Systems and methods for implementing a uniflow fluid engine having at least one cylinder having at least one high-pressure input and at least one low-pressure output. In some embodiments, the engine includes piston-operated valves that are related to the piston shaft and pistons that may also act as exhaust valves. In some embodiments, a valve is slidably positioned within the cylinder on the piston shaft, the valve being movable between a first position allowing input fluid to be conveyed through a first passage and blocking input fluid from a second passage, and a second position allowing input fluid to be conveyed through the second passage and blocking input fluid from the first passage.