GB2111591A - Two-stroke engine with adjustable crankcase inlet valve timing - Google Patents

Abstract

A member 19 with an aperture 21 is movable relative to the mixture inlet to vary the instants at which a cutaway portion of the crankshaft driven valve disc 13 permits mixture to enter the crankcase 9. The member 19 may be operated separately or in conjunction with the carburettor throttle valve and may in fact provide the carburettor throttle valve, Fig. 6 (not shown). The disc 9 may be adjustable relative to the crankshaft 7 by fly-weights (49), Fig. 9 (not shown). <IMAGE>

Description

SPECIFICATION Improvements relating to two-stroke engines The present invention relates to a two-stroke engine in which the inlet port timing is variable.

For years two-stroke internal combustion engines have overcome the symmetry of port timing by harnessing the natural resonances in the engine to provide for asymmetrical flow characteristics. However, for improved performance and/or fuel economy it is now necessary for further variations in engine timing to be provided. This applies to both road and racing bikes. All the work carried out on fuel injection has also led to a similar conclusion, in that you can either have power and poor fuel economy, or low power and good fuel economy.

Clearly any improvement in timing adjustment can improve the power band of the engine and will therefore improve the power relationship with economy.

Such timing adjustment has previously been contemplated relative to the exhaust valve cycle of the engine. However another important available area is the inlet cycle and it is in this latter direction that the present invention is pointed.

With piston ported engines, some inlet length is needed to maintain the inertia of the charge to overcome the disadvantages of symmetrical inlet timing.

However, a disc valve inlet arrangement with its asymmetrical timing does not have the same requirements. Indeed it would appear from tests that the shorter the inlet length the more power that can be generated by the engine through better trapping of the initial charge. The main disadvantage with a disc inlet valve is that the asymmetric timing is fixed and not variable with regard to engine speed.

Tests have shown that there would be an improvement to both the power for a racing machine and particularly economy to a road machine, if the closing period of a disc inlet valve could be varied by between 250 and 35 over the engine speed range. The tests also show that the closing variation required would not be linear but would, in fact, be logarithmic and that for the last quarter throttle opening the disc timing needs to be virtually the same, whereas from initial opening to threequarters throttle, there needs to be a more linear and gradual increase.

The aim of the present invention is to provide a two-stroke engine with variable inlet timing in a simple and efficient manner.

According to the present invention there is provided a two-stroke engine comprising an inlet port which is opened and closed to a fuel/air mix intake by means of a partially cutaway disc which is rotatable about its central axis by means of the engine crankshaft, an adjustable member being arranged to be moved relative to the disc and inlet port, said adjustable member having an aperture which extends at least partially over said inlet port.

Thus, by virtue of the present invention the partially cutaway disc and said adjustable member, together, control the opening and closing of the inlet port. With said adjustable member stationary the rotating disc will open and close the inlet port at regular intervals. However, by adjusting the adjustable member and thus the overlap of the aperture in the adjusting member, with said inlet port, the instant of initial opening and the closing point of the inlet port can be adjusted with respect to the rotational cycle of the disc. This movement of the adjustable member may be made automatically by suitable means or by mere operation of the engine throttle, the adjusting member being connected by a linkage to the throttle. Alternatively a separate manual or foot-operated control may be provided.

In a preferred embodiment of the present invention the adjustable member is in the form of a flat slide which is mounted in a channel in the engine housing in the region of the inlet port and adjacent to said partially cutaway disc, the channel extending preferably at 600 to the horizontal plane passing through the central axis of the disc. This angle provides a timing variation i.e. inlet opening curve variation, which is perfect for racing machines. However, within the scope of the present invention the slide can of course be at any desired angle to provide the desired performance. Preferably the flat slide is located on the opposite side of the cutaway disc to the inlet port. The flat slide is preferably connected by a linkage e.g.Bowden cable, to the throttle control so that the slide is moved as the throttle operates to thus provide the desired timing variation relationship with engine speed. Alternatively the flat slide may be connected to any other independent control or may be automatically controlled in dependence upon a sensed parameter e.g. engine operating characteristic.

