From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The elimination of secondary vibrations and engine roughness in various engine configurations.
| Identifier | ExFiles\Box 126\4\ scan0139 | |
| Date | 1st April 1934 | |
| RESILIENT MOUNTINGS 137 Elimination of Secondaries The development of the six-cylinder engine was among the earliest attempts to eliminate secondaries and also the earliest producer of torsional vibration. More than twenty years ago Lanchester developed mechanical antidotes for these vibrations; namely, the “bob-weight” counterbalance for secondaries and the torsional damper for long six-cylinder crankshafts. The “bob-weight” counterbalance was used in this country, but not extensively. The cure was considered by some to be worse than the disease. Dampers are in general use today. The battle of the secondaries, however, went on for many years. The development of the 90-deg.-crankshaft V-Eight, as used in the Cadillac for years, was the result of that company’s efforts to eliminate the vibration caused by secondaries from their original 180-deg.-crankshaft V-Eight. The battle against this form of vibration was not confined to our industry. An outstanding and interesting example is to be found in the electric-refrigeration industry. The possibility of elimination of secondaries by mounting began to interest engineers. Three outstanding examples appeared. The electric-refrigeration industry produced a complete commercial result. The compressor and its driving unit were set on opposite ends of a frame, which frame was very flexibly supported at the center of gravity of the whole unit on a base. Rocking was limited by flexible bumpers. This unit operated with commercial freedom from secondaries; however, the unit rocked violently as a whole about its mountings when starting. Engineers became imbued with the idea that the 90-deg. crank for V-Eights, with its attendant heavy and costly counterweights, could be eliminated, as well as its now corrected inherent distribution weakness. Engines with 180-deg. cranks were mounted to permit movement about the polar axis of the engine. These mountings were fairly flexible, but by no means excessively so. In conjunction with the mountings was a mechanical device designed to eliminate the effect of transverse movement of the powerplant. This unit was designed to provide zero change in load on the frame by means of a cam-operated plunger moving at twice engine speed, and was attached to the frame. Direction of movement was phased to offset engine movement. This device was originally developed for four-cylinder engines by C. E.{Mr Elliott - Chief Engineer} Summers and R.{Sir Henry Royce} K.{Mr Kilner} Lee, at that time of the General Motors Research Corp., and was adapted by Summers to the Oakland Light Eight which was quite as smooth as the 90-deg.-crank-shaft V-Eight. Another interesting example was a four-cylinder-engine solution of secondaries, this being the next to appear. Here again the unit was mounted to swing or move about or around the polar axis. In this case the mountings were extremely flexible, the engine moving considerably, making necessary special consideration for connections of various pipes and leads to the engine. We must admire the courage of the organization which commercialized extreme flexibility and produced a fair commercial result in the business of smoothing out the four-cylinder engine. Three examples of the commercial elimination of secondaries have been cited, but they do not exist today. The electric refrigerator uses a new type of compressor that is inherently smooth. The Oakland Co. uses a Line-Eight engine that is inherently smooth. The four-cylinder proponent uses a six-cylinder engine that is inherently smooth. Each, however, has applied his mounting knowledge to his inherently smooth unit with considerable success. Merchandizing expediency is quite often more potent than engineering ingenuity. Engine Roughness A few of the contributing causes of engine roughness are: (1) Dynamics; including running balance or out-of-balance (2) Combustion Roughness; including rate of pressure rise and high brake mean effective pressure (3) Torsional Roughness (4) Structural Weakness Consideration must be given to all of the foregoing factors if an engine is to be considered inherently smooth. At best we can only expect engine mountings to add that portion of “silk” thought necessary in modern motor-car operation. To overburden the mountings with an inherently rough engine will mean excessive softness for the mountings and a low durability. Engine roughness during car operation can be divided into two ranges; (a) below 20 m.p.h., or torque reaction, and (b) above 20 m.p.h., or range roughness. These conditions or elements may be treated structurally to a certain extent. Torque reaction, being a function of the engine impulses per car mile, can be washed out of the picture by increasing the number of cylinders. Range roughness, though more complex, does react to treatment along the lines of rigidity, correct balance and low reciprocating values. However, in spite of all we may do, shock-absorbing mountings are necessary. Two apparent facts stand out as a result of careful observation. They are: (1) Low resistance to rotation about the principal axis provides low torque-reaction frequencies. (2) High lateral freedom—transverse of the crankshaft axis—provides the maximum of smoothness throughout the speed range. The first can be readily supported mathematically. The following data were developed by William Samuels, of the Chevrolet engineering department. “A six-cylinder engine may be assumed to be a three-cylinder engine having all cylinders in one central plane, and having in each cylinder reciprocating weights of twice actual value, with cranks spaced at 120 deg. “Superimposed upon each other a regular torque wave is produced which for all practical purposes may be considered a regular sine curve with three full waves per engine revolution. The resulting frequency is therefore three times the number of revolutions. “The frequency of engine movement can be determined as follows, reference being made to Fig. 1. “The weight of the rocking body (engine and transmission assembly, Confederate, 1932) is 660 lb. The term ‘body’ refers to the rocking body, that is, engine and transmission assembly. The moment of inertia of the body is I1 = 25,087 lb.-in.2 I = I1/g = 25,087 lb.-in.2 / 386 in. per sec.2 = 65 lb.-in. sec.2 “Radius R = 11.25 in. and R2 = 126.6 in.2 “The rate of each spring is 50 lb. per 1/16 in., or 800 lb. per in. “The total spring constant, c, equals 2 X 800, or 1600 lb. per in. April, 1934 | ||