TWO-STROKE INTERNAL COMBUSTION ENGINE

Abstract: Crankcase scavenged two-stroke internal combustion engine (1) in which a piston ported air passage is arranged between an air inlet (2) and the upper part of a number of transfer ducts (3, 3'). The air passage is arranged from an air inlet (2) equipped with a restriction valve (4), controlled by at least one engine parameter, for instance the carburettor throttle control. The air inlet extends via at least one connecting duct (6, 6') to at least one connecting port (8, 8') in the engine's cylinder wall (12). The connecting port (8, 8') is arranged so that it in connection with piston positions at the top dead centre is connected with flow paths (10, 10') embodied in the piston (13), which extend to the upper part of a number of transfer ducts (3, 3'). Each flow path through the cylinder and piston is to a great extent arranged in the cylinder's lateral direction, on the one hand in that the connecting port (8, 8') and adjacent scavenging port (31, 31') of the cylinder are shifted sideways in relation to each other along the periphery of the cylinder wall (12), and on the other hand in that the transfer ducts (3, 3') of the cylinder are running essentially in the cylinder's lateral direction away from each transfer port (31, 31') respectively, i.e. tangentially in relation to the circumference of the cylinder wall (12).

Description: TWO-STROKE INTERNAL COMBUSTION ENGINE Technical field The subject invention refers to a two-stroke crankcase scavenged internal combustion engine, in which a piston ported air passage is arranged between an air inlet and the upper part of a number of transfer ducts. Fresh air is added at the top of the transfer ducts and is intended to serve as a buffer against the air/fuel mixture below. Mainly this buffer is lost out into the exhaust outlet during the scavenging process. The fuel consumption and the exhaust emissions are thereby reduced. The engine is foremost intended for a handheld working tool. Background of the invention Combustion engines of the above mentioned kind are known. They reduce the fuel consumption and exhaust emissions, but it is difficult to control the air/fuel ratio in such an engine. US 5,425,346 shows an engine with a somewhat different design than the above mentioned. In this case, channels are arranged in the piston of the engine, which at specific piston positions are aligned with ducts arranged in the cylinder. Fresh air, as shown in figure 7, or exhaust gases can thereby be added to the upper part of the transfer ducts. This only happens at the specific piston positions where the ducts in the piston and the cylinder are aligned. This happens both when the piston moves downwards and when the piston moves upwards far away from the top dead centre. To avoid unwanted flow in the wrong direction in the latter case, check valves are arranged at the inlet to the upper part of the transfer ducts. The amount of fresh air that can be added is utmost limited owing to the short total time or period for the supply of air and the flow resistance in the check valves. This type of check valves, usually called reed valves, has however a number of other disadvantages. They have frequently a tendency to come into resonant oscillations and can have difficulties to cope with the high rotational speeds that many two-stroke engines can reach. Besides, it results in added cost and increased number of engine components. The amount of fresh air added is varied by means of a variable inlet, i.e. an inlet that can be advanced or retarded in the work cycle. This is however a very complicated solution. The international patent application W098/57053 shows a few different embodiments of an engine where air is supplied to the transfer ducts via L-shaped or T-shaped recesses in the piston. Thus, there are no check valves. In all embodiments the piston recess has, where it meets the respective transfer duct, a very limited height, which is essentially equal to the height of the actual transfer port. A consequence of this embodiment is that the passage for the air delivery through the piston to the transfer port is opened significantly later than the passage for the air/fuel mixture to the crankcase is opened by the piston. The period for the air supply is consequently significantly shorter than the period for the supply of air/fuel mixture, where the period can be counted as crank angle or time. This could complicate the control of the total air-fuel ratio of the engine. This also means that the amount of air that can be delivered to the transfer duct is significantly limited, since the underpressure driving this additional air has decreased a lot, because the inlet port has already been open during a certain period of time when the air supply is opened. This implies that both the period and the driving force for the air supply are small. Furthermore, the flow resistance in the L-shaped and the T-shaped ducts as shown becomes relatively high, partly because the cross section of the duct is small close to the transfer port and partly because of the sharp bend created by the L-shape or T-shape. When the air has just passed through the transfer port it is forced to change direction abruptly away from the lateral direction of the cylinder