Two-stroke engine

Abstract

A two-stroke engine (1) has a cylinder (2) in which a piston (5) is mounted to move reciprocally. The two-stroke engine (1) comprises an intake port (9) which opens into a crankcase interior (14) via an intake port opening (10), and an air port (11) which opens into a cylinder bore (13) of the cylinder (2) via at least one air port opening (12). The two-stroke engine (1) comprises at least one transfer port (17, 18). The piston (5) has at least one piston pocket (16). A fuel supply device (15) is provided. The at least one piston pocket (16) has at least one section (23) which is designed and arranged such that during the downward stroke of the piston (5) there is a fluidic connection between the at least one piston pocket (16) and the at least one air duct opening (12) up to a crankshaft angle (α) of at least 40° before bottom dead center (BDC).

Description

[0001] The invention relates to a two-stroke engine of the type specified in the preamble of claim 1.

[0002] German patent DE 10 2010 045 016 A1 discloses a hand-held work device with a two-stroke engine of the type. Fuel is supplied via a carburetor depending on the vacuum in the intake manifold.

[0003] It has been shown that in known two-stroke engines, insufficient fuel supply can occur, particularly in certain speed ranges. This has been especially evident at high speeds.

[0004] The invention is based on the objective of creating a two-stroke engine of the generic type with which a sufficient fuel supply can be achieved in all operating conditions.

[0005] This problem is solved by a two-stroke engine with the features of claim 1.

[0006] It has been shown that insufficient fuel supply can result from excessively high pressure in the intake manifold at least within a certain engine speed range and for at least a certain period. This can prevent the necessary pressure differential between the fuel supply system and the intake manifold, particularly in fuel systems operating at low or near-zero pressure, from being present. It has been observed that this slight pressure differential can occur in known two-stroke engines before the point in an engine cycle when the intake manifold connects to the crankcase interior via the intake manifold opening, i.e., during the timing of the intake manifold opening.

[0007] To reduce the pressure in the intake manifold before it connects to the crankcase interior via the intake manifold opening, the invention provides for maintaining the connection between the piston pocket and the air duct opening for a longer period than usual during the piston's downward stroke. It has been shown that closing the piston pocket early during the piston's downward stroke can result in a comparatively high pressure within the piston pocket. During the piston's downward stroke, the transfer ports to the piston pocket are typically closed first, followed by the air duct opening to the piston pocket. The pressure at the air duct opening is the same as the pressure in the air duct at that time. This can be the same pressure as the pressure in the intake manifold.

[0008] The comparatively high overpressure that can exist in the piston pocket during the downward stroke of the piston in known two-stroke engines when the piston pocket closes can lead to a pressure increase in the air duct when the piston pocket reopens to the air duct. This increased pressure can, for example, be transmitted into the intake manifold via a fluidic connection between the intake manifold and the air duct.

[0009] According to the invention, the fluidic connection between the air duct opening and the piston pocket is maintained until a crankshaft angle of at least 40° before bottom dead center. In the region of bottom dead center, and even before, the air duct and the intake port are typically at approximately ambient pressure. By maintaining the connection between the piston pocket and the air duct opening for a longer period than usual, the overpressure in the piston pocket can be gradually reduced at an early stage. When the piston pocket reopens to the air duct during the subsequent upward stroke, this reduces the pressure in the air duct and, in particular, the pressure in the intake port. This pressure reduction can be sufficient to lower the pressure level in the intake port to such an extent that fuel can be supplied to the intake port at low pressure.

[0010] Fuel is supplied, in particular, via a carburetor or a fuel valve. The fuel supplied to the intake manifold is under low pressure. Specifically, the fuel overpressure is at most 1 bar, and in particular, at most 500 mbar above atmospheric pressure. Specifically, the fuel overpressure is at most 200 mbar, and in particular, between 50 mbar and 150 mbar above atmospheric pressure.

[0011] Fuel systems that meter fuel under very low pressure can be simple in design. In particular, a fuel pump in the system is driven by the rotating crankshaft of the two-stroke engine. This results in a simple design. Specifically, the two-stroke engine includes a carburetor, and fuel is drawn into the intake manifold due to the vacuum created there. The carburetor may have an electrically actuated valve, especially an electromagnetic valve, to further control the fuel supply. Alternatively, the two-stroke engine may have a fuel valve, for example, an electromagnetic valve, that delivers the fuel.

