This application claims priority of German patent application no. 10 2015 013 786.7, filed Oct. 20, 2015, the entire content of which is incorporated herein by reference.
U.S. Pat. No. 8,671,897 describes a two-stroke engine of the generic type. In the crankcase interior there is arranged a flow guide element which diverts the fuel/air mixture flowing into the crankcase interior through the inlet window toward the piston base.
It is an object of the invention to provide a two-stroke engine of the generic type in which the lubricating action during operation is improved.
The two-stroke engine of the invention includes: a cylinder defining a longitudinal cylinder axis and having a combustion chamber formed therein; a piston arranged in the cylinder so as to carry out a reciprocating back and forth movement therein; the piston delimiting the combustion chamber; a crankcase defining an interior; an inlet window controlled by the piston for supplying a fuel/air mixture to the crankcase; an outlet window leading out of the combustion chamber; at least one transfer channel via which combustion air flows from the interior of the crankcase into the combustion chamber; a crankshaft rotatably journaled in the crankcase for rotating about a rotational axis; a connecting rod connected to the piston via a piston pin; the piston driving the crankshaft via the connecting rod; the connecting rod being journaled on a crankpin of the crankshaft; the piston having a piston base defining a lower end facing toward the crankcase; a flow guide element arranged in the interior of the crankcase so as to extend adjacent to the inlet window; and, the flow guide element having at least a first inflow surface configured to divert at least a first partial quantity of the fuel/air mixture flowing in through the inlet window in a direction toward the crankshaft.
It has been found that inadequate lubrication of the crankpin bearing at which the connecting rod is mounted on the crankshaft may arise if, in the crankcase interior, there is arranged a flow guide element which diverts the inflowing mixture toward the piston base. To achieve adequate lubrication of the crankpin bearing during operation, provision is now made for at least one first inflow surface to be provided which diverts at least a first partial quantity of the fuel/air mixture flowing in through the inlet window in the direction of the crankshaft. In this way, adequate lubrication of the crankshaft, in particular of the crankpin bearing, can be ensured. Here, the first inflow surface is oriented such that, during a piston stroke, the inflowing fuel/air mixture strikes at least one region of the crankshaft, that is, normally the crankpin and/or at least one of the crank webs arranged at both sides on the crankpin and/or at least one bearing section, which is mounted in the crankshaft bearing, of the crankshaft. The two-stroke engine is a mixture-lubricating engine. The fuel accordingly contains oil for the lubrication of the moving parts in the crankcase. The oil is supplied into the crankcase interior together with the fuel and the combustion air in the form of a fuel/air mixture.
According to an aspect of the invention, at least one first inflow surface diverts a first partial quantity of the fuel/air mixture flowing in through the inlet window in the direction of the crankshaft, and at least one second inflow surface diverts a second partial quantity of the fuel/air mixture flowing in through the inlet window in the direction of a bottom side of the piston base. The first and the second inflow surface cause the inflowing fuel/air mixture to be split up in the direction of the crankcase and in the direction of the bottom side of the piston base. In this way, it is possible to ensure good lubrication and cooling of the piston pin bearing and of the crankpin bearing. Through suitable configuration of the inflow surfaces, it is possible to set the magnitude of the fraction of the fuel/air mixture flowing to the crankshaft in relation to the total quantity of the fuel/air mixture flowing into the crankcase interior. In this way, very good lubrication of the two-stroke engine is ensured in a simple manner. It may however also be advantageous for the two-stroke engine to have only a first inflow surface, and for adequate lubrication of the piston pin bearing to be ensured in some other way. The first inflow surface directs the inflowing fuel/air mixture downward in the direction of the crankshaft bearing. The two inflow surfaces may advantageously be provided on a common flow guide element. According to another aspect of the invention, a separate flow guide elements may be provided for the two inflow surfaces.