to instead follow the transfer duct outwards and then downwards, i.e. two curves of 90° in rapid succession. This is due to the fact that the transfer ducts of the engine are running in a radial direction to the cylinder. In all, this contributes to increasing the flow resistance and to reducing the amount of air that can be delivered to the transfer ducts, which reduces the possibilities to reduce the fuel consumption and the exhaust emissions by means of this arrangement. The purpose of the invention The purpose of the subject invention is to significantly reduce the above-mentioned problems and to achieve advantages in many respects. Summary of the invention The above-mentioned purpose is achieved by a two-stroke combustion engine in accordance with the invention showing the characteristics of the appended patent claims. The combustion engine in accordance with the invention is thus essentially characterized in that the air passage is arranged from an air inlet equipped with a restriction valve, controlled by at least one engine parameter, e.g. the carburettor throttle control, the mentioned air inlet is via at least one connecting duct channelled to at least one connecting port in the cylinder wall of the engine, which is arranged so that it, in connection with piston positions at the top dead centre, is connected with flow paths embodied in the piston, which extend to the upper part of a number of transfer ducts, and each flow path in the cylinder and piston is to a great extent arranged in the cylinder's lateral direction in that, on the one hand the connecting port and adjacent scavenging port of the cylinder are shifted sideways in relation to each other along the periphery of the cylinder wall, and on the other hand the transfer ducts of the cylinder are essentially running in the cylinder's lateral direction away from each scavenging port respectively, i.e. tangentially in relation to the circumference of the cylinder wall. By this arrangement a flow of air through the cylinder with very few and moderate curves is achieved, so that the flow resistance becomes low. Because at least one connecting port in the engine's cylinder wall is arranged so that it in c jnnection with piston positions at the top dead centre is connected with flow paths embodied in the piston, the supply of fresh air to the upper part of the transfer ducts can be arranged entirely without check valves. This can take place because that at piston positions at or near the top dead centre there is an underpressure in the transfer duct in relation to the ambient air. Thus a piston ported air passage without check valves can be arranged, which is a big advantage. Because the air supply has a very long period, a lot of air can be delivered, so that a very high exhaust emissions reduction effect can be achieved. Control is applied by means of a restriction valve in the air inlet, controlled by at least one engine parameter. Such control is of a significantly less complicated design than a variable inlet. The air inlet has preferably two connecting ports, which in one embodiment are so located that the piston is covering them at its bottom dead centre. The restriction valve can suitably be controlled by the engine speed, alone or in combination with another engine parameter. These and other characteristics and advantages are clarified in the detailed description of the different embodiments, supported by the enclosed drawing figures. Brief description of the drawing The invention will be described in closer detail in the following by means of various embodiments thereof with reference to the accompanying drawing figures. For parts that are symmetrically located on the engine, the part on the one side has been given a numeric designation while the part on the opposite side has been given the same designation but with a '-symbol. In the drawings the parts with a '-symbol are located above the plane of the paper and are therefore not visible. Figure 1 shows a side view of an engine according to the invention. The cylinder is shown in a cross section, as well as the piston, which is shown at the top dead centre. Description of embodiments In figure 1, numeral reference 1 designates an internal combustion engine according to the invention. It is of two-stroke type and has transfer ducts 3, 3'. The latter is not visible since it is located above the plane of the paper. The engine has a cylinder 15 and a crankcase 16, a piston 13 with a connecting rod 17 and a crank mechanism 18. Furthermore, the engine has an inlet tube 22 with an inlet port 23 and an, to the inlet tube connected, intermediate section 24, which in turn connects to a carburettor 25 with a throttle valve 26. Fuel 37 is supplied by way of the carburettor. Usually the carburettor connects to an inlet muffler with a filter. These are not shown for the sake of clarity. The same applies for the exhaust port, the exhaust duct and the muffler of the engine. They are totally conventional and located on the opposite side of the cylinder compared to the inlet. The piston has a plane upper side without any step or similar, so that it co-operates equally with the cylinder ports wherever they are located around the periphery. The height of the engine body is therefore approximately unchanged