Two-stroke internal combustion engine with improved transfer system

Abstract

A two-stroke internal combustion engine (100) is described as comprising: an engine body (150) defining an internal volume, at least one piston (135), which is slidably ac-commodated in a cylindrical portion (151) of said internal volume, to subdivide it into at least two chambers of variable volume, one of which is a pumping chamber (160) and one is a combustion chamber (155), a crankshaft (105), rotatably coupled to the engine body (150) and rotating about a predetermined rotation axis (R), which carries at least one crank (120) within the pumping chamber (160), a connecting rod (140) connected between said crank (120) and the piston (135), to transform a linear reciprocating movement of the piston (135) into a rotary movement of the crankshaft (105), and at least one transfer system for fluidically connecting said pumping chamber (160) with said combustion chamber (155); wherein said transfer system comprises: an inlet port (205) located on a plane internal surface (165) of the engine body (150), which plane internal surface (165) delimits the pumping chamber (160) and is orthogonal to the rota-tion axis (R) of the crankshaft (105), at least one outlet port (210) formed on an internal surface of the engine body (150) delimiting said cylindrical portion (151) of the internal volume, and at least one transfer duct (215) adapted to put the inlet port (205) in com-munication with the outlet port (210); wherein the inlet port (205) has an arcuate shape extending angularly about the rotation axis (R) of the crankshaft (105) with an angular extension (Ω) greater than or equal to 50 sexagesimal degrees, for example greater than or equal to 90 sexagesimal degrees.

Description

[0001] TWO-STROKE INTERNAL COMBUSTION ENGINE WITH IMPROVED TRANSFER

[0002] SYSTEM

[0003] Technical field

[0004] The present invention relates to a two-stroke internal combustion engine, in particular of the type adapted to be installed in portable work tools, for example brush cutters, lawn mowers, chain saws and other similar gardening tools.

[0005] Prior art

[0006] Two-stroke internal combustion engines generally comprise an engine body, wherein there is at least one cylinder adapted to accommodate a piston that subdivides the internal volume into at least two chambers, one of which is a combustion chamber and one is a pumping chamber.

[0007] Inside the cylinder, the piston is movable reciprocating between a bottom dead centre, where the volume of the combustion chamber is maximum and the volume of the pumping chamber is minimum, and a top dead centre, where the volume of the combustion chamber is minimum and the volume of the pumping chamber is maximum.

[0008] A crankshaft is accommodated in the pumping chamber, which is connected to the piston by means of a connecting rod, so as to create a crank mechanism which transforms the aforesaid reciprocating movement of the piston into a rotation of the crankshaft.

[0009] In order to enable the reciprocating movement of the piston, the engine body is provided with a suction duct, through which a fresh charge of combustible mixture enters the pumping chamber, with at least one transfer duct connecting the pumping chamber with the combustion chamber, and with an exhaust duct to evacuate burnt gases from the combustion chamber.

[0010] The operation of a two-stroke engine provides that, when the piston is at the top dead centre, the suction duct is open to allow the mixture to enter the pumping chamber, while the transfer duct and exhaust duct are closed by the piston itself.

[0011] During the next stroke towards the bottom dead centre, the piston closes the suction duct, simultaneously opening the exhaust duct and then the transfer duct.

[0012] Thus, the mixture previously sucked into the pumping chamber is pushed through the transfer duct into the combustion chamber, also facilitating the discharge of the burnt gases produced during the previous operating cycle.

[0013] Upon reaching the bottom dead centre, the piston moves back to the top dead centre, first closing the transfer duct and then the exhaust duct, so as to compress the mixture in the combustion chamber.

[0014] When the piston is close to the top dead centre, a spark plug generates the spark necessary to ignite the combustion of the mixture compressed in the combustion chamber, thus producing rapidly expanding burnt gases that push the piston back to the bottom dead centre and repeat the cycle.

[0015] A known problem with these engines is that, for a certain time lapse, when the piston is close to the bottom dead centre, the transfer duct and exhaust duct are both open at the same time, so that part of the mixture pumped into the combustion chamber through the transfer duct can be directly expelled (unburnt) through the exhaust duct.

[0016] As a result, during this step, the engine can release large amounts of fuel and lubricating oil into the environment, which are health-threatening pollutants.

[0017] In addition, the loss of unburnt mixture through the exhaust duct inevitably causes a decrease in efficiency, hence an increase in consumption, compared to an ideal cycle wherein all the mixture remains confined in the combustion chamber.

[0018] One solution known to the person skilled in the art to reduce the mixture consumption is to decrease the passage cross-section of the transfer duct, so as to reduce as much as possible the period during which it is open at the same time as the exhaust duct.

