An engine (10) which can deliver mechanical work for an extended duration in comparison to the duration of energy input into the engine. The engine (10) includes a drive shaft (11) through which it delivers mechanical work. The drive shaft (11) is drivingly connected to an electrodynamic machine (13) for generating electrical energy. The drive shaft (11) is driven by a drive system (15) which incorporates various drive mechanisms (16), power means (17) for operating the drive mechanisms (16) and gearing (18) for drivingly connecting the drive mechanisms (16) to the drive shaft (11). The various drive mechanisms (16) are operated in a pre-determined sequence by the power means (17). Each drive mechanism (16) is in the form of a rack and pinion mechanism. The power means (17) for operating the drive mechanisms (16) comprises spring structure associated with each rack of the rack and pinion mechanisms. The power means (17) also comprises a hydraulic ram for loading each of the spring structures.