The preferred embodiment of the present invention may be modified to also form a carburettor, the flat slide being used to control the air delivery to the primary crankcase of the engine, and carrying a flow control needle which engages in an orifice i.e. jet, at the end of the channel for the slide. Thus, movement of the flat slide moves the needle in the jet, which arrangement can be constructed to function as per a carburettor. By this latter design an extremely space saving arrangement for the carburettor is achieved, aiding bike design in many ways, with the attendant power and/or fuel economy advantages of the present invention. For example, as previously mentioned, tests have shown that the shorter the inlet length when using inlet valve disc timing and control, the more power can be generated by the engine through better trapping of initial charge.By using the variable disc timing flat slide as part of a carburettor, it is possible to reduce the initial length by approximately two inches. This should result in a power improvement of between one and two horse power per engine cylinder. In addition the traditional problems of a four cylinder engine, namely the width across the carburettors, would, of course, be drastically reduced by four inches overall and this arrangement when fitted to a square four engine would result in a much slimmer fairing shape, better penetration and greater top speed.

As an alternative to the flat slide any suitable adjusting member may be used. One such adjusting member is in the form of a disc which has a central aperture and which is arranged to be moved along a curved path centred on the axis of the cutaway timing disc and within the cutaway timing disc periphery. This adjusting member may also be moved by any suitable means.

In addition to the above the cutaway timing disc may be adjustable relative to the crankshaft on which it is supported and by which it is driven.

This adjustment may be effected by the action of flyweights mounted between the timing disc and the crankshaft and will clearly be dependent upon engine speed.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic cross-sectional view through part of a prior art two-stroke engine having rotational disc inlet port timing; Fig. 2 shows diagrammatically the operation of the timing disc of Fig. 1; Fig. 3 is a diagrammatic cross-sectional view through part of one embodiment of a two-stroke engine constructed according to the present invention; Figs. 4a, 4b and 4c schematically show various positions of the timing disc, adjusting member and inlet port in the engine of Fig. 3; Fig. 5 is a plan view of part of a two-cylinder two-stroke engine constructed according to the present invention; Fig. 6 is a diagrammatic cross-sectional view through part of a further embodiment of the present invention;; Figs. 7a, 7b and 7c schematically show various positions of the timing disc, adjusting member etc. of the embodiment of Fig. 6; Figs. 8a and 8b schematically illustrate a still further possible embodiment of the present invention; and Fig. 9 shows a modified form of cutaway timing disc suitable for use in the present invention.

Figs. 1 and 2 of the accompanying drawings illustrate a prior art two-stroke internal combustion engine having disc timing. The engine as illustrated comprises a carburettor 1 mounted by a rubber mounting 3 on an engine block generally designated 5. A crankshaft 7 is rotatably supported in a crankcase 9 by bearings 11, and is keyed to a valve timing disc 13, best seen in Fig.

2. Valve timing disc 13 is partially cutaway at 15 and overiaps cylinder inlet port 1 7 (Fig. 2). As the disc 1 3 rotates with crankshaft 7 so the inlet port 1 7 is opened and closed, the frequency and period of this action being dependent upon crankshaft speed, i.e. engine speed, and being a fixed parameter.

A preferred embodiment of the present invention is shown in Figs. 3 and 4, parts equivalent to parts in Figs. 1 and 2, being identified by the same reference numerals. The main difference between the prior art embodiment of Figs. 1 and 2 and the embodiment of the present invention shown in Figs. 3 and 4, is the provision of an adjusting member in the form of an elongate slide 19 which has a circular aperture 21 formed therein towards one end region of the slide, the slide 1 9 being axially movable in a channel, to overlap the inlet port 1 7 to varying degrees. The slide 19 is movable preferably by means of a connection (not shown) to the engine throttle, so that the slide 1 9 is movable in dependence upon engine speed.Thus by moving slide 19 and varying the degree of overlap between the aperture 21 and inlet port 17, the instant at which the inlet port 17 is opened by rotating disc 13, is varied. This is clearly shown in Figs. 4a, 4b and 4c. In Fig. 4a the aperture 21 and inlet port 17 overlap completely and the inlet port 17 is opened by edge 23 of disc 13 at angle 0,. In Fig. 4b slide 19 has been moved so that aperture 21 and inlet port 17 overlap only as at 25. Thus inlet port 19 only opens when edge 23 of disc 13 reaches point 27. This corresponds to angle O2 which is smaller than 8,. Similarly in Fig. 4c the overlap between aperture 21 and inlet port 17 is adjusted to 29 and only opens when edge 23 of disc 13 reaches point 31.Thus the inlet timing is variable by the axial movement of elongate slide 19.

