Abstract:
The invention related to a two-stroke internal combustion engine ( 1 ), comprising a crankcase ( 2 ), which has an air intake ( 3 ), and at least one cylinder ( 4 ), which is connected to the crankcase ( 2 ) and which, together with a piston ( 5 ) arranged therein, defines a combustion chamber ( 6 ). The cylinder ( 4 ) has at least one air inlet ( 7 ), which via an air duct ( 8 ) communicates with the crankcase ( 2 ), in which a mixture of air and lubricating oil and fuel, if any, is contained. The air duct ( 8 ) is connected to the crankcase ( 2 ) via a separator ( 10 ) for cleaning of air coming from the crankcase ( 2 ) from lubricating oil and fuel, if any. The separator ( 10 ) has a centrifugal chamber ( 11 ), in which the lubricating oil and the fuel, if any, in a circulating motion are thrown outwards to a circumferential wall ( 12 ) and thus substantially cleaned air is passed, through a chamber outlet ( 13 ), which with respect to said circulating motion is a radially inner outlet, to said air duct ( 8 ) to be supplied to the combustion chamber ( 6 ) through the air inlet ( 7 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a two-stroke internal combustion engine, comprising a crankcase, which has an air intake, and at least one cylinder, which is connected to the crankcase and which, together with a piston arranged therein, defines a combustion chamber. The cylinder has at least one air inlet, which via an air duct communicates with the crankcase, in which a mixture of air and lubricating oil and fuel, if any, is contained. 
       BACKGROUND ART 
       [0002]    U.S. Pat. No. 4,481,910 discloses a two-stroke combustion engine as described by way of introduction. In this engine, what is referred to as stratified charge is provided by supplying to the combustion chamber more or less clean air and a fuel-air mixture from a carburettor which is connected to the crankcase. The carburettor comprises a valve, which opens at a negative pressure in the crankcase and closes at a positive pressure in the same. When a positive pressure occurs in the crankcase, fuel-air mixture is forced out of the crankcase through a short suction duct and a long air duct. Since, during a preceding engine cycle with a negative pressure in crankcase, the air duct has been filled with clean air through an extra air shutter, almost clean air is forced through the air inlet into the cylinder of the engine and, in this way, the cylinder is scavenged with a minimum of fuel leakage. This results in improved fuel economy as well as cleaner exhaust gases. 
       OBJECT OF THE INVENTION 
       [0003]    The stratified-charged engine, which is known from U.S. Pat. No. 4,481,910, suffers from two drawbacks which the invention is to eliminate. The first drawback is that the prior art solution makes cold starting difficult, requiring a rich fuel-air mixture since the extra air shutter takes in outdoor air which is quite unaffected by a possibly richer fuel-air mixture in the crankcase. The second drawback is that the prior art solution is relatively bulky. This is due not only to the function-related long air duct but also to the air, which flows into it through the extra air shutter, first having to be cleaned. This requires either an extra air filter in addition to an ordinary air filter for the engine, or an extra air conduit from an ordinary air filter to said air shutter. 
         [0004]    Taken together, this makes the engine according to U.S. Pat. No. 4,481,910 less suitable for use as a drive unit especially in machines, in which weight and outer dimensions are important, such as in hand-held power tools. The object of the invention is to eliminate the drawbacks of the prior art solution while at the same time the advantages thereof are maintained. 
       SUMMARY OF THE INVENTION 
       [0005]    According to the invention, this object is achieved by a two-stroke internal combustion engine as described by way of introduction by the air duct being connected to the crankcase via a separator for cleaning of air coming from the crankcase from lubricating oil and fuel, if any, said separator having a centrifugal chamber, in which the lubricating oil and the fuel, if any, in a circulating motion are thrown outwards to a circumferential wall and thus substantially cleaned air is passed, through a chamber outlet, which with respect to said circulating motion is a radially inner outlet, to said air duct to be supplied to the combustion chamber through the air inlet. 
