The Fiat-SRM Hydraulic Transmission - A Typical City Bus Application Employing an Electronic Control Unit

The Fiat-SRM hydraulic transmission in a version suitable for city buses is described with regard to its basic features and to the different operational modes. The transmission uses a two-stage torque converter with fixed and rotating reactor, which provides two phases of hydraulic drive in the medium- and low-speed range. In the latter, the reaction torque on the reactor is added to the output torque by planetary gears, obtaining exceptionally high ratios at the stall point. A third phase in direct mechanical drive is used in the high-speed range, using a clutch bypassing the converter.

The different modes of operation are automatically controlled by an electronic unit, which is described with particular regard to its characteristics and to the possibility of obtaining high continuous decelerations on steep gradients, using the hydraulic braking offered by the transmission.