Engine braking system

An engine braking system comprises an internal combustion engine 1, a turbocharger 6 with a variable geometry turbine (VGT) 7, and an exhaust brake valve 9 in the exhaust path 5 downstream of the turbine wheel 10. A variable size exhaust gas inlet passage 19 is defined, in one embodiment, between a movable member 21 and a facing wall 22 of the turbine housing 11. The movable member 21 is movable in an annular cavity 31 to alter the size of the inlet passageway 19 between fully open and restricted positions. In the restricted position a bypass flow path may be provided for delivering gas to pressurize the cavity 31 such that the gas applies a force to the movable member 21 in a direction towards the facing wall 22. When the exhaust brake valve 9 is actuated to restrict the exhaust gas flow the speed of rotation of the turbine 7, and therefore the compressor 8, tends to reduce. This is mitigated by restricting the size of the inlet passageway 19 so that the gas flow can maintain the rotation of turbocharger shaft 14 so that the pressure in the compressor housing 13 does not reduce to an undesirable level where oil leakage into the housing may occur.