TWO-STROKE ENGINE

Description

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

[0002] In the prior art, designs for two-stroke engines with comparatively long transfer ports are known, for example from DE 10 2009 059 143 A1, EP 2 017 446 A1, or DE 10 2020 000 989 A1. It has been shown that low emissions can be achieved with long transfer ports. However, it has also been shown that two-stroke engines with comparatively long transfer ports experience a drop in power at high engine speeds.

[0003] The US 4,955,333 A design results in a two-stroke engine featuring short transfer ports. The crankcase volume of this engine is designed to be as small as possible in order to maximize the pressure ratio of the crankcase compression.

[0004] The publication Xutao et al: "Simulation of a two-stroke dimethyl-ether free piston engine operating on HCCL combustion", TRANSPORTATION, MECHANICAL, AND ELECTRICAL ENGINEERING (TMEE), 2011 INTERNATIONAL CONFERENCE ON IEEE, December 16, 2011, pages 2106-2110, discloses an internal combustion engine operating with homogeneous compression ignition.

[0005] The DE 31 08 519 A1 reveals a model engine with short transfer ports.

[0006] The JP S61-268820 A reveals a two-stroke engine whose combustion chamber roof has a dome on the side furthest from the exhaust port.

[0007] The invention is based on the objective of creating a two-stroke engine of the generic type that enables low emission values ​​with favorable performance development.

[0008] This problem is solved by a two-stroke engine with the features of claim 1.

[0009] To achieve favorable exhaust emission values, it is stipulated that at least one transfer port, and in particular all transfer ports, must have an average length that is at least 1.5 times the piston stroke. The average length of the transfer port is measured through the geometric center of the port cross-section in every cross-section perpendicular to the flow direction. The average length is measured from the outlet opening to the transfer window.

[0010] It has been shown that particularly with transfer ports whose average length relative to the stroke is at least 1.5, low exhaust emissions result, but that a drop in power was observed in previous two-stroke engines at high engine speeds. It has been shown that this drop in power is caused by combustion disturbances in the combustion chamber. During the piston's downward stroke, exhaust gases flow into the transfer ports due to the increased residual pressure in the combustion chamber. Because of the incoming exhaust gas, the gas volume in the transfer port is initially accelerated towards the crankcase. This movement of the gas volume must first be reversed due to the increasing pressure in the crankcase. This delays the flow of air and the fresh air-fuel mixture into the combustion chamber. In previous two-stroke engine designs, this resulted in insufficient filling of the combustion chamber with the fresh air-fuel mixture and thus combustion disturbances.

[0011] To at least partially compensate for this delay, the crankcase volume is designed to be relatively small compared to the engine displacement. Experience has shown that for optimal power delivery, the crankcase volume should ideally be no more than 3.1 times the engine displacement. This ensures sufficient pre-compression of the fuel-air mixture in the crankcase and a sufficiently high pressure at the point when the transfer port opens into the combustion chamber. This allows for a relatively early inflow of fresh air-fuel mixture into the combustion chamber, despite the considerable length of at least one transfer port. In this way, adequate combustion chamber filling with fresh air-fuel mixture can be achieved even with relatively long transfer ports. This, in turn, reduces the power drop-off at high engine speeds.

[0012] The average length of at least one, and especially all, overflow channels is particularly comparatively large.

[0013] The average length of at least one, and in particular each, overflow channel is in particular at least 50 mm, and in particular at least 80 mm. The average length of at least one, and in particular each, overflow channel is in particular at least 90 mm, and in particular at least 100 mm.

[0014] The ratio of the mean length of the transfer port to the piston stroke is, in particular for at least one transfer port, and in particular for each transfer port, at least 1.5, and in particular at least 2. In particular, the ratio of the mean length of at least one intake-adjacent transfer port to the piston stroke is at least 2.5, and in particular at least 3. The piston stroke corresponds to the distance the piston travels from its bottom dead center to its top dead center. In particular, the outlet of the at least one transfer port into the crankcase is arranged on the side of a transverse plane of the two-stroke engine on which an exhaust port from the combustion chamber is also arranged. The transfer port of the at least one transfer port is arranged, in particular, on the side of a transverse plane of the two-stroke engine on which at least one intake port, in particular a mixture intake, is also arranged.

[0015] In particular, the displacement of the two-stroke engine is from 50 cm 3< to 100 cm 3< .