The actual inlet timing can be further varied at the design stage by the selection of the angle of the slide 1 9 to the vertical or horizontal plane passing through the central axis of the timing disc 1 3. For power e.g. for racing engines, an angle of 600 to the vertical provides a perfect opening curve variation. However, any other angle within 3600 can be selected as desired.

Fig. 5 of the accompanying drawings shows part of a two-cylinder two-stroke engine constructed according to the present invention.

Each cylinder 33 has a channel 35 within which a slide 37 can be axially moved to overlap the inlet port to a varying degree and thus alter the inlet timing as described with reference to Figs. 4a, 4b and 4c. The respective timing discs are not shown in Fig. 5.

Figs. 6 and 7 show a modified form of the engine of Figs. 3 and 4 and again like parts are identified by the same reference numerals. The main modification is the inclusion of the carburettor in the adjusting member i.e. slide 19, of the present invention, thus reducing the inlet length and increasing the available power of the engine, as proven by tests. The slide 1 9 carries a needle 39 which engages in a jet (not shown) at one end of the channel in which the slide is slidably mounted, and thus the fuel flow is controlled by movement of the slide and needle, slide 19 also controlling the air flow to the crankcase. The operation of this construction is shown schematically in Figs. 7a, 7b and 7c in a similar manner to Figs. 4a, 4b and 4c.

By using the variable disc flat timing slide 19 as part of a carburettor, it is possible to reduce the inlet length by approximately two inches in comparison to a conventional carburettor arrangement. This should result in a power improvement of between one and two horse power per cylinder. In addition to that the traditional problems of a four cylinder engine, namely the width across the carburettors, would, of course, be drastically reduced by four inches overall and this system fitted to a square four engine would result in a much slimmer fairing shape, better penetration and greater top speed.

An alternative embodiment of the present invention is briefly diagrammatically illustrated in Figs, 8a and 8b. This embodiment has a disc shaped adjusting member 41 which has a central aperture 43, the member 41 being movable by suitable means along a curved path centred on the axis of rotation of timing disc 45 and within the periphery of disc 45. Thus the aperture 43 can overlap the inlet port 47 to varying degrees and so adjust the inlet timing.

Fig. 9 shows a modified connection between crankshaft 7 and timing disc 1 3 whereby spring loaded flyweights 49 can vary the relative rotational positions of the crankshaft 7 and the control edge 23 of timing disc 1 3 in dependence upon the crankshaft speed i.e. engine speed.

The present invention thus provides a twostroke engine which has been very simply modified to provide for a variation in inlet timing in dependence upon any desired parameter or purely arbitrary manual operation, to so improve the "power band" of the engine.

Claims (Filed on 29.10.82) 1. A two-stroke engine comprising an inlet port which is opened and closed to a fuel/air mix intake by means of a partially cutaway disc which is rotatable about its central axis by means of the engine crankshaft, an adjustable member being arranged to be moved relative to the disc and inlet port, said adjustable member having an aperture which extends at least partially over said inlet port.

2. A two-stroke engine as claimed in claim 1, in which the adjustable member is movable by operation of the engine throttle.

3. A two-stroke engine as claimed in claim 1, in which the adjustable member is movable by a manually or foot operated control separate from the engine throttle.

4. A two-stroke engine as claimed in any one of claims 1 to 3, in which the adjustable member is in the form of a flat slide which is mounted in a channel in a housing of the engine, in the region of the inlet port and adjacent to said partially cutaway disc.

5. A two-stroke engine as claimed in claim 4, in which the said channel extends at any angle of up to 60C to the horizontal plane passing through the central axis of the disc.

6. A two-stroke engine as claimed in claim 4 or 5, in which the flat slide is located on the opposite side of the cutaway disc to the inlet port.

7. A two-stroke engine as claimed in any one of claims 4 to 6 when dependent upon claim 1, in which the slide is controlled in dependence upon a sensed engine operating parameter.

