TWO-CYLINDER ENGINE WITH A DEVICE FOR BALANCING THE ALTERNATING INERTIA FORCES

Abstract: A two-cylinder internal combustion engine with parallel cylinders and crank pins (16b) at 360° has a counter-rotating eccentric mass (30) supported by a sleeve (24) rotatable on the crankshaft (16). This eccentric mass is located between the two crank pins (16b) and is rotated by the crankshaft (16).

Description: Two-cylinder engine with a device for balancing the alternating inertia forces. The present invention relates to a two-cylinder internal combustion engine including a crankshaft, two parallel cylinders in which there are reciprocatingly slidable two pistons having respective connecting rods connected to the crankshaft in correspondence with respective crank pins disposed at 3600, and counter-rotating eccentric mass means for balancing the alternating first-order forces during operation of the engine. In engines of this type, the eccentric counter rotating mass means are constituted by a shaft rotating in the opposite sense from the crankshaft, parallel thereto, and carrying eccentric masses able to create a centrifugal force which is added to that generated by the counterweights on the crankshafts when the pistons are dead centre but is in opposition when the pistons are mid-stroke, giving rise to a zero resultant. Thus, the alternating first-order forces are balanced. The use of counter-rotating shafts to solve the problem of vibrations is not without disadvantages. In fact, in addition to the increased weight, bulk and cost, it must be remembered that the counter-rotating shafts annul the effects of the inertia forces at the level of the supports for the engine on its supporting structure but do not eliminate the stresses which they discharge on the crankshaft (and hence on its main journals) within the engine. The object of the present invention is to provide an engine which does not have these disadvantages and which is simple and cheap to manufacture. According to the invention, this object is achieved by virtue of the fact that the eccentric mass means are supported directly by the crankshaft and by the fact that drive means are provided which are driven by the shaft to effect the counter-rotation of the eccentric mass means at the same speed of rotation as the crankshaft. By virtue of these characteristics, the alternating first-order forces are balanced 2'within" the crankshaft without stressing the bearings of the shaft; thus, there is a more reliable operation and a lower probability of breakdown. Moreover, the balancing system of the present invention has a small bulk and thus allows an engine to be made which is more compact and lighter. Preferably, the eccentric mass means comprise a single eccentric mass carried by a sleeve member rotatable coaxially on a central portion of the crankshaft between the two crank pins. Thus, the couples due to the alternating first-order forces are completely balanced, giving a further reduction in the forces on the crankshaft bearings. Further advantages and characteristics of the engine according to the present invention will become clear from the detailed description which follows, purely by way of non-limiting example, with reference to the appended drawings, in which: Figure 1 is a partially-sectioned side view of the engine of the invention, Figure 2 is a view taken on the arrow II of Figure 1, Figure 3 is a detail of Figure 1 on an enlarged scale, and Figure 4 is a section taken on the line IV-IV.of Figure 3. With reference to the drawings, a two-cylinder four-stroke engine with parallel cylinders is generally indicated 10 and has pistons 12, connecting rods 14 and a crankshaft 16. The shaft 16, whose axis is indicated X-X in the drawings, has two cranks 16a at 3600 and a corresponding pair of crank pins 16b. The crankshaft 16 also has two main journals, 16c and 16d respectively, at its ends rotatably supported by the structure of the engine, indicated 18, and two counterweights 20 interposed between the crank pins 16b. Between the counterweights 20, the crankshaft 16 has a central cylindrical portion 16e coaxial with the axis X-X, on which a sleeve member 24 is rotatably mounted with the interposition of a plain bearing 22. As shown in Figure 4, the member 24 includes two halfshells, 24a and 24b respectively, joined by fixing screws 26. The sleeve member 24 also has an external ring gear 28 and an eccentric mass 30 fixed to the half-shell 24b and provided with a reference notch 32. The structure 18 of the engine 10 rotatably supports an auxiliary shaft 34 parallel to the crankshaft 16 and having ends 34a and 34b respectively. A gear wheel 36 is keyed onto the end 34a of the shaft 34 and meshes with the ring gear 28 of the sleeve member 24. The gear 36 has an axial reference projection 38 corresponding and complementary to the notch 32 of the eccentric mass 30. A toothed pulley 40 is keyed onto the end 34b of the driving shaft 34 and is rotated, during operation of the engine, by a toothed belt 42 driven in its turn by a toothed pulley 44 keyed onto an end portion 46 of the driving shaft 16 adjacent the main journal 16d. The toothed belt 42 is tensioned by a tensioning roller 48 and also drives a toothed pulley 50 for rotating the timing shaft (not illustrated) The toothed pulleys 40, 44, 50 and the toothed belt 42 are enclosed in a protective casing 52. A pulley 54 is also keyed onto the end portion 46 of the crankshaft 16, outside the casing 52, and drives an alternator 58 through a V-belt 56. The crankshaft 16 has internal passages 60 for the circulation of lubricating oil for lubricating the crank pins 16b, the main journals 16c and 16d, and the plain bearing 22 of the sleeve member 24. Moreover, the ring gear 28 of the member 24 has the same number of teeth for each half-shell 24a and 24b, the distance between the half-shells being such that the line of separation corresponds to the gap between two consecutive teeth. During operation of the engine, the crankshaft 16 rotates the auxiliary shaft 34 on which the gear 36 is keyed through the toothed pulley 44, the toothed belt 42 and the toothed pulley 40. The auxiliary shaft 34 rotates in the same sense as the crankshaft 16, while the sleeve member 24 rotated by the ring gear 28 meshing with the gear 36 rotates in the opposite sense to the crankshaft 16. The transmission ratio between the crankshaft 16 and the sleeve member 24 is such that the sleeve member and the eccentric mass 30 fixed thereto rotate at the same speed of rotation as the crankshaft 16. With regard to the balancing of the alternating first-order forces, the member 24 and the eccentric mass 30 behave substantially identically to the eccentric masses of the prior-art counter- rotating shafts. The correct angular positioning of the eccentric mass 30 relative to the counterweights 20 of the driving shaft 16 is achieved by the projection 38 and the corresponding notch 32 which enable the synchronization of the eccentric mass 30 relative to the alternating masses of the crankshaft 16, as well as by the normal references on the toothed pulleys 40 and 44 (like those used for the phasing of the timing shaft).

