A four-stroke engine cylinder 10 has two separate intake ports 20a, 20b controlled by respective port throttles 22a, 22b and leading to respective intake valves 18a, 18b which are arranged in close proximity to the associated intake valves. A compressed air storage tank 30 is connected by respective lines 24, 32 to the intake ports 20a, 20b, each line 24, 32 incorporating a control valve 26, 32 and being connected to the associated intake port 20a, 20b at a point lying between the port throttle 22a, 2b and the intake valve 18a, 18b. In addition to a normal mode of operation in which fuel is burnt to generate power, the engine is operable in an air driven motor mode in which compressed air is supplied to the engine cylinder 10 from the compressed air storage tank 30 during the intake stroke of the engine by way of one of the intake ports 20a, 20b and is discharged from the engine cylinder during the ensuing compression stroke of the engine by way of the other intake port. Each port throttle valves 22a, 22b may be pivotally mounted (figs. 7,8).
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