Patent Publication Number: US-10767550-B2

Title: Two-stroke internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention regards two-stroke internal combustion engines, in particular two-stroke engines intended to actuate small work equipment such as for example chainsaws, string trimmers and the like. 
     PRIOR ART 
     As known, two-stroke engines may comprise at least one cylinder within which there is slidably received a piston that divides the interior volume of the cylinder into two distinct chambers; a combustion chamber and a pumping chamber. 
     An air and fuel mixture, whose combustion produces exhaust gases in rapid expansion causing the movement of the piston before flowing out through an exhaust duct, is periodically introduced into the combustion chamber. 
     The comburent air is generally supplied through an intake duct, which places the pumping chamber in communication with the external environment. 
     The amount of air introduced into the pumping chamber can be regulated by means of a power valve, typically a butterfly valve, which is arranged in the intake duct and it can be activated from the external so as to vary the opening degree thereof. 
     The fuel is normally supplied by means of a carburettor, which comprises a venturi pipe arranged along the intake duct, typically upstream of the power valve, and a dispensing nozzle which terminates into the venturi pipe and which is in communication with a fuel tank. 
     Thus, the airflow which flows along the venturi pipe generates a depression which, through the dispensing nozzle, intakes the fuel from the tank and it with the air directed towards the pumping chamber. 
     The air and fuel mixture which gathers in the pumping chamber is subsequently pushed into the combustion chamber thanks to the movement of the piston, which forces it to flow through a series of scavenging ducts which place the pumping chamber in communication with the combustion chamber. 
     These scavenging ducts are generally open at the end of the expansion stroke of the piston, when the exhaust duct is open, so that the air and fuel mixture facilitates the cleaning of the cylinder, i.e. it pushes the combustion gases to flow out of the exhaust duct. 
     Each scavenging duct generally comprises an initial portion exiting from the pumping chamber, extending parallel to the cylinder, and a terminal portion terminating in the combustion chamber, extending transversely towards the cylinder. 
     Generally, the cross-section of the terminal portion is substantially constant or slightly convergent towards the cylinder, so as to accelerate and direct the air and fuel mixture towards the interior of the combustion chamber. 
     However, it was observed that small amounts of the air and fuel mixture can be directly suctioned into the exhaust duct flowing from the engine unburnt. 
     DESCRIPTION OF THE INVENTION 
     An object of the present invention is to overcome or at least reduce this drawback of the prior art through a solution that is simple, rational and inexpensive. 
     These and other objects are attained by the characteristics of the invention, which are outlined in the independent claim  1 . The dependent claims outline preferred and/or particularly advantageous aspects of the invention. 
     In particular, an embodiment of the present invention provides a two-stroke internal combustion engine comprising:
         a cylinder having a pre-set central axis,   a piston slidably coupled to the cylinder and suitable to divide the interior volume of the cylinder into two distinct chambers; a combustion chamber and a pumping chamber,   an intake duct communicating with the crank case,   an exhaust duct communicating with the combustion chamber, and   at least one scavenging duct suitable to place the pumping chamber in communication with the combustion chamber,
 
wherein said scavenging duct comprises a terminal portion terminating in the combustion chamber ( 150 ) which extends with configuration diverging from an inlet section up to an outlet section ( 205 ) obtained on a lateral surface of the cylinder.
       

     In other words, the terminal portion of the scavenging duct widens progressively moving from the inlet section towards the outlet section. 
     Thanks to this solution, it was observed that the flow of the air and fuel mixture fuel flowing into the combustion chamber, though maintaining an optimal directionality, tends to separate from the lateral surface of the scavenging duct and reach very high speeds. 
     This enables improving the cleaning of the cylinder and simultaneously reducing the amount of air and fuel mixture flowing from the exhaust duct unburnt. 
     According to an aspect of the invention, the inlet section of the terminal portion may define a choke in the scavenging duct. 
     In other words, the inlet section may be narrower not only as regards all the passage sections of the terminal portion, but also all the passage sections of at least the scavenging duct that is immediately upstream with respect thereto (with respect to the direction of flow of the mixture). 
     Thus, the flow of the air and fuel mixture that flows along the scavenging duct is accelerated at the initial section of the terminal portion i.e. the choke, and then projected into the combustion chamber at a higher speed. 
     According to another aspect of the invention, the inlet section of the terminal portion may be the narrowest passage section of the entire scavenging duct. 
     In other words, the area of the inlet section of the terminal portion may be smaller than the area of all the other passage sections of the scavenging duct. 
     Thus, the acceleration of the air and fuel mixture can be advantageously obtained without excessively increasing pressure drop. 
     According to a different aspect of the invention, the projection of the terminal portion of the scavenging duct on a median plane containing the axis of the cylinder may be fully contained in the projection of the outlet section on the same median plane. 
     Thanks to this solution, the terminal portion of the scavenging duct has no undercut surface with respect to the aforementioned median plane, thus enabling obtaining the engine by means of a casting process, in a relatively simple and inexpensive manner. 
     In particular, this solution enables obtaining the engine by means of a die-casting process, which advantageously allows reducing costs, thickness and tolerances with respect to the normal casting processes, for example sand casting. 
     Even more in particular, the aforementioned solution enables obtaining the interior wall of the terminal portion of the scavenging duct by means of a core (or insert) which can be advantageously extracted, at the end of the casting process, through just one straight motion in the perpendicular direction with respect to the median plane. 
     According to an aspect of the invention, the aforementioned median plane may be a symmetry plane of the exhaust duct and/or a symmetry plane of the intake duct. 
     These aspects enable rationalising the engine design, thus simplifying the die-casting process. 
     According to a different aspect of the invention, the outlet section of the terminal portion of the scavenging duct may be substantially rectangular-shaped. 
     This embodiment enables a more accurate opening/closure of the scavenging duct by the piston sleeve, during the various engine duty cycles. 
     According to a further aspect of the invention, a cross-section of the terminal portion of the scavenging duct, carried out according to a section plane orthogonal to the central axis of the cylinder, may be substantially trapezoidal-shaped. 
     This embodiment enables a better distribution of the air and fuel mixture in the combustion chamber. 
     In particular, the aforementioned cross-section may have a side substantially orthogonal to the median plane 
     This solution facilitates the manufacturing of the engine by means of the die-casting process and it also enables nearing the terminal portion of the scavenging duct to the exhaust opening. 
     According to an aspect of the invention, the engine may comprise at least one pair of said scavenging ducts (which are shaped and arranged in a mutually symmetrical manner with respect to said median plane. 
     This solution enables making the cleaning of the cylinder as well as the loading of the air and fuel mixture into the combustion chamber more uniform and efficient. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Further characteristics and advantages of the invention will be apparent from reading the following description—provided by way of non-limiting example—with reference to the figures illustrated in the attached drawings. 
         FIG. 1  is an axonometric view of a two-stroke internal combustion engine according to an embodiment of the present invention. 
         FIG. 2  is a section of the engine of  FIG. 1  carried out according to a plane section containing the central axis of the cylinder and the rotational axis of the crank shaft. 
         FIG. 3  is the section of  FIG. 2  solely regarding the cylinder head, showed in enlarged scale. 
         FIG. 4  is section IV-IV of  FIG. 3 . 
         FIG. 5  is the detail V of  FIG. 4  shown in enlarged scale. 
         FIG. 6  is section VI-VI of  FIG. 5 . 
         FIG. 7  is an axonometric view of the cylinder head of  FIG. 3  shown capsized. 
     
    
    
     DETAILED DESCRIPTION 
     The aforementioned figures reveal an internal combustion engine  100 , in particular a two-stroke internal combustion engine intended to actuate small work equipment such as for example chainsaws, string trimmers, blowers and the like. 
     The engine  100  comprises an outer body which can be made up of a casing  110  and a cylinder head  115  which is fixed to the casing  110 , for example by means of screws. 
     As shown in  FIG. 2 , inside the casing  110  there is defined a crank chamber  120 , in which there is received a crank  125  suitable to rotate around a pre-set rotational axis A. 
     The crank  125  may be made in a single piece with the rotating shaft  130 , which is generally referred to as a crankshaft and whose axis coincides with the rotational axis A. 
     Inside the cylinder head  115  there is defined a cylinder  135 , which terminates at one end in the crank chamber  120  while it is closed at the opposite end by a head wall  140 . 
     The cylinder  135  extends longitudinally along a central axis B, which can be orthogonal and/or coplanar with the rotational axis A of the crank  125 . 
     Inside the cylinder  135  there is slidably received a piston  145 , which divides the interior volume into two separate chambers, including a combustion chamber  150  defined between the piston  145  and the head wall  140 , and a pumping chamber  155  defined on the opposite side of the piston  145  and communicating with the crank chamber  120 . 
     The separation between the combustion chamber  150  and the pumping chamber  155  may be improved by one or more sealing rings coaxially interposed between the sleeve of the piston  145  and the interior surface of the cylinder  135 . 
     On the head wall  140  of the cylinder  135  there may be installed a spark plug (not illustrated), which may be inserted into a reception hole  105  and it is capable of igniting a spark in the combustion chamber  150 . 
     In the cylinder head  115  there may be provided an intake duct  160 , which is suitable to supply an air and fuel mixture into the pumping chamber  155 . 
