Balancing of High-Speed Four-Stroke Engines

Abstract

The problem of balancing the rotating and reciprocating components of high-speed engines has engaged the attention of engineers for many years with the result that a considerable literature is now available on this subject. Generally, the need to minimize external and internal force and couple unbalance is well recognized, since only in this way is it possible to avoid excessive vibration of the engine and its surroundings as well as undesirable fluctuating loads on the engine and its mounting structure. Furthermore, an engine which is dynamically ‘quiet’ is generally more silent and durable than one which is not.

Unbalanced rotating forces may be readily dealt with by the addition of counterweights to the crankshaft although extravagant use of these is to be avoided. Not only do they increase engine weight and cost, but the additional rotational inertia which they produce may lead to difficulties in dealing with the problem of torsional vibration. It is pointed out in the paper that where such weights are a necessity from the balance aspect, they may be designed as pendulum vibration absorbers so producing an engine which is smoother in a torsional sense.

Unbalanced inertia forces are normally dealt with by selecting an engine assembly which provides collective balance of the forces at the individual cylinders, and in presenting this paper, the author considers that one of the best methods of studying these effects is by means of reverse rotation vectors. This method enables a clear picture to be obtained of the resultant unbalance effect of different engine arrangements and provides a ready means of determining the best solution. The representation of reciprocating forces by means of reverse rotation vectors is not new and, indeed, was the basis of the Lanchester second-order balancer for four-cylinder in-line engines.

Some of the engine balancing arrangements in current use in the automobile industry are reviewed and discussed and mention is also made of some of the more important practical problems encountered when carrying out actual balancing operations on crankshafts and their associated components. Variations in the balance of identical engines may be traced to assembly tolerances and hence there is an increase in the practice of assembly balancing. As a logical conclusion it is to be expected that final balancing operations will eventually be made on the complete engine on the test bed.