[0012] The section of the piston pocket that establishes the fluidic connection between the piston pocket and the air duct opening serves solely to allow pressure equalization between the piston pocket and the air duct up to a desired point during the downward stroke of the piston. For this purpose, this section has a particularly small flow cross-section.

[0013] In particular, the intake duct and air duct are fluidically connected.

[0014] In particular, the intake manifold and air duct are controlled via a common throttle element. In the area of ​​the throttle element, the ducts are fluidically connected to each other, at least outside of full load conditions.

[0015] The section has a width that is less than 50%, and in particular less than 80%, of the maximum width of the piston pocket. Both the width of the section and the width of the piston pocket are measured in a developed section of the piston perpendicular to the longitudinal center axis of the cylinder bore. The width is measured in the circumferential direction of the piston or in the circumferential direction of the cylinder bore.

[0016] The section has, in particular, a height measured parallel to the longitudinal center axis that corresponds to at least 3%, and in particular at least 5%, of a piston stroke.

[0017] This section is specifically designed not to alter the timing of the connection between the piston pocket and the transfer ports during the piston's upward stroke. To this end, it is specifically designed that this section lies within a circumferential region of the piston that is not in contact with any transfer port in any piston position.

[0018] A simple design is achieved when the section extends along the upper edge of the piston pocket. In particular, the section is designed as a groove.

[0019] This section forms, in particular, a pilot groove. Specifically, this section modifies the timing of the connection between the piston pocket and the air duct without altering the timing of the connection between the piston pocket and the transfer ports.

[0020] The flow cross-section of the fluidic connection produced by the section is in particular less than 50%, and in particular less than 80%, of the maximum flow cross-section of the connection between the piston pocket and the air duct opening.

[0021] In particular, the at least one air duct opening and the at least one piston pocket are designed and arranged such that the fluidic connection between the at least one piston pocket and the at least one air duct opening is closed during the downward stroke of the piston at a time when, in at least one operating condition, ambient pressure or a pressure lower than ambient pressure prevails in the air duct, in particular at least at full load.

[0022] In particular, the fuel is supplied via a fuel valve. The fuel supply occurs, at least partially, in at least one operating condition at a time when the intake manifold opening is not connected to the crankcase interior.

[0023] The piston has, in particular, at least one piston ring groove. The piston pocket has a distance to the piston ring groove that is, in particular, less than 8 mm, and especially less than 5 mm. This distance is measured parallel to the longitudinal center axis of the cylinder bore. The distance is, in particular, as small as possible.

[0024] The two-stroke engine features, in particular, an air filter into which the intake manifold and the air intake duct open. Specifically, the air for the operation of the two-stroke engine is drawn in through the air filter.

[0025] In particular, the intake duct and the air duct are fluidically connected to each other in a cleanroom of the air filter.

[0026] The intake duct and the air duct have a connection, particularly downstream of the air filter, through which the intake duct and the air duct are fluidically connected. Specifically, the intake duct and the air duct share a common duct section in which a controllable throttle element is arranged, controlling the free flow cross-section of the intake duct and the air duct. The fluidic connection between the intake duct and the air duct exists in the region of the controllable throttle element. The fluidic connection exists at least when the throttle element is fully closed and / or only partially open. When the throttle element is fully open, the fluidic connection between the intake duct and the air duct can be closed by the throttle element. Specifically, the air duct and the intake duct are separated from each other by a partition. The throttle element is, in particular, arranged in an opening of the partition.In particular, when the throttle element is fully open, it closes the opening in the partition.

[0027] The fuel supply device is, in particular, a fuel valve. Specifically, the fuel supply device is a fuel valve operating at an overpressure of at most 1 bar, in particular at most 500 mbar, in particular at most 200 mbar, in particular 50 mbar to 150 mbar above atmospheric pressure.

[0028] Exemplary embodiments of the invention are explained below with reference to the drawing. The drawing shows: Fig. 1 a schematic representation of a two-stroke engine, Fig. 2 a perspective view of a piston of the two-stroke engine made of Fig. 1 , Fig. 3 a development of piston and cylinder bore of the two-stroke engine made of Fig. 1 at the bottom dead center of the piston, Fig. 4 a partial development according to Fig. 3 For an alternative embodiment of the piston of the two-stroke engine, Fig. 5 shows a partial sectional view of the two-stroke engine in the area of ​​the piston and air duct opening, Fig. 6 shows a schematic diagram showing a possible pressure profile in the intake duct of a two-stroke engine according to the prior art and of a two-stroke engine according to the invention.

