citroën technical training zf 4hp20 automatic transmission

41Chapter 7THE MECHANISMI -INTRODUCTIONThe transmission mechanism has two roles:• it provides 4 forward gears and 1 reverse gear,• it automatically changes the forward gears.The first function is provided by two epicyclic gear trains which receive themovement from the turbine shaft, perform the demultiplications and transmit themovement to the step-down gear.Automatic changes are performed by hydraulic, electric and electroniccomponents.Note: The mechanism of the 4HP20 automatic transmission does not have a freewheel since gear changes are performed in a very precise manner andunder the best possible conditions due to the hydraulic unit and the ECUstrategies.Reminder of free wheels: in previous automatic transmissions, free wheelsallowed gears to be changed without any interruption in the traction force.Removing the free wheels has the following advantages:• compact and lighter transmission,• less mechanical coupling components,• better efficiency by reducing friction losses,• lower torque peaks on the components and the moving items.ZF 4HP20 AUTOMATIC TRANSMISSION© AUTOMOBILES CITROËN Toute reproduction ou traduction même partielle sans l'autorisation écrite d'AUTOMOBILES CITROËN est interdite et constitue une contrefaçon