         [0006]    The invention thus makes it possible to run the engine with a single air intake and use a short air duct for the air to the cylinder. This means that only one ordinary air filter is required, and no extra air conduit, and also that the air duct between the crankcase and the cylinder can be very short. 
         [0007]    The centrifugal chamber of the separator suitably has, with respect to said circulating motion, an axially first end portion, which by an inlet duct is connected to the crankcase. The connection at an end portion of the centrifugal chamber significantly facilitates efficient production of said circulating motion in the same, particularly if the inlet duct also connects substantially tangentially to the centrifugal chamber. Then a mixture of air and lubricating oil and fuel, if any, flowing into the chamber is in a very natural way put in a circulating motion along the circumferential wall of the chamber, and in a radially inner portion of the chamber a very lean fuel-air mixture is obtained to be discharged through the radially inner chamber outlet. 
         [0008]    In an alternative embodiment, the inlet duct, however, connects substantially axially to the centrifugal chamber. Here a turbulator is arranged in the first end portion of the centrifugal chamber so that a mixture of air and lubricating oil and fuel, if any, which flows into the chamber, is put in the required circulating motion along the circumferential wall of the chamber. 
         [0009]    In a variant of the engine with fuel injection, in which variant there is no fuel in the crankcase, the inlet duct is arranged to drain, by gravity, said lubricating oil back to the crankcase. 
         [0010]    Preferably, the centrifugal chamber of the separator has, with respect to said circulating motion, an axially second end portion, in which the radially inner chamber outlet is formed. This solution adds to the compact design of the centrifugal chamber and can be further improved if a collar extends into the chamber from said second end portion, which collar surrounds the radially inner chamber outlet. The collar effectively prevents oil and fuel, if any, from flowing along a chamber end wall into this chamber outlet. 
         [0011]    In a variant of the engine, in which variant there is fuel in the crankcase, said second end portion has, with respect to said circulating motion, a radially outer chamber outlet, which by a suction duct is connected to a suction port in the cylinder, which suction duct, due to the dynamics of liquids, suitably connects substantially tangentially to the centrifugal chamber. 
         [0012]    The centrifugal separator which is employed according to the invention can thus be efficiently used both for supply of air or a very lean, fuel-air mixture to the air inlet of the engine and a rich and, thus, ignitable fuel-air mixture to the suction port of the engine. 
         [0013]    In that case, air inlets are preferably formed in the cylinder on both sides of the suction port, which air inlets are connected by air ducts to at least one separator. This is advantageous since a plurality of air inlets can significantly improve the so-called scavenging of the combustion chamber, which promotes considerably cleaner exhaust gases. 
         [0014]    The air ducts are suitably each connected to a separator, which by a suction duct of its own is connected to one and the same suction port. Such a solution involving preferably two separators allows an extremely compact engine design with separators on both sides of the crank mechanism in the crankcase. 
         [0015]    In a variant of the engine, in which variant fuel is not supplied to the cylinder through the separator, said second end portion has, with respect to said circulating motion, a radially outer chamber outlet, which by a return duct is connected to the crankcase. In this way, separated oil will be effectively returned to the crankcase. Preferably the return duct has an opening, which in the crankcase is formed in an eccentric part of a crank mechanism. In this way the rotation of the crank mechanism will throw the mixture of oil and air radially outwards from the centre of the crank mechanism and, thus, produce suction in the return duct which promotes increased return of oil to the crankcase. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention will in the following be described in more detail with reference to the accompanying schematic drawings, in which 
           [0017]      FIG. 1  shows, with partly broken-away portions, an engine according to the invention in a variant with a carburettor; 
           [0018]      FIG. 2  shows, with partly broken-away portions, an engine according to the invention in a variant with direct injection; 
           [0019]      FIG. 3  shows, with partly broken-away portions, an engine according to the invention in a variant with return of oil to the crankcase; and 
           [0020]      FIG. 4  shows, with partly broken-away portions, parts of an engine according to the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    For simplified reading, equivalent components are provided with the same reference numerals, FIGS.  1 , 2  and  3  showing two-stroke internal combustion engines  1  which have a crankcase  2  and a cylinder  4  connected to the crankcase  2 , and  FIG. 4  showing a cylinder  4  intended for such an engine  1 . 
