Device for two-stroke internal combustion engines

Technical field:

The present invention relates to a device for two-stroke internal combustion engines and refers , more closely defined , to the suction system of such an engine .

Background:

A normal type of internal combustion engine is two-stroke engines with crankcase scavenging, i. e. the movement of the piston, or in the case of multiple cylinder engines , of the pistons, is used simul¬ taneously during the compression of the cylinder space when the piston moves to the top position to suck the air-petrol-mixture into the crankcase and, during the movement of the piston from the top position, to compress the gas sucked into the crankcase and to press it into the cylinder space. For the transfer of the compressed air from the crankcase to the cylinder space transfer ducts are used which end in the cylinder wall and are exposed by the piston during its downward movement. The exhaust ports of the piston are con¬ trolled in a similar way.

Also the gas sucked in must be controlled so that it can enter the crankcase when the piston moves upward, but is prevented from flowing back again into the suction tube of the engine when the pis¬ ton moves downward compressing the gas in the crankcase for its transfer to the cylinder space. Also for this control the movement of the piston is used in many types of engines , when the piston skirt exposes and covers respectively one or several ports in the cylinder wall, which in exposed position have connection with the crankcase.

A control of this type, however, must always be a compromise as , when determining a crankshaft angle at which the port begins to be exposed, the same angle will apply for the closing of the port during the piston movement toward the crankcase when the piston has passed the top dead centre . Optimum adaptation of the exposure results in a non-optimum angle for the closing of the port and vice versa, the reason why an optimum angle has been renounced in both

instances - hence a compromise. This leads to inconveniences , such as the pressing back of the gas sucked in through the port when the piston starts moving downward which , in turn, interferes with the gas flow in the suction tube causing a reduced flow into the crankcase. The result is insufficient gas-filling of the crankcase and hence an insufficient engine charge with reduced power and an un¬ favourable influence on the torque as a consequence.

Several solutions have been suggested to avoid these disadvan¬ tages . Thus , it is normal to use a clack valve on smaller, fast run¬ ning engines, frequently a so called Reed- valve; a valve with leaf springs as a valve element. However, this also causes disadvantages , an automatic valve always hampers the flow to a certain degree and the Reed-valve is known to be able to cause engine breakdowns by the spring leaf brealdng and being sucked into the engine.

Another method proposed, the solution of which is concurrent with the present invention, is to mount a rotating valve in a branch of the suction tube which ends directly in the crankcase. The main branch of the suction tube, on the other hand, ends in the cylinder wall and is controlled by the piston. The rotating valve controlled by the crankshaft is used to allow gas to flow into the crankcase during a period when the suction port is kept closed by the piston. In the present case it is assumed that the valve determines the start of the suction process by opening, while the piston determines the end of the suction process by closing the suction port. Hereby it is possible to utilize maximally the crank angle at disposal for suction without delaying the closing, so that a virtually evenly flowing gas flow "flux" is formed in the suction tube.

Technical problem:

The solutions known up to now according to this principle, however, have not proved to result in such a great improvement in power and torque that they have justified the more complicated design of the engine.

Solution:

At the two-stroke combustion engine with crankshaft- scavenging which the invention refers to and at which the skirt of the cylinder

piston opens and closes a suction port ending in the crankcase in step with the piston movement to and from the crankcase is according to the invention the suction duct of the engine , which close to the engine divides into two branches , of which one of the branches ends in the suction port the opening and closing of which is controlled by the piston in its other branch for controlling the closing and opening of its connection with the crankcase provided with a rotating valve, the axis of rotation of which runs mainly in the direction of the engine crankshaft and which axial section in the area of the suc¬ tion duct- has the shape of the segment of a circle which is located on the one side of the axis of rotation of the valve . The valve can be formed by a recess in a shaft running parallel with the crank¬ shaft where the shaft mentioned is arranged, with such a distribution of mass at least partly obtained by the one-sided recesses mentioned that it during rotation at least partly counteracts the free forces generated by the other movable parts of the engine, the cranshaft, the piston and the connecting rod.

