If automobile builders had available a variable transmission that was capable of giving any ratio between the upper and the lower limits and that substantially was equally satisfactory at all ratios from the viewpoint of efficiency of transmission, wear and quietness of operation, a comparatively large reduction-ratio would be used most of the time, because that would assure the greatest fuel-economy.
Several types of continuously variable gears have been used on automobiles, or merely suggested for such use. Of these, the systems employing belts and friction discs or wheels need hardly be considered at present, because of their bulkiness and comparatively low efficiency. Much work has been done on hydraulic transmissions, chiefly with a view to their use on trucks and other heavy types of vehicle, and to increase the average efficiency of transmission it has been suggested recently to combine a direct drive with the hydraulic gear, so that the loss due to the double conversion need be sustained only under conditions of rapid acceleration and heavy traction. The form of hydraulic gear referred to is automatically variable in accordance with the torque load.
Variable-throw-type transmissions have been invented in great numbers but, so far, little commercial success has been attained. The most advanced design seems to be the Lavaud, developed in France, which at present is being given an endurance test by the French automobile builder, Voisin. With this design, the torque reaction on the rear-axle housing changes the angularity of a swash-plate and thereby varies the ratio. The weakest element of a variable-throw transmission seems to be the ball or roller ratchet, which must be made comparatively large for the amount of power to be transmitted.
The Weiss transmission, which was demonstrated at the 1924 Semi-Annual Meeting, is in a class by itself; hence, a simple explanation of the principle involved is given. In England, Constantinesco has been working on a transmission combining the variable-throw and the inertia principles. In inertia-type transmissions, the energy supplied by the driving shaft is stored up in reciprocating or oscillating weights and is then given up to the driven shaft at the same rate at which it was absorbed but with the relation between torque and speed changed. Although it is possible theoretically to make use of the inertia principle alone, without having recourse to a variable-throw device, no practical mechanism of this type seems to have been evolved as yet.