1. Field of the Invention
The present invention relates to a drive unit, and, more particularly, to a drive unit for a motor vehicle having an internal combustion engine with a cooling system having a fan and a hydrodynamic retarder having a stator, a rotor and a housing. Such a drive unit is known from DE 37 13 580 C1.
2. Description of the Related Art
Retarders are primarily employed in heavy vehicles to absorb and convert to heat the kinetic braking energy which accrues most notably during high speed braking actions (adaptation braking). Retarders are also well suited for situations involving sustained braking, for example, maintaining a constant speed of 30 km/h on an incline of 7%. Oil normally serves as the operating fluid. The heat transferred in the retarder to the operating fluid must be delivered, by means of a specific heat exchanger, to a coolant or the ambient air.
U.S. Pat. No. 3,720,372 discloses a retarder which in one embodiment is powered by the crankshaft and constantly flooded by the coolant of the cooling system. The rotor of the retarder serves as circulating pump directing heated coolant to a heater core within the passenger compartment of the automobile. The purpose of this system is to heat the coolant by means of the retarder and thereby heat the passenger compartment. There is also a control system arranged on the retarder which controls the distribution of the coolant depending on its temperature.
Also known, from DE-PS 33 01 560, is a retarder which, by way of a clutch, is connected to the crankshaft of the drive engine and to the driven wheels of the vehicle. It is not the purpose of this retarder, however, to absorb the high kinetic braking energy of the vehicle and convert it to heat. Instead, the retarder is operated exclusively as a heater, with the available operating energy input controlling the heating output. The coolant of the engine also serves as the operating fluid of the retarder.
A retarder known from DE-AS 1 946 167 (U.S. Pat. No. 3,650,358) is powered by the crankshaft of an internal combustion engine whose coolant also serves as the operating fluid for the retarder. The advantage of this mode of operation is that the accruing heat develops directly in the coolant passed to the radiator and a heat exchanger between the two fluids is unnecessary. The rotor is mounted on an antifriction bearing and the seal between the frame and rotor shaft is established by two lip seals.
It is desirable to keep the overall axial dimensions and weight of the drive units of this type as low as possible, especially when the drive unit will be used in a motor vehicle. The drive units known heretofore have failed to satisfy this goal.