[0029] Fig. 1 Figure 1 schematically shows a two-stroke engine 1. The two-stroke engine 1 is, in particular, the drive motor in a power tool such as a chainsaw, an angle grinder, a brush cutter, a blower, or the like. The power tool could also be, for example, a lawnmower. The power tool is, in particular, a hand-held power tool.

[0030] The two-stroke engine 1 comprises a cylinder 2 in which a cylinder bore 13 is formed. The cylinder bore 13 has a longitudinal center axis 29. A combustion chamber 3 is formed in the cylinder bore 13. A spark plug 28 projects into the combustion chamber 3. The combustion chamber 3 is bounded by a piston 5. The piston 5 is mounted to reciprocately in the cylinder bore 13. The piston 5 drives a crankshaft 7 via a connecting rod 6. The crankshaft 7 is rotatably mounted in a crankcase 4 about an axis of rotation 8. The crankcase 4 has a crankcase interior 14.

[0031] In Fig. 1 The piston 5 is shown at its bottom dead center (BDC). At BDC, the piston 5 is at its minimum distance from the axis of rotation 8 of the crankshaft 7. BDC represents one end position of the piston 5's reciprocating motion. The other end position of the piston 5 is its top dead center (TDC). At TDC, the piston 5 is at its maximum distance from the axis of rotation 8 of the crankshaft 7. Between BDC and TDC, the piston 5 completes a stroke h. The stroke h is measured parallel to the longitudinal center axis 29.

[0032] In the region of top dead center (TDC), the crankcase interior 14 is connected to the combustion chamber 3 via transfer ports 17 and 18. In the exemplary embodiment, two intake-adjacent transfer ports 17 and two exhaust-adjacent transfer ports 18 are provided, of which 17 and 18 are shown in the sectional view. Fig. 1 Only one is shown at a time. Each inlet-adjacent transfer channel 17 opens into the combustion chamber 3 via a transfer window 19. Each outlet-adjacent transfer channel 18 opens into the combustion chamber 3 via a transfer window 20.

[0033] In the exemplary embodiment, all transfer channels 17 and 18 are combined and open into the crankcase interior 14 at a common outlet 21. In this embodiment, the outlet 21 is located approximately below the exhaust opening 36 when viewed along the longitudinal center axis 29. An alternative arrangement of one or more outlets 21 for the transfer channels 17 and 18 may also be advantageous.

[0034] An exhaust opening 36 leads from the combustion chamber 3. The two-stroke engine 1 includes an air filter 30. The air filter 30 has filter material 31. The air filter 30 has a clean chamber 32 into which air filtered by the filter material 31 enters.

[0035] The two-stroke engine 1 comprises an intake duct 9 and an air duct 11. The intake duct 9 and the air duct 11 lead out of the cleanroom 32. The intake duct 9 opens into the cylinder bore 13 via an intake port 10. In the region of the top dead center (TDC) of the piston 5, the intake port 10 is fluidically connected to the crankcase interior 14. The intake port 10 is controlled by the piston 5 and is open or closed depending on the position of the piston 5.

[0036] The air duct 11 terminates with at least one air duct opening 12 ( Fig. 3 ) at the cylinder bore 13. In the exemplary embodiment, the air duct 11 divides into two branches, which open at two air duct openings 12 on both sides of the intake duct opening 10, as shown. Fig. 3 shows.

[0037] How Fig. 1 As shown, a fuel supply device 15 is arranged on the intake port 9, which is designed to supply fuel into the intake port 9. In the exemplary embodiment, a fuel valve is provided as the fuel supply device 15. The fuel valve operates, in particular, with an overpressure of at most 1 bar, and more specifically, at most 500 mbar above atmospheric pressure. In particular, the overpressure is at most 200 mbar, and more specifically, 50 mbar to 150 mbar above atmospheric pressure. Alternatively, the fuel supply device 15 can also be a carburetor.

