What is solenoid valve?

The hydraulic excavator has a very interesting solenoid valve at the heart of its operation(refer fig 1a). The solenoid valve is so crucial that even minor wear and tear on its disc can cause the excavator to jam, leading to its failure. They are even used in sensor taps. The other major industrial applications of the solenoid valve are shown in (refer fig 1b). In this article we are going to explore the impressive engineering behind solenoid valves starting from the simplest one.

Fig 1a : Hydraulic excavator has a solenoid valve
Fig 1b : Applications of 2/2 solenoid valve

Need of solenoid valve

Here is an interesting question for you. Some commonly used motorized valves in the industry are working perfectly, then why do we need solenoid valves? The answer is, the solenoid valves are insanely fast. A normal plate motorized valve takes 1 second for its operation, while the solenoid valve finishes its task in 0.03 seconds as shown in fig 2.

Fig 2 : Motorized valve vs solenoid valve

Design 1 : basic design of 2-by-2 solenoid valve

Let’s see the construction of the simplest possible solenoid valve. I took an iron armature that is kept perpendicular (which is free to move) inside the pipe(refer fig 3a). An electromagnet is placed around the armature and also attach a spring to the fixed plate at the top of the armature. The spring is compressed here and it always tries to keep the armature at the bottom as shown in fig 3b.

Fig 3a : Armature is perpendicular inside the pipe
Fig 3b : Spring is trying to keep armature at the bottom

When I energize this electromagnet with DC current, strong magnetic fields are generated around it. You can see in the fig 4 the armature will experience a force causing it to move upwards and to align perfectly to the center of the coil. The valve is now open, and the water flows(refer fig 4a). Let me de-energized the coil, and see what will happen now, obviously the compressed spring have closed the valve immediately. With this design, we have achieved a simple 2-by-2 solenoid valve. It is called 2-by-2 because of its two ports - input and output and two operation states - ON and OFF(refer fig 4b).

Fig 4a : Valve is open and water flows
Fig 4b : Solenoid valve in OFF state

Solenoid valve in OFF state

I noticed small issue with this design. In OFF state, if the fluid pressure increases, the armature can bend, which results in fluid leakage. Please have a look at fig 5a, it will help you to better understand this. To solve this issue let’s make some design modifications. We can try to shift the coil and the armature to the top and instead add a solid thick barrier(refer fig 5b) in the flow path. Now the valve is closed, and the fluid won’t be able to cross this region. This design will has ensure that the armature does not bend due to water pressure.

Fig 5a : Armature is bent due to high fluid pressure
Fig 5b : Valve barrier - OFF state of valve

Solenoid valve in ON state

Lets energizes the coil, will you can see in fig 6a the armature moved up and the valve is open now, allowing fluid to easily flow through the gap. Now that the armature is safe from bending, but the high fluid pressure can still push the armature upwards which is resulting in fluid leakage(refer fig 6b).

Fig 6a : 2/2 Solenoid valve with barrier- ON state
Fig 6b : In OFF state, the high fluid pressure pushes the armature upwards

Now we have seen, this design has a small issue. But good to hear that our smart engineers have solved this issue. Engineer came up with a perfect solution: just keep the same fluid pressure on both sides of the armature(refer fig 7a). Let’s see how this new design is achieved practically.Engineers used a flexible rubber diaphragm with two holes(refer fig 7b). Let’s fix this diaphragm with the valve body.

Fig 7a : Keeping same fluid pressure on both side of the armature
Fig 7b : Diaphragm with the valve body

Coil de-energised in OFF state

The tiny red hole on the diaphragm plays a major role here. You can see how the fluid reaches both the sides of the armature due to this small red hole. This means that, when the valve is closed, the fluid pressure acting on the armature gets canceled. This perfectly solves the previous problem. (refer fig 8a).

Fig 8a : Fluid pressure over armature from chamber and valve body

Coil Energized in ON state

Now the coil is energized, the armature is moving up, and the fluid inside the armature chamber escapes via the central hole. Here, the fluid in the armature region escapes and causes a sudden pressure drop. This pressure drops because the outflow via the central hole is higher than the inflow as illustrated in fig below(refer fig 8b). Because of this pressure difference, the diaphragm will bend as shown in fig 8c. Now the fluid can escape directly via the bottom of the diaphragm.

Fig 8b : High pressure outflow via the central hole is higher than the inflow
Fig 8c : Diaphragm has bent due to high fluid pressure

Now, our simple 2 by 2 solenoid valve design is almost over.

However, if you tear down a real 2 by 2 valve, you can see an additional component - an iron cap at the top of the cover. Why is it needed? Now, let’s see which design achieves that small distance first! (refer fig 9a) In the given FEA result, you can see that the solenoid valve with the extra iron cap reaches that distance in half the time of the other case. This reduces the response time of the solenoid valve. The reason for the fast movement is that the iron cap produces a strong magnetic pole. Now a quick and efficient design of 2/2 solenoid valve is ready.

Fig 9a : Solenoid valve with diagram and iron cap
Fig 9b : FEA simulation representing the solenoid valve with iron cylinder

I hope you have a good understanding of 2-2 solenoid valves. Thanks for reading.


Yogeshwari S Gaddam

Yogeshwari S Gaddam, B.E in electrical engineering, currently she is working at Lesics Engineers Pvt. Ltd. as a manager. Each day she encounters new challenges and loves the complexity that each project requires. Yogeshwari has done projects such as Tesla model-3's motor(IPM-SynRM), RMF, SynRM motors, Etc. To know more about the author check this link.