         [0022]    The crankcase  2  has at the bottom (with reference to the situation shown in the drawings) an air intake  3 , which by means of a valve flap  22  is opened at a negative pressure in the crankcase  2  and is closed at a positive pressure in the same. In the crankcase  2  there is in prior art manner a crank mechanism  23 , which is connected to a piston  5 . 
         [0023]    The piston  5  is slidingly arranged in the cylinder  4  and, during sliding, opens and closes the openings that are formed in the cylinder  4 . The openings are an exhaust gas port  24 , air inlets  7  and, in the engine  1  in  FIG. 1 , a suction port  21 . 
         [0024]    The exhaust gas port  24  is in prior art manner connected to an exhaust pipe (not shown). The other openings are, however, in a way unique to the invention connected to a separator  10 , which will be described below in more detail first for the engine variant shown in  FIG. 1  and subsequently for the engine variant shown in  FIG. 2 . 
         [0025]    The engine  1  in  FIG. 1  is a two-stroke engine and is supplied with fuel by means of a carburettor  25 . The carburettor  25  is connected to the air intake  3  and supplied with lubricated fuel through a fuel line  26 . When in a first engine cycle (upward stroke of the piston  5 ) a negative pressure occurs in the crankcase  2 , lubricated fuel, together with air, is sucked into the crankcase  2  and forms a mist therein consisting of air and atomised droplets of fuel and lubricating oil. When in a second engine cycle (downward stroke of the piston) a positive pressure occurs in the crankcase  2 , the mist is forced out of the crankcase  2  through an inlet duct  15  to the separator  10 . 
         [0026]    In the embodiment in  FIG. 1 , the separator  10  comprises a circular-cylindrical centrifugal chamber  11 , which at the bottom has an axially first end portion  14  and at the top has an axially second end portion  17 , axially of course referring to the axial direction of the centrifugal chamber  11 . Between the end portion  14 ,  17  extends the circumferential wall  12  of the chamber  11  in the form of a smooth homogeneous cylinder wall surface. 
         [0027]    The inlet duct  15  connects substantially tangentially to the centrifugal chamber  11 , which, as is evident from  FIG. 1 , does not prevent the inlet duct  15  from reaching the chamber  11  at an angle other than perpendicular to the axial direction of the chamber  11 . The mist flowing into the centrifugal chamber  11  is, due to the tangential connection, put in a circulating motion inside the chamber  11 , the heavier fuel and oil droplets being thrown by centrifugal forces outwards to the circumferential wall  12  of the chamber  11  and the lighter air remaining in the centre of the chamber  11 . 
         [0028]    It will be appreciated that the thus provided separation of fuel/oil and air is most distinct at a distance from the inlet duct  15 , that is adjacent the axially second portion  17  of the centrifugal chamber  11 . In this portion  17 , two outlets are arranged, a radially inner or axial chamber outlet  13  and a radially outer chamber outlet  19 . 
         [0029]    The radially outer chamber outlet  19  is connected to a suction duct  20 , which first extends substantially tangentially away from the centrifugal chamber  11  along an upward curve. This is a natural continuation of a helical line which extends helically along the circumferential wall  12  of the chamber  11  and which starts in the inlet duct  15  which is substantially tangential to the chamber  11 . The other end of the suction duct  20  opens in the cylinder  4  adjacent the above-mentioned suction port  21 , which, due to the separation of fuel/oil and air, is reached by a fairly rich, and thus ignitable, fuel-air mixture. 