[0038] Air duct 11 and intake duct 9 are separated over part of their length by a partition 26. Air duct 11 and intake duct 9 share a common duct section 33 in which they are routed together. In the exemplary embodiment, the partition 26 does not extend into the common duct section 33.

[0039] A throttle element 25 is provided to control the amount of combustion air supplied to the two-stroke engine 1. In the exemplary embodiment, the throttle element 25 is pivotally mounted. In this exemplary embodiment, the throttle element 25 is a throttle valve. The throttle element 25 is pivotally mounted in the common channel section 33 of intake duct 9 and air duct 11. Alternatively, the throttle element can be arranged in a connecting opening of the partition 26 if air duct 11 and intake duct 9 do not have a common channel section 33.

[0040] In the Fig. 1 In the fully open position of the throttle element 25 shown, the throttle element 25 largely separates the air duct 11 and the intake duct 9 in the area of ​​the common duct section 33. If the throttle element 25 is partially open, as shown in Fig. 1 As shown schematically with a dashed line for a partial load position of the throttle element 25, air duct 11 and intake duct 9 are fluidically connected to each other in the common duct section 33. The partially open position of the throttle element 25 is shown in Fig. 1 marked with the reference number 25'.

[0041] In the exemplary embodiment, the piston 5 has two piston ring grooves 34 and 35. Alternatively, the piston 5 can have only one piston ring groove 34 and no piston ring groove 35. A piston ring 24 is arranged in each piston ring groove 34 and 35.

[0042] How Fig. 1 As also shown, the piston 5 has a piston crown 38 that defines the combustion chamber. In the exemplary embodiment, the piston crown 38 is flat. An uneven design of the piston crown 38 can also be advantageous.

[0043] How Fig. 2 As shown, the piston 5 has two piston pockets 16. The piston pockets 16 are, in particular, symmetrical about a central plane 42 of the cylinder 2 ( Fig. 3 ) formed. The median plane 42 is the cutting plane in Fig. 1 The median plane 42 lies perpendicular to the axis of rotation 8 of the crankshaft 7 and contains the longitudinal center axis 29 of the cylinder bore 13.

[0044] How Fig. 1 As shown, the piston 5 is pivotally mounted on the connecting rod 6 via a piston pin 43. The piston pin 43 is supported in the piston 5 in two piston pin bores 44, of which in Fig. 2 one is visible. How Fig. 2 As shown, the piston pin eye 44 is completely arranged in the piston pocket 16 in the exemplary embodiment.

[0045] During the operation of the two-stroke engine 1, air is supplied to the crankcase interior 14 via the intake port 9 and the intake port opening 10 during the upward stroke of the piston 5. Fuel is metered to this air via the fuel supply device 15, so that a fuel / air mixture enters the crankcase interior 14. In the region of top dead center, the intake port 11 is closed via the air port openings 12 and piston pockets 16 of the piston 5, which are located in Fig. 2 The transfer ports 17 and 18 are fluidically connected to the transfer channels 17 and 18. The piston pockets 16 are located on the cylinder bore 13 in overlap with the transfer ports 19 and 20. During the upward stroke of the piston 5, a vacuum exists in the crankcase interior 14. Due to this vacuum, fuel / air mixture is drawn from the intake port 9 into the crankcase interior 14, and air is drawn from the air duct 11 via the transfer ports 17 and 18 into the crankcase interior 14. The upward stroke of the piston is the movement of the piston 5 from bottom dead center (BDC) to top dead center (TDC). At top dead center (TDC), the intake port opening 10 to the crankcase interior 14 is fully open. The air duct 11 is fluidically connected to the transfer ducts 17 and 18 via the at least one piston pocket 16 and the transfer windows 19 and 20.

[0046] During the subsequent downward stroke of piston 5, i.e., during its movement from top dead center (TDC) to bottom dead center (BDC), the fuel / air mixture in the crankcase interior 14 is compressed. As soon as the transfer ports 19 and 20 to the combustion chamber 3 are opened by piston 5, the air drawn from air duct 11 into transfer ports 17 and 18 flows into combustion chamber 3 and expels exhaust gases from the previous engine cycle. Fresh fuel / air mixture then flows from the crankcase interior 14 into combustion chamber 3 via transfer ports 17 and 18. At bottom dead center (BDC), the air duct opening 12 and the transfer ports 19 and 20 are completely aligned with the piston pocket 16.After the transfer ports 19 and 20 and the exhaust port 36 are closed by the piston 5 during its upward stroke, the fuel / air mixture in the combustion chamber 3 is compressed and ignited by the spark plug 28 in the region of top dead center (TDC). The subsequent combustion accelerates the piston 5 back towards bottom dead center (BDC).

