An engine for use in an air hybrid vehicle comprises at least one cylinder having a piston (20) defining a variable volume working chamber (10) and intake (12, 14) and exhaust (16) valves controlling the flow of air into and out of the working chamber. The cylinder is operable in any one of at least two modes, namely a first mode in which power is generated by burning fuel in the working chamber (10), and a second mode in which the cylinder acts to compress air drawn into the working chamber (10) and to store the compressed air in an air tank (36). The engine further comprises a non-return valve (32) in an intake port leading to an intake valve (12) of the cylinder so as to define an auxiliary chamber (30) in the intake port between the intake valve (12) and the non-return valve (32). A passage (24) connecting the auxiliary chamber (30) to the air tank (36) contains a valve (34) for controlling the flow of compressed air between the auxiliary chamber (30) and the air tank (36). In the invention, a stop (40) is provided which is operative to prevent full closure of the intake valve (12) when the engine is operating in the second mode, whereby the air compressed in the working chamber by the movement of the piston is admitted through the open intake valve (12) into the auxiliary chamber (30).
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