[0019] However, this solution is detrimental from an efficiency perspective, since a reduction in the passage cross-section causes an increase in the pressure losses suffered by the mixture when passing through the transfer duct, which implies an increase in the energy absorbed by the piston to pump the mixture into the combustion chamber.

[0020] Another solution that has been adopted to reduce the mixture consumption is to fit a movable bulkhead (or rotary valve) on the crankshaft, inside the pumping chamber, adapted to keep the transfer duct closed for longer while the exhaust duct is open.

[0021] However, this solution has the drawback of introducing a non-negligible construction complication, resulting in higher production costs and an increase in the overall dimensions, as well as a decrease in efficiency due to the need to also drive the additional mass of the movable bulkhead in rotation.

[0022] Disclosure of the invention

[0023] In the light of the foregoing, one object of the present invention is to reduce the amount of fresh mixture that is expelled unburnt from a two-stroke internal combustion engine, in order to decrease the environmental impact and increase efficiency.

[0024] A further object of the invention is to achieve the mentioned purpose within the context of a simple, rational and relatively cheap solution.

[0025] These and other objects are achieved by the characteristics of the invention reported in the independent claim. The dependent claims outline preferred and / or particularly advantageous aspects of the invention.

[0026] In particular, an embodiment of the invention makes available a two-stroke internal combustion engine comprising:

[0027] - an engine body defining an internal volume,

[0028] - at least one piston, which is slidably accommodated in a cylindrical portion of said internal volume, to subdivide it into at least two variable-volume chambers, one of which is a pumping chamber and one is a combustion chamber,

[0029] - a crankshaft, rotatably coupled to the engine body and rotating about a predetermined rotation axis, which carries at least one crank inside the pumping chamber,

[0030] - a connecting rod connected between said crank and the piston, to transform a linear reciprocating movement of the piston into a rotary movement of the crankshaft, and

[0031] - at least one transfer system adapted to put the pumping chamber in fluidic communication with the combustion chamber, wherein said transfer system comprises:

[0032] - an inlet port placed on a plane internal surface of the engine body, which plane internal surface delimits the pumping chamber and is orthogonal to the rotation axis of the crankshaft,

[0033] - at least one outlet port formed on an internal surface of the engine body delimiting said cylindrical portion of the internal volume, and

[0034] - at least one transfer duct connecting the inlet port and the outlet port, wherein the inlet port has an arcuate shape that extends angularly about the rotation axis of the crankshaft with an angular extension greater than or equal to 50 sexagesimal degrees, e.g. greater than or equal to 90 sexagesimal degrees, and preferably but not necessarily lower than or equal to 115 sexagesimal degrees.

[0035] Thanks to this unique geometry of the inlet port, an increase in engine efficiency is advantageously achieved, even without using rotary valves or other devices. The explanation for this effect, experimentally proven, lies in the fact that the new geometry of the inlet port, which also leads to an increase in the overall length of the transfer system, improves the vaporisation of the mixture.

[0036] This improved vaporisation involves that the mixture is broken down into finer particles with a lower inertia, which spread more evenly in the combustion chamber and take longer to reach the exhaust duct, ensuring both a more complete combustion and a lower consumption of unburnt mixture.

[0037] All of this, as said, with no need for rotary valves or other devices, and therefore as part of a simpler, cheaper and dimensionally more compact solution than the known ones. According to one aspect of the invention, the inlet port may be subdivided, by a transverse plane orthogonal to a central axis of the cylindrical portion and containing the rotation axis of the crankshaft, into a first portion, located on the same side of the combustion chamber, and a second portion, having a smaller angular extension than the first portion and located on the opposite side.

[0038] In particular, the angular extension of the second portion of the inlet port can be lower than or equal to 10 sexagesimal degrees, e.g. lower than or equal to 5 sexagesimal degrees.

[0039] Thus, the largest portion of the inlet port is located in the “upper” part of the pumping chamber, i.e. the one proximal to the combustion chamber, facilitating the flow of the mixture in the transfer system towards the combustion chamber itself.

[0040] However, it cannot be ruled out that, in some embodiments, the inlet port may be entirely located on the same side of the combustion chamber, with respect to a transverse plane orthogonal to the central axis of the cylindrical portion and containing the rotation axis of the crankshaft, or that it may be entirely located in the “upper” part of the pumping chamber.

[0041] In that case, the aforesaid transverse plane could, for example, be tangent to the inlet port.

[0042] According to another aspect of the invention, the inlet port may be subdivided, by a longitudinal plane containing a central axis of the cylindrical portion and the rotation axis of the crankshaft, into a primary portion, located on the same side of an engine suction duct, and a secondary portion, having a smaller angular extension than the primary portion and located on the opposite side. In particular, the angular extension of the secondary portion of the inlet port can be lower than or equal to 20 sexagesimal degrees, e.g. lower than or equal to 15 sexagesimal degrees.

