formed in the crankcase member 14. This induction system includes a throttle body 25 in which a flow-controlling throttle valve 26 is positioned. The throttle valve 26 is connected in an appropriate manner to a suitable throttle actuator. A reed-type check valve assembly 27 is interposed between the throttle valve 26 and the intake port 24, and is preferably in close proximity to the intake port 24. The reed-type check valve 27 permits the air flow to enter the crankcase chamber 23 when the piston 16 is moving upwardly in the cylinder bore. However, as the piston 16 moves downwardly to compress the charge in the crankcase chamber 23, the check valve 27 will close so as to permit this compression to take place and to prevent reverse flow through the induction system. A scavenging system, best shown in Figures 2 and 4, is provided for transferring the charge which has been compressed in the crankcase chamber 23 to the combustion chamber formed by the head of the piston 16, the cylinder bore 13, and the cylinder head 15. This charge is then further compressed in the combustion chamber as the piston 16 continues its upward stroke toward the cylinder head 15. This scavenging system includes a center, main scavenge passage 28 that is formed in the cylinder block 12 and which extends from an inlet opening in the crankcase chamber 23 to a scavenge port 29 that opens through the cylinder bore 13. This scavenge passage 28 and scavenge port 29 are configured so as to create a flow pattern in the combustion chamber, indicated by the shaded arrow 31 in Figure 4. This flow moves generally axially toward the cylinder head 15 along the side of the cylinder bore 13, across the cylinder bore axis, and then downwardly toward an exhaust port 32 formed in the cylinder bore 13 in opposing relationship to the center scavenge port 29. This exhaust port 32 communicates with an exhaust passage 33 that extends through the cylinder block 12 and delivers the exhaust gases to the atmosphere through a suitable exhaust system, which is not shown. In addition to the center, main scavenge passage 28, there are provided a pair of side, main scavenge passages 34 and 35. These scavenge passages 34 and 35 are positioned in diametrically opposed relationship and are configured so as to open into the crankcase chamber 23 at their inlet ends. These passages 34 and 35 terminate in side, main scavenge ports 36 and 37, which are disposed in opposite relationship to each other and on opposite sides of both the center, main scavenge port 29 and the exhaust port 32. These side, main scavenge passages 34 and 35, and their exit scavenge ports 36 and 37, are configured so as to provide a flow pattern, as indicated by the shaded arrow 38 in Figure 4, which also flows generally axially along the cylinder bore 13 upwardly toward the cylinder head 15. The flow then passes across the cylinder bore toward the side where the exhaust port 32 is formed and then downwardly so as to exit this port. The construction of the engine 11 as thus far described may be considered to be conventional. In accordance with the invention, there is provided a tumble scavenge passage 39 (auxiliary scavenging passage 39 in the claims) which also extends from an inlet end in the crankcase chamber 23 and parallel to the axis of the cylinder bore 13 at one side thereof and which exits into the combustion chamber through a tumble port 41 (auxiliary scavenging port 41 in the claims). This tumble port 41 and the tumble passage 39 are configured so as to create a flow pattern in the combustion chamber that travels first across the cylinder bore axis toward the opposite side of the cylinder bore, then axially upwardly along the side of the cylinder bore 13 toward the cylinder head 15. This charge then passes back across the cylinder bore and flows axially downwardly back toward the tumble port 41, as seen by the arrow 42 in Figure 4. Hence, this tumble passage 39 and the tumble port 41 create turbulence in the combustion chamber. This flow may also somewhat cause a skew to the flow action from the center scavenge passage 28 and the side scavenge passages 34 and 35 so as to further improve the tumbling action and the turbulence in the combustion chamber, particularly at low speeds. In accordance with one embodiment of the invention, a fuel injector 43 is mounted in the side of the cylinder block 12 so as to have a spray pattern 44 that flows with the air flow through the tumble passage 39 and port 41 so as to improve fuel mixing in the combustion chamber. This fuel pattern is also shown in Figure 1 by the same shaded line 42. The resulting flow pattern also stratifies the mixture in the combustion chamber to form a richer and stoichiometric charge in proximity to a spark plug 45 mounted in the cylinder head 15. Alternatively, the fuel injector may be mounted at the location 43-1, which is axially above the tumble passage 41, and thus will still be intersected by the flow pattern 42 to promote homogenous mixing of the fuel charge and/or stratification under some running conditions in proximity to the spark plug 45 that is mounted in the cylinder head 15 and which is fired in a known manner. As a result, there will always be a stoichiometric mixture present at the gap of the spark plug 45 at the time of ignition, and thus lean-burn running can be accomplished under low-speed, low-load conditions. Alternatively, the fuel injector may be mounted at the location 43-2 in the cylinder head 15 so as to spray downwardly into the cylinder bore 13. Again, however, the spray location is such that the fuel will be congregated at the gap of the spark plug 45 at the time of firing so as to provide stratification and operation which permits lean burn. Figure 3 shows the timing events for this embodiment. The events are, however, basically the same for all embodiments. The exhaust port 32 is opened by the movement of the piston 16 at the crank angle A. The scavange ports 28, 36, 37 and 41 all open at the crank angle B and then close at the angle C. At any time during this rotation, the fuel injection may occur. The exhaust port 32 closes at the crank angle D and it is best to inject no fuel between the angles C and D to insure against the escape of fuel into the exhaust. Figure 5 shows another embodiment of the invention which is the same as the first embodiment described in Figure 1, except that in this embodiment the fuel injector 51 is positioned so that its spray pattern does not go in the same direction as the tumbling air flow 44, but rather sprays in a pattern 52 which is transverse to this air flow pattern. Hence, the spray 52 is confined primarily within the tumble passage 39 adjacent the tumble port 41. This will ensure more complete mixing of the fuel with the tumbling air flow, and thus can further improve stratification. Figure 6 shows another embodiment of the invention. This embodiment differs from the previously described embodiments in that the tumble passage and port are actually formed as a part of one of the side scavenge passages, this being the passage 101 indicated in this figure. The remaining passages, which are the same as those previuosly described, have been identified by the same reference numerals, and hence these passages will not be described again. However, it should be seen that the scavenge passage 101 has what constitutes an extension, as indicated by the area indicated by the reference numeral 102, which is defined toward the center scavenge passage 28 from an imaginary line 103, which would be the peripheral edge of a conventional Schnurle-type side scavenge passage. Hence, the area 102 actually functions as the tumble passage, and the tumble port also has this same circumferential extent so as to create the flow pattern 44. In this embodiment a fuel injector 104 is provided so it sprays in a pattern 105, which is generally aligned with the portion of the tumbling air flow which enter the combustion chamber through the tumbling portion 102 of the passage 101. Hence, a stratification will also be achieved with this embodiment. This embodiment otherwise has the same advantages as have already been described, and thus further description of this embodiment is not believed to be necessary to permit those skilled in the art to practice the invention. Figure 7 shows another embodiment which is like the embodiment of Figure 6 in that the passage 101 serves the combined function of acting as a combined Schnurle-type scavenge passage as well as a tumble passage. In this embodiment, however, the opposite scavenge passage, indicated by the reference numeral 151, is configured so as to have a somewhat modified scavenging flow on the opposite side of the cylinder bore. The scavenge passage 151 has greater circumferential extent than the previous side scavenge passages 34, but has less width so as to provide substantially the same air flow volume. In all of the embodiments as thus far described, the engine has been described as being of the crankcase compression type. Figure 8 shows another embodiment of the invention which functions both as a crankcase compression engine, but also as a supercharged engine. This embodiment employs a tumble passage of the type shown in Figure 5, and therefore the components of this embodiment, which are the same as that embodiment, have been identified by the same reference numerals and will not be described again, except as to understand the construction and operation of this embodiment. In this embodiment the tumble flow can be augmented by providing, rather than crankcase compression flow, flow from a supercharger, indicated generally by the reference numeral 201. The supercharger 201 has an outer casing 202 that contains a pair of intermeshing rotors 203 and 204, which are driven by the crankshaft 22 through a suitable gear train. Although a positive displacement, Roots-type supercharger is employed, it will be apparent to those skilled in the art that other types of compressors may be employed. An atmospheric air inlet 205 admits air to the casing 202 for compression and delivery to the tumble passage 39. By controlling the speed or amount of boost by the supercharger 201, the degree of tumbling action can be controlled. From the description of Figure 8, it should be apparent that at times it may be desirable not to have the tumble action. As has been previously noted, the tumble action is particularly desirable in order to improve turbulence and in some instances achieve stratification under low-speed, low-load condition. Figure 9 shows another embodiment of the invention which is basically the same as the embodiment of Figure 1. In this embodiment, however, a tumble control valve 251 is positioned in the tumble passage 39 between its inlet end and the tumble port 41. By controlling the position of the tumble valve 251 from the open tumbling position, as shown in phantom lines, to the closed, non-tumbling position shown in solid lines, it is possible to selectively utilize the tumble action or disable it. Generally, the strategy would be such to introduce tumbling action under low-speed, low-load conditions and eliminate it under other running conditions. Figure 10 shows the scavenging flow in relation to engine load. With a two-stroke cycle engine according to the invention, since the air is blown out in the crossing direction of the exhaust gas direction, the vertical vortex is produced in the cylinder bore to restrict blow-by of fuel to the exhaust port. With the two-stroke cycle engine according to the invention, since the auxiliary scavenging passage is formed and the cylinder bore opening portion of the passage is provided with the straight portion extending in the crossing direction of the exhaust gas direction, the vertical vortex is produced in the cylinder bore as the exhaust gas flows through the straight portion. With a further embodiment of the invention, since the shapes or areas of the pair of main scavenging ports are made asymmetric each other, differences in the strength and direction of the scavenging flows are produced when they blow from the main scavenging ports into the cylinder bore. As a result, the vertical vortex is produced in the cylinder bore and blow-by of fuel is prevented with a simple structure. With a still further embodiment of the invention, since the blower is disposed in an air flow passage connected to the cylinder bore, which passage is perpendicular to a plane defined by the cylinder axis and the center of the exhaust port, air is strongly blown in the crossing direction of the exhaust gas direction and the vertical vortex is formed more reliably. As a result, blow-by of fuel is more reliably restricted. Further, since fuel is injected in the outflow direction of air into the cylinder bore, fuel flows together with the vertical vortex and blow-by of fuel is prevented. In addition , since fuel is injected into the vertical vortex according to another embodiment , since fuel is injected during the period of time from the closing of the exhaust port to the closing of the main scavenging ports, the period of time during which fuel may be injected is elongated and the dynamic range is enlarged accordingly and atomization of fuel is accelerated. With a further embodiment of the invention, since the air outflow strength is controlled with the throttling valve, the strength of the vertical vortex formed in the cylinder is controlled, and as with a still further embodiment of the invention, in which the throttling amount of the throttling valve is controlled according to the state of the engine load, blow-by of fuel is adjusted according to the operating state of the engine as by keeping the vertical vortex in a strong state even during a low load operation in which the scavenging amount is small.

Claims: A two-stroke cycle internal combustion engine (11) comprising a cylinder block (12) defining at least one cylinder bore (13), a piston (16) reciprocatably received within the cylinder bore (13) and a scavenging system including an exhaust port (32) and at least one scavenging port (29;36;37), wherein a plane is defined by the cylinder axis of the cylinder bore (13) and the center of said exhaust port (32), wherein the exhaust port (32) is formed in one side wall of the cylinder bore (13) and is operable by said piston (16), wherein one or several scavenging passages terminating at scavenging ports (29;36;37) in the side wall of said cylinder bore (13) are provided for generating a main scavenging flow from said at least one scavenging port to the exhaust port (32), said main scavenging flow passing through a top area of the cylinder bore (13), said main scavenging ports being arranged opposite said exhaust port and/or on either side of said plane opposite to each other, and wherein an auxiliary scavenging passage (39) terminating at a scavenging port (41) at the side wall of said cylinder bore (13) is provided for generating a secondary scavenging flow directed toward the opposite side wall of the cylinder bore (13), characterized in that the gas flow through the auxiliary scavenging port (41) for forming a tumbling secondary scavenging flow is directed to pass across said plane. Two-stroke cycle internal combustion engine according to claim 1, characterized in that the auxiliary scavenging port (41) is disposed in an area between the main scavenging port or ports and the exhaust port (32). Two-stroke cycle internal combustion engine according to claim 1 or 2, characterized in that a first scavenging port (29) is configured to create a first main scavenging air flow that moves axially along said cylinder bore (13) toward a cylinder head (15), across said cylinder bore (13), and down a side of said