Claims: CLAIMS 1. Two-cylinder internal combustion engine including a crankshaft 116), two parallel cylinders in which there are reciprocatingly slidable two pistons (12) having respective connecting rods (14) connected to the crankshaft (16) in correspondence with respective crank pins (16b) disposed at 3600, and counter-rotating eccentric mass means for balancing the alternating first-order forces during operation of the engine, characterised in that the eccentric mass means (24, 30) are carried directly by the crankshaft (16) and in that drive means (44, 42, 40, 34, 36, 28) are provided which are driven by the shaft (16) to effect the counter-rotation of the eccentric mass means (24, 30) at the same speed of rotation as the crankshaft (16). 2. Engine according to Claim 1, characterised in that the eccentric mass means comprise a single eccentric mass (30) carried by a sleeve member (24) rotatable coaxially on a central portion (16a) of the crankshaft (16) between the two crank pins (16b). 3. Engine according to Claim 1 or Claim 2, characterised in that the means for driving the counterrotation of the sleeve member (24) comprise a peripheral ring gear (28) formed on the member (24) and meshing with a complementary gear (36) carried by an auxiliary shaft (34) parallel to the crankshaft (16) and driven thereby. 4. Engine according to Claim 3, characterised in that the auxiliary shaft (34) is rotated by the crankshaft (16) through a toothed-belt transmission (44, 42, 40). 5. Engine according to Claim 3 or Claim 4, characterised in that the gear (36) has a lateral axial reference projection (38) corresponding to a complementary notch (32) in the mass (30) for ensuring that the mass is in exact angular phase with the crankshaft (16).

Inventors: CUCCATO, UBALDO
Applicants: FIAT AUTO S.P.A.
Application year: 1987
Main classification: F16F15
IPC codes: F16F15, F02B75, F02B67