in comparison with a conventional engine. The transfer ducts 3 and 3' have ports 31 and 31 ' in the engine's cylinder wall 12. The engine has a combustion chamber 32 with a spark plug, which is not shown. All of this is conventional and is therefore not further commented. What is special is that an air inlet 2 equipped with a restriction valve 4 is arranged so that fresh air can be supplied to the cylinder. The air inlet 2 has a connecting duct 6 channelled to the cylinder, which is equipped with an outer connecting port 7. By connecting port is from now on meant the port of the connection on the inside of the cylinder, while its port on the outside of the cylinder is called outer connecting port. The air inlet 2 suitably connects to an inlet muffler with a filter, so that cleaned fresh air is taken in. If the requirements are lower, this is of course not necessary. The inlet muffler is not shown for the sake of clarity. A connecting port 6 is thus connected to an outer connecting port 7. This is an advantage. At or after this port the duct divides into two branches 11, 11 ' which lead to a connecting port 8, 8' each. These are located symmetrically and the parts with a '-symbol are as mentioned lying above the plane of the paper. The outer connecting port 7 is thus located under the inlet tube 22, which means a number of advantages such as lower air temperature and a better utilizing of space for a handheld working tool, which usually has a fuel tank 33 located as shown in the figure. However, the outer connecting port 7 could also be located above the inlet tube 22, which then is directed more horizontally. Wherever they are located two outer connecting ports 7, 7' could be used. They could then also be located on each side of the inlet tube 22. Flow paths 10, 10' are arranged in the piston so that they, in connection with piston positions at the top dead centre, connect the respective connecting port 8, 8' to the upper part of the transfer ducts 3, 3 '. The flow paths 10, 10' are made by means of local recesses in the piston. The piston is simply manufactured, usually cast, with these local recesses. Usually the connecting ports 8, 8' are so located in the axial direction of the cylinder that the piston covers them when it is located at its bottom dead centie. Thereby exhaust gases cannot penetrate into the connecting port and further on towards an eventual air filter. But it is also possible that the connecting ports 8, 8' are located so high up that they to some part are open when the piston is located at its bottom dead centre. This is adapted so that a desirable amount of exhaust gases will be supplied into the connecting duct 6. A highly located connecting port could also reduce the flow resistance of air at the changeover from connecting port to scavenging port 31. The period of air supply from the connecting ports 8, 8' to the scavenging port 31, 31 ' is very important and is to a great extent determined by the flow paths in the piston, i.e. the recess 10, 10' in the piston. Preferably the upper edge of the recess is located so high that it, when the piston is moving upwards from the bottom dead centre, reaches up to the lower edge of the respective port 31, 31 ' at the same time or earlier than the lower edge of the piston reaches up to the lower edge of the inlet port. Thereby the air connection between the connecting ports 8, 8' and the scavenging ports 31, 31 ' is opened at the same time or earlier than the inlet is opened. When the piston moves down again after being at the top dead centre then also the air connection will be shut off at the same time or later than the inlet. Thereby the air supply has an essentially equally long or longer period than the inlet has, counted in crank angle or in time. This will reduce its flow resistance. Often it is desirable that the inlet period and the air period are essentially equally long. Preferably the air period should be 90 % - 1 10 % of the inlet period. Because both these periods are limited by the maximum period during which the pressure is sufficiently low in the crankcase to enable a maximal inflow. Both periods are preferably maximised and equally long. The position of the upper edge of the recess 10, 10' will thus determine how early the recess will connect with each scavenging port 31, 31 ' respectively. Consequently, preferably the recess 10, 10' in the piston that meets each port 31, 31 ' respectively, has an axial height locally at this port that is greater than 1,5 times the height of the respective scavenging port, but preferably greater than 2 times the height of the scavenging port. This provided that the port has a normal height so that the upper side of the piston, when located in its bottom dead centre, is level with the underside of the scavenging port, or is protruding only a few millimetres. The recess is preferably downwards shaped in such a way that the connection between the recess 10, 10' and the connecting port 8, 8' is maximised, since it reduces the flow resistance. This means that when the piston is located at its top dead centre, the recess 10, 10' preferably reaches so far down that it does not cover the connecting port 8, 8' at all, as shown in figure 1. As a whole, this means that the recess 10, 10' in the piston that