A direct acting solenoid valve is one in which the action of the
solenoid alone switches the valve gate. Most solenoid valves are
"piloted", which means that a relatively small solenoid valve is
used to allow pressurized air to actuate the valve gate
mechanism.
Because the solenoid does all of the work in a direct acting
valve, the solenoid must therefore be larger than that of a piloted
valve in order to generate the switching force. Piloted solenoid
valves have the advantage of being more compact and consuming less
current than a direct acting solenoid valve.
Direct acting solenoid valves are useful when switching low
pressure fluids.