cylinder bore (13) toward said exhaust port (32), and that said secondary tumbling flow flows into said cylinder bore (13) and across said cylinder bore (13), axially along said cylinder bore (13) toward said cylinder head (15), and transversely back across said cylinder bore (13). Two-stroke cycle internal combustion engine according to claim 3, characterized in that the auxiliary scavenging port (41) is disposed closer to the first scavenging port (29) than to the exhaust port (32). Two-stroke cycle internal combustion engine according to claim 3 or 4, characterized in that a pair of side scavenging ports (36,37) is disposed in the area between the first scavenging port (29) and the exhaust port (32) on diametrically opposite sides of the cylinder bore (13), the flow through said side scavenging ports (36,37) being directed axially along the cylinder bore (13) toward the cylinder head (15) and then across the cylinder bore (13) toward the side thereof where the exhaust port (32) is positioned and then axially downwardly toward said exhaust port (32). Two-stroke cycle internal combustion engine according to claim 5, characterized in that the auxiliary scavenging port is formed by an extension (102) of one of the side scavenging ports (101). Two-stroke cycle internal combustion engine according to at least one of the preceding claims 3 to 5, characterized in that the auxiliary scavenging port is formed separately from the remaining ports. Two-stroke cycle internal combustion engine according to claim 7, characterized in that a charge-forming device for introducing an air charge into the combustion chamber is provided in proximity to the cylinder bore, said air charge generating said tumbling secondary flow. Two-stroke cycle internal combustion engine according to claim 7 or 8, characterized in that a supercharger (201) is arranged to deliver air to the auxiliary scavenging port (41). Two-stroke cycle internal combustion engine according to at least one of the preceding claims 2 to 9, characterized in that it comprises means (251) for controlling the amount of flow through the auxiliary scavenging port. Two-stroke cycle internal combustion engine according to claim 10, characterized in that the means (251) for controlling the amount of flow through the auxiliary scavenging port comprises a compressor for delivering compressed air to the auxiliary scavenging port. Two-stroke cycle internal combustion engine according to claim 10 or 11, characterized in that the means (251) for controlling the air flow through the auxiliary scavenging port (41) comprises a throttle valve in said auxiliary scavenging passage (39). Two-stroke cycle internal combustion engine according to at least one of the preceding claims 1 to 12, characterized in that it comprises a fuel injection device (43;51;104) adapted to inject fuel into the secondary flow by spraying in the direction or traverse to the direction of the secondary flow. Method of charging a cylinder bore (13) of an internal combustion engine of the two-stroke cycle type with a fresh air/fuel mixture, wherein a main scavenging flow is generated in the cylinder bore (13) for discharging burnt mixture through an exhaust port (32), said main scavenging flow passes through a top area of the cylinder bore (13), and wherein a secondary scavenging flow, starting at a secondary scavenging port and directed toward the opposite side wall of the cylinder bore is generated in said cylinder bore (13), said secondary scavenging flow being a tumbling flow, characterized in that said tumbling flow is directed to pass across a plane defined by the longitudinal axis of the cylinder bore (13) and the center of the exhaust port (32). Method according to claim 14, characterized in that a first main scavenging air flow moves axially along said cylinder bore (13) toward a cylinder head (15), across said cylinder bore (13), and down a side of said cylinder bore (13) toward said exhaust port (32), and that the tumbling secondary flow is a tumbling air flow that flows into said cylinder bore (13) and across said cylinder bore (13), axially along said cylinder bore (13) toward said cylinder head (15), and transversely back across said cylinder bore (13). Method according to claim 14 or 15, characterized in that a second main scavenging flow is induced by a pair of side scavenging ports (36,37) disposed in the area between the first scavenging port (29) and the exhaust port (32) on diametrically opposite sides of the cylinder bore (13), the flow through said side scavenging ports (36,37) being directed axially along the cylinder bore (13) toward the cylinder head (15) and then across the cylinder bore (13) toward the side thereof where the exhaust port (32) is positioned and then axially downwardly toward said exhaust port (32). Method according to at least one of the preceding claims 14 to 16, characterized in that fuel is injected into the tumbling secondary flow by spraying in the direction or traverse to the direction of the secondary flow.

Inventors: OTOME KIMITAKE, TAMURA OSAMU, OTOME, KIMITAKE, TAMURA, OSAMU
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1996
Main classification: F02B25
IPC codes: F02B31, F02B17, F02B23, F02B75, F02B25, F02M69