Tensioner and engine

The tensioner comprises a piston, a piston cavity, a tensioning wheel, a magneto-rheological substance, an elastic component and a coil, the coil is used for generating a magnetic field, one end of the piston is inserted into the piston cavity, the tensioning wheel is installed on the piston, the piston cavity is filled with the magneto-rheological substance, and the magneto-rheological substance is arranged in the piston cavity. The elastic component is arranged between the piston and the cavity wall of the piston cavity defined by the piston in a compressed mode and used for participating in balancing the dynamic load transmitted from the to-be-tensioned mechanism. The tensioner can rapidly change the tension degree of a timing belt when the load of an engine changes, the problem of response lag of the tensioner is solved, meanwhile, the dynamic load transmitted by a self-tensioning wheel can be balanced through elastic potential energy of an elastic component and damping of magnetorheological substances, t