     As illustrated in  FIG. 3 , this intake duct  160  may extend longitudinally according to a central axis C which can be coplanar and/or orthogonal to the central axis B of the cylinder  135 . 
     In particular, the intake duct  160  may extend with an almost constant passage section up to an outlet section  165 , which may be substantially rectangular-shaped and can be obtained on interior surface of the cylinder  135 . 
     The air and fuel mixture may be supplied through a conventional carburettor system (not illustrated), which can be connected to the intake duct  160 . The carburettor may have the characteristics explained and described in the introduction. 
     As illustrated in  FIG. 4 , on the cylinder head  115  there may also be obtained an exhaust duct  170 , which is suitable to convey the combustion gases produced in the combustion chamber  150  to the external. 
     This exhaust duct  170  extends longitudinally according to a central axis D which can be coplanar and/or orthogonal to the central axis B of the cylinder  135 . 
     In particular, the exhaust duct  170  may extend with an almost constant passage section starting from an inlet section  175 , which may be substantially rectangular-shaped and can be obtained on interior surface of the cylinder  135 . 
     The inlet section  175  of the exhaust duct  170  is generally at least partly positioned at a greater height, i.e. closer to the head wall  140  of the cylinder  135 , with respect to the outlet section  165  of the intake duct  160 . 
     In addition, in the cylinder head  115  there is also obtained a first pair of scavenging ducts  180 , which can be configured and arranged in a perfectly symmetrical manner with respect to a median plane M containing the central axis B of the cylinder  135  and, in the embodiment illustrated in the attached figures, also containing the central axes C and D respectively of the intake duct  160  and the exhaust duct  170 . 
     Basically, the median plane M may be a symmetry plane not only of the cylinder  135 , but also of the intake duct  160  and/or of the exhaust duct  170 . 
     As illustrated in  FIG. 6 , each of these scavenging ducts  180  comprises a first section  185  exiting from the pumping chamber  155 , generally through the crank chamber  120  (also see  FIG. 2 ), which may extend in the direction substantially parallel to the central axis B of the cylinder  135 , and a second portion  190  terminating in the combustion chamber  150 , which may extend in the direction substantially transverse with respect to the central axis B. 
     In particular, the first portion  185  may be configured as a blind cavity which extends starting from an inlet section  195  towards an opposite closed end. 
     The inlet section may be obtained on a surface of the cylinder head  115  that is orthogonal to the central axis B of the cylinder  135  and which is suitable to remain exposed in the crank chamber  120  when the cylinder head  115  is joined to the casing  110 . 
     The first portion  185  may have a converging configuration in which the area of the passage section, made with respect to a plane orthogonal to the central axis B of the cylinder  135 , decreases progressively starting from the inlet section  195  towards the closed end. 
     The second portion  190  laterally derives from the first portion  185 , for example at the closed end of the latter, and extends up to the inner surface of the cylinder  135 . 
     As illustrated in  FIG. 4 , the second portion  190  has an inlet section  200  which is defined at the intersection between the second portion  190  and the first portion  185 , and an opposite outlet section  205  which is directly obtained on the interior surface of the cylinder  135 . 
     The second portion  190  may have a diverging configuration in which the area of the passage section increases progressively starting from the inlet section  200  towards the outlet section  205 . 
     In particular, the projection of the second portion  190 , i.e. of the inlet section  200  thereof, on the median plane M is fully contained in the projection of the outlet section  205  on the same median plane M. 
     Thus, the second section  190  of the scavenging duct  180  does not have any undercut surface with respect to the median plane M, enabling obtaining the cylinder head  115  through a casting process, for example a die-casting process. 
     In particular, this solution enables obtaining the interior wall of the terminal portion of the scavenging duct by means of a core (or insert) which can be advantageously extracted, at the end of the casting process, through just one straight motion in the perpendicular direction with respect to the median plane M. 
     More in detail, the outlet section  205  of the second portion  190  may be substantially rectangular-shaped and it can be positioned at a higher height, i.e. closer to the head wall  140  of the cylinder  135 , with respect to the outlet section  165  of the intake duct  160 , for example at a height comprised between the outlet section  165  of the intake duct and the inlet section  175  of the exhaust duct  170 . 
     The inlet section  200  of the second portion  190  may be dimensioned according to the bore or displacement capacity of the engine  100  and it can be the narrowest passage section of the entire scavenging duct  180 . 
     In other words, the inlet section  200  may be smaller than the area of all the other passage sections of the scavenging duct  180 , both in the first portion  185  and in the second portion  190 . 
     Thus, the inlet section  200  creates some sort of choke of the scavenging duct  180  hence enabling accelerating the mass of air and fuel that traverses it. 