[0016] In particular, it is stipulated that at least one transfer port, and especially all transfer ports, must have an average length of at least 2 mm / cm³ relative to the engine displacement. The average length is divided by the engine displacement. For each transfer port, the average length divided by the engine displacement must be at least 2 mm / cm³. The displacement of the two-stroke engine corresponds to the area of ​​the cylinder bore multiplied by the piston stroke. The piston stroke is the distance the piston travels from its top dead center to its bottom dead center. It has been shown that low exhaust emissions result, particularly with transfer ports whose average length is at least 2 mm / cm³ relative to the engine displacement.

[0017] In particular, the ratio of the volume of all transfer ports to the engine displacement is at least 1. In particular, the volume of all transfer ports is greater than the engine displacement. In particular, the ratio of the volume of all transfer ports to the engine displacement is at least 1.3, in particular at least 1.5, in particular at least 1.6. The volume of all transfer ports is the combined volume of all transfer ports.

[0018] The flow of exhaust gases from the combustion chamber into the at least one transfer port occurs due to the pressure difference between the combustion chamber and the crankcase interior. It has been shown that by lowering the residual pressure in the combustion chamber, the delay in the flow of fresh air-fuel mixture from the at least one transfer port into the combustion chamber can be further reduced. A reduction in the residual pressure in the combustion chamber can be achieved by the fastest possible mixture conversion, i.e., by rapid combustion. It has been shown that a geometric compression ratio of at least 10.0 in the combustion chamber is advantageous for this purpose. The geometric compression ratio is the volume of the combustion chamber at bottom dead center of the piston divided by the volume of the combustion chamber at top dead center of the piston. A compression ratio of at least 10.0 allows for a comparatively high temperature of the mixture in the combustion chamber at the ignition point.This promotes good and rapid combustion in the combustion chamber.

[0019] At the piston's top dead center, a flat gap forms between the piston and the combustion chamber roof over a portion of the piston's surface. In this gap, the distance between the piston and the combustion chamber roof, measured along the cylinder's longitudinal axis, is minimal. Adjoining this flat gap may be a region where the combustion chamber roof is inclined at up to 5° towards the piston crown. The area of ​​the combustion chamber roof that defines this flat gap and the region where the roof is inclined at up to 5° towards the piston crown is the cylinder's squish area. Particularly high compression and turbulence of the mixture are achieved at the cylinder's squish area. The mixture is moved at high speed towards the cylinder head. Due to this high speed and turbulence, as well as the high pressure resulting from the high compression, excellent mixture preparation is achieved.To promote good combustion in the combustion chamber, the proportion of the squish area to the projected area of ​​the cylinder bore is in particular at least 40%, and in particular 40% to 50%.

[0020] A small squish clearance is advantageous for high mixture compression. The squish clearance corresponds to the distance between the piston at top dead center and the combustion chamber roof at the flat space between the squish surfaces. Specifically, the squish clearance is less than 0.7 mm.

[0021] To easily achieve a relatively large squish area and simultaneously ensure a favorable mixture distribution in the combustion chamber, the dome of the combustion chamber roof is positioned off-center. The geometric center of the dome is spaced from the longitudinal center axis of the cylinder. This space is, in particular, at least 3% of the diameter of the cylinder bore. Preferably, the space is at least 2 mm, and more preferably at least 5 mm.

[0022] The dome is preferably positioned closer to the intake side of the cylinder than to the exhaust side. This is particularly advantageous when the transfer ports are oriented towards the intake side of the cylinder, so that the flow direction from the transfer ports is towards the intake and, in particular, also towards the combustion chamber roof. The intake side of the cylinder is the side where an intake port opening into the crankcase is located. The exhaust side of the cylinder is the side of the cylinder where the exhaust port is located.

[0023] The dome is arranged in particular such that the minimum distance of the dome to the cylinder bore on the intake side in the direction of the cylinder longitudinal axis is in particular 0 mm to 10 mm, in particular 2 mm to 6 mm.

[0024] The cylinder has a median plane that contains the cylinder's longitudinal axis and bisects the exhaust port. The cylinder also has a transverse plane that contains the cylinder's longitudinal axis and is perpendicular to the median plane. The geometric center of the cylinder head is located on the side of the transverse plane furthest from the exhaust port.

