External meshing gear retarder and braking system

The utility model provides an external meshing gear retarder and a braking system, and relates to the technical field of automobile parts, the retarder comprises a shell, a gear transmission assembly, magnetorheological fluid and an electromagnetic generator; a working cavity is formed in the shell, and the working cavity is filled with magnetorheological fluid; the gear transmission assembly comprises a gear assembly and a rotating shaft; the gear assembly is arranged on the rotating shaft and is arranged in the working cavity; the rotating shaft can be in transmission connection with an output shaft of a vehicle gearbox, the shell is provided with a liquid inlet and a liquid outlet, the liquid inlet is connected with a liquid inlet pipe, and the liquid outlet is connected with a liquid outlet pipe; and an electromagnetic generating piece is arranged in the liquid inlet pipe and/or the liquid outlet pipe. When speed reduction is needed, the magneto-rheological fluid can be affected by a magnetic field by ope