[0043] Thus, the largest portion of the inlet port is located in the part of the pumping chamber proximal to the suction duct from which the fresh mixture enters, further facilitating the flow of said mixture into the transfer system towards the combustion chamber.

[0044] Even in this case, however, it cannot be ruled out that, in some embodiments, the inlet port may be located entirely on the same side of an engine suction duct, relative to a longitudinal plane containing the central axis of the cylindrical portion and the rotation axis of the crankshaft.

[0045] In that case, the aforesaid longitudinal plane could, for example, be tangent to the inlet port.

[0046] Another aspect of the invention provides that the inlet port may have a radial extension entirely within a circular crown, centred in the rotation axis of the crankshaft, whose internal diameters Dinn and external diameters Dout satisfy the following relationships:

[0047] Dinn — CT ■ S

[0048] DOut = P ' S wherein a is a coefficient with a value between 1.25 and 1.5 (extremes included), (3 is a coefficient with a value between 1 .55 and 1 .9 (extremes included), while S is a full stroke of the piston from the bottom dead centre to the top dead centre.

[0049] Fulfilment of these dimensional relations makes it advantageously possible to keep the dimensions of the internal combustion engine very compact, without detriment to efficiency, specifically without making the inlet port so small (in relation to the piston stroke and thus the amount of mixture to be transferred) as to impair an adequate flow of the mixture towards the combustion chamber.

[0050] According to another aspect of the invention, the inlet port may be made available by a recess formed into said plane internal surface of the engine body and in communication with the transfer duct.

[0051] This recess may have a maximum depth lower than or equal to 10 mm, e.g. lower than or equal to 6 mm. It should be noted herein that the “depth” of the recess means the size / width of the recess in the direction orthogonal to the plane internal surface on which it is formed.

[0052] Another aspect of the invention provides that the recess may have an increasing depth (i.e. rising) from a first angular end of the inlet port to an opposite second angular end. Since the inlet port has an arcuate shape, an “angular end” means one end of the arc (or curve) defining the shape of the inlet port.

[0053] That being said, the second angular end of the inlet port may, for example, be the one proximal to the combustion chamber, i.e. the one that is closer to the combustion chamber than the other, which may therefore be the first angular end.

[0054] Thus, the depth of the recess increases as one proceeds towards the combustion chamber.

[0055] According to another aspect of the invention, the aforesaid recess may be in communication with the transfer duct by means of a connecting duct, which is formed in the engine body, in a position retracted from the plane internal surface on which the inlet port is placed.

[0056] “Retracted position” means on the opposite side of the recess relative to the inlet port, i.e. a position such that the recess is substantially interposed between the inlet port and the connecting duct, along a direction orthogonal to the plane internal surface on which the inlet port is located.

[0057] In order to put the recess in communication with the transfer duct, the connecting duct may intersect at least a portion of the recess, preferably only a (limited) portion thereof. Thus, the intersection between the connecting duct and the aforesaid portion of the recess defines an opening that puts them directly in communication with each other.

[0058] Alternatively or as a special case, at least a portion of the recess, preferably only a (limited) portion thereof, can be opened at the bottom (e.g. completely without a bottom) so as to flow into the connecting duct.

[0059] The “bottom” of the recess (or of a portion thereof) obviously means a surface of the recess (or of the portion thereof) which, if present, is opposite to the inlet port along a direction orthogonal to the plane internal surface on which the inlet port is placed.

[0060] According to an aspect of the invention, the portion of the recess which is intersected by the connecting duct and / or which is open at the bottom may be that portion which subtends the angular end of the inlet port which is proximal to the combustion chamber. Thus, the connecting duct is shorter and constructively simpler, helping to keep the size of the engine limited.

[0061] Another aspect of the invention provides that the connecting duct may extend parallel to a central axis of the cylindrical portion wherein the piston is accommodated, e.g., that it may comprise at least one cross-sectional segment along an axis parallel to said central axis.

[0062] Thanks to this solution, the mixture coming from the pumping chamber is efficiently diverted and conveyed to the combustion chamber through the inlet port and the relative recess.

[0063] According to another aspect of the invention, the connecting duct may be positioned so as to be intercepted by a longitudinal plane containing both a central axis of the cylindrical portion wherein the piston is accommodated and the rotation axis of the crankshaft.

[0064] In other words, the connecting duct may be positioned so that said longitudinal plane divides it into two portions, preferably two portions of comparable size.

[0065] Thus, the connecting duct is substantially positioned on the centreline of the internal combustion engine, helping to keep its dimensions small and compact.

[0066] A different aspect of the invention provides that the transfer system may comprise at least two transfer ducts that are separate from each other and both communicating with the inlet port through the same connecting duct.