[0047] The position of piston 5 is given here as the crankshaft angle α. The crankshaft angle α denotes the rotational position of the crankshaft 7 about the axis of rotation 8 of the crankshaft 7. The in Fig. 1 The depicted position of bottom dead center (BDC) corresponds to a crankshaft angle α of 0°, and the position of the piston 5 at top dead center (TDC) corresponds to a crankshaft angle α of 180°. At bottom dead center (BDC) and top dead center (TDC), there is a pivot axis 41 about which the connecting rod 6 is pivotably mounted on the crankshaft 7; in the exemplary embodiment, this pivot axis lies on the longitudinal center axis 29 of the cylinder bore 13.

[0048] The piston pocket 16 has an upper edge 22, as Fig. 2 The upper edge 22 is the side of the piston pocket 16 closest to the piston crown 38, which, during the stroke of the piston 5, overlaps the transfer ports 19 and 20. The upper edge 22 thus determines the timing of the connection between the piston pocket 16 and the transfer ports 19 and 20. How Fig. 2 As shown, a section 23 of the piston pocket 16 is arranged at the upper edge 22, extending from the upper edge 22 towards the piston base 38.

[0049] Section 23 establishes a fluidic connection between the piston pocket and the air duct opening 12 before the upper edge 22 comes into contact with the air duct opening 12. Section 23 has a height c measured parallel to the longitudinal center axis 29. The height c is measured to the upper edge 22, i.e., to a region of the piston pocket that comes into contact with a transfer window 19, 20 during the piston stroke.

[0050] The section 23 has a parallel to the longitudinal center axis 29 towards the piston ring groove 35, which is located away from the piston base 38 ( Fig. 1 The measured distance d is measured. The distance d is advantageously as small as possible. The distance d is in particular less than 8 mm, and in particular less than 5 mm. The height c is in particular chosen to be as large as possible. The height c is in particular at least 3%, and in particular at least 5%, of the stroke h of the piston 5.

[0051] The height c is designed in particular such that the piston pocket 16 has a distance d to the at least one piston ring groove 34, 35 which is greater than 1% of the piston stroke.

[0052] How Fig. 2 As shown, the piston 5 has additional pockets 37 between the piston pockets 16 and the piston base 38. The additional pockets 37 are located in the Fig. 3 und 4 Not shown. The additional pockets 37 are arranged such that, during the stroke of the piston 5, they are only in contact with one functional opening in the cylinder bore 13. In the exemplary embodiment, the additional pockets 37 are in contact with the transfer ports 20. The additional pockets 37 serve to reduce the weight of the piston 5. The additional pockets 37 do not affect the valve timing of the two-stroke engine 1.

[0053] Fig. 3 Figure 1 shows a development of cylinder bore 13 and piston 5 at bottom dead center (BDC) of piston 5. The piston pocket 16 has a width b. The width b is measured in the development and perpendicular to the longitudinal center axis 29. The section 23 has a width a. The width a is measured in the development and perpendicular to the longitudinal center axis 29. The width a and the width b are measured in the same direction on the circumference of the piston 5 and adjacent to the cylinder bore 13. It is provided that the width a is smaller than the width b. In particular, the width a is less than 50%, and especially less than 80%, of the width b. The width b is the maximum width of the piston pocket 16 in the development in the direction perpendicular to the longitudinal center axis 29.

[0054] Section 23 is designed and arranged such that during the downward stroke of the piston 5, up to a crank angle α of at least 40° before bottom dead center UT, a fluidic connection exists between the piston pocket 16 and the air duct opening 12.

[0055] Fig. 4 shows an alternative embodiment. Section 23 demonstrates, in the exemplary embodiment, that... Fig. 4 a height c'. The height c' is greater than the height c. The height c' is chosen such that section 23 is also connected to the air duct opening 12 at the bottom dead center UT.