The crankcase advantageously has an inlet end and an outlet end, which are separated by a transverse plane. The transverse plane of the two-stroke engine is in this case the plane that encompasses the longitudinal cylinder axis and the axis of rotation of the crankshaft. The inlet end is that side of the crankcase at which the inlet window opens out when the piston is at top dead center. The outlet end is that side of the transverse plane which is averted from the inlet end. To realize good lubrication of the crankshaft, in particular of the crankpin bearing, it is advantageously provided that the first inflow surface diverts the first partial quantity of the inflowing combustion air to the inlet end of the crankcase. In this way, good lubrication of the crankshaft bearings is also realized. The crankshaft bearings are the bearings by which the crankshaft is rotatably mounted in the crankcase. The rotational direction of the crankshaft is advantageously selected such that the crankpin bearing, at the inlet end, moves in the direction of the combustion chamber and, at the outlet end, moves away from the combustion chamber. By virtue of the fact that the first partial quantity of the inflowing fuel/air mixture is diverted to the inlet end, the fuel/air mixture flows toward the piston pin bearing. The first partial quantity of the inflowing fuel/air mixture forms, at the inlet end in the crankcase interior, a cloud through which the crank pin bearing moves during the upward stroke of the piston. Adequate lubrication of the crankpin bearing can be ensured in this way. Here, the first inflow surface is advantageously arranged on the inlet side of the crankcase. According to another aspect of the invention, the entire flow guide element is arranged on the inlet side. The cloud has a high concentration of oil and fuel, because the mixture formation takes place at least partially in the crankcase. This is the case in particular if fuel and oil are supplied via a carburetor. In the case of fuel being supplied via a carburetor, only a partial mixture formation takes place in the intake channel.
In a section plane which encompasses the longitudinal cylinder axis and which is perpendicular to the axis of rotation of the crankshaft, the first inflow surface is advantageously arranged so as to lie at least partially between an extension of a top side and an extension of a bottom side of an inlet channel which opens out at the inlet window. A part of the inflowing fuel/air mixture thus flows directly onto the first inflow surface and is conducted directly into the crankcase interior. The first inflow surface has an inflow edge situated so as to face toward the inlet window. The inflow edge is that region of the first inflow surface which is impinged on first by the inflowing fuel/air mixture. The expression “inflow edge” is in this case to be interpreted broadly. In this context, an inflow edge may also be a rounded region, which is impinged on first by the inflowing fuel/air mixture, of the inflow surface.
The crankcase has a crankshaft plane which encompasses the axis of rotation of the crankshaft and which is perpendicular to the longitudinal cylinder axis. The bottom edge of the inlet window denotes that region of the inlet window which has the smallest spacing, measured parallel to the longitudinal cylinder axis, to the crankshaft plane. Provision is advantageously made for the bottom edge of the inlet window to be situated closer than the inflow edge of the first inflow surface to the crankshaft plane. The spacing, measured parallel to the longitudinal cylinder axis, of the inflow edge to the crankshaft plane is accordingly advantageously greater than the spacing of the bottom edge of the inlet window to the crankshaft plane. The inflow edge is accordingly arranged closer than the lower edge of the inlet window to the combustion chamber. In this way, the inflow edge is impinged on directly by the combustion air flowing in through the inlet window. The inflow edge and the bottom edge of the inlet window advantageously have a spacing, measured parallel to the longitudinal cylinder axis, which amounts to no more than 50% of a height, measured parallel to the longitudinal cylinder axis, of the inlet window. The spacing between the inflow edge and the bottom edge of the inlet window is in this case measured in a section plane lying perpendicular to the axis of rotation of the crankshaft. The spacing between the inflow edge and the bottom edge of the inlet window preferably amounts to 25% to 50% of the height of the inlet window. The inlet window may for example be of circular, elliptical or flattened form, or may have an irregular shape. Here, the inflow edge is advantageously further remote than the lower edge from the crankshaft plane, such that the inflow edge is situated opposite a middle to lower region of the inlet window.
The first inflow surface advantageously has a separation edge at its side averted from the inlet channel. Here, the separation edge need not be a sharp edge, but rather denotes the region at which the fuel/air mixture flowing in through the inlet window separates from the first inflow surface. The two-stroke engine advantageously has an imaginary central plane which encompasses the longitudinal cylinder axis and which is perpendicular to the axis of rotation of the crankshaft. The imaginary central plane, the transverse plane and the crankshaft plane are thus oriented perpendicular to one another. The separation edge is advantageously arranged such that, in a viewing direction perpendicular to the imaginary central plane, in particular in a sectional illustration through the imaginary central plane, a tangent to the first inflow surface, the tangent running through the separation edge, intersects the longitudinal cylinder axis at an intersection point which has a spacing of less than 2 cm from the axis of rotation of the crankshaft. The tangent preferably intersects the longitudinal cylinder axis on that side of the crankshaft plane which faces toward the combustion chamber. It is achieved in this way that the fuel/air mixture that is diverted by the first inflow surface is conducted into a region through which the crankpin bearing passes during the upward stroke of the piston. The tangent through the separation edge encloses an angle with the longitudinal cylinder axis which advantageously amounts to 5° to 40°. The angle particularly advantageously amounts to 10° to 25°.
