The present invention relates to combustion engines. The present invention is more specifically directed to the combustion engine design of the preamble of claim 1.

A design based on a piston and a crankshaft is usually used in combustion en- gine designs for converting the energy obtained from a fuel into mechanical en- ergy. The idea is to make use of the pressure generated by the volumetric ex- pansion of a burning fuel as a piston movement in a cylinder. The piston move- ment is transmitted by means of a connecting rod to the crankshaft, where the movement is converted into a rotating motion.

A combustion engine design of the kind mentioned above with a variable com- pression ratio is previously known from French patent application FR-2489450.

It is previously known that the piston is connected to one or two connecting rods by means of a piston pin. The connecting rod is further connected by means of a crankshaft bearing to a crankshaft. There is thus typically a piston pin between the connecting rod and the piston, which piston pin serves as a shaft of the bear- ing of the upper end of the connecting rod. In fairly large machines the piston can be connected by means of a fixed rod to a bearing of a connecting rod, which bearing moves in a sleeve. The main problem with the known arrangements has to do with supporting the piston with respect to lateral tilting. When the connect- ing rod is connected to the piston by means of a bearing it does not provide any tilting-preventing support for the piston in a direction perpendicular to the axis of the piston pin. The rotating motion also tends to tilt the piston in the direction of the movement. Because of what is mentioned above it is common for the piston skirt to wear and, as a result, become loose. And at the same time the tilting also wears the cylinder walls so that the cylinder becomes oval. Also, in this kind of engine designs it is not possible to change the path of travel of the piston i. e. the compression in the cylinder.

An object of this invention is to provide an improved engine design that makes it possible to change the compression ratio and piston displacement and with which the disadvantages of the above-mentioned known combustion engine arrange- ments can be avoided or at least substantially minimized and which thus im- proves the operability and long-term use of the engine.

More specifically, the typical characteristics of the combustion engine design ac- cording to the invention are defined in the characterizing part of independent claim 1.

By means of the design according to the invention the tilting of the piston and the resulting wear are prevented. When, in the engine design, the piston is con- nected to the connecting rods by means of an intermediate part and two support points, both of which have a separate connecting rod for two separate crank- shafts, the piston support is stable. This design makes it possible to use a simpler piston design and lighter structures in the connecting rods and in the crankshaft.

Also, the contrary motions of the connecting rods and the effect of the two crank- shafts can be made to function so that the inertial forces diminish and the engine runs more smoothly.

The invention renders the structure of the engine durable and makes it possible to avoid vibration and the resulting disadvantages caused by the inertial forces.

At the same time the use of two connecting rods and crankshafts makes it possi- ble to change the stroke volume and compression ratio of the engine during the operation of the machine. This procedure and design enables optimal adjustment of the operation of the engine in a wide range of loading.

The invention will now be described in more detail with reference to the accom- panying figures, of which Figure 1 is a cross-sectional view of the engine design according to the invention seen from the end of the crankshafts.

The combustion engine design according to the invention comprises an engine frame 1 connected to a cylinder part 2. In the frame of the engine the crankshaft bearing arrangement is provided by means of movable bearing units 17. The bearing units are attached to the engine frame so as to be slidable. A moving mechanism 12 is connected to the bearing units 17, which moving mechanism moves the crankshaft bearings 17 at the same time towards each other or further away from each other. The distance between the crankshafts is controlled by means of a drive unit 12 included in the moving mechanism. Connecting rods 6 and 7 are connected by means of a crankshaft bearing to crankshafts 4 and 5.

The connecting rods 6 and 7 are connected, at each cylinder, to the same piston 3. Piston pins 9 and 10 are, in this embodiment, disposed directly in the piston, or a separate intermediate part 8 rigidly fixed to the piston is used. The intermediate part 8 is connected to the piston with a pin 18, which is angular and prevents the attachment part from moving relative to the piston. The two crankshafts 4 and 5 included in the structure of the engine are connected to each other by means of gearing, which synchronizes the rotating motion of the crankshafts 4 and 5 rela- tive to each other. The structure of the gearing 14,15 is accomplished so that the movement of the shafts relative to each other moves, at the same time, a gear wheel 15 of the gearing. The gear wheel 15 is arranged to slide axially along an intermediate shaft 16 and arranged to move together with a main bearing 17 of the crankshaft.

The figures and the description relating to them are intended only to illustrate the present invention. The details of the combustion engine design may vary within the scope of the inventive idea disclosed in the claims. Further, it is obvious to a person skilled in the art that the connecting rods may be fixed by means of differ- ent kinds of intermediate parts. What is essential is that two support points ac- cording to the claim are used. The crankshaft connection with synchronizing gearing can in principle also be provided by means of a toothed belt or chain gearing.