[0056] Fig. 5 Figure 1 shows the arrangement of section 23 in a position where section 23 establishes a connection between the piston pocket 16 and the air duct opening 12. In this position of the piston 5, the upper edge 22 of the piston pocket 16 is not yet in overlap with the air duct opening 12. Due to section 23, the piston pocket 16 remains connected to the air duct opening 12 for a longer period during the downward stroke of the piston 5. During the upward stroke of the piston 5, section 23 establishes a fluidic connection between the piston pocket 16 and the air duct 11 at an earlier time.

[0057] Section 23 is designed as a groove. Section 23 forms a pilot groove. Section 23 has a small flow cross-section. Section 23 is arranged in a circumferential region 40 of the piston 5, as shown. Fig. 3 The upper edge 22 is arranged in a circumferential region 39 adjoining it. How Fig. 3 As shown, the circumferential section 40 is laterally offset from the transfer windows 19 and 20. Therefore, during the stroke of the piston 5, section 23 only overlaps with the air duct opening 12. Section 23 does not overlap with either of the transfer windows 19 or 20 in any position of the piston 5.

[0058] Section 23 establishes a fluidic connection between air duct opening 12 and piston pocket 16. The flow cross-section of this fluidic connection is, in particular, less than 50%, and especially less than 80%, of the maximum flow cross-section of the connection between piston pocket 16 and air duct opening 12. The maximum flow cross-section of the connection between piston pocket 16 and air duct opening 12 is, in particular, determined by the flow cross-section of the air duct opening 12.

[0059] Fig. 6 Figure 1 schematically shows the pressure p in the intake manifold 9 during one revolution of the crankshaft 7. The pressure p is shown as curve 50 for a two-stroke engine not according to the invention. Fig. 6 The mean effective pressure pm in the crankcase interior is also shown schematically in 14. Fig. 6 A pressure p1 is also shown, which corresponds to the pressure of the fuel being supplied. The pressure p1 is in Fig. 6 The pressure p1 is specified as a constant pressure. However, the pressure p1 can fluctuate depending on the fuel system.

[0060] During a time interval 51, the fuel supply device 15 is open. This is particularly relevant when the fuel supply device 15 includes a fuel valve that is opened and closed by a control unit of the two-stroke engine. Alternatively, the fuel supply can also be provided via a carburetor and be open for the entire revolution of the crankshaft. The fuel supply can also be provided via a carburetor with an electric valve, where the time intervals during which fuel can be drawn in can be controlled by opening and closing the valve.

[0061] The time interval 51 extends from time t1 to time t5. At time t1, the pressure p in the intake port 9 is below the pressure p1, and fuel can be supplied due to the pressure difference. The pressure in the crankcase interior 14 rises between bottom dead center (BDC) and top dead center (TDC) to a pressure p2. Between time t2 and time t3, the pressure in the crankcase interior 14 is above the fuel pressure p1. During this time interval, fuel supply to the intake port 9 cannot be guaranteed in a two-stroke engine not according to the invention.

[0062] At time t4, which is later than time t3, the intake port opening 10 opens. The pressure in the intake port 9 then drops sharply and is minimal in the region of top dead center (TDC) before rising again. At time t5, the fuel supply device 15 closes. At time t6, the intake port opening 10 closes. For a time interval of 52, the intake port opening 10 is fluidically connected to the crankcase interior 14. With further downward stroke of the piston 5, the pressure in the intake port 9 rises again and then falls once more.

[0063] In conventional two-stroke engines 1, the connection between the piston pocket 16 and the air duct opening 12 is interrupted, for example, at time t 7. According to the invention, it is now provided that the time of closing of the piston pocket 16 is postponed to a later time t 8, as schematically indicated by arrow 53 in Fig. 6 This is shown. All other timing of the two-stroke engine 1 remains unchanged. For a description of the other timing of the two-stroke engine 1 according to the invention, reference is made to the preceding description of the two-stroke engine 1 according to the non-invention.

[0064] Between time t 7 and time t 8, the pressure in the intake duct 9 decreases. In particular, the pressure in the air duct 11 decreases accordingly, for example, due to a common duct section 33 or due to a fluidic connection between air duct 11 and intake duct 9 in the cleanroom 32 of the air filter 30. By maintaining the connection between piston pocket 16 and air duct opening 12 for a longer period than usual, allowing pressure equalization, the pressure level in the piston pocket 16 can be reduced. This allows the pressure level in the intake duct 9 to be reduced after the piston pocket 16 opens to the air duct opening 12, which can occur, for example, at approximately time t 1, as indicated by arrow 54 in the figure. Fig. 6 This is illustrated. An exemplary resulting pressure curve for the pressure in the intake manifold 9 with a two-stroke engine 1 according to the invention is shown with a dashed line in Fig. 6 indicated by a curve 55.

