What additional function may a cylinder unloader have besides controlling capacity?

The additional function of assisting with compressor startup is critical in certain compressor designs. Cylinder unloaders are typically used in variable-capacity compressors to manage the amount of refrigerant circulated based on the system's demand. By unloading or loading cylinders, the unloader can limit the number of active cylinders during startup, thus preventing the compressor from drawing excessive power and ensuring a smoother start-up process. This feature helps in reducing mechanical stress on the components, extending equipment life, and improving overall system reliability.

The other functions, while important in other contexts, do not align as closