An engine has two pressure vessels arranged as a diametrically opposed pair. Each pressure vessel has an operating pressure sufficient to hold gas at a pre-defined pressure. At least one gas compressor is in communication with each pressure vessel, and the gas compressor is capable of compressing a gas in each pressure vessel to the pre-defined pressure. A pressure relief mechanism is in communication with each pressure vessel. The pressure relief mechanism is capable of returning the gas in each vessel to atmospheric pressure.