Abstract: Arrangement and method for operating a drive unit for motor vehicles, particularly for agricultural and construction machinery, with a controllable internal combustion engine (2), a continuously variable hydromechanical power split transmission (6) and a control device (3) which in order to start and to operate the vehicle at low speeds and under load, at first reduces the rotational speed of the internal combustion engine to less then the nominal speed, reaches a first speed by reducing the delivery volume of the adjusting pump (13) of the hydrostatic branch (15), a further increased speed by increasing the rotational speed of the internal combustion engine and as a result the further increase in the vehicle speed by controlling the transmission ratio of the hydrostatic-mechanical power split transmission (6) is achieved.

Abstract: An automatic control system for a hydrostatic transmission having a control circuit receiving a control flow from a control pump which represents a speed of a prime mover and valve components including a single adjustable valve which creates a delta pressure signal to control the transmission ratio which controls the speed of the prime mover.