meets each connecting port 8, I' respectively, has an axial height locally at this port that is greater than 1.5 times the height of the respective connecting port, but preferably greater than 2 times the height of the connecting port. The relative location of the connecting port 8, 8' and the scavenging port 31, 31' can be varied considerably provided that the ports are shifted sideways, i.e. in the cylinder's tangential direction, as shown in figure 1. Figure 1 illustrates a case where the connecting port and the scavenging port 31, 31' have an axial overlap, i.e. that the upper edge of each connecting port respectively is located as high or higher in the cylinder's axial direction as the lower edge of each scavenging port respectively. One advantage is that the two ports are more aligned with each other in an arrangement of this kind, which reduces the flow resistance when air is being transported from the connecting port to the scavenging port. Consequently, more air can be transported, which can enhance the positive effects of this arrangement, i.e. reduced fuel consumption and exhaust emissions. For many two-stroke engines, the piston's upper side is level with the lower edge of the exhaust outlet and the lower edge of the scavenging port, when the piston is at its bottom dead centie. However, it is also quite common for the piston to extend a millimetre or a few above the scavenging port's lower edge. If the lower edge of the scavenging port is further lowered, an even greater axial overlap will be created between the connecting port and the scavenging port. When air is supplied to the scavenging duct, the flow resistance is now reduced, both due to that the ports are more level with each other and also due to the greater surface area of the scavenging port. Above is pointed out the importance of having a long period of air supply in order to achieve a low flow resistance at the changeover between cylinder and piston. Furthermore is pointed out the advantage that the connecting port is located as high or higher in the cylinder's axial direction as the lower edge of each scavenging port respectively. This provided that the connecting port/scavenging port are shifted sideways in relation to each other along the periphery of the cylinder wall. Hereby the transition from port 8 to port 31 via the piston can occur in a slightly upward direction in relation to the cylinder's lateral direction. If the port 8 instead had been located right below port 31, then the transition had occurred in a straight upward direction. The result had been that the flow would at first turn upwards and then after reaching the scavenging port turn into a horizontal direction, i.e. two sharp turns in succession. Owing to the fact that the ports are shifted sideways this enables a slightly upwards flow with small turns. As mentioned it is a big advantage that the transfer ducts 3, 3' are arranged essentially in the cylinder's lateral direction. The result will be that the slightly upwards flow from port 8 to port 31 will turn slightly and then continue in a straight lateral direction out in the transfer duct. In the shown case the transfer duct is thus running in the cylinder's lateral direction until it is in height level with the cylinder wall where a soft turn takes place, so that the transfer duct connects to the crankcase where it has its mouth 38. Preferably each branch 11, 11' leading to each connecting port 8, 8' respectively is embodied so that it is directed in the cylinder's lateral direction, or slightly upwards from this. Hereby the advantageous main flow direction, which is arranged through the cylinder and piston, is emphasized. In the shown embodiment each branch arrives obliquely from below from an outer connecting port 7, so that the branch first turns upwards after the outer connecting port and then continues upwards and turns into a lateral direction up to the connecting port 8, 8' in the cylinder wall 12. At the transition from cylinder to piston is therefore created a slightly upward direction of the flow which then will be slightly interrupted and turn into a straight lateral flow direction in the transfer duct. Since the connecting port 8 must be located at a lower level than each scavenging port 31 respectively, this is a natural arrangement. But it is also possible to place one or two outer connecting ports 7 above the inlet 22-25. If so, this inlet is preferably angled more in the cylinder's lateral direction than in the shown case. In this case this could be arranged so that each branch 11, 11' is directed essentially in the cylinder's lateral direction up to each connecting port 8, 8' respectively. We are considering that we can see the flow from above as from the outer connecting port 7 to the connecting port 8 and over to the scavenging port 31 and further on to the transfer duct 3. Then it becomes apparent that the transfer duct 3 up to the scavenging port 31 is rtinning in an essentially tangential direction in relation to the cylinder and the same condition is to a great extent also valid for the first part of the branch 11 from the connecting port 8. Thereby the changes of direction will become small when the air passes from the branch 11 over to the piston recess 10 and into the transfer duct 3.

Claims: PATENT CLAIMS 1. Crankcase scavenged two-stroke internal combustion engine (1), in which a piston ported air passage is arranged between an air inlet (2) and the upper part of a number of transfer ducts (3, 3'), characterized in that the air passage is arranged from an air inlet (2) equipped with restriction valves (4) controlled by at least one engine parameter, for example the carburettor throttle control, the air inlet extends via at least one connecting duct (6, 6') to at least one connecting port (8, 8') in the cylinder wall (12) of the engine, which connecting port is arranged so that it, in connection with piston positions at the top dead centre, is connected with flow paths (10, 10') embodied in the piston (13), which extend to the upper part of a number of transfer ducts (3, 3'), and each flow path in the cylinder and the piston is to a great extent arranged in the cylinder's lateral direction in that, on the one hand the connecting port (8, 8') and the adjacent scavenging port (31, 31') of the cylinder are shifted sideways in relation to each other along the periphery of the cylinder wall (12), and on the other hand the transfer ducts (3, 3') of the cylinder are running essentially in the cylinder's lateral direction away from each scavenging port (31, 31') respectively, i.e. tangentially in relation to the circumference of the cylinder wall (12). 2. Crankcase scavenged internal combustion engine (1) according to claim 1, characterized in that each branch (11, 11') which leads to each connecting port (8, 8') respectively is directed in the cylinder's lateral direction, or slightly upwards from this. 3. Crankcase scavenged internal combustion engine (1) according to any one of the claims 1-2, characterized in that the flow paths are so arranged that the recess (10, 10') in the piston that meets the respective scavenging port (31, 31') is arranged so that the air supply is given an essentially equally long or longer period, counted as crank angle or time, in relation to the inlet. 4. Crankca.'e scavenged internal combustion engine (1) in accordance with any of t le preceding claims, characterizedin that the period of the air supply is greater than 90 % of the inlet period but smaller than 110 % of the inlet period. 5. Crankcase scavenged internal combustion engine (1) in accordance with any of the preceding claims, characterizedin that the recess (10, 10') in the piston that meets the respective scavenging port (31, 31') has an axial height locally at this port that is greater than 1.5 times the height of the respective scavenging port (31, 31'), preferably greater than 2 times the height of the scavenging port. 6. Crankcase scavenged internal combustion engine (1) in accordance with any of the preceding claims, characterized in that the upper edge of the respective connecting port (8, 8') is located as high or higher in the cylinder's axial direction than the lower edge of the respective scavenging port (31 , 31 '). 7. Crankcase scavenged internal combustion engine (1) in accordance with any of the preceding claims, characterized in that the air inlet (2) has at least two connecting ports (8, 8') in the engine's cylinder wall (12). 8. Crankcase scavenged internal combustion engine (1) in accordance with any of the preceding claims, characterized in that the connecting port(s) (8, 8') in the engine's cylinder wall (12) are so located that the piston (13) covers them when it is located at its bottom dead centre. 9. Crankcase scavenged internal combustion engine (1) in accordance with any one of the claims 1-4, characterizedin that the connecting port(s) (8, 8') in the engine's cylinder wall (12) are so located that the piston (13) does not cover them when it is located at its bottom dead centre, but exhaust gases from the cylinder can penetrate into the air inlet. 10. Crankcase scavenged internal combustion engine (1) in accordance with any of the preceding claims, characterizedin that the flow paths (10, 10') in the piston at least partly are arranged in the form of at least one recess (10, 10') in the periphery of the piston.

Inventors: CARLSSON BO, EKDAHL ROY, STROEM HANS, CARLSSON, BO, EKDAHL, ROY, STROEM, HANS
Applicants: CARLSSON BO, EKDAHL ROY, ELECTROLUX AB, STROEM HANS, AKTIEBOLAGET ELECTROLUX, CARLSSON, BO, EKDAHL, ROY, STROEM, HANS
Application year: 2000
Main classification: F02B25
IPC codes: F02B25, F02B75