An air solenoid valve is an electrically controlled valve. This solenoid valve has an electric coil with a movable ferromagnetic core (piston) in its center. In the rest position the piston closes . When current flows through the solenoid valve, the coil is energized and creates a magnetic field. This creates a magnetic attraction with the piston and thus moves it and it opens. This is the basic principle used to open and close air solenoid valves.