Voith DIWA

The Voith DIWA automotive transmission was specifically designed by Voith primarily for city-buses and suburban buses. The transmission utilizes a Differential-Wandler (Converter) or DIWA which replaces two gears which would be used on a conventional transmission. It has several generations - all of the automatic transmission type with many variants.

During Starting, the proven DIWA power split principle comes into action. As soon as the vehicle begins to move, power is divided steplessly between the hydrodynamic and the mechanical transmission of torque. In 1st gear, the DIWA range, input clutch and turbine brake are closed. The engine torque is directly transmitted to the outer rim of the differential. The impeller and the pump shaft is driven in opposite rotary directions via the planetary gears of the differential. During this process, the output and hence the planetary carrier remain stationary. In the converter, oil delivered by the impeller flows through the guide wheel to the turbine wheel in a closed circuit. The torque that is delivered and converted, that is increased, is transmitted to the planetary carrier via the sun gear of the turbine gear. The planetary gear shaft transmits both the torque of the input differential, that is the purely mechanical share, and the torque of the turbine gear, that is the hydrodynamic share, directly to the transmission output.

From 2nd to 4th gear, in contrast to the 1st gear, power transmission in 2nd to 4th gear occurs purely mechanically. The transition from 1st gear into 2nd gear occurs automatically, independent on the driving speed and the driving peddle position. The hydrodynamic power transmission is switched off in 2nd gear. This occurs by the pump brake being closed, which stops the impeller. At the same time, the turbine wheel is not transmitting torque to the turbine gear due to the open turbine brake. The transmission ratio of the 2nd gear corresponds to the input differential. When changing from 2nd into 3rd gear, the input clutch opens and the lockup clutch, is shut. As a result, the drive shaft is, by the outer rim of the locked up clutch, directly connected with the planetary gear shaft, and thus the output shaft. The ratio is now 1:1. Depending on the position of the accelerator pedal, the vehicle load and the topography, the shift from 3rd to 4th gear occurs automatically. During this transition, the locked up clutch opens while the coupling for the 4th gear, the fast release coupling, closes. The 4th gear represents the overdrive gear, with the ratio of 0.7 or 0.73 depending on the differential.

In order to activate the retarder in the mechanical gears 2 to 4, the reverse gear brake, is closed. As a result, the turbine wheel is accelerated by a correspondingly high ratio and driven in 1st gear against the rotary direction. The turbine wheel now acts as an axial pump which delivers oil against the guide wheel and the braked or stationary impeller. During this process, kinetic energy is converted into heat which is dissipated by the heat exchanger. The vehicle is braked.

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