     As illustrated in  FIG. 5 , a cross-section of the second portion  190  of the scavenging duct  180 , carried out according to a section plane orthogonal to the central axis B of the cylinder  135 , may substantially be perimeter shaped to form trapezium, whose greater base coincides with the profile of the outlet section  205  while the smaller base coincides with the profile of the inlet section  200 . 
     The side of the trapezium proximal to the exhaust duct  170  may be orthogonal to the median plane M or it may be inclined with respect to the orthogonal so as to be non-undercut with respect to the median plane M, for example about 1°. 
     This solution enables maintaining the outlet section  205  of the scavenging duct  180  very close to the inlet section  175  of the exhaust duct  170 , without interference. 
     The side of the trapezium distal from the exhaust duct  170  may instead have a greater inclination than the proximal side, but still in the non-undercut direction with respect to the median plane M. 
     The cross-section of the first portion  185 , carried out according to the same section plane, may be generally rectangular-shaped having a greater side from which the second portion  190  derives. 
     The width of this greater side of the first portion  185  is greater than the width of the inlet section  200  of the second portion  190 , so that between them there is defined a triangular notch projecting on the opposite side with respect to the exhaust duct  170 . 
     The angle at the vertex of this notch, that is, the angle formed by the side of the notch facing the second portion  190  of the scavenging duct  180  and the side of the notch facing the first portion  185 , may be an acute angle, for example comprised between 40° and 70°, preferably equal to 55°. 
     In addition, the side of the notch facing the first portion  185  of the scavenging duct  180  may have a substantially arched section profile with its center on the central axis B of cylinder  135 , or may have a substantially rectilinear section profile but tangent to an imaginary circumference centered on the central axis B of cylinder  135 . 
     Back to  FIG. 4 , in the cylinder head  115  of the engine  100  may also be obtained in a second pair of scavenging ducts  210 , which are arranged distant from the exhaust duct  170  with respect to the scavenging ducts  180 . 
     Even the scavenging ducts  210  may be configured and arranged in a perfectly symmetrical manner with respect to the median plane M. 
     Each of these further scavenging ducts  210  may have shapes and dimensions partly different from the scavenging ducts  180  but they reproduce all previously described technical characteristics thereof, which will not be repeated herein but will still be deemed valid also as concerns the second scavenging ducts  210 . 
     When the engine  100  is running, the fresh air and fuel mixture coming from the intake duct  160  is intaken into the pumping chamber  155  due to the depression created by the piston  145  each time the latter performs an ascent stroke towards the head wall  140  which closes the cylinder  135  (see  FIG. 2 ). 
     While the piston  145  performs this ascent stroke, the air and fuel mixture already present in the combustion chamber  150  is simultaneously compressed. When the piston  145  is in proximity of the top dead centre, i.e. the position where the volume of the combustion chamber  150  is minimum, the spark plug is controlled to generate a spark that ignites the combustion of the air and fuel mixture. 
     Upon generating the spark, the combustion of the air and fuel mixture produces exhaust gases in rapid expansion that push the piston  145  to perform a descent stroke moving away from the head wall  140  of the cylinder  135 . 
     During this descent stroke, the piston  145  firstly opens the inlet section  175  of the exhaust duct  170  to enable the exhaust gases to flow out towards the external environment, then it opens the outlet section  205  of the scavenging ducts  180  and  210 , and then it closes the outlet section  165  of the intake duct  160 . 
     Thus, in the last part of the descent stroke, the piston  145  pumps the mixture previously intaken into the pumping chamber  155  into the scavenging ducts  180  and  210  and from there to the combustion chamber  150 . 
     Thanks to the particular configuration of the second portion  190  of the scavenging ducts  180 , this mixture flow is accelerated at the choke defined by the inlet section  200  and projected into the combustion chamber  150  at a high speed. 
     In particular, it was observed that the mixture flow maintains a compact front and strong directionality, separating from the diverging walls of the second portion  190  of the scavenging duct  180 . 
     Thus, the mixture flow penetrates deeply into the combustion chamber  50 , reducing the amount of fuel that could flow out directly from the exhaust duct  170  unburnt. 
     Upon reaching the bottom dead centre position, i.e. the position where the volume of the combustion chamber  150  is maximum, the piston  145  starts a new ascent stroke. 
     During this ascent stroke, the piston  145  firstly closes the outlet section  205  of the scavenging ducts  180  and  210  and the exhaust duct  170  and then progressively reduces the volume of the combustion chamber  150 , compressing the air and fuel mixture contained therein, so that the cycle can restart anew. 
     Obviously, the engine  100 , as described above, may be subjected—by a man skilled in the art—to numerous technical/application modifications, without departing from the scope of protection of the invention as claimed below.