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).
Explaining the operation and benefits of an automotive overdrive system, its effect on gear ratios, and the resulting improvements in engine longevity.
| Identifier | ExFiles\Box 125\2\ scan0113 | |
| Date | 2nd November 1936 guessed | |
| accelerator which allows the mechanism to automatically disengage into direct drive. This arrangement is particularly advantageous for it automatically gives the driver a free-wheeling condition with easy shifting at speeds required in city driving and definitely locks out the free-wheeling action at high road speeds so that the engine can be used as a brake in cross-country driving. There is an overdrive control button on the instrument board. This button in its forward or “in” position allows the overdrive to function as described above. In its outward or “pulled out” position it locks the transmission so that overdrive cannot be obtained and free wheeling is not functioning in any gear. This is a position which might be desirable for mountain driving in slippery weather. This button should be operated at speeds below 35 miles per hour and the clutch pedal should be depressed in either pushing the button in or pulling it out. The use of an overdrive on the Airflow Chrysler makes it possible to select two high speed ratios, one permitting better acceleration and better hill climbing in high gear, the other with the overdrive in operation adapts the gear ratio to high speed work with smoothness and economy. Using one ratio for these two conditions is in reality a compromise. Since the overdrive engagement is governed by car speed, the overdrive may be used with second gear. Accelerate to 45 miles per hour in second gear, release the accelerator as before and the overdrive engages and you are now in overdrive second and the free-wheeling function is locked out. A silent drive with an overall ratio of 4.5 to 1 is now effective which will give remarkable hill climbing and acceleration performance. The following overall gear ratios can be obtained under the various conditions described above : Ratios Direct Overdrive High 4.3—1 3.03—1 2nd 6.41—1 4.51—1 • 4 • per hour at 4,000 engine revolutions per minute will, in overdrive, have a top speed of ninety miles per hour, and an engine speed of 3,000 revolutions per minute. At first glance it appears paradoxical that the engine should be slowed down and the maximum car speed increased, yet the change in gear ratio is the simple explanation for this performance. The power required to propel a car at a given speed is fixed by such considerations as rolling friction, and wind resistance. What we do when we drive at a constant speed is to adjust the power output of the engine by means of the throttle until it equals the power required to propel the car at that speed. If we open the throttle wide, the car will accelerate until the power developed and the power required again come into balance. It is safe to say that the life of a given engine is inversely proportional to the square of the operating speed. That is, at 4,000 revolutions per minute the life would be roughly ¼ of what it would be at 2,000 revolutions per minute. The reason for this is that the loads on the bearing surfaces increase as the square of the speed. The higher the load on a bearing the greater is the friction. Friction not only means waste of power but manifests itself as heat. Resistance of a bearing surface to wear and abrasion decreases rapidly as the temperature increases. Rapid increase in bearing friction with increased load and speed means a terrific acceleration in the rate of wear. AT 4000 R.P.M. AN ENGINE WILL LAST THIS LONG AT 2000 R.P.M. THE SAME ENGINE WILL LAST THIS LONG The reduction in engine speed brought about by the overdrive also means a tremendous gain in piston, piston ring, and cylinder bore life. It is easy to lose sight of the fact that in the last analysis, the car is actually propelled by the pistons of the engine. Thus, there is a definite relation between the distance travelled by the pistons and that travelled by the car. In conventional drive each piston travels over four miles for each ten miles of car travel or a total piston travel for an eight-cylinder engine of over thirty-two miles for each ten miles of car travel. Considering the tremendous forces acting on the pistons, one begins to appreciate the possibilities of wear in this vital engine part, also the tremendous advantage of - 9 - | ||