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).
Article from the S.A.E. Journal discussing the principles of resilient engine mountings and their relationship to combustion smoothness.
| Identifier | ExFiles\Box 126\4\ scan0144 | |
| Date | 1st April 1934 | |
| 142 S.A.E. JOURNAL (Transactions) harshness created by combustion; but, certainly, mountings cannot be expected to wash out all of the possible bumps that can be created by combustion. The distribution of the combustion-chamber volume is still of extreme importance. Fig. 7 represents a series of simple comparative curves that, in my opinion, should be used by all engineers before calling a job done. These curves give the percentage volume of the chamber in various locations as we progress across the chamber; they were relatively simply obtained without calculation, and eliminate the necessity for developing the resultant time-pressure or combustion curve which depends for accuracy on a varying constant. A simple cutting machine was rigged that could cut spherically. Segments of a plaster cast were progressively cut out, and the remaining volume checked each time. Curve A is from an L-head combustion-chamber and is ideal, being relatively smooth and indicating that the volume is distributed correctly, meaning that it is proportioned to give a smooth combustion-pressure rise throughout. This particular result has been repeatedly checked in cars and does give smooth operation. Curve B is from an overhead valve engine and is also ideal. There was a time when it was thought impossible to obtain combustion smoothness in an overhead-valve engine. It was assumed to be a prerogative of L-head engines. However, Curve B is proof enough. Curves C, D and E, all show possible rough spots which could be eliminated with a little consideration. Curve E is from an inherently rough engine, although it has been materially helped by mountings. No doubt there may be other causes for the roughness of the engine represented by Curve E.{Mr Elliott - Chief Engineer} However, the combustion curve certainly indicates need of adjustment of the combustion-chamber-volume distribution. [DIAGRAMS] Fig. 9—Means for Approaching the Ideal for Minimum Shift of the Principal Axis Mountings at Principal-Axis Centers Are Shown at A, and Mounting Tangential to the Principal Axis at B. These Two Groups Cover the Majority of Designs. CENTER OF GRAVITY PRINCIPAL AXIS POINT OF PRINCIPAL AXIS TANGENTIAL tion. Incidentally, this curve is from an L-head engine. Fig. 8 is an interesting picture of a characteristic light-beam chart of engine movement with high compression. Note the wavy condition of the record. This condition did not exist with low compression. The lines were clear and smooth. Torsional vibration is being adequately met by modern harmonizers with proper tuning. The types of harmonizers used vary, their desirability being a matter of opinion. Structural weakness of crankshaft and of crankcase are broad problems, and require complete treatment in a paper on this subject. Resonance of either case or shaft will make itself felt and heard through almost any type of mounting. With this remark we will leave the subject until a more propitious occasion. Engine Mountings From the foregoing it is seen that what is desired for engine mountings is: (1) Low resistance to rotation about the longitudinal principal axis. (2) Low resistance to rotation about the vertical axis through the center of gravity. (3) Minimum shift of effective principal axis and vertical axis. There are several means of approaching the ideal for minimum shift of the principal axis. Fig. 9 shows A, mountings at the principal-axis centers and B, mounting tangential to the principal axis. These two groups cover the majority of designs. Tangential mountings are being used fairly broadly, and we will confine our discussion and analysis to this type. They must either be capable of compound action, or must be located so that its one directional freedom will permit a rocking movement about the longitudinal axis, and also rotational movement about the vertical axis. Tangential mountings at the front end are divided into two classes as in Fig. 10, two points as in A, or single point as in B. Undoubtedly, from the slow speed, the two or the single-point front can be equally effective. However, the effect during high speed must be considered, since it is under these conditions that rotation around the vertical axis is in force. Fig. 8 Characteristic Light-Beam Chart of Engine Movement with High Compression Vol. 34, No. 4 RESILIENT MOUNTINGS 143 As stated, it is obvious that the detail construction can be compounded as in C, Fig. 10, or the position may be compromised as shown in D with somewhat increased softness. We may use a support that is so soft as to be universal. With the latter, engine flop during rough-road operation may be the limiting factor. The single-point mounting with its single direction satisfies both low-speed rocking and high-speed rotation. Being corrected, no compromise in softness need be made. In D, Fig. 10, is shown an old type of mounting that incorporated a 5.5:1 ratio of horizontal movement to vertical, adequately restraining the engine against flop and providing relative freedom in a plane capable of satisfying the compound demand. There are many other designs that may do just as well. Rear Engine-Mountings Rear engine-mountings are more complex and single-point mountings for the rear have been lacking in durability although, like the front, they satisfy both rotations. The transmission support in A, Fig. 11, is a support for the rear, located under the transmission. However, for durability, we find that two mountings at the rear have been necessary. The position of the rear mountings varies from ahead of the flywheel housing to the rear of the transmission. In the majority of cases they are located tangential to the longitudinal principal axis. However, this position does not satisfy the high-speed demand unless, like the front, they are compounded or oversoft or in some other way made universal. [DIAGRAMS] 1 SMOOTH BUT NOT DURABLE 2 NOT SMOOTH BUT DURABLE 3 NOT SMOOTH BUT DURABLE 4 SMOOTH AND DURABLE Fig. 11—Rear Engine-Mountings A support for rear engine-mounting, located under the transmission, is shown in Diagram A, while Diagram B indicates a series of rear mounts made to obtain high-speed smoothness and located as shown in Diagram C In B, Fig. 11, is indicated a series of rear mountings made to obtain high-speed smoothness. They were located as indicated in C. Only the designs permitting sideways movement were smooth. However, the slow-speed effect was not satisfactory. This series of experiments does serve to prove the importance of rotational movement about the vertical axis; in fact, in these experiments, we learned that it was more important than the rocking about the longitudinal axis. To substantiate our thoughts further on the necessity for rotational freedom, the Inland Manufacturing Co. furnished us with a transmission support incorporating a large measure of friction as shown in D, Fig. 11. This definitely upset things by making the job rough through both the high and the low-speed range. Looking at this whole problem we are forced to agree that we must provide relative movement about two axes which pass through the center of gravity. Then it is universal rotation at the center of gravity that is required, and the ideal would be a universal support right at the center of gravity. This is impossible, but a fair compromise can be made. The resistance to rotation should be balanced about the center of gravity. The relatively free plane of the mounts should be about the center of gravity, and the resistance should be in proportion to the distance from the center of gravity. In this way we not only provide the necessary direc- [DIAGRAMS] PRINCIPAL AXIS MAX. TANGENTIAL FREEDOM 2-POINT FRONT-END MAX. TANGENTIAL FREEDOM SINGLE-POINT FRONT-END HIGH-SPEED DIRECTION FREE TRANSVERSELY HERE CLAMPED TIGHTLY HERE MOVEMENT .034 PER 100 LB. Fig. 10—Tangential Mountings at the Front End A Two-Point Mounting Is Shown in Diagram A and a Single-Point Mounting in Diagram B. The Detail Construction can be compounded as shown in Diagram C or the position may be compromised as shown in Diagram D with somewhat increased softness. Diagram B represents an old type of mounting that incorporated a 5.5:1 ratio of horizontal movement to vertical April, 1934 | ||