[0025] The ratio of the largest to the smallest extent of the cap, when projected along the longitudinal axis of the cylinder, is in particular at most 1.1. It is specifically provided that the largest extent of the cap is parallel to the transverse plane and the smallest extent of the cap is parallel to the median plane. In projection along the longitudinal axis of the cylinder, the cap has a circular or approximately circular shape.

[0026] The transfer ports are oriented in an end section adjacent to the transfer windows such that the mixture from the transfer ports enters the combustion chamber with a directional component directed away from the exhaust port. This results in improved scavenging of the exhaust gases from the previous engine cycle from the combustion chamber, as well as improved mixture combustion within the combustion chamber. The mixture flowing into the combustion chamber is moved in the direction in which the dome is positioned.

[0027] In particular, the two-stroke engine features an air channel for pre-positioning air in at least one transfer port. This improves the combustion engine's emissions. Because exhaust gases from the combustion chamber are scavenged by the pre-positioned air in the transfer ports, the proportion of exhaust gases from the previous engine cycle in the combustion chamber can be reduced. This allows for faster combustion in the combustion chamber.

[0028] In particular, at least one circumferential segment containing the outlet of a transfer channel lies, at least partially, and in particular completely, overlapping with the circumferential segment of the cylinder containing the exhaust port, as viewed along the longitudinal axis of the cylinder. The circumferential segments in which the outlet and exhaust ports extend are circumferential segments of the cylinder around the longitudinal axis. The outlet extends, in particular, over a first circumferential segment of the cylinder, and the exhaust port extends, in particular, over a second circumferential segment. The first circumferential segment and the second circumferential segment overlap, in particular, at least partially. It may be provided that the second circumferential segment lies completely within the first circumferential segment. Alternatively, it may be provided that the first circumferential segment lies completely within the second circumferential segment.In particular, the outlet opening and the muzzle opening are arranged symmetrically to the central plane of the cylinder.

[0029] In particular, at least one transfer port runs helically around the cylinder bore. The transfer port therefore does not run parallel to the longitudinal center axis of the cylinder, but rather is inclined to the longitudinal center axis of the cylinder when viewed from the side. Most importantly, at least two transfer ports, and in particular all transfer ports of the two-stroke engine, open into the crankcase interior through a common opening.

[0030] Exemplary embodiments of the invention are explained below with reference to the drawing. The drawing shows: Fig. 1 a schematic sectional view of a two-stroke engine, Fig. 2 a schematic view of the cylinder of the two-stroke engine, Fig. 3 a partial view of the cylinder from the crankcase to the combustion chamber roof, Fig. 4 the view from Fig. 3 , where the squish area is hatched, Fig. 5 a sectional view of the cylinder with the piston schematically drawn along line VV in Fig. 1 , Fig. 6 a section along line VI-VI in Fig. 5 , Fig. 7 a representation of the combustion chamber roof of a cylinder of another embodiment of a two-stroke engine when viewed from the crankcase to the combustion chamber roof.

[0031] Fig. 1 Figure 1 schematically shows a two-stroke engine 1. The two-stroke engine 1 is a single-cylinder engine. The two-stroke engine 1 can be used, for example, in a handheld power tool such as an angle grinder, a brush cutter, a chainsaw, a blower, a lawnmower, or the like. The two-stroke engine 1 has a cylinder 2 in which a combustion chamber 3 is formed. The combustion chamber 3 is bounded by a piston 5. The piston 5 is movably mounted in a cylinder bore 22 of the cylinder 2. The piston 5 drives a crankshaft 8 via a connecting rod 4. The crankshaft 8 is rotatably mounted in a crankcase interior 7 of a crankcase 6. The crankshaft 8 is rotatably mounted about an axis of rotation 38.

[0032] In the cylinder bore 22, an exhaust port 13, controlled by the piston 5, is arranged from the combustion chamber 3, to which an exhaust channel 27 is connected. The exhaust channel 27 leads into an exhaust silencer 28.

[0033] The two-stroke engine 1 has an air filter 32. Filter material 34 is arranged in the air filter 32, separating a cleanroom 51 of the air filter 32 from the environment. The air filter 32 has an air filter base 33, to which an intake duct 23 is connected. In the exemplary embodiment, a section of the intake duct 23 is formed in a fuel supply device 25. The fuel supply device 25 can, for example, be a carburetor. Alternatively, the fuel supply device 25 can have a fuel valve 21 for metering fuel. The fuel valve 21 can be an electromagnetic valve. It can be provided that the fuel valve 21 merely meters the amount of fuel to be supplied, and the fuel is drawn into the intake duct 23 due to the vacuum prevailing in the intake duct. Alternatively, the fuel valve 21 can be an injector.The fuel supply device 25 includes a throttle element 24, for example a throttle valve. The free flow cross-section in the intake channel 23 can be adjusted by an operator via the throttle element 24.

