The invention provides an apparatus for providing forced aspiration to an internal combustion engine. The apparatus includes a first displacement device for being driven by exhaust gas from an internal combustion engine to which the apparatus is in use mounted and, a second displacement device operable to compress combustion gas for provision to an engine to which the apparatus is in use mounted. The first and second displacement devices are coupled such that when in use the first displacement device is driven and causes the second displacement device to operate.