What is the fundamental difference between a hydrostatic transmission and a hydrokinetic transmission?

The fundamental difference between a hydrostatic transmission and a hydrokinetic transmission lies in how they transmit power from the engine to the wheels or driven components: hydrostatic transmissions use fluid pressure and displacement, while hydrokinetic transmissions use fluid velocity and kinetic energy. A hydrostatic transmission consists of a hydraulic pump and a hydraulic motor. The pump, driven by the engine, converts mechanical energy into hydraulic energy by displacing fluid under pressure. This pressurized fluid is then directed to the hydraulic motor, which converts the hydraulic energy back into mechanical energy to drive the wheels or other components. The amount of power transmitted is controlled by varying the displacement of the pump or motor, which changes the flow rate and pressure of the hydraulic fluid. A hydrokinetic transmission, on the other hand, uses a fluid coupling or torque converter to transmit power. The engine drives an impeller, which throws fluid against a turbine. The turbine is connected to the transmission's output shaft. The kinetic energy of the fluid transfers power from the impeller to the turbine. Torque multiplication can occur in a hydrokinetic transmission due to the design of the impeller, turbine, and stator (in a torque converter). However, the primary mechanism of power transfer is through fluid velocity, not fluid pressure. Hydrostatic transmissions offer infinitely variable speed control and are often used in applications requiring precise speed and torque control, such as skid-steer loaders and combine harvesters. Hydrokinetic transmissions are commonly found in automatic transmissions in cars and trucks where smoother starts and shifting are desired, but precise control isn't the primary concern. For example, a combine harvester uses a hydrostatic transmission to precisely control its ground speed while harvesting, regardless of the crop load, while a road truck uses a hydrokinetic automatic transmission for comfortable gear changes and efficient highway driving.