[0067] Thanks to this solution, wherein the connecting duct also acts as a manifold for the transfer ducts, it is possible to feed the combustion chamber with several separate flows of mixture, improving the distribution thereof and thus the subsequent combustion.

[0068] Turning to more properly constructive aspects, one aspect of the invention provides that the inlet port, recess and connecting duct may be made in a single-piece body.

[0069] This advantageously reduces the number of components to be mounted to assemble the engine, making this production step simpler and faster.

[0070] A further aspect of the invention provides that the engine body may consist of at least two separate components assembled together, of which a crankcase (or pump casing) wherein at least the inlet port, recess and connecting duct are made, and a head wherein at least the transfer duct and outlet port are made.

[0071] The crankcase (or pump casing) may in turn be formed by at least two separate shells joined together along a plane orthogonal to the rotation axis of the crankshaft and containing the axis of the cylindrical portion of the engine body.

[0072] According to another aspect of the invention, the engine may comprise two transfer systems, each having the characteristics outlined above, which are arranged in a mirror-like manner with respect to a plane of symmetry orthogonal to the rotation axis of the crankshaft.

[0073] This further improves the filling of the combustion chamber with the mixture from the pumping chamber.

[0074] Brief description of the figures

[0075] Further features and advantages of the invention will be more apparent after reading the following description provided by way of non-limiting example, with the aid of the accompanying drawings.

[0076] Figure 1 is a longitudinal section of a two-stroke internal combustion engine according to an embodiment of the present invention.

[0077] Figure 2 is section ll-ll of Figure 1 .

[0078] Figure 3 is the section of Figure 2, shown with the piston in the bottom dead centre position.

[0079] Figure 4 is an enlarged detail of Figure 2.

[0080] Figure 5 is the section of Figure 2 wherein the crankshaft and the connecting rod have been removed to highlight certain aspects of the invention.

[0081] Figure 6 is a perspective view of one of the shells forming the crankcase (or pump casing) of the engine of Figure 1 .

[0082] Figure 7 is the section of the shell of Figure 6 sectioned according to the plane VII-VII of Figure 5.

[0083] Figure 8 is section VIII-VIII of Figure 5.

[0084] Detailed description

[0085] In the figures, a two-stroke internal combustion engine has been globally referred to as 100, e.g. an engine that can be installed in portable work tools, such as brush cutters, lawn mowers, chain saws and similar gardening tools.

[0086] The engine 100 can be fuelled by a mixture consisting of a fuel (e.g. petrol) and lubricating fluid, which is dispersed into an air stream by means of a suitable mixing system, e.g. comprising a carburettor.

[0087] The engine 100 comprises a crankshaft 105 adapted to rotate about a rotation axis R and through which the drive force generated by the engine itself is drawn.

[0088] It should be noted that crankshaft means a shaft carrying at least one crank, i.e. an eccentric element which, as a result of the rotation of the shaft, is adapted to perform a full rotation movement about the rotation axis R.

[0089] For example, the crankshaft 105 may comprise a first cylindrical section 110 coaxial to the rotation axis R, a second cylindrical section 115 opposite to the first cylindrical section 110 and also coaxial to the rotation axis R, and a crank 120, which is interposed between the first cylindrical section 110 and the second cylindrical section 115 and is rigidly rotationally integral with them (with no residual degrees of freedom).

[0090] The crank 120 may comprise a pivot 125 having the axis parallel and eccentric to the rotation axis R, i.e. arranged at a certain (non-zero) distance from the rotation axis R.

[0091] In the embodiment shown, the pivot 125 is interposed between two arms which extend and project radially from the cylindrical segments 110 and 115, moving away from the rotation axis R, and which together with pin 125 globally define the crank 120.

[0092] The crankshaft 105 may also comprise a flywheel mass 130 with which it is rotationally integral.

[0093] For example, the flywheel mass 130 protrudes radially from the first cylindrical section 110 and the second cylindrical section 115, preferably in a radial direction opposite the crank 120.

[0094] In the embodiment shown, the flywheel mass 130 comprises a pair of disc sectors, each of which is rotationally integral with a respective cylindrical section 110 and 115.

[0095] Each of these disc sectors can be made in a single body, i.e. as a one-piece body, with a respective arm of the crank 120 and the corresponding cylindrical segment 110 or 115. The engine 100 further comprises at least one piston 135, which is connected to the crankshaft 105, more precisely to the crank 120, by a connecting rod 140.

[0096] In particular, the connecting rod 140 may have one end hinged to the piston 135, for example to a pin 145 of the piston 135, and an opposite end hinged to the crank 120, for example to the pivot 125, possibly by interposing a bearing.

[0097] Thus, the crank 120, piston 135 and connecting rod 140 together form a pushing crank mechanism adapted to transform a linear reciprocating movement of the piston 135 into a rotary movement of the crank 120 and crankshaft 105 in general.

