ROUGHNESS in the operation of engines has increased in seriousness with increase in the compression ratio and in the provisions for inducing turbulence in the combustion chamber, both of which factors tend to increase the rate of pressure rise in the engine. In this paper the thesis is maintained that this roughness consists of synchronous transverse vibration of the crankcase, due to variations in gas pressure and inertia forces. By synchronous vibration is meant a vibration which passes through a cycle in exactly the same time as the periodic force which produces it, so that the amplitude of the vibration builds up from cycle to cycle until the damping forces become equal to the exciting force.
Owing to the angularity of the connecting rod in all except the dead-center positions, the gas pressure produces an alternating horizontal force on the crankcase at the main bearings. This alternating horizontal force may be resolved into an endless series of harmonics of successively increasing frequencies. The actual frequencies naturally rise and fall with the speed of the engine, and when one of the lower harmonics corresponds in frequency to the natural frequency of the crankcase as an elastic vibrating body, serious lateral vibration is set up, which we know as engine roughness.
Gas pressures tend to cause lateral vibration in a horizontal plane only (in a vertical engine), but inertia forces tend to produce both horizontal and vertical vibrations. For each harmonic of the horizontal vibrating force except the first there is an harmonic of the vertical vibrating force, and if there happens to be approximate phase equality between the horizontal and vertical harmonics they will combine to produce vibration in an inclined plane. As the plane of vibration shifts from the horizontal, the natural frequency of vibration of the crankcase is likely to increase gradually, which is probably one reason why roughness generally is not confined to a narrow speed range but covers the whole range above a certain minimum critical speed.
The best safeguard against trouble from rough operation is to so design the crankcase that it is quite stiff in both the horizontal and vertical planes.
As the stiffness increases, so does the natural frequency of vibration of the crankcase, and if the latter is sufficiently stiff, only the higher harmonics of the transverse forces can come into synchronism with it. These higher harmonics are of such small magnitude that they cannot do much harm.