Hydromechanical transmissions are characterized by a use of hydraulic pumps and motors to transfer the energy from a combustion engine to the wheels in a driveline. This enables a continuously variable transmission gear ratio, which facilitates exact control of the tractive force and allows the combustion engine to operate in its most efficient point. By adding a planetary gear train and clutches to the transmission, its efficiency and versatility may be increased further. The machine’s fuel consumption may be lowered even further by including a hydraulic accumulator to the transmission, which enables recuperation and reuse of energy in hybrid concepts.
With increased complexity of the transmissions, the need for fast, accurate control increases as well. A challenge in this area is that the transmissions are very versatile which makes it difficult to find a general control strategy that is applicable to more than one specific concept. This in turn presents another challenge in how to test the control strategies early in the design process to evaluate the feasibility of both the control strategy and the transmission concept itself.