[0098] The crank 120, piston 135 and connecting rod 140 are contained within the internal volume of an engine body 150.

[0099] In particular, the piston 135 is coaxially and slidably coupled to a cylindrical portion 151 of the internal volume of the engine body 150, also referred to as a “cylinder”, whose central axis Q is preferably orthogonal and coplanar to the rotation axis R of the crankshaft 105.

[0100] Thus, the piston 135 subdivides the internal volume of the engine body 150 into at least two variable-volume chambers, of which one is a combustion chamber 155 and one is a pumping chamber 160.

[0101] Specifically, within the cylinder, the piston 135 is able to slide between a top dead centre position (shown in Figure 2), wherein the volume of the combustion chamber 155 is minimum and that of the pumping chamber 160 is maximum, and a bottom dead centre position (shown in Figure 3), where the volume of the combustion chamber 155 is maximum and the volume of the pumping chamber 160 is minimum.

[0102] While the combustion chamber 155 can house a spark plug (not shown) to ignite the mixture combustion, the pumping chamber 160 can house the crank 120 and the connecting rod 140.

[0103] In particular, along the direction of the rotation axis R, the crank 120 can be interposed between two internal surfaces of the engine body 150 that delimit the pumping chamber 160, a first internal surface 165 and a second internal surface 170.

[0104] Each of these internal surfaces 165 and 170 is substantially plane and orthogonally oriented to the rotation axis R.

[0105] The first internal surface 165 can be crossed by the first cylindrical segment 110 of the crankshaft 105, which can be rotatably coupled to (and supported by) the engine body 150 by means of a bearing which is housed in a seat formed on said first internal surface 165.

[0106] Similarly, the second internal surface 170 can be crossed by the second cylindrical section 115 of the crankshaft 105, which can be rotatably coupled to (and supported by) the engine body 150 by means of an additional bearing which is housed in a seat on said second internal surface 170.

[0107] The engine body 150 may consist of a head 175 that defines the combustion chamber 155 and a crankcase (or pump casing) 180 that at least partially defines the pumping chamber 160. In particular, the seats for the bearings supporting the crankshaft 105 can be fully formed in the crankcase 180.

[0108] The head 175 and the crankcase 180 may be manufactured as separate components and may be assembled together.

[0109] Furthermore, while the head 175 may be formed from a single-piece body, made for example by a forming process, the crankcase 180 may in turn be subdivided, for example along a plane orthogonal to the rotation axis R of the crankshaft 105 and containing the central axis Q of the cylinder, into two separate shells which that may be assembled together, a first shell 185 and a second shell 190.

[0110] Each of these first and second shells 185 and 190 can be produced as a single-piece body, e.g. by a forming process.

[0111] The first shell 185 can make available the first plane internal surface 165, while the second shell 190 can make available the second plane internal surface 170, wherein the seats for the crankshaft 105 bearings can be made.

[0112] In the engine body 150, for example in the head 175, there is also a suction duct 195, which is adapted to feed a flow of mixture into the pumping chamber 160.

[0113] As shown in Figure 2, this suction duct 195 can extend longitudinally along a central axis coplanar and / or orthogonal to the cylinder central axis Q.

[0114] The suction duct 195 can flow onto a portion of the internal surface of the cylinder such as to be occluded by the piston 135, when the latter is at bottom dead centre (see Figure 3), but such as to be open and in communication with the pumping chamber 160, when the piston 135 is at top dead centre (see Figure 2).

[0115] In the engine body 150, for example in the head 175, there is also an exhaust duct 200, which is adapted to convey the combustion gases produced inside the combustion chamber 155 to the outside.

[0116] This exhaust duct 200 may also extend longitudinally along a central axis coplanar and / or orthogonal to the central axis Q of the cylinder, but on the opposite side of the suction duct 195, relative to a plane containing the central axis Q of the cylinder and orthogonal to the rotation axis R of the crankshaft 105.

[0117] The exhaust duct 200 may open on a portion of the internal surface of the cylinder such as to be occluded by the piston 135, when the latter is at top dead centre (see Figure 2), but such as to be open and in communication with the combustion chamber 155, when the piston 135 is at bottom dead centre (see Figure 3).

[0118] The exhaust duct 200 may therefore open onto a portion of the cylinder internal surface located at a greater distance from the rotation axis R than that onto which the inlet duct 195 flows.

[0119] Finally, the engine 100 may comprise at least one transfer system, made for example in the engine body 150, which is adapted to place the pumping chamber 160 in fluidic communication with the combustion chamber 155 at certain steps of the operating cycle.

[0120] More preferably, the engine 100 comprises two of said transfer systems, which may be shaped and arranged in a mirror-like manner relative to a median plane containing the central axis Q of the cylinder and perpendicular to the rotation axis R of the crankshaft 105.

[0121] Since these two transfer systems are mirror-like, only one will be described in the following, it being understood that every feature described will also apply to the other.

[0122] Well, as shown in Figure 4, the transfer system comprises an inlet port 205 located on the plane internal surface 165 of the engine body 150, at least one outlet port 210 formed on an internal surface of the cylinder, and at least one transfer duct 215 adapted to put the inlet port 205 in communication with the outlet port 210.

[0123] More preferably (see Fig. 8), each transfer system may comprise at least one pair of transfer ducts 215, separated from each other and provided with respective outlet ports 210 (see Fig. 3), but both communicating with the same inlet port 205.

[0124] Turning now to Figure 5, it can be appreciated that the inlet port 205 has an arcuate shape extending angularly about the rotation axis R of the crankshaft 105, for example from a first angular end 220 to a second angular end 225.

[0125] Since the inlet port 205 has an arcuate shape, “angular end” means one end of the arc (or curve) defining the shape of the inlet port 205.

[0126] The first angular end 220 of the inlet port 205 may be distal from the combustion chamber 155, while the second angular end 225 may be proximal to the combustion chamber 155. In other words, the distance between the second angular end 225 and the combustion chamber 155 may be lower than the distance between the latter and the first angular end 220.

[0127] The total angular extension O of the inlet port 205, i.e. from the first angular end 220 to the second angular end 225, is greater than or equal to 50 sexagesimal degrees, e.g. greater than or equal to 90 sexagesimal degrees, and preferably but not necessarily lower than or equal to 115 sexagesimal degrees. In the embodiment herein shown, the total angular extension 0 of the inlet port 205 is, for example, approximately equal to 103 sexagesimal degrees.

[0128] The arrangement of the inlet port 205 may be such that it can be subdivided, from a transverse plane A orthogonal to the central axis Q of the cylinder and containing the rotation axis R of the crankshaft 105, into a first portion 205A, which is located on the same side of the combustion chamber 155, and a second portion 205B, having an angular extension y smaller than the angular extension 5 of the first portion 205A, preferably much smaller, which is located on the opposite side.

[0129] For example, the angular extension y of the second portion 205B of the inlet port 205 may be lower than or equal to 10 sexagesimal degrees, preferably lower than or equal to 5 sexagesimal degrees.

[0130] In the embodiment herein shown, the angular extension y of the second portion 205B of the inlet port 205 is, for example, approximately equal to 3 sexagesimal degrees, so that the angular extension 5 of the first portion 205A is approximately equal to 100 sexagesimal degrees.

[0131] However, it is not excluded that, in other embodiments, the inlet port 205 may be located, relative to the transverse plane A, entirely on the same side of the combustion chamber 155, or that it may be entirely located in the “upper” part of the pumping chamber 160, for example so that the transverse plane A is tangent to the inlet port 205 (angular extension y equal to zero).

[0132] The inlet port 205 may also be positioned so as to be subdivided, by a longitudinal plane B containing both the central axis Q of the cylinder and the rotation axis R of the crankshaft 105, into a primary portion 205C, located on the same side of the inlet port 195, and a secondary portion 205D, having an angular extension <p smaller than the angular extension s of the primary portion 205C, preferably much smaller, which is located on the opposite side.

[0133] For example, the angular extension <p of the secondary portion 205D of the inlet port 205 may be lower than or equal to 20 sexagesimal degrees, preferably lower than or equal to 15 sexagesimal degrees.

[0134] In the embodiment herein shown, the angular extension <p of the secondary portion 205D of the inlet port 205 is, for example, approximately equal to 10 sexagesimal degrees, so that the angular extension £ of the primary portion 205C is approximately equal to 103 sexagesimal degrees.

[0135] Even in this case, however, it cannot be excluded that, in certain embodiments, the inlet port 205 may be located, relative to the longitudinal plane B, entirely on the same side of the inlet duct 195, for example so that said longitudinal plane B is tangent to the inlet port 205 (angular extension <p equal to zero).

[0136] Turning to the radial extension, the inlet port 205 may have a radial extension such that it is entirely enclosed (contained) in a circular crown of the internal plane surface 165, centred in the rotation axis R of the crankshaft 105, whose internal diameters Dinn and external diameters Dout satisfy the following relationships:

[0137] ^inn — CT ■ S

[0138] DOut = P ' S wherein a is a coefficient with a value between 1.25 and 1.5 (extremes included), (3 is a coefficient with a value between 1 .55 and 1 .9 (extremes included), while S is a full stroke of the piston 135 from the bottom dead centre position to the top dead centre position.

[0139] Generally, the stroke S is equal to twice the radial distance between the rotation axis R of the crankshaft 105 and the axis of the pivot 125 with which the crankshaft 105 is jointed to the connecting rod 140.

