A buoyancy engine is a device that alters the buoyancy of a vehicle or object in order to either move it vertically, as in the case of underwater profiling floats and stealth buoys, or provide forward motion (therefore providing variable-buoyancy propulsion) such as with underwater gliders and some autonomous aircraft.[1][2]
For underwater applications, buoyancy engines typically involve a hydraulic pump that either inflates and deflates an external bladder filled with hydraulic fluid, or extends and retracts a rigid plunger. The change in the vehicle's total volume alters its buoyancy, making it float upwards or sink as required.[1] Alternative systems employing gas obtained from water electrolysis, rather than hydraulic fluid, have also been proposed,[2] as have systems which pump ambient water into and out of a pressure vessel.[3]