[0034] It may also be provided that instead of the fuel supply device 25 only a throttle housing is provided for the storage of the throttle element 24 and that the fuel is supplied directly into the crankcase interior 7 via a schematically illustrated fuel valve 21'.

[0035] In the exemplary embodiment, the intake duct 23 has a partition wall that divides the intake duct 23 into a mixture channel 9 and an air channel 11. Downstream of the throttle element 24, the intake duct 23 is divided by a partition wall section 26 into air channel 11 and mixture channel 9. Additionally, a partition wall section 37 can be provided upstream of the throttle element 24. The mixture channel 9 opens at a mixture channel opening 10 on the cylinder bore 22. The air channel 11 opens at the cylinder bore 22 via two air channel openings 12, which are also located in Fig. 2 are shown.

[0036] The amount of fuel supplied is controlled by a control unit 30 when the fuel is supplied via a fuel valve 21, 21'. The control unit 30 actuates the fuel valve 21. In particular, the two-stroke engine 1 has a pressure sensor 29 that measures the pressure prevailing in the crankcase interior 7. The pressure sensor 29 is also specifically connected to the control unit 30.

[0037] The piston 5 is movable in the cylinder bore 22 between bottom dead center (BDC) and top dead center (TDC). Between bottom dead center (BDC) and top dead center (TDC), the piston 5 travels a stroke h.

[0038] In the area of ​​bottom dead center (BDC) of piston 5, which is located in Fig. 1 As shown, the crankcase interior 7 is fluidically connected to the combustion chamber 3 via transfer ports 14 and 15. Transfer ports 14 open into the combustion chamber 3 via transfer windows 16. Transfer ports 15 open into the combustion chamber 3 via transfer windows 17. The transfer windows 16 are located closer to the exhaust port 13 than the transfer windows 17. In this embodiment, transfer ports 14 and 15 share a common transfer port section 19. Transfer ports 14 and 15 are thus combined. In this embodiment, a transfer port 14 and a transfer port 15 are arranged on each side of the cylinder 2. Transfer ports 14 and 15 are initially combined on each side of the cylinder, and then all transfer ports 14 and 15 are routed into the common transfer port section 19. The common overflow channel section 19 opens into the crankcase interior 7 via an outlet opening 18.

[0039] The transfer ports 14 and 15 are comparatively long. The crankcase 6 has a crankcase plane 44. The crankcase plane 44 runs perpendicular to a longitudinal center axis 50 of the cylinder 2 and contains the axis of rotation 38 of the crankshaft 8. In the exemplary embodiment, the outlet opening 18 is arranged entirely on the side of the crankcase plane 44 furthest from the cylinder 2.

[0040] The crankcase interior 7 has a lowest region 57. The lowest region 57 of the crankcase interior 7 is the region of the crankcase interior 7 on the side of the crankcase plane 44 furthest from the cylinder 2, which has a greatest distance k to the crankcase plane 44.

[0041] In a preferred embodiment, the longitudinal center axis 50 of the cylinder 2 runs through the lowest region 57. However, a different arrangement of the lowest region 57 may also be advantageous.

[0042] The outlet opening 18 has a distance m to the lowest region 57, measured parallel to the longitudinal center axis 50 of the cylinder 2, which is in particular less than 20 mm, and in particular less than 10 mm. In particular, the distance m is less than 7 mm, and in particular less than 5 mm. The distance m can be 0 mm. A favorable design for the manufacture of the crankcase 6 is achieved if the distance m of the outlet opening 18 to the lowest region 57 is not less than 0.5 mm, so that even with unfavorable tolerances, a small distance between the outlet opening 18 and the lowest region 57 is maintained. In particular, the outlet opening 18 lies entirely on one side of a plane, wherein the plane contains the axis of rotation 38 of the crankshaft 8 and the longitudinal center axis 50 of the cylinder 2. This plane can coincide with a transverse plane 52 described in more detail below.

