Which part of a hydraulic accumulator stores a volume of fluid under pressure?

The cylinder is the component of a hydraulic accumulator that stores a volume of fluid under pressure. It acts as a reservoir where hydraulic fluid can be contained and maintained at a certain pressure, which is essential for the operation of hydraulic systems. The pressure in the fluid stored within the cylinder is what allows hydraulic systems to function effectively, providing the necessary force to operate various components such as actuators, valves, and other machinery.

The diaphragm, although it can be found in some types of accumulators, primarily separates different fluids or compartments and does not serve as the main storage area for the fluid. The piston is a critical part of some accumulator designs, as it can move within the cylinder to adjust fluid volume or pressure, but it does not solely serve the purpose of fluid storage. The cap is primarily a structural component that helps seal the accumulator but does not play a role in fluid storage under pressure. Thus, the cylinder is indeed the correct answer for where fluid is stored under pressure in a hydraulic accumulator.