Buoyancy engine is a device which changes net buoyancy of an underwater vehicle by attracting and expelling water. Underwater gliders equipped with buoyancy engines can be driven by net buoyancy forces to travel in the vertical plane. Existing underwater gliders, such as Seaglider, Spray and SLOCUM are configured with single buoyancy engine and a longitudinally mass shifting device by which pitch angle are varied by moving their center of gravity. The buoyancy engine on Seaglider,and Spray include a high-pressure reciprocating pump and an external bladder [1,2]. For SLOCUM Battery, buoyancy engine is a piston driven by a motor. Modeling and control of ocean gliders driven by buoyancy engines and mass-shifting device can be found in [3]. Nonlinear