[0043] The distance m of the deepest area 57 to the mouth opening 18, measured parallel to the longitudinal axis 50 of the cylinder 2, is in particular less than 30%, in particular less than 20%, in particular less than 10% of the stroke h of the piston 5.

[0044] The distance m is measured in particular to an area of ​​the muzzle opening 18 that is located away from the combustion chamber 3.

[0045] The transfer ports 14 have an average length a1. The transfer ports 15 have an average length a2. The average length a1, a2 of the transfer ports 14, 15 is measured from the outlet opening 18 to the transfer window 16, 17 in each cross-section of the respective transfer port 14, 15 perpendicular to the flow direction through the geometric center of the port cross-section. The average length a1, a2 divided by the displacement is, for at least one, in particular for each, transfer port 14, 15, in particular at least 2 mm / cm³. The displacement of the two-stroke engine 1 is the area of ​​the cylinder bore 22 multiplied by the stroke h of the piston 5.

[0046] In particular, the displacement of the two-stroke engine is 1 from 50 cm 3< to 100 cm 3< .

[0047] The mean length a1, a2 of at least one, in particular each, transfer channel 14, 15 is in particular at least 50 mm, in particular at least 80 mm. The mean length a1, a2 of at least one, in particular each, transfer channel 14, 15 is in particular at least 90 mm, in particular at least 100 mm. In particular, at least the mean length a2 of the transfer channels 15, whose transfer windows 17 are arranged near the mixture inlet 10, is at least 110 mm, in particular at least 120 mm. In particular, the ratio of the mean length a2 of at least one of the transfer channels 15, whose transfer windows 17 are arranged near the mixture inlet 10, to the stroke h of the piston 5 is at least 2.5, in particular at least 3.

[0048] The ratio of the mean length a 1 , a 2 of at least one, in particular each, overflow channel 14, 15 to the stroke h of the piston 5 is at least 1.5, in particular at least 2, in particular at least 2.5, in particular at least 3.

[0049] In Fig. 1 A schematic cross-section Q is shown in which the cross-sectional area of ​​the overflow channel 15 is minimal. The smallest cross-sectional area is the area of ​​the overflow channel 15 in this cross-section Q. The smallest cross-sectional area of ​​the overflow channel 14 or 15 can, for example, range from 50 mm² to 100 mm².

[0050] In Fig. 1 The spark plug opening 35 and the decompression valve opening 36, in which the spark plug and the decompression valve are to be arranged, are also shown. How Fig. 1 As also shown, the piston 5 has a piston base 43 which limits the combustion chamber 3.

[0051] In Fig. 2 The cylinder 2 is shown schematically from the crankcase 4 into the cylinder bore 22. The piston 5 has two piston pockets 20. How Fig. 2 As shown, the air duct 9 divides into two branches 39 and 40. Branches 39 and 40 each open with an air duct opening 12 in the area of ​​one of the piston pockets 20 of the piston 5 at the cylinder bore 22. In the area of ​​top dead center TDC ( Fig. 1 The air channel 11 of piston 5 is connected to the transfer channels 14 and 15 via the piston pockets 20. In the region of the top dead center (TDC) of piston 5, air can thus be pre-positioned in the transfer channels 14 and 15.

[0052] In the Fig. 1 and 2 The longitudinal center axis 50 of cylinder 2 is also shown. How Fig. 2 As shown, cylinder 2 has a central plane 53. The central plane 53 divides the exhaust port 13 in the middle and contains the longitudinal center axis 50 of the cylinder. Cylinder 2 also has a transverse plane 52, which runs perpendicular to the central plane 53. The transverse plane 52 also contains the longitudinal center axis 50 of the cylinder.

[0053] How Fig. 2 As shown, the outlet opening 18 is arranged in a first circumferential segment 55 of the cylinder 2. The first circumferential segment 55 is a segment of the cylinder 2 around the longitudinal center axis 50. The outlet opening 13 is arranged in a second circumferential segment 56. In the exemplary embodiment, the circumferential segments 55 and 56 coincide. In particular, the circumferential segments 55 and 56 overlap at least partially. It can be provided that the first circumferential segment 55, over which the outlet opening 18 extends, is larger than the second circumferential segment 56 of the outlet opening 13. However, it can also be provided that the second circumferential segment 56 of the outlet opening 13 is larger than the first circumferential segment 55 of the outlet opening 18.