         [0030]    The radially inner or axial chamber outlet  13  is connected to an air duct  8 , which extends away from the centrifugal chamber  17  in a direction that is substantially axial thereto. The air duct  8  has at its chamber opening a collar  18  which extends into the chamber  11  and which is adapted to serve as a further barrier against fuel and oil. The other end of the air duct  8  connects to said air inlet  7  in the cylinder  4 . In the version shown, two such air inlets  7  are arranged, of which, however, the front one in the figure is broken away and indicated only by a branch of the air duct  7 , which branch is shown in the form of a dashed oval. 
         [0031]    In operation of the engine  1  in  FIG. 1 , the described separator  10  has the function of supplying to the cylinder chamber  6  through the suction port  21  a rich fuel-air mixture which can easily be ignited by a sparking plug  27  in the top of the cylinder  4  while simultaneously supplying through the air inlets  7  a very lean fuel-air mixture, which helps, with very small fuel losses, to flush out exhaust gases from a preceding combustion cycle through the exhaust gas port  24 . 
         [0032]    The engine  1  in  FIG. 2  is also of a two-stroke type. In contrast to the engine in  FIG. 1 , in which the fuel-air mixture is ignited by means of a sparking plug  27 , the engine  1  in  FIG. 2  operates, however, with compression ignition. For this purpose, there is in the top of the cylinder  4  a fuel injector  28 , which is arranged to atomise, in a prior art manner, fuel supplied through a fuel line  26 . 
         [0033]    Analogously to the solution in  FIG. 1 , there is however at the bottom of the crankcase (referring to the situation shown in the drawings) an air intake  3 , through which air is sucked into the engine  1  during a first engine cycle (upward stroke of the piston  5 ) when a negative pressure occurs in the crankcase  2 . In the engine  1  in  FIG. 2 , this air is, however, not mixed with fuel, but only to a certain extent with lubricating oil, which in a manner that is known and therefore not shown is injected into the crankcase for lubricating the engine  1 . In the next engine cycle (downward stroke of the piston) a positive pressure occurs in the crankcase  2  and a mist of air and lubricating oil is forced out of the crankcase  2  through an inlet duct  15  to the separator  10 . 
         [0034]    Also in the embodiment in  FIG. 2 , the separator  2  comprises a circular-cylindrical centrifugal chamber  11 , which at the bottom has an axially first end portion  14  and at the top an axially second end portion  17 , axially once more relating to the axial direction of the centrifugal chamber  11 . Between the end portions  14 ,  17  extends the circumferential wall  12  of the chamber  11  in the form of a smooth homogeneous cylinder wall surface, which continuously passes into an axial inlet duct  15 , which is also circular-cylindrical and opens in the crankcase  2 . 
         [0035]    To produce a desirable circulating motion along the circumferential wall  12  of the chamber  11 , the inlet duct  15  in  FIG. 2  comprises a turbulator  16 , which in arranged in the axially first end portion  14  of the centrifugal chamber  11  and substantially consists of a number of inclined guide plates. The circulating motion causes the heavier oil droplets to be thrown outwards by centrifugal forces to the circumferential wall  12 , while the lighter air remains in the centre of the chamber  11 . 
         [0036]    The thus provided separation of oil and air increases with the distance from the inlet duct  15 , that is closer to the axially second portion  17  of the centrifugal chamber  11 . In this portion  17  there is in the embodiment in  FIG. 2  a radially inner or axial chamber outlet  13  and, around the same, only a circular closed chamber end wall. Separated lubricating oil is collected at this end wall, forms an oils film and flows down along the circumferential wall  12  of the centrifugal chamber  11  and the inlet duct  15  back to the crankcase  2 . 
