Hydrostatic Transmission
Hydrostatic transmissions with two displacement machines working as a pump or engine, enable the variation of the speed of the combustion engine independent of load. By shifting the hydraulic machines, e.g. axial piston pump or engine, the negative velocity of flow can be set in any direction between zero and the maximum value. Thus, in hydrostatic transmissions, neither starting clutches nor gear clusters are required as back up. Transmission that have these characteristics, are called IVTs (Infinitely Variable Transmissions). The figure shows the conversion characteristic of such a transmission. It essentially corresponds to the principle of the aforementioned mechanical CVT.
The disadvantage here is that each of the two hydraulic machines have to transfer the entire driving power and thus they have to be dimensioned correspondingly large. This has a particular negative effect on the efficiency of the transmission. Further disadvantages of hydrostatic transmissions include the unfavorable specific power output, high production costs, and the noise development. Therefore we do not deal with this transmission any further. Such transmissions are often used in construction and agricultural machinery, partly as components of hi-tech transmissions in which a mechanical branch is responsible for the improvement of efficiency.