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A variable geometry turbine in which a turbine wheel is mounted to rotate about a pre-determined axis within a housing. A sidewall is displaceable relative to a surface of the housing to control the width of a gas inlet passage defined adjacent the wheel between the sidewall and the housing surface.

A variable geometry turbine in which a turbine wheel is mounted to rotate about a pre-determined axis within a housing. A sidewall is displaceable relative to a surface of the housing to control the width of a gas inlet passage defined adjacent the wheel between the sidewall and the housing surface. The sidewall is supported on rods extending parallel to the wheel rotation axis, and the rods are displaced to control the displacement of the sidewall relative to the housing. The housing defines a chamber into which the rods extend such that one or more piston and cylinder arrangements are defined. The pressure within the chamber is controlled to control the axial position of the piston, the sidewall being displaced as a result of displacement of the piston.

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1. A variable geometry turbine comprising a housing, a turbine wheel mounted to rotate about a pre-determined axis within the housing, a sidewall which is displaceable relative to the housing to control the width of a gas inlet passage defined adjacent the wheel between a first surface defined by th

1. A variable geometry turbine comprising a housing, a turbine wheel mounted to rotate about a pre-determined axis within the housing, a sidewall which is displaceable relative to the housing to control the width of a gas inlet passage defined adjacent the wheel between a first surface defined by the sidewall and a second surface defined by the housing, and displacement control means for controlling displacement of the sidewall relative to the housing, wherein the housing defines at least one chamber forming an annular cylinder which receives a piston comprising an annular member coupled to and defined by the sidewall, the sidewall is displaced as a result of displacement of the piston, and the displacement control means comprise means for controlling the pressure within the said at least one chamber to control the position of the sidewall relative to the housing said sidewall being supported on guide rods parallel to the wheel rotation axis, said guide rods being biased by at least one spring away from the second surface. 2. A variable geometry turbine according to claim 1, wherein each rod is biased by at least one spring away from the second surface. 3. A variable geometry turbine according to claim 2, wherein the said at least one spring has a variable spring rate such that the rate of change of spring force with gas flow passage width increases as the sidewall approaches the second surface. 4. A variable geometry turbine according to claim 3 wherein each rod extends through a respective compression spring bearing against the housing and the rod, and a further compression spring is arranged to bear against the end of each rod, the said further spring being compressed only when the sidewall approaches the second surface.