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An American View of

24th October 1947
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The Torque Converter

BY affording a better performance for a given power-toweight ratio, the hydraulic torque converter enhances speed and economy, and reduces maintenance. A variety of benefits results from its employment, and time, energy and material—both mechanical and human—are conserved.

The question might well be asked, "Why did the torque converter not gain a foothold earlier?" The idea is not revolutionary. Rather, it is an evolutionary development which started in 1905, when Foettinger obtained basic patents on both the hydraulic clutch and the torque converter, It was developed in connection with high-speed steam turbines for ships and, odd as it may seem, was prompted by the need for a high-capacity reduction drive, because the art of gear reduction at that time had not reached an advanced state.

Its application to marine engineering was much more simple than to road vehicles, as the normal cruising speed of a ship is fixed. Much further research was necessary before the adaptation could come about, and this was work for a succeeding generation. During the development period Foettinger surrounded himself with young engineers who were later to become known for their achievements in this field.

To apply the torque converter to motor vehicles a broadening of the high-efficiency speed range was necessary. This alone has not sufficed, because, despite the broader efficiency, the curve still starts at zero, rising to its maximum range and then dropping to zero again. Zero efficiency was never associated with the conventional gear train, yet close observation shows that this state exists as a clutch slips into positive engagement. Once engaged, the gear train is a closer approach to complete efficiency than is any other type, but disadvantages exist, inasmuch as the torque multiplication is always equal to the eear combination into which it is meshed. Unfortunately, Ccigirie torque and fueleconomy curves drop off at low speeds, when the demand for high torque is the greatest. Thus, what is needed is a transmission that incorporates the features of the torque converter, by providing torque multiplication with decreasing speeds, and that permits the engine to operate in its most efficient range.

Lysholin, applying his knowledge of steam turbine theory to the torque converter, effected a broadening of the high-efficiency speed range and cut out the action at certain periods, so as to by-pass the converter action in the highspeed low-efficiency range. Schneider's contribution consisted of a new type of blading, which made the simple single-stage converter economical for vehicle use, in combination with two or more operating speed ranges. This made possible a combination of the advantages of the hydraulic torque converter and the hydraulic coupling in a self-contained unit, by means of a free-wheeling reaction member permitting a hydraulic drive throughout all speed ranges.

The coach industry has provided the real stimulus for America's development of successful hydraulic transmission, prominent in this field to-day being the Spicer Converter and the White Hydrotorque. The Spicer Converter is doing a good job in the passenger field and, because of its construction, produces a relatively high initial torque increase, which may approach a ratio of 6:1.

The point where torque input equals the output occurs at a relatively low speed ratio, that is, the ratio of speed of the turbine output shaft to the speed of the engine. In order to preserve efficiency of operation, it is desirable at this point to engage a mechanical drive between the engine and transmission. This is accomplished automatically by a transmission governor, as is the change back to torque converter. With the Spicer device, no gearbox is used, other than for the provision of reverse gear.

The White Hydrotorque drive has more recently made its commercial appearance. This device performs the functions of an automatic transmission by combining the features of a hydraulic torque converter and hydraulic coupling with a two-speed gearbox which extends the converter range of torque multiplication and provides exceptional acceleration at low speeds. Automatic controls deteimine the occurrence of gear change. The hydraulic unit of the torque converter consists primarily of a pump, a turbine and a reaction member. The pump is in principle like any other centrifugal fluid pump and the turbine operates in much the same manner as impulse hydraulic turbines. The pump discharges directly into the turbine and the turbine discharges directly back to the pump suction after the direction of the oil flow has been changed by the reaction member.

This close-coupled arrangement eliminates a source of loss and the reaction member makes a torque increase possible. The member is arranged so that in one direction of rotation it is firmly anchored to the transmission case, but is free to rotate in the other direction. Essentially a fulcrum point, the reaction member absorbs the difference between the torque delivered by the engine and that available for transmission to the propeller shaft. If there were no reaction member, or if it could rotate in both directions, the White Hydrotorque would be a hydraulic coupling, similar io those often used on private cars, and incapable of torque increase.

The characteristic of a torque converter, as distinct from hydraulic transmission, is that it provides torque multiplication and a relatively high efficiency of conversion of energy from the engine to the transmission when the speed ratio between turbine and engine is decreased, such as when the vehicle moves from a standstill to a moderate speed. In this range, a hydraulic coupling's efficiency is relatively low and when accelerating from a low speed there are advantages in the employment of the torque converter. When the speed ratio between turbine and engine is increased, or when the vehicle is accelerated to a moderate speed on level roads, the torque converter's efficiency falls off and it is desirable to use the characteristics of the hydraulic coupling. /hese functions are carried out in the White Hydrotorque In the one-way clutch reaction member. When the vehicle is being accelerated from low speeds, the angle of force of the oil issuing from the turbine to the reaction member causes that member to rotate in a direction counter to that of the turbine. This tendency is resisted by the one-way clutch, which locks the reaction member to the crankcase. As the vehicle is accelerated to a moderate speed, the angle of force of oil automatically changes until, at a point where the engine torque equals turbine torque, the force on the reaction member moves to the opposite side of the reactionmember blades, thus causing that member to rotate in the same direction as the turbine. This the one-way clutch permits, and so the torque converter becomes a hydraulic coupling.

To obtain good acceleration and climbing power, a twospeed gearbox has been provided which also incorporates the reversing mechanism; the automatically controlled change from low to high gear is accomplished by a piston actuated by compressed air and the highto-low movement by means of a spring. Selection of forward, neutral or reverse gear is manually controlled by the driver. In the case of airpressure failure, the vehicle would be able to proceed in low gear.

It would seem that the use of the torque converter will open up new possibilities in engine design. The low-speed, low-torque and low-efficiency parts of the engine performance are made unnecessary by the converter, and for this reason the engine designer would concentrate on improving the upper ranges of engine performance, without having to concern himself with the sometimes conflicting low-speed requirements. Various means suggest themselves, including mechanically driven or exhaust-driven superchargers.

A mechanical supercharger drive, making use of the speed differential across the converter, appears possible by use of a differential drive. This type of drive would provide engine supercharging in accordance with the vehicle's own torque demand, rather than in accordance with engine speed. Even with the advent of new types of prime mover, such as the gas turbine, the hydraulic torque converter Will continue to attract major interest as a possible means for supplementing the characteristics of these new devices. [Experience gained in Britain before 1939 in the use of torque converters for passenger transport vehicles, showed that although theygaite smoothness in performance, the running costs were high. On certain routes, fuel-consump tion figures rose by 10 per cent, above the normal, and certain weaknesses appeared in the component parts of the converter. Difficulties created by the war have hindered further development. —ED.'

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