[0140] As visible in Figure 6, the inlet port 205 can be made available by a recess 230 formed into the plane internal surface 165 of the engine body 150, which can have a maximum depth lower than or equal to 10 mm, e.g. lower than or equal to 6 mm.

[0141] In particular, the recess 230 may have an increasing depth (i.e. rising) from the first angular end 220 of the inlet port 205, wherein the depth may be substantially zero, towards the opposite second angular end 225.

[0142] This recess 230 may be in communication with each transfer duct 210 by means of a connecting duct (or manifold) 235, which is formed in the engine body 150, in a retracted position from the plane internal surface 165 on which the inlet port 205 is placed.

[0143] Retracted position means on the opposite side of the recess 230 from the inlet port 205, i.e., a position such that the recess 230 is substantially interposed between the inlet port 205 and the connecting duct 235, along a direction orthogonal to the plane internal surface 165 on which the inlet port 205 is located.

[0144] The connecting duct 235 may extend parallel to the direction of the central axis Q of the cylinder, i.e. it may comprise at least one segment with a constant cross-section relative to an axis parallel to said central axis Q of the cylinder.

[0145] In particular, the connecting duct 235 may be positioned on the centreline of the engine 100 or, more precisely, so as to be intercepted by a longitudinal plane containing both the central axis Q of the cylinder and the rotation axis R of the crankshaft 105, thereby being subdivided by said longitudinal plane into two portions, preferably of comparable dimensions.

[0146] As visible in Figure 7, in order to put the recess 230 in communication with the transfer duct 215, the connecting duct 235 may intersect a portion of the recess 230, so that the intersection 240 between the connecting duct 235 and the aforesaid portion of the recess 230 defines an opening that puts them directly in communication with each other.

[0147] In the specific example, this means that said portion of the recess 230 is open at the bottom (i.e. completely without a bottom) so that it flows into connecting duct 235.

[0148] "Bottom" of the recess (or of a portion thereof) means that surface of the recess (or portion thereof) which, if present, is opposite to the inlet port 205 along a direction orthogonal to the plane surface 165 on which the inlet port 205 is placed.

[0149] Preferably, the portion of the recess 230 intersected by the connecting duct 235 or open at the bottom is a limited portion, from which it follows that the recess 230 also has another portion which is not intersected by the connecting duct 235 and which, in the example shown herein, is actually closed at the bottom by a surface 245 adapted to define the depth thereof.

[0150] Among these two portions, the portion of the recess 230 which is intersected by the connecting duct 235 or which is open at the bottom is preferably the one which subtends / de- fines the second angular end 225 of the inlet port 205, i.e. the one proximal to the combustion chamber 155.

[0151] As visible in the figures, the inlet port 205, recess 230 and connecting duct 235 are preferably formed in the crankcase 180 of the engine 100, while each transfer duct 215 with the relative outlet port 210 are made in the head 175.

[0152] In particular, the inlet port 205, recess 230 and connecting duct 235 are preferably made in a single-piece body, in this case in the first shell 185 forming the crankcase 180.

[0153] Obviously, the inlet port 205, recess 230 and connecting duct 235 of the other transfer system (if present), are preferably formed in the second shell 190, with the inlet port 205 positioned on the plane internal surface 170.

[0154] The operation of the engine 100 can be described from the instant when the piston 135 is in the top dead centre position, as show in Figure 2.

[0155] In this position, the piston 135 occludes the exhaust duct 200 and the outlet port 210 of each transfer duct 215, while leaving the inlet port 195 open to allow the mixture to enter the pumping chamber 160.

[0156] During the next stroke towards the bottom dead centre (see Fig. 3), the piston 135 closes the suction duct 195 while simultaneously opening the exhaust duct 200 and then the outlet port 210 of each transfer duct 215.

[0157] Thus, the mixture previously sucked into the pumping chamber 160 is pushed, through each transfer system, into the combustion chamber 155, also favouring the discharge of the burnt gases produced during the previous operating cycle.

[0158] Upon reaching the bottom dead centre, the piston 135 moves back to the top dead centre, first closing the outlet port 210 of each transfer tube 215 and then the exhaust duct 200, in order to compress the mixture in the combustion chamber 155.

[0159] When the piston 135 is near the top dead centre, the spark plug (not shown) generates a spark that ignites the combustion of the mixture compressed in the combustion chamber 155, thus producing rapidly expanding burnt gases that push the piston 135 back to the bottom dead centre and repeat the cycle.

[0160] The invention thus conceived is susceptible to several modifications and variations, all falling within the scope of the inventive concept. Moreover, all the details can be replaced by other technically equivalent elements. In practice, the materials used, as well as the contingent shapes and sizes, can be whatever according to the requirements without for this reason departing from the scope of protection of the following claims.

