ELI5: How does this water (solenoid) valve work?

Question

I took apart this hose faucet timer:

When unpowered the valve is closed. When powered (either manually or by timer) it opens allowing water to flow through. I am attempting to learn how the open/close valve mechanism works. From some research I believe it is called a solenoid valve, but the youtube videos I've watched on the matter, don't seem to cover this particular design. Pictured is the valve assembly, minus the circuit board.

From some research I believe the silver device on the right is a solenoid. I could tell this because when it was unpowered, a piston sat inside, and when it received power, the piston/rod would pop out (I lost the actual piston before taking pictures). The solenoid screws into a hard plastic harness with a single hole in it, which I dont believe the piston even touches or covers. On the other side of that plastic housing is what I believe is referred to as a diaphragm which depresses.

The water flows from the top inlet, through a larger outer ring (top arrow in the picture below). My presumption is that when the diaphragm is pushed back the water can then continue to flow through the middle ring (bottom arrow) and out of the bottom.

What I am trying to understand is two fold:

1. Why isn't this valve normally open? If the water from the inlet is directly pushing against the diaphragm, wouldn't that pressure be enough to push the diaphragm back and allow the water to flow through?
2. How does the solenoid "pull" the diaphragm back? Wouldn't the solenoid pushing out, cause positive pressure, directly pushing the diaphragm away from the solenoid, further sealing the valve?

I put together this video showing a breakdown of the valve.

p.s. Incase its not clear, I am a laymen when it comes to mechanical/electrical engineering, so please dumb down your explanation.

Answer 1

A fairly clear video explaining the operation of the valve is provided by Rain Bird Irrigation™. It shows how the inlet water is providing pressure against the diaphragm, sealing the exit opening. That inlet water also travels to the solenoid valve, which when closed, keeps the pressure on the diaphragm.

The diaphragm has a greater surface area on the inlet side than on the exit side. When the solenoid is activated, some of the inlet pressure is released, allowing the diaphragm to open.

The solenoid force is minimal and is only that which is required to seal the transfer opening.

This type of valve is common in sprinkler systems, which use low voltages for safety reasons. I have seen modifications of this valve used in massive air dump systems such as those used for launching projectiles (pumpkins) as it provides for great volumes of air to be released in a short time.

Answer 2

Usually there would be a spring to hold the valve closed. Then the solenoid would "pull" against the spring to open the valve.

As for how the water flows, the water comes out of the larger annulus and through the small hole to the outlet. The spring would be assisted by the inlet water pressure holding the valve shut, but then there would need to be a pressure limit based on the force that the solenoid can apply.