What is the difference between a direct acting solenoid valve and a solenoid-pilot operated valve?
In a direct acting valve, the main body or poppet is moved using magnetic force generated by the electrical coil to shift the valve.
In a solenoid-pilot operated valve, the main body or plunger is moved using force developed from the available pilot pressure. The coil in this case is used to control the pilot for the pilot piston.