Electric vacuum solenoid valve perform an environmentally and economically important function in the engines of most cars.
First off, the fact that electric vacuum solenoids prevent unburned fuel particles and several other forms of gaseous and particulate waste that engines produce from escaping into the air and thence to the atmosphere means that cars without them are bound to leave you with a very swollen-looking carbon footprint.
Secondly, the fact that the exhaust gas recirculation (EGR) systems of cars that require electric vacuum solenoids or switching solenoids raise fuel efficiency is a great boon to just about anyone these days. With fuel prices soaring higher by the day, every little bit, even teeny-weeny particles and gas-huffs, really does count.
EGR valves work by the sucking up of exhaust gases before they can leave the car. Very frequently, they're activated by an increase in the vacuum produced by the engine, a factor that's directly proportional to how hard the engine is exerting itself. When the valve's transduction mechanism gets activated in this way, the electric vacuum solenoid is also activated, and the valve opens, allowing exhaust gases to return to the intake manifold, which normally has the pleasing side effect of cooling the cylinder.
The switching solenoid at work in an EGR system is the same as any other solenoid - an unimpressive-looking spool of engine wire (often wound up to a thousand times) which, when it has a current passed through it, produces an electromagnetic field. This field will, depending on the direction in which the wire coils, either produce a suctioning, 'pulling' force or a repulsive, 'pushing' force on any object placed within it.
Solenoids just like the electric vacuum solenoid appear in several other parts of your car. There's one connected to your car's ignition and, when you turn the key, for the car to grumble to life the starter solenoid has to move two heavy contacts together. When those contacts meet, they allow electricity to chart a straight course from the car's battery to the engine, thus starting the car.
The principle in operation with starter solenoids is similar in some ways to how most solenoid valve work, the only difference being that, where valves are concerned, you're dealing with the flow of liquid or gas, not electricity, and that often the movement of the solenoid is to remove it from being a barrier to the flow, not to bring two conduits together.
Simply described, the 'pin' that blocks the secondary conduit between the pressurized central chamber and the outlet of the valve is actually the armature of the solenoid. When removed, pressure is immediately reduced in the central chamber, allowing the burgeoning pressure pushing against the diaphragm blocking the central chamber from the entry conduit to push the diaphragm up. This then allows for a straight flow of air or liquid to run from the inlet to the outlet of the valve.
As soon as the armature pin is replaced, pressure builds up again in the central chamber, and the force of the fluid allowed to flow through the tiny hole in the centre of the diaphragm builds up sufficiently to push the diaphragm back into place, closing the pneumatic valve.