A system for balancing a four-stroke internal combustion engine, particularly with five cylinders in line

Abstract: A system for balancing an internal combustion engine, particularly a four-stroke engine with five cylinders in line, in which two of the counterweights (7) for balancing the crankshaft (1) have asymmetric geometry defined by an additional mass (8) in each case. The respective centres of gravity (m′ s ) of the two additional masses (8) are situated in the plane (P) containing the resultant moment of the rotary components of the first-order alternating forces and the centrifugal forces of the rotating masses.

Description: The present invention relates in general to internal combustion engines of the type indicated in the pre-characterizing portion of the annexed claim 1. In engines of this type, there is a problem in balancing the resultant moments of the centrifugal forces of the rotating masses and the rotary components of the first-order alternating forces. Balancing systems are known for this purpose which have counterweights designed so that each has a static moment opposed to that of the respective crank pin. DE-A-3 447 048 discloses a system for balancing a four-stroke internal combustion engine, wherein each crank pin of the crankshaft comprises a pair of respective balancing counterweights. In the case of odd number of cylinders is necessary to add supplementary masses to the counterweights in order to compensate the rising of external moments. This solution, however, involves the use of very large counterweights which are sometimes incompatible with the space available for the installation of the engine in the vehicle. In fact, in order to balance the said moments, which are indicated M c below, it is necessary for the counterweights to achieve a static moment m s given by the formula: M c = a . i . m s . ω² where a is a coefficient which depends on the engine type (for example a = 0.449 for five-cylinder, in-line engine), and i is the interaxial spacing of the cylinders. m s is therefore large since a and i are small. The object of the present invention is to avoid the aforesaid problem and to provide a system for balancing an engine of the type defined above which enables the resultant moment of the centrifugal forces and the rotary components of the first order alternating forces to be balanced by means of light-weight drive shaft counterweights of low bulk, without the need for separate balancing of the flywheel and the pulley associated with the engine. According to the invention, this object is achieved by virtue of the features set forth in the characterizing portion of claim 1. As is well known, the plane containing the resultant moment of the torques generated by the rotary components of first-order alternating forces and the centrifugal forces of the rotating masses can be identified in precise geometric terms by calculation and its position is characteristic of the number of cylinders in the engine. In the case of an engine with five cylinders, the plane intersects the vertical plane passing through the axis of the crankshaft at an angle of 54°. To advantage, the two additional masses are carried by the balancing counterweights associated with the ends of the shaft. This leads to the two additional masses having very low static moments: in fact the moment to be balanced thus becomes: M c = (n i ) m' s ω² where n i is the lever arm expressed as a multiple of the interaxial spacing: clearly m' s = a m s n <<m s Moreover by calibrating the values of m' s one can easily adapt the same drive shaft to different alternating masses, that is, for example to Otto-cycle and Diesel-cycle versions of the same engine: it is in fact sufficient to provide corresponding cut lines at different depths for the different versions on the additional masses m' s . The invention will now be described in detail with reference to the appended drawings, provided purely by way of non-limiting example, in which: Figure 1 is a schematic perspective view of a crankshaft provided with a balancing system according to the invention, and Figure 2 is a schematic elevational view of one end of the shaft. With reference to the drawings, the crankshaft of a four-stroke motor-vehicle internal combustion engine with five cylinders in line is schematically and generally indicated 1. The crankshaft 1 is formed with five crank pins 2-6 each associated, in known manner, with a connecting-rod-piston-cylinder unit, not illustrated, of the engine. The crank pins are angularly spaced around the axis A of the shaft 1 by equal phase angles of 72° in a definite keying sequence. The crank arms or cheeks of the shaft 1 between the crank pins 2-6 are provided, in generally known manner, with balancing counterweights of which only that corresponding to one of the end crank arms of the shaft is shown at 7 in Figure 2. According to the invention, the end counterweight 7 has asymmetric geometry defined by an additional lateral mass 8 whose centre of gravity, indicated m' s , is situated in the plane P containing the resultant moment of the rotary components of the first-order alternating forces and the centrifugal forces of the rotating masses. The position of this plane is characteristic for each engine type and can be determined by calculation in a manner well known to an expert in the art. According to the invention, the balancing counterweight associated with the crank arm at the other end of the shaft 1 also has similar asymmetric geometry due to the presence of a respective additional lateral mass 8 whose centre of gravity is also situated in the said plane P. The two additional masses 8 effectively balance the torques generated by the centrifugal forces and by the rotating components of the first-order alternating forces but with little weight and bulk: indeed, with the arrangement described above, the static moment of the two additional masses 8 may be extremely small. Moreover, the provision of cut planes at different depths of the two additional masses 8 enables the same drive shaft 1 to be adapted easily to different versions (for example Otto-cycle and Diesel-cycle) of the engine with which the shaft is used.

Claims: A system for balancing a four-stroke internal combustion engine, particularly with five cylinders in line, including a crankshaft (1) with crank pins (2, 3, 4, 5, 6) which are angularly spaced around the axis (A) of the shaft (1) and provided with balancing counterweights (7), wherein at least two of the balancing counterweights (7) have an additional mass, characterized in that two of the balancing counterweights (7) have asymmetric geometry defined by an additional lateral mass (8) for each, the respective centres of gravity (m' s ) of the two additional masses (8) being located on opposite sides of the axis (A) of the crankshaft (1) and in the plane (P) containing the resultant moment of the rotary components of the first-order alternating forces and the centrifugal forces of the rotating masses. A system according to Claim 1, characterized in that the two balancing counterweights (7) with the additional masses (8) are associated with the ends of the shaft (1).

Inventors: MAIORANA GIOVANNI, MAIORANA, GIOVANNI
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1990
Main classification: F16C3
IPC codes: F02B75, F16F15, F16C3