The air supply system has an air intake passage (11) for air to flow to the engine (8). A control (12) varies the air flow. A driver actuated mechanism varies the air flow in response to driver demand. - There is a second air intake passage (13) through which air can flow to the engine. A second control (5) varies the air flow through the secondary passage. An actuator responds to sensed engine conditions to operate the second control to vary air flow through the second passage. The first and second controls are operable independently, so that air may be supplied solely through one of the passages or simultaneously through both.