[0054] The transfer ports 14 and 15 each have an end section 41, 42. The end sections 41, 42 are designed such that air and mixture from the transfer ports 14 and 15 flow into the combustion chamber 3 in an inflow direction 48. The inflow direction 48 can be different for each transfer port 14, 15. However, the inflow directions 48 for each transfer port 14, 15 have a directional component 49 that is directed towards the mixture channel opening 10, i.e., away from the outlet opening 13. The directional component 49 is a directional component perpendicular to the transverse plane 52.

[0055] During the operation of the two-stroke engine 1, the mixture channel opening 10 is opened during the upward stroke of the piston 5, and a fuel / air mixture is drawn into the crankcase interior 7. If a fuel valve 21' is present on the crankcase 6, air is drawn in through the mixture channel opening 10, and fuel is supplied to the crankcase interior 7 via the fuel valve 21'. At the top dead center (TDC) of the piston 5, air from the air channel 11 is drawn into the transfer ports 14 and 15 via the piston pockets 20 ( Fig. 2 ) upstream. During the downward stroke of piston 5, the mixture is compressed in the crankcase interior 7. When piston 5 opens the transfer ports 16 and 17, upstream air and then the mixture flow into combustion chamber 3 from transfer channels 14 and 15. The incoming air scavenges exhaust gases from combustion chamber 3. During the upward stroke of piston 5, the fuel / air mixture in combustion chamber 3 is compressed and ignited by a spark plug located in the spark plug hole 35 near the top dead center of piston 5. During the downward stroke of piston 5, the exhaust port 13 opens first, allowing exhaust gases to escape from combustion chamber 3. Subsequently, transfer ports 16 and 17 open, and air and mixture for the next engine cycle flow into combustion chamber 3.

[0056] The transfer ports 14 and 15 are designed to have a comparatively large mean length a1, a2. The mean length a1, a2 of each transfer port 14, 15 is at least 2 mm / cm³ relative to the displacement. To ensure a rapid flow of fresh mixture into the combustion chamber 3 despite the large length of the transfer ports 14, 15, the volume of the crankcase interior 7, including the volume of all transfer ports 14, 15, is designed to be comparatively small. Specifically, the volume of the crankcase interior 7 is at most 3.1 liters relative to the displacement.

[0057] The volume of the transfer ports 14 and 15 is comparatively small, especially when the transfer ports 14 and 15 are of considerable length. In particular, the ratio of the volume of all transfer ports 14 and 15 to the engine displacement is at least 1. In particular, the volume of all transfer ports 14 and 15 is greater than the engine displacement. In particular, the ratio of the volume of all transfer ports 14 and 15 to the engine displacement is at least 1.3, in particular at least 1.5, in particular at least 1.6. The volume of all transfer ports 14 and 15 is the total volume of all transfer ports 14 and 15. The volume of all transfer ports 14 and 15 is the sum of the volumes of all transfer ports 14 and 15.

[0058] To achieve good filling of combustion chamber 3, the geometry of combustion chamber 3 is adapted to ensure rapid combustion and consequently a comparatively low residual exhaust gas pressure. This allows the amount of exhaust gas flowing from combustion chamber 3 into transfer ports 14 and 15 to be kept low. To achieve rapid combustion, the geometric compression ratio of combustion chamber 3 is specifically designed to be at least 10.0.

[0059] In the Fig. 3 und 4 Figure 45 shows the design of the combustion chamber roof of combustion chamber 3. As the Fig. 3 und 4 As shown, the combustion chamber roof 45 has a dome 46.

[0060] The dome 46 has a geometric center 47, which is arranged at a distance c from the longitudinal center axis 50 of the cylinder 2. The distance c is, in particular, 3% of a diameter f of the cylinder bore 22. In particular, the distance c is at least 2 mm, and more specifically, at least 5 mm. The geometric center 47 of the dome 46 is, in particular, arranged on the side of the transverse plane 52 on which the mixture channel opening 10 is also arranged (see figure). Fig. 1 The geometric center 47 of the calotte 46 and the outlet opening 13 are arranged on opposite sides of the transverse plane 52, as well as Fig. 1 The spark plug opening 35 is arranged symmetrically to the central plane 53 in the exemplary embodiment. The decompression valve opening 36 is arranged on the side of the transverse plane 52 on which the geometric center 47 of the dome 46 is also located in the exemplary embodiment.