8. A two-stroke engine as claimed in any one of claims 4 to 6 when dependent upon claim 1 or 2, in which the flat slide carries a fuel flow control needle which engages in a jet at the end of the channel for the slide, thus acting as a carburettor.

9. A two-stroke engine as claimed in any one of claims 1 to 3, in which the adjusting member is in the form of a disc which has a central aperture, the disc being arranged to be moved along a curved path which is centred on the axis of the cutaway timing disc and within the periphery of the cutaway timing disc.

10. A two-stroke engine as claimed in any one of the preceding claims, in which the cutaway timing disc is rotationally adjustable relative to the crankshaft by which it is driven.

11. A two-stroke engine as claimed in claim 10, in which the rotational adjustment between the timing disc and crankshaft is controlled by flyweights.

1 2. A two-stroke engine constructed substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. shown schematically in Figs. 7a, 7b and 7c in a similar manner to Figs. 4a, 4b and 4c. By using the variable disc flat timing slide 19 as part of a carburettor, it is possible to reduce the inlet length by approximately two inches in comparison to a conventional carburettor arrangement. This should result in a power improvement of between one and two horse power per cylinder. In addition to that the traditional problems of a four cylinder engine, namely the width across the carburettors, would, of course, be drastically reduced by four inches overall and this system fitted to a square four engine would result in a much slimmer fairing shape, better penetration and greater top speed. An alternative embodiment of the present invention is briefly diagrammatically illustrated in Figs, 8a and 8b. This embodiment has a disc shaped adjusting member 41 which has a central aperture 43, the member 41 being movable by suitable means along a curved path centred on the axis of rotation of timing disc 45 and within the periphery of disc 45. Thus the aperture 43 can overlap the inlet port 47 to varying degrees and so adjust the inlet timing. Fig. 9 shows a modified connection between crankshaft 7 and timing disc 1 3 whereby spring loaded flyweights 49 can vary the relative rotational positions of the crankshaft 7 and the control edge 23 of timing disc 1 3 in dependence upon the crankshaft speed i.e. engine speed. The present invention thus provides a twostroke engine which has been very simply modified to provide for a variation in inlet timing in dependence upon any desired parameter or purely arbitrary manual operation, to so improve the "power band" of the engine. Claims (Filed on 29.10.82)

1. A two-stroke engine comprising an inlet port which is opened and closed to a fuel/air mix intake by means of a partially cutaway disc which is rotatable about its central axis by means of the engine crankshaft, an adjustable member being arranged to be moved relative to the disc and inlet port, said adjustable member having an aperture which extends at least partially over said inlet port.

2. A two-stroke engine as claimed in claim 1, in which the adjustable member is movable by operation of the engine throttle.

3. A two-stroke engine as claimed in claim 1, in which the adjustable member is movable by a manually or foot operated control separate from the engine throttle.

4. A two-stroke engine as claimed in any one of claims 1 to 3, in which the adjustable member is in the form of a flat slide which is mounted in a channel in a housing of the engine, in the region of the inlet port and adjacent to said partially cutaway disc.

5. A two-stroke engine as claimed in claim 4, in which the said channel extends at any angle of up to 60C to the horizontal plane passing through the central axis of the disc.

6. A two-stroke engine as claimed in claim 4 or 5, in which the flat slide is located on the opposite side of the cutaway disc to the inlet port.

7. A two-stroke engine as claimed in any one of claims 4 to 6 when dependent upon claim 1, in which the slide is controlled in dependence upon a sensed engine operating parameter.

8. A two-stroke engine as claimed in any one of claims 4 to 6 when dependent upon claim 1 or 2, in which the flat slide carries a fuel flow control needle which engages in a jet at the end of the channel for the slide, thus acting as a carburettor.

9. A two-stroke engine as claimed in any one of claims 1 to 3, in which the adjusting member is in the form of a disc which has a central aperture, the disc being arranged to be moved along a curved path which is centred on the axis of the cutaway timing disc and within the periphery of the cutaway timing disc.

10. A two-stroke engine as claimed in any one of the preceding claims, in which the cutaway timing disc is rotationally adjustable relative to the crankshaft by which it is driven.

11. A two-stroke engine as claimed in claim 10, in which the rotational adjustment between the timing disc and crankshaft is controlled by flyweights.

1 2. A two-stroke engine constructed substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.