Claims

CLAIMS1. A two-stroke (100) internal combustion engine comprising:- an engine body (150) defining an internal volume,- at least one piston (135), which is slidably accommodated in a cylindrical portion (151 ) of said internal volume, to subdivide it into at least two chambers of variable volume, one of which is a pumping chamber (160) and one is a combustion chamber (155),- a crankshaft (105), rotatably coupled to the engine body (150) and rotating about a predetermined rotation axis (R), which carries at least one crank (120) within the pumping chamber (160),- a connecting rod (140) connected between said crank (120) and the piston (135), to transform a linear reciprocating movement of the piston (135) into a rotary movement of the crankshaft (105), and- at least one transfer system adapted to put the pumping chamber (160) in fluidic communication with the combustion chamber (155), wherein said transfer system comprises:- an inlet port (205) placed on a plane internal surface (165) of the engine body (150), which plane internal surface (165) delimits the pumping chamber (160) and is orthogonal to the rotation axis (R) of the crankshaft (105),- at least one outlet port (210) formed on an internal surface of the engine body (150) delimiting said cylindrical portion (151 ) of the internal volume, and- at least one transfer duct (215) connecting the inlet port (205) with the outlet port (210), wherein the inlet port (205) has an arcuate shape extending angularly about the rotation axis (R) of the crankshaft (105) with an angular extension (Q) greater than or equal to 50 sexagesimal degrees, for example greater than or equal to 90 sexagesimal degrees.

2. An engine (100) according to claim 1 , wherein said angular extension (Q) of the inlet port (205) is lower than or equal to 115 sexagesimal degrees.

3. An engine (100) according to claim 1 or 2, wherein the inlet port (205) is subdivided, by a transverse plane orthogonal to a central axis (Q) of the cylindrical portion (151 ) and containing the rotation axis (R) of the crankshaft (105), into a first portion (205A), located on the same side of the combustion chamber (155), and a second portion (205B), havinga smaller angular extension than the first portion (205A) and located on the opposite side.

4. An engine (100) according to claim 3, wherein the angular extension (y) of the second portion (205B) of the inlet port (205) is lower than or equal to 10 sexagesimal degrees.

5. An engine (100) according to any one of the preceding claims, wherein the inlet port (205) is subdivided, by a longitudinal plane containing a central axis (Q) of the cylindrical portion (151 ) and the rotation axis (R) of the crankshaft (105), into a primary portion (205C), located on the same side of a suction duct (195) of the engine, and a secondary portion (205D), having a smaller angular extension than the primary portion and located on the opposite side.

6. An engine (100) according to claim 5, wherein the angular extension (E) of the secondary portion (205D) of the inlet port (205) is lower than or equal to 20 sexagesimal degrees.

7. An engine (100) according to any one of the preceding claims, wherein the inlet port (205) has a radial extension entirely within a circular crown, centred in the rotation axis (R) of the crankshaft 105, whose internal diameters Dinn and external diameters Dout satisfy the following relationships:^inn — CT ■ S DOut = P ' S where a is a coefficient with a value between 1 .25 and 1 .5, (3 is a coefficient with a value between 1.55 and 1.9, and S is a full stroke of the piston (135) from a bottom dead centre to a top dead centre.

8. An engine (100) according to any one of the preceding claims, wherein the inlet port (205) is made available by a recess (130), which is formed on said plane internal surface (165) of the engine body (150) and is in communication with the transfer duct (215).

9. An engine (100) according to claim 8, wherein said recess (230) has a maximum depth lower than or equal to 10 mm.

10. An engine according to claim 8 or 9, wherein said recess (230) has an increasing depth from a first angular end (220) of the inlet port (205) towards an opposite second angular end (225).

11. An engine (100) according to claim 10, wherein the second angular end (225) of theinlet port (205) is proximal to the combustion chamber (155).

12. An engine (100) according to any one of claims 8 to 11 , wherein said recess (230) is in communication with the transfer duct (215) by means of a connecting duct (235), which is formed in the engine body (150) in a retracted position relative to the plane in- ternal surface (165) on which the inlet port (205) is located, and extends parallel to a central axis (Q) of the cylindrical portion (151 ) in which the piston (135) is accommodated.

13. An engine (100) according to claim 12, wherein the transfer system comprises at least two transfer ducts (215) separated from each other and both communicating with the inlet port (205) via the same connecting duct (235).

14. An engine (100) according to claim 12 or 13, wherein the inlet port (205), the recess(230) and the connecting duct (235) are made in a single-piece body (185).

15. An engine (100) according to any one of claims 12 to 14, wherein the engine body (150) is made up of at least two separate components assembled together, of which a crankcase (180) wherein at least the inlet port (205), the recess (230) and the connecting duct (235) are made, and a head (175) wherein at least the transfer duct (215) and the outlet port (210) are made.

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