[0061] The ratio of the mean length a 1 , a 2 of the transfer channel 14, 15 to the diameter f of the cylinder bore 22 is, in particular for at least one transfer channel 14 or 15, in particular for each transfer channel 14, 15, at least 1.5, in particular at least 2, in particular at least 3.

[0062] How Fig. 3 As shown, the combustion chamber roof 45 has a flat area 58 that extends almost to the cylinder bore 22. The piston crown 43 also has an inclined area 59, which in the exemplary embodiment lies between the flat area 58 and the spherical cap 46.

[0063] Cylinder 2 has an inlet side 60 and an exhaust side 61, which are located in Fig. 2 The intake side 60 of cylinder 2 is the side where the mixture port opening 10 of the mixture port 9 into the crankcase 6 is located. The exhaust side 61 of cylinder 2 is the side of cylinder 2 where the exhaust port 13 is located.

[0064] How Fig. 6 As shown, the dome 46 is arranged closer to the intake side 60 than to the exhaust side 61. The dome 46 is arranged such that the minimum distance g of the dome 46 to the cylinder bore 22 on the intake side 60, viewed along the longitudinal center axis 50 of the cylinder 2, is from 0 mm to 10 mm, particularly from 2 mm to 6 mm. The distance i of the dome 46 to the cylinder bore 22 on the exhaust side 61 is particularly greater than the distance g on the intake side 60. Both the distance i and the distance g are measured along the longitudinal center axis 50, particularly perpendicular to the longitudinal center axis 50.

[0065] As the Fig. 5 und 6 As shown, the combustion chamber roof 45 runs perpendicular to the longitudinal center axis 50 of the cylinder 2 in the flat area 58. A squish clearance s is formed between the flat area 58 and the piston crown 43 at top dead center (TDC). The squish clearance s is less than 0.7 mm. This causes the mixture to be displaced very quickly from this area into the area of ​​the cylinder head 46, resulting in high turbulence in the combustion chamber 3. This ensures rapid mixture combustion in the combustion chamber 3 and reduces the residual pressure in the combustion chamber 3, which exists during the downward stroke of the piston 5 at the time the transfer ports 16, 17 open.

[0066] In the inclined area 59, the combustion chamber roof 45 is inclined relative to the piston crown 43 by an angle α. In the exemplary embodiment, the piston crown 43 runs perpendicular to the longitudinal center axis 50. The angle α can be up to 5°.

[0067] The crushing area A of the combustion chamber roof 45 is in Fig. 4 The squish area A comprises areas 58 and 59, i.e., all areas where the combustion chamber roof 45 is inclined at up to 5° towards the piston crown 43. The proportion of the squish area A of the combustion chamber roof 45 to the area B of the cylinder bore 22 is, in particular, at least 40%, and more specifically, 40% to 50%. This results in accelerated combustion.

[0068] In the exemplary embodiment, the spherical cap 46 is round in a projection in the direction of the longitudinal center axis 50, as is particularly evident from the Fig. 4 and 5 show. Fig. 7 Figure 1 shows another embodiment in which the spherical cap 46 has a non-circular cross-section. The spherical cap 46 in Fig. 7 The spherical cap 46 has an approximately elliptical shape. The geometric center 47 of the spherical cap 46 is at a distance c from the longitudinal center axis 50. The spherical cap 46 has a maximum extent d, which in the exemplary embodiment is aligned parallel to the transverse plane 52. The spherical cap 46 also has a minimum extent e, which in the exemplary embodiment is measured perpendicular to the maximum extent d. In the exemplary embodiment, the minimum extent e is aligned in the direction of the median plane 53. The maximum extent d to the minimum extent e is, in particular, at most 1.1. Fig. 3, 4 and 7 The common section 19 of the overflow channels 14, 15 is also shown.

