| January/February 1998

Hit-or-Miss Arrangement

Fig. 7

The following is the third in a series of articles from the 1923 edition of Modern Mechanical Engineering, on the subject of gas engines. The original articles were sent to us by Jan vander Gugten, 2633 Ware Street, Abbotsford, B.C., Canada V2S 3E2, who thought our' readers would find them of interest. The first two installments appeared in our April and May 1997 issues.


Engines in general are required to maintain a considerable degree of uniformity in their speed under normal working conditions, and in this respect the single-cylindered single-acting four-stroke engine is at a serious disadvantage, as a working impulse occurs only, at best, once in every four strokes; the momentum of massive flywheels thus has to maintain rotation during at least three-fourths of the running time. This inherent drawback is, however, reduced to a practically negligible amount: (a) by using multi-cylindered single-acting engines; (b) by using tandem or twin-tandem double-acting engines; and (c) by using two-stroke single-or double-acting engines; and in nearly all cases with the addition of a massive flywheel.

Speed Fluctuation. The coefficient of fluctuation of speed of an engine is defined as the ratio of the difference between the maximum and minimum angular velocity of the crank-shaft per cycle to its mean angular velocity. The permissible value of this coefficient depends upon the nature of the work performed by the engine; usual values in a number of typical cases are given here-under:


Approximate Permissible

Nature of Service

Coefficient of Speed Fluctuation.

Driving pumps


Driving machine tools


Driving textile machinery


Driving C.C. dynamos


Driving spinning machinery


Driving direct-coupled alternators in parallel


Cf. R. E. Mathot, Construction and Working of I.C. Engines (Constable), or Clerk and Burls, The Gas, Petrol, and Oil Engine, Vol. II (Longmans).

For a full account see Clerk and Burls, Vol. II, Chapter IV.

So long as the external resistances overcome by the engine remain unchanged, cyclic speed fluctuation can be reduced to any desired extent by providing sufficient flywheel inertia; for a detailed consideration of this question reference must be made to the larger special treatises.