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As far as I can tell, solenoid valves can be divided into three main categories based on their principle of operation (namely: direct-acting, step-by-step direct-acting, and pilot-operated). Furthermore, due to differences in valve disc structure, materials, and operating principles, they can be further divided into six subcategories (direct-acting diaphragm structure, step-by-step diaphragm structure, pilot-operated diaphragm structure, direct-acting piston structure, step-by-step piston structure, and pilot-operated piston structure). (1) Direct-acting solenoid valve: Principle – When electric current is applied, the electromagnetic coil generates an electromagnetic force that lifts the closing element away from the valve seat, thereby opening the valve ； When the power is cut off, the electromagnetic force disappears, and the spring force presses the closing element against the valve seat, causing the valve to close. Features: It can operate normally under vacuum, negative pressure, or zero pressure, but the typical diameter does not exceed 25 mm. (II) Step-type direct-acting solenoid valve: Principle: It combines a direct-acting mechanism with a pilot-operated mechanism. When the pressure difference between the inlet and outlet is ≤0.05 Mpa, when power is applied, the electromagnetic force lifts the pilot valve and the closing element of the main valve upward sequentially, thereby opening the valve. When the pressure difference between the inlet and outlet is >0.05 Mpa and power is applied, the electromagnetic force first opens the pilot valve; as a result, the pressure in the lower chamber of the main valve rises while the pressure in the upper chamber drops, and this pressure difference is used to push the main valve upward ； During a power outage, the pilot valve and the main valve use spring force or medium pressure to push the closing element downward, thereby closing the valve. Features: It can operate reliably at zero pressure difference or in vacuum as well as under high pressure, but it requires a large power supply and must be installed vertically. (III) Pilot-operated solenoid valve: When powered, the electromagnetic force opens the pilot hole, causing the pressure in the upper chamber to drop rapidly. This creates a pressure difference with higher pressure below and lower pressure above the closing element, pushing it upward and thus opening the valve ； When power is lost, the spring force closes the pilot hole; the inlet pressure enters the upper chamber rapidly through the bypass hole, creating a pressure difference with lower pressure below and higher pressure above the valve closing element, which pushes the closing element downward to close the valve. Feature: The upper limit of the fluid pressure range is very high, but the fluid pressure difference condition must be met