[0065] In particular, the piston pocket 16 and the air duct opening 12 are designed and arranged such that the fluidic connection between the piston pocket 16 and the air duct opening 12 is closed during the downward stroke of the piston 5 at a time t 8 at which there is a negative pressure in the air duct 11, i.e. a pressure of 0 bar or less.

[0066] In particular, the two-stroke engine 1 is designed such that the opening of the air path and the opening of the mixture path occur approximately simultaneously. The connection of the two transfer ports 17 and 18 to the piston pocket 16 and the connection of the intake port opening 10 to the crankcase interior 14 are made, in particular, at crankshaft angles α that are spaced apart from each other by no more than 10° crankshaft angle α, and in particular by no more than 5° crankshaft angle α. It can be advantageous for the transfer ports 17 and 18 to be connected to the piston pocket 16 before the intake port opening 10 opens into the crankcase interior 14. Alternatively, it can also be provided that the intake port opening 10 is connected to the crankcase interior 14 before the transfer ports 17 and 18 are connected to the piston pocket 16.

Claims

1. Two-stroke engine with one cylinder (2) in which a piston (5) is mounted to move reciprocally, wherein the piston (5) defines a combustion chamber (3) formed in the cylinder (2), wherein the piston (5) rotatably drives a crankshaft (7) rotatably mounted in a crankcase (4), with an intake port (9) opening into a crankcase interior (14) via an intake port opening (10), and with an air port (11) opening into a cylinder bore (13) via at least one air port opening (12), with at least one transfer port (17, 18) opening into the cylinder bore (13) of the cylinder (2) via at least one transfer window (19, 20) and fluidically connecting the crankcase interior (14) to the combustion chamber (3) in the region of the bottom dead center (BDC) of the piston (5), wherein the piston (5) has at least one piston pocket (16) exhibitswhich connects at least one air duct opening (12) at least in the region of the top dead center (TDC) of the piston (5) with at least one transfer port (19, 20), with a fuel supply device (15) for supplying fuel into the intake manifold (9), , characterized by the fact that the at least one piston pocket (16) has at least one section (23) which is designed and arranged such that during the downward stroke of the piston (5) there is a fluidic connection between the at least one piston pocket (16) and the at least one air duct opening (12) up to a crankshaft angle (α) of at least 40° before bottom dead center (BDC).

2. Two-stroke engine according to claim 1, characterized by the fact thatthe section (23) has a width (a) measured in a development of the piston (5) perpendicular to a longitudinal central axis (29) of the cylinder bore (13) which is less than 50%, in particular less than 80%, of the maximum width (b) of the piston pocket (16) measured in a development of the piston (5) perpendicular to the longitudinal central axis (29).

3. Two-stroke engine according to claim 1 or 2, characterized by the fact that the section (23) has a height (c) measured parallel to the longitudinal center axis (29) which corresponds to at least 3%, in particular at least 5%, of a stroke (h) of the piston (5).

4. Two-stroke engine according to one of claims 1 to 3, characterized by the fact that the section (23) lies in a circumferential area (40) of the piston (5) which is not in overlap with a transfer window (19, 20) in any piston position.

5. Two-stroke engine according to one of claims 1 to 4, characterized by the fact that the section (23) extends along an upper edge (22) of the piston pocket (16).

6. Two-stroke engine according to one of claims 1 to 5, characterized by the fact that Section (23) is designed as a groove.

7. Two-stroke engine according to one of claims 1 to 6, characterized by the fact that the flow cross-section of the fluidic connection produced by section (23) is less than 50%, in particular less than 80%, of the maximum flow cross-section of the connection between piston pocket (16) and air duct opening (12).

8. Two-stroke engine according to one of claims 1 to 7, characterized by the fact that the piston (5) has at least one piston ring groove (34, 35) to which the piston pocket (16) has a distance (d) measured parallel to the longitudinal center axis (29) of the cylinder bore (13) which is less than 8 mm, in particular less than 5 mm.

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