Claims

1. Two-stroke engine having a cylinder (2) in whose cylinder bore (22) a combustion chamber (3) is formed, wherein the combustion chamber (3) is bounded by a piston (5) which is mounted so as to move in a reciprocating manner, wherein the piston (5) drives a crankshaft (8) which is mounted rotatably in a crankcase (6), wherein a crankcase interior (7) of the crankcase (6) is connected fluidically to the combustion chamber (3) in at least one position of the piston (5) via at least one transfer channel (14, 15), wherein the at least one transfer channel (14, 15) opens out into the crankcase interior (7) by way of a mouth opening (18), and wherein the at least one transfer channel (14, 15) opens out at the cylinder bore (22) by way of at least one transfer window (16, 17), wherein the two-stroke engine (1) has an intake channel (23) and a fuel feed device (25) for feeding fuel into the intake duct (23) or the crankcase interior (7) and has an outlet opening (13) from the combustion chamber (3), wherein all the transfer channels (14, 15) have an average length (a1, a2) measured from the mouth opening (18) to the transfer window (16, 17), wherein the average length (a1, a2) of at least one transfer channel (14, 15) in relation to the stroke (h) of the piston (5) is at least 1.5, wherein the combustion-chamber roof (45) has a spherical cap (46), wherein the cylinder (2) has a central plane (53) which contains the longitudinal central axis (50) of the cylinder (2) and which divides the outlet opening (13) centrally, and wherein the cylinder (2) has a transverse plane (52) which contains the longitudinal center axis (50) of the cylinder (2) and which is perpendicular to the central plane (53), characterized in that the volume of the crankcase interior (7), with all the transfer channels (16, 17) included, in relation to the displacement is at most 3.1, in that the geometrical centre (47) of the spherical cap (46) is at a distance (c) from the longitudinal central axis (50) of the cylinder (2), wherein the geometrical centre (47) of the spherical cap (46) is positioned on that side of the transverse plane (52) which is remote from the outlet opening (13), and in that, in an end portion (41, 42) adjacent to the transfer windows (16, 17), the transfer channels (14, 15) are oriented in such a way that mixture flows from the transfer channels (14, 15) into the combustion chamber (3) with a directional component (49) that is directed away from the outlet opening (13).

2. Two-stroke engine according to Claim 1, characterized in that the ratio of the average length (a1, a2) of at least one, in particular each, transfer channel (14, 15) to the stroke (h) of the piston (5) is at least 2.

3. Two-stroke engine according to Claim 1 or 2, characterized in that the average length (a1, a2) of each transfer channel (14, 15) in relation to the displacement of the two-stroke engine (1) is at least 2 mm / cm3.

4. Two-stroke engine according to one of Claims 1 to 3, characterized in that the average length (a1, a2) of at least one, in particular each, transfer channel (14, 15) is at least 50 mm, in particular at least 80 mm.

5. Two-stroke engine according to one of Claims 1 to 4, characterized in that the mouth opening (18) of at least one transfer channel (14, 15), in particular the at least one mouth opening (18) of all the transfer channels (14, 15), extends at least partially, in particular completely, on a side, remote from the cylinder (2), of a crankcase plane (44), wherein the crankcase plane (44) extends perpendicularly to a longitudinal central axis (50) of the cylinder (2) and contains an axis of rotation (38) of the crankshaft (8).

6. Two-stroke engine according to one of Claims 1 to 5, characterized in that the geometrical compression ratio of the combustion chamber (3) is at least 10.0.

7. Two-stroke engine according to one of Claims 1 to 6, characterized in that the proportion of the squish surface (A) of the combustion chamber roof (45) at the surface (B) of the cylinder bore (22) is at least 40%, in particular 40% to 50%.

8. Two-stroke engine according to one of Claims 1 to 7, characterized in that the squish clearance (s) at the top dead centre (OT) of the piston (5) is smaller than 0.7 mm.

9. Two-stroke engine according to one of Claims 1 to 8, characterized in that the ratio of the largest extent (d) to the smallest extent (e) of the spherical cap (46) in a projection in the direction of the longitudinal central axis (50) of the cylinder (2) is at most 1.1.

10. Two-stroke engine according to one of Claims 1 to 9, characterized in that the two-stroke engine (2) has an air channel (11) for pre-storage of air in at least one transfer channel (14, 15).

11. Two-stroke engine according to one of Claims 1 to 10, characterized in that the mouth opening (18) of the transfer channel (14, 15) extends over a first peripheral segment (55) of the cylinder (2), and in that the outlet opening (13) extends over a second peripheral segment (56) of the cylinder (2), wherein the first peripheral segment (55) and the second peripheral segment (56) at least partially overlap.

12. Two-stroke engine according to one of Claims 1 to 11, characterized in that at least one transfer channel (14, 15) extends helically around the cylinder bore (22).

13. Two-stroke engine according to one of Claims 1 to 12, characterized in that at least two transfer channels (14, 15), in particular all the transfer channels (14, 15), open out into the crankcase interior (7) at a common mouth opening (18).