An active chamber engine, includes at least one piston (2) slidingly mounted in a cylinder (1) and operating according to a three-phase thermodynamic cycle including an isobaric and isothermal transfer, a polytropic expansion with work and an exhaust at ambient pressure, which is preferably supplied with compressed air contained in a high-pressure storage tank (12), in which the volume of the cylinder (1) swept by the piston is divided into an active chamber (CA) and an expansion chamber (CD), and in which the compressed air is used to move the intake valve (9) in order to open and then close the intake duct, making it possible to supply the active chamber of the engine, the compressed air having been used for the actions then being reused in the engine to produce additional work.