         [0037]    The radially inner or axial chamber outlet  13  is, in the embodiment in  FIG. 2 , connected to an air duct  8 , which extends away from the centrifugal chamber  11  in a direction that is substantially axial to the same. Again the air duct  8  has, at its chamber opening, a collar  18 , which extends into the chamber  11  and is adapted to serve as a further barrier, in this case only against oil. The other end of the air duct  8  is branched and connects to a plurality of air inlets  7  in the cylinder  4 , of which a first is positioned opposite the exhaust gas port  24  and the others between the exhaust gas port  24  and the first air inlets  7 . (Once more, the air inlet is not to be seen in the broken-away part of the cylinder  4 ). 
         [0038]    In operation of the engine  1  in  FIG. 2 , the described separator  10  has the function of supplying to the cylinder chamber  6  through the air inlets  7  a very lean mixture consisting of air and, due to the separation in the separator, an extremely small amount of lubricating oil. The fuel, such as diesel, is in this engine  1  supplied by means of the fuel injector  28  and ignited when, during an upward stroke of the piston  5  in the cylinder chamber  6 , it reaches by compression a temperature which exceeds the autoignition temperature of the fuel. Before the piston  7  reaches this ignition position, exhaust gases from a preceding combustion cycle will, however, be flushed out through the exhaust gas port  24  with very small oil losses. 
         [0039]    The engine  1  in the embodiment in  FIG. 3  is of essentially the same type as the engine in  FIG. 2 , but is, in contrast to the engine in  FIG. 2 , provided with a separator  10 , which in the same way as in the engine  1  in  FIG. 1 , is provided with a radially outer chamber outlet  19 . The chamber outlet  19  is connected to a return duct  30 , which is adapted to return a mixture of oil and air to the crankcase  2 . 
         [0040]    The return duct  30  extends through the material of the cylinder  4  and the crankcase  2  to a first bearing for a first end of the crank shaft  31  included in the crank mechanism  23 . The return duct  30  further extends along a path in the crank shaft  31  to an opening  32  toward the crankcase  2  positioned on a portion of the boundary surface of the crank shaft  31 , said portion being eccentrically located relative to the axis of rotation of the crank shaft  31 . In other words, the opening  32  may, as illustrated, be formed in a balancer  33  on the crank shaft  31  or in a crank arm  34  opposite the balancer  33 . It will be appreciated that the returning effect increases with an increasing radial distance between the opening  32  of the return duct  30  in the crankcase  2  and the axis of rotation of the crank shaft  31 . 
         [0041]    Of course, the embodiments described above may be varied in different ways with the scope of the claims. For instance in an alternative to the separator  10  in  FIG. 3 , the return duct can thus be eliminated, and instead a return flow can be used in the same way as shown for the embodiment in  FIG. 2 , but through an inlet duct  15  of the type shown in  FIGS. 1 and 3 . Correspondingly, for instance also the separator  10  in  FIG. 2  may be used for the engine  1  in  FIG. 1  or  3 , although, of course, supplemented with a suction duct  20  or return duct  30  connected to the separator  10 . 
         [0042]    In addition, it is conceivable to arrange in all separators  10  according to the invention a turbulence-damping means in the air duct  8 , for instance in the form of small vanes  35  in the vicinity of the inlet  13  of the air duct  8  (see  FIG. 3 ). The turbulence-damping means promotes more favourable flow conditions in the air duct  8  and the subsequent cylinder chamber  6  and, thus, a marginally improved efficiency of the engine  1 . 
         [0043]    It will be appreciated that it is also possible to arrange more than one separator  10 , such as one on each side of the crankcase  3 , thus on both sides of the crank mechanism  23 . This is illustrated in  FIG. 4 , in which a solution with two separators  10  is shown, in section along a parting line across the cylinder  4 . The separators  10  shown in  FIG. 4  are of the same type as the separators in  FIGS. 1 and 3 , but have, in contrast to them, an axis A which is lying relative to the cylinder  4  and, besides, is curved around the cylinder chamber  6 , so that an extremely compact solution is obtained. 
         [0044]    It goes without saying that it is also possible to design the air intake  3  of the engine  1  in a manner other than shown and, if desired, arrange a plurality of air inlets  7  and suction ports  2 .