Advantages :

The purpose of the present invention is to effect a device which enables the utilization of an optimum crank angle for suction of gases and prevents them from being pressed back again so that it is possible to utilize, to a high degree, the theoretically available capacity of the engine.

A further purpose of the invention is to effect a suction device with a rotating suction valve which may also be used to balakce the engine for equalizing of free forces .

Brief description of drawings :

Apparent from the enclosed drawings is one embodiment of the invention. Fig. 1 shows a section along the line I-I in Fig. 2 through an engine where the device according to the invention is applied; Fig. 2 shows a section through the same engine along a line II-II in Fig. 1; and Fig. 3 shows a diagram of the crankshaft angles .

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Best mode of carrying out the invention :

The engine shown in Fig. 1 has chiefly the design used for cranskcase-scavenged two-stroke engines and therefore the engine ^" has not been described in detail other than in respect to the device according to the invention . For additional information please refer to the introductory description of the embodiment and method of operation of such an engine.

According to the invention the engine comprises a suction tube 1 which in the case of an internal combustion engine as assumed, is connected to a carburetter with its external outlet. Close to the en¬ gine the suction tube is divided into two branches , one branch 2 which ends in a port in the cylinder wall and one branch 3 in which a rotating valve 4 is mounted after which branch 3 continues to an outlet 5 in the engine crankcase 8. The piston 6 of the engine has a piston skirt 7 the edge of which is facing the crankcase passes port 9 with which branch 2 ends in cylinder wall 10 whereby port 9 is kept open when piston 6 is in its upper position. In Fig. 1 the piston is shown in its bottom dead centre and in this position port 9 is closed by the piston skirt 7.

Branch 3, 5 of the suction duct lies , as is apparent from Fig. 1, obliquely in relation to the axis of rotation of valve 4 which is located in the centre of the arc formed by a part of the circumfer¬ ence of the valve. The oblique position of the duct and the design of the rotating valve as a segment of a circle causes valve 4, when the part forming the chord of the arc is in a certain position, to be parallel with the longitudinal direction of branch 3, 5 keeping the branch fully open while it, during that part of its revolution when the arc-formed periphery passes the connecting openings of branch 3, 5 keeps the flow closed. Fig. 1 shows the valve when it is just about to open branch 5. After this position the valve will keep the flow open during a crankshaft angle of about 180° with consideration to the covering of both duct 3 and duct 5 which takes place during the turning of the valve. The valve turns in the direction indicated by the adjacent arrow and with the same angular velocity as the cranskshaft.

According to Fig. 2 valve 4 is formed by a recess 12 in a cylin¬ drical valve shaft 13 which is mounted in bearings of the cylinder-

IjURE. OMPI

block 14 of the engine . Its axis of rotation is situated , as already mentioned, in the centre of the arc which valve 4 displays in Fig .

1. Recess 12 is located exactly opposite branches 3 and 5 of the suc ^" - tion duct and the shape of its cross section is adapted in such a way, that the least possible resistance to the flow is caused in the duct when the valve is open . If the engine has several cylinders , as indicated in Fig. 2, there is a recess in shaft 13 exactly opposite the suction duct of each cylinder. The angle between these recesses is determined by how the top dead centres of the various cylinders are displaced in relation to each other by the displacement of the angles between the crank elbows of the cranskshaft 15 of the engine.

Recesses 12, as apparent from the description in connection with

Fig. 1, are made from one direction and are so deep, that only one segment of the circle of the cross section of the shaft is left. The bottom of the recesses does not have to be plane as indicated, but can be concave, convex or have another shape to adapt the valve for optimum function.

Shaft 13 is driven from crankshaft 15 via a transmission which according to Fig. 2 can consist of a V-belt 16 and have a transmis¬ sion ration of 1: 1.

