A pump for pressurizing a fluid includes an engine portion including a first pressure vessel, a first piston movable inside the first pressure vessel, at least two pressurant entrance valves connected to the first pressure vessel, and at least two pressurant exit valves connected to the first pressure vessel. The valves are configured to be opened and closed automatically and directly as a function of a position of the first piston inside the first pressure vessel. The valves are also configured to be automatically opened and closed out of phase with each other. The pump also includes a pump portion including a second pressure vessel, a second piston connected to the first piston and movable inside the second pressure vessel, at least two fluid entrance valves connected to the second pressure vessel, and at least two fluid exit valves connected to the second pressure vessel.