The width, measured parallel to the axis of rotation of the crankshaft, of the first inflow surface is advantageously smaller than the width, measured parallel to the axis of rotation of the crankshaft, of the inlet window. In this way, only a part of the inflowing fuel/air mixture is guided into the crankcase interior by the first inflow surface. The width of the first inflow surface is in this case advantageously measured in a section plane perpendicular to the longitudinal cylinder axis. The width of the first inflow surface advantageously amounts to 10% to 50% of the width of the inlet window. The width of the first inflow surface particularly advantageously amounts to 20% to 40% of the width of the inlet window. The width of the inlet window and the width of the first inflow surface is in this case in each case the greatest width of inflow surface and inlet window.
To realize good cooling of the piston and good lubrication of the piston pin bearing, it is advantageously provided that the flow guide element has at least one second inflow surface which diverts a second partial quantity of the fuel/air mixture flowing in through the inlet window in the direction of the bottom side of the piston base. To realize expedient flow guidance and a low flow resistance, it is advantageously provided that the first inflow surface and the second inflow surface are concavely curved. The first and/or the second inflow surface may however also be formed as planar surfaces. By way of the two inflow surfaces, it is possible to realize a good distribution of the inflowing fuel/air mixture, such that a first partial quantity is diverted in the direction of the crankcase interior and a second partial quantity is diverted in the direction of the bottom side of the piston base. The concave curvature results in a low flow resistance and a streamlined diversion. The first inflow surface and the second inflow surface advantageously abut against one another at a flow divider. The flow divider is preferably formed as a straight edge running parallel to the axis of rotation of the crankshaft. A rounded or curved form or a suitable arrangement of the flow divider may however also be advantageous. The first partial quantity of the inflowing fuel/air mixture, which is diverted in the direction of the crankcase interior, is preferably smaller than the second partial quantity, which is diverted in the direction of the bottom side of the piston base. This can be easily realized by virtue of the width, measured parallel to the axis of rotation of the crankshaft, of the second inflow surface amounting to at least 1.5 times, in particular at least 2 times, the width of the first inflow surface. The second inflow surface advantageously has a cutout which is adjoined by the first inflow surface. An expedient arrangement of the first inflow surface is realized in this way.
The flow divider is advantageously arranged so as to stand in the fuel/air mixture flowing in through the inlet window. In the case of a central arrangement of the inlet window, it is advantageously provided that the flow divider is partitioned by an imaginary central plane. The second inflow surface is advantageously larger than the first inflow surface. In this way, it is possible for adequate cooling of the piston base and lubrication of the piston pin bearing to be realized.
The invention will now be described with reference to the drawings wherein:
For the supply of fuel/air mixture, a carburetor 65 is provided in the embodiment, which carburetor is schematically illustrated in
The cylinder 2 has a longitudinal cylinder axis 17. In the section plane shown, the longitudinal cylinder axis 17 coincides with a transverse plane 41 which encompasses the longitudinal cylinder axis 17 and the axis of rotation 8 of the crankshaft 7.
The transverse plane 41 divides an inlet end 42 of the crankcase 4 from an outlet end 43. At the inlet end 42, the inlet window 10 opens out, at the top dead center of the piston, into the crankcase interior 9. The connecting rod 6 is pivotably mounted, by way of a piston pin bearing 19, on a piston pin 18 of the piston 5. The other end of the connecting rod 6 is mounted by way of a crankpin bearing 21 on a crankpin 20 of the crankshaft. The crankpin 20 connects two crank webs 46 which are arranged to both sides of the connecting rod 6. During operation, the crankshaft 7 rotates in a rotational direction 50 which is oriented such that, during the upward stroke of the piston, the crankpin 20 moves in the direction of the combustion chamber 3 at the inlet end 42, and during the downward stroke of the piston 5, the crankpin moves away from the combustion chamber 3 at the outlet end 43.
