Paragraphs

Claims

Gear [20] of sleeve [28] is located outside the engine unit [1] and is coupled, at the same rotational speed, to the long-stroke crankshaft [4] by means of a gear [19] rigidly mounted on the latter and an intermediate gear [21] located between both aforesaid gears [19] and [20].

The drive shaft [17] comprises, on the side of the bearing housing [22] facing the bearing plate [15], helical splines [31] onto which the sliding tube [32] is engaged.

The inner surface of said sliding tube [32] comprises splines [33] mated to the helical splines [31], so as to permit the sliding tube [32] to travel helically along drive shaft [17] and provide an angular displacement between said first and third members.

The outer surface of the sliding tube [32] also comprises helical splines [34], the helix of which is contrary to that of the splines [33] on the inner surface of the sliding tube [32].

The inner surface of sleeve [28] comprises helical splines [35] mated to the outer helical splines [34] of the sliding tube [32], so as to permit the latte to travel helically in sleeve [28] and provide an angular displacement between said second and third members, at the same time as the helical travel of the sliding tube [32] along drive shaft [17].

The sleeve [28] rotates again with shaft [17] when the sliding tube [32] no longer travels axially.

The length of the sliding tube [32] is established inside sleeve [28] when the end of said sliding tube [32] is located at the stop point defined by the surface of the bearing housing [22], the other end of the sliding tube [32] is free outside sleeve [28], passes through gear [20] and emerges from the engine unit [1], so as to permit, through appropriate means, the inner ring of the double-row angular contact bearing [36] to be mounted and secured.

Said inner ring of bearing [36] rotates with the sliding tube [32], whereas the outer ring of bearing [36] does not rotate and is rigidly connected to the holding member [37].

A decision-making memory of the compression ratio programme, acting by means of a hydraulic control system, permits the holding member [37] and the sliding tube [32] to be shifted, so as to alter the lead angle between the two crankshafts [4] and [5].

The start-of-travel of the variably timed transmission is arranged so that the sliding tube [32] is at the travel-out stop position ( not shown ) of sleeve [28] ( low torque ), which corresponds to the minimum lead angle between the crank of the short-stroke crankshaft [5] and the crank of the long-stroke crankshaft [4].

The end-of-travel of the variably timed transmission is arranged so that the sliding tube [32] is at the travel-in stop position ( not shown ) of sleeve [28] ( high torque ), which corresponds to the maximum lead angle between the crank of the short-stroke crankshaft [5] and the crank of the long-stroke crankshaft [4].

According to the invention, to define and facilitate the coupling of both crankshafts [4] and [5] between the variably timed transmission, the number of teeth of gear [20] is even when the number of mated splines [34] and [35] of sliding tube [32] and sleeve [28] respectively, of mated splines [31] and [33] of shaft [17] and sliding tube [32] respectively, and of abuting splines between both shafts [17] and [18] is uneven and vice versa.

According to an alternative embodiment of the invention, the shaft [17] of the variably timed transmission comprises, on the side of the bearing housing [22] facing the bearing plate [15], straight splines [38] instead of helical splines [31], onto which the sliding tube [32] is engaged, and the inner surface of the sliding tube [32] comprises straight splines [39] instead of helical splines [33], mated to the straight splines [38] of shaft [17].

According to the invention, the minimum and maximum compression ratios selected for the type of engine to be designed, are determined based on the dimensions of the different engine members, i.e. on the one hand, the ratio between the displacements of the paired cylinders [2] and [3] and, on the other hand, the ratio between the total displacement of these cylinders [2] and [3] and the clearance space [40], these ratios being defined so that the maximum lead angle between the crank of the short-stroke crankshaft [5] and the crank of the long-stroke crankshaft [4], defined by the end-of-travel position of the variably timed transmission, determines at the end of the compression phase ( top dead center of piston [6] ), the position of piston [8] with respect to the additional volume required for the clearance space [40] to define said minimum compression ratio of the engine, with an angle of at least 90° between the connecting rod [9] and the crank of the short-stroke crankshaft [5].

Claims