BACKGROUND ART In conventional four-stroke internal combustion engines, the crankshaft crank bearings are journal bearings having bearing shells fitted to the connecting rod big end and running on a steel shaft. The piston is attached to the cran bearing by means of a piston rod with the distance between th piston and the crankshaft crank being fixed by the piston rod. Accordingly movement of the piston during the compression phase is egual to the movement of the piston during the exhaust phase.

It would be advantageous for the piston to have a greater stroke during the exhaust phase in order to more full clear the combusion chamber of exhaust gases. A device aimed at achieving this advantage is described in U.S. Patent No 3,686,972. However, the gear mechanism described therein is generally complex whereby adding.considerably to the cost of manufacture and detracting from the overall life and reliability of the engine. It is the object of the present invention to overcome o substantially ameliorate the above disadvantages.

DISCLOSURE OF INVENTION There is disclosed herein a crankshaft assembly for a four-stroke internal combustion engine having at least one piston and a piston rod extending between the piston and the crankshaft assembly, said assembly comprising a shaft providing a crank extending radially from the axis of rotatio of the shaft, a first bearing surface formed adjacent the extremity of said crank and providing a first rotational axis parallel to the axis of the shaft, an eccentric piston rod support rotatably mounted on said first bearing surface so as to be rotatably supported thereby, said eccentric support providing a second bearing surface to rotatably support said

piston rod, said second bearing surface providing a second rotational axis parallel to the axis of said shaft, but spaced radially from said first rotational axis, drive means coupling said crank and support to drive said support about said first rotational axis at half the rotational speed of said shaft, said drive means consisting of a first gear fixed to said crank and having its longitudinal axis coaxial with said first axis of rotation, and a second gear fixed to said support so as to have its longitudinal axis coaxial with respect to said second rotational axis and meshingly engaged with said first gear, a third gear rotatably fixed ^" to said second gear so as to rotate therewith, and a fourth gear having its longitudinal axis coaxial with respect to said second rotational axis, fixed to said crank and meshingly engaged with said first gear The present invention as described above, differs from the device of US Patent 3,686,972 by having the gear train driven between the crank and the piston (connecting) rod. This greatly simplifies the device and therefore substantiall overcomes the above disadvantages. In US Patent 3 686 972, the eccentric support is driven by a gear train extending fro around the crank shaft.

BRIEF DESCRIPTION OF DRAWINGS Preferred forms of the present invention will now be described by way of example with reference to the accompanyin drawings, wherein:

Fig. 1 is a schematic part-sectioned perspective view of a portion of a crank shaft with a crank assembly extending therefrom attached to a piston rod of a four-stroke internal combustion engine; Fig. 2 is a schematic end elevation of the crank assembly and piston rod of Fig. 1;

Fig. 3 is a schematic part-sectioned front elevation of the crank assembly and piston rod of Fig. 1; and

Figs. 4 to 6 are schematic part-sectioned side elevations of alternative gear trains which may be employed with the crank assembly and piston rod of Fig. 1. MODES FOR CARRYING OUT THE INVENTION In Fig. 1 there is schematically depicted a crankshaft

10 of an internal combustion engine having at least one piston coupled to a piston rod 11, which piston rod 11 is attached to the shaft 10 by means of a crank assembly 12. The crank assembly 12 comprises a crank 13 extending radially from the longitudinal axis of the shaft 10 and being provided at its outer end with a bearing surface 14. The bearing surface 14 engages bearing cups 15 which in turn rotatably support an eccentric piston rod support 16. The support 16 has a bearing surface 17 which rotatably supports the piston rod 11. The crank assembly 12 is held together by means of a pair of bolts 18.

The bearing surface 14 provides a first rotational axis parallel to the rotational axis of the shaft 10, while the bearing surface 17 provides a second rotational axis, parallel to the rotational axis of the shaft 10 and the rotational axis of the bearing surface 14, however it is spaced radially from the rotational axis of the bearing surface 14.

Attached to the crank 13 is a first gear 19 which is meshingly engaged with a second gear 20 rotatably supported by the support 16. The gear 20 is coupled to a further gear 21 by means of a shaft 22 so as to rotate therewith. The gear 21 meshingly engages with a gear 23 fixed to the piston rod 11. The gear ratios in respect of the gear train consisting of the gears 19, 20, 21 and 23 are arranged so that the support 16 has half the average rotational speed, about the axis of the bearing surface 14, of the shaft 10.

The above described preferred embodiment of the present invention is arranged with the valve timing such that the piston passes through its maximum stroke during the exhaust stroke of the engine. This has the advantage that substantially all of the exhaust gases are expelled from the combustion chamber thereby greatly eliminating the need for long valve timing overlap. Still further, this would enable considerable reduction of, and possibly total elimination of, a combustion recess to be formed in the cylinder head of the engine. These advantages stem from the piston providing a greater swept volume during the exhaust stroke in comparison

to the compression stroke.

In the above described preferred embodiment the compression ratio of the engine is dependent on the size of the support 16. The compression ratio may be decreased by using an eccentric support which has a higher eccentricity, or increased by using an eccentric support having a lower eccentricity. The maximum piston displacement for a given eccentric support occurs when the rotational axes of the shaft 10, bearing surface 14, and bearing surface 17 are in a common plane, and the surface 17 being spaced radially outward from the surface 14 relative to the shaft 10. However, in the above discussed preferred embodiment, the rotational axes of the surfaces 14 and 17 may have a phase difference of approximately 45$ and still be effective. Such a phase difference might be useful in making slight alterations to compression ratio and to diminish stress on the gear train consisting of the gears 19, 20, 21 and 23.

In Figs. 4 to 6 there is 'schematically depicted alternative gear trains to that employed in the embodiment of Figs. 1 to 3. In Figs. 4 to 6 like parts to those used in the embodiment of Figs. 1 to 3, have been given the same numeral.

In the embodiments of Figs. 1 to 3 and 4, gears 19, 20 and 21 have external teeth, while gear 23 has an internal ring gear. The gears 20 and 21 are located on opposite sides of the support 16 and are joined by shaft 22.

In Fig. 5, the gears are substantially the same as that of Fig. 4, however the gears 20 and 21 are located adjacent each other on the same side of the support 16 and are mounted on the shaft 22, which is a "stub axle".

In Fig. 6, the gear chain is substantially the same as that depicted in Fig. 5, however the gears 20 and 21, and the shaft 22 (stub axle) are located in a shaped recess 24 formed in the support 16.