As apparent when studying Fig. 1, the masses of the shaft 13 are unbalanced where recesses 12 have been made. This means that free forces will form during the rotation of the shaft and the direc¬ tion of these forces at the successive position of rotation depends on the position of the angle of recess 12 and as this angle is speci¬ fic for each cylinder, a composite system of free forces will be ob¬ tained in the case of multiple cylinder engines . Free forces will also form during the rotation of the crankshaft and the up and down movement of the piston in the cylinder as well as during the compli¬ cated movement of the connecting rod. Such free forces generate vibrations which, in general, should be reduced as much as possible . In the i tance of so called in-line engines , this can only be done to a limited extent by the relatively simple device of counter weights on the crankshaft. If a more complete balance should be required, it would be necessary to mount special balance axles which are driven from the crankshaft .

By making the valve shaft unbalanced in the invention , a valve device has been created which at the same time can be used to bal¬ ance the engine. It has , in fact, been releaved that it is possible ^* - to place recesses 12 in such a way that a better balance of the crank¬ shaft can be obtained than in the case with only cranskshaft counter wights . However, in the design according to the figures , the duct is wide and the recesses have a considerable width so that the un¬ balanced mass is considerable. To complete the balancing system even further, shaft 13., as indicated in Fig. 2, may carry special balance weights 17 at one or at both ends . It is also possible to make the shaft unbalanced between recesses 12 within areas 18.

What would be expected to be a disadvantage, namely that the valve shaft adds to the unbalance of the engine, can in fact be turned into an advantage. To this can be added that the design with the valve made only as a segment of a circle provides good adaption possibilities for the valve in regard to its opening and closing times and gives short periods when the valve is partly closed which is advantageous for the gas flow.

Assuming that the engine is in its bottom dead centre, as shown in Fig. 1, valve 4 will immediately open when the crankshaft rotates so that gas is sucked into crankcase 8 during the upward movement of piston 6. This is illustrated in Fig. 3 where arrow 20 indicates how the valve begins to open at 0° in order to close at about 180° . When piston 6 moves upward, piston skirt 7 will gradually expose the lower edge of suction port 9. This occurs , according to the dia¬ gram in Fig. 3, 100° after the bottom dead centre and is shown by arc 21. When port 9 begins to open, the air in suction duct 1 has obtained a certain speed through suction via branch 3, 5, and the air flow is now partly deflected and sucked in through port 9.

180° after the bottom dead centre and thus 80° after port 9 has started to open, valve 4, as mentioned before, closes while port 9 is still open. As the opening angle of port 9 is symmetrically placed in relation to the top dead centre, it is open until about 160° and will not be closed before 260° after the bottom dead centre.

Any of the branches of the suction duct is thus open from the moment the piston starts moving from the bottom dead centre and suction can start and continue during the entire upward stroke as

well as below 80° of the downward stroke . It would be expected that the piston would start pressing out the air after having passed the top dead centre . This , however, is not the case as the air, owing ^" - to its impetus , will continue to rush into the crankcase even when the actual sucking effect from the piston has stopped . Hereby it is important that the air has had time to accelerate as much as possible before the downward stroke of the piston commences . By ensuring that the suction process starts as early as possible by means of valve 4, the air can attain high speed whereby the pressure increase caused by the downward movement of the piston is counteracted. Because of this, the closing of the suction port can be postponed. Thus , due to the invention, the suction time is increased both at its beginning and at its end.

To be able to utilize the air acceleration achieved when valve 4 has opened, it is important that the air does not have to be deflected too much when port 9 opens . Therefore, the design shown in Fig. 1 where the duct branches are connected with each other at about 30° in Y-form is favourable.

The diagram shown in Fig. 3 regarding opening and closing times constitutes only one example and the angles may be varied according to the size of the engine, the intended range of revolutions and other operational conditions as well as the design of the engine details .

What has been described constitutes more the principles of the invention than its design in detail. The latter one depends namely on a number of factors , some of which have been mentioned above . For each type of engine, where the invention is applicable and in many cases for condition of operation of a specific kind for which this type of engine is intended, a certain determination of such fac¬ tors are requires , like the ration of angle of the valve flow to the crankshaft in its different positions of operation, the direction of rotation for the crankshaft and the valve shaft in relation to each other, the design and position of the branches and so on in order to reach an optimum of the qualities of the engine during the spe¬ cific conditions of operation . This determination can be achieved partly by means of theoretical considerations and partly by means of testings .