In the embodiment, the two-stroke engine 1 has two transfer channels 12 close to the inlet and two transfer channels 13 close to the outlet, which are arranged symmetrically with respect to the section plane in
Two-stroke engines preferably run at very high rotational speeds. The nominal rotational speed may for example lie in the range from approximately 10,000 to 16,000 revolutions per minute. The lubrication of the piston pin bearing 19, of the crankpin bearing 21 and of the crankshaft bearing 46 during operation is realized by way of two-stroke oil which is admixed to the fuel/air mixture flowing in through the inlet window 10. Here, a mixture ratio of oil to fuel of 1:25 to 1:50 is normally provided. The fuel/air mixture flowing in through the inlet window 10 has only been partially prepared to form a combustible mixture and has a high concentration of oil and fuel, because the complete preparation to form a combustible mixture takes place for the first time in the crankcase interior 9. To realize adequate cooling and lubrication during operation, a flow guide element 22 is arranged in the crankcase interior 9. The flow guide element 22 extends at least partially adjacent to the inlet window 10. On the flow guide element 22 a first inflow surface 23 is formed which diverts a first partial quantity of the fuel/air mixture flowing in through the inlet window 10 into the crankcase interior 9 in the direction of the crankshaft 7. The first partial quantity of the fuel/air mixture is in this case diverted into a region through which at least one element of the crankshaft 7 moves during operation. In the embodiment, the first inflow surface 23 is oriented such that the first partial quantity is diverted to the inlet end 42 of the crankcase 4. Here, the first partial quantity of the fuel/air mixture advantageously flows into a region through which the crankpin bearing 21 moves during the upward stroke of the piston 5. During the upward stroke of the piston 5, the inlet window 10 is opened by the piston skirt, such that fuel/air mixture can flow into the crankcase interior 9 through the inlet window 10. The orientation of the first inflow surface 23 is advantageously selected such that the crankpin bearing 21 passes through the mixture that is conducted into the crankcase interior 9 by the first inflow surface 23. Good lubrication of the crankpin bearing 21 is realized in this way.
As shown in
When, during operation, the piston 5 moves upward toward the combustion chamber 3 from the bottom dead center shown in
In the embodiment, the first inflow surface 23 and the second inflow surface 24 abut against one another at a flow divider 31. In the embodiment, the flow divider 31 is in the form of a straight edge running parallel to the axis of rotation 8 of the crankshaft 7. The flow divider 31 forms the inflow edge for the first inflow surface 23 and the second inflow surface 24, that is, the region at which the fuel/air mixture flowing in through the inlet window 10 impinges on the inflow surfaces (23, 24). Here, the inflow edge need not be in the form of a sharp edge, but rather may also be of rounded form. The flow divider 31 splits up the inflowing flow between the two inflow surfaces 23 and 24. The flow divider 31 is arranged approximately centrally in the extension 47 of the inlet duct 44. The flow divider 31 has a spacing (e), measured parallel to the longitudinal cylinder axis 14, to the bottom edge 38, the spacing advantageously being less than 50% of the height (h) of the inlet window 10. The spacing (e) advantageously amounts to 25% to 50% of the height (h) of the inlet window 10. The spacing (e) is selected such that the flow divider 31 is arranged considerably below the top edge 39 of the inlet window 10. The bottom edge 38 has a spacing (f) to the crankshaft plane 40, the spacing being smaller than a spacing (g) of the flow divider 31 to the crankshaft plane 40. As also shown in
In
As shown in
As shown in
The two-stroke engine 1 has an imaginary central plane 25 which is perpendicular to the axis of rotation 8 of the crankshaft 7 and to the transverse plane 41 and which encompasses the longitudinal cylinder axis 17. As shown in
As is also shown in
The fastening of the intermediate part 51 to the crankcase 4 is shown in
The flow guide element 22 has two arms 61 which are connected, at their side facing toward the combustion chamber 3, by way of the inflow surface 24. In the embodiment, the arms 61 run approximately parallel to the flow direction. Provision may however also be made for the arms 61 to be formed as a nozzle or diffuser. In the embodiment shown, in every section plane perpendicular to the longitudinal cylinder axis 17, the arms 61 run parallel to the imaginary central plane 25. The flow divider 31 is also visible in
In the embodiment, the flow guide element 22 is provided on an intermediate part 51 which is formed separately from the crankcase 4 and from the cylinder 2. It may however also be expedient for the flow guide element 22 and/or the flow guide element 58 to be formed on the crankcase 4, that is, formed in one piece with components of the crankcase 4. Some other suitable fastening, for example by way of welding, may also be advantageous.
It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.
Number | Date | Country | Kind |
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10 2015 013 786.7 | Oct 2015 | DE | national |