An engine with an active chamber, having at least one piston (2) mounted in a cylinder (1) in a sliding manner and driving a crankshaft (5) via a slider-crank device (3, 4) and operating according to a four-phase thermodynamic cycle includes: an isothermal expansion without work; a transfer—slight so-called quasi-isothermal expansion with work; a polytropic expansion with work; and an exhaust at ambient pressure, preferentially supplied by compressed air contained in a high-pressure storage tank (12), through a buffer capacity, called a working capacity (11), which is expanded at an average pressure, called a working pressure, in a working capacity, preferentially through a dynamic pressure-reducing device (13), wherein the active chamber is included in the engine cylinder, the cylinder volume being swept by the piston and divided into two separate parts, a first part forming the active chamber (CA) and a second part forming the expansion chamber (CD).