One type of pressure-reducing valve is the regulator shown in figure 13-29, in which com-pressed air, sealed in a dome, furnishes the regulating pressure that actuates the valve. The compressed air in the dome performs the same function as a spring used in a more common type of regulating valve.

The dome is tightly secured to the valve body, which is separated into an upper (low-pressure outlet) and a lower (high-pressure inlet) chamber by the main valve. At the top of the valve stem is another chamber, which contains a rubber diaphragm and a metal diaphragm plate. This chamber has an opening leading to the low-pressure outlet chamber. When the outlet pressure drops below the pressure in the dome, air in the dome forces the diaphragm and the diaphragm plate down on the valve stem. This partially opens the valve and permits high-pressure air to pass the valve seat into the low-pressure out-let and into the space under the diaphragm. As soon as the pressure under the diaphragm is equal to that in the dome, the diaphragm returns to its normal position, and the valve is forced shut by the high-pressure air acting on the valve head.

When the dome-type regulator is used in the air start system for a diesel engine dur-ing the starting event, the regulator valve con-tinuously and rapidly adjusts for changes in air pressure by partially opening and par-tially closing to maintain a safe, constant starting pressure. When the engine starts and there is no longer a demand for air, pres-sure builds up in a low-pressure chamber to equal the pressure in the dome, and the valve closes completely.

Figure 13-29.-Pressure-reducing (regulator) valve.