Drive assistance apparatus for a vehicle having a main transmission that is mechanical

Drive assistance apparatus for a vehicle having a mechanical main transmission for driving displacement members of the vehicle, the apparatus comprising a main source of fluid under pressure suitable for being driven by first drive means to deliver fluid under pressure, and at least one hydraulic motor suitable for being fed with fluid under pressure by the main source of fluid to drive at least one displacement member of the vehicle, said motor having a stator and a rotor suitable for rotating relative to the stator about an axis of rotation, the motor comprising a cinder block which belongs to a first one of the two elements constituted by the stator and the rotor, which is disposed inside a case, and which presents a plurality of cylinder-and-piston assemblies disposed radially relative to the axis of rotation, together with a reaction member for the pistons which belongs to the second one of said two elements, the motor also having a main fluid feed duct and a main fluid exhaust duct suitable for being put into communication with the cylinders, and an auxiliary duct connected to an internal space provided inside the case around the cylinder block and suitable for being put into communication with a fluid return tank, the motor being suitable for adopting a working configuration in which the pistons co-operate with the reaction member and are suitable for generating outlet torque under the effect of the fluid delivered by the main fluid source, and also a free-wheel configuration in which the pistons are maintained in the retracted position inside their cylinders so that the rotor is free to rotate relative to the stator without generating any outlet torque,

• the apparatus comprising means for performing the successive stages of a sequence for causing the motor to pass from its free-wheel configuration into its working configuration, said sequence comprising an initialization stage in which the auxiliary duct is isolated from the fluid return tank and is connected to a service duct which is fed with fluid under pressure such that a service fluid pressure exists inside the internal space of the case, and in which the main feed and exhaust ducts are placed at a common filling pressure which is less than the service pressure but greater than the pressure of the fluid return tank, the sequence further comprising, after the initialization stage, a clutch engagement stage during which the auxiliary duct is put into communication with the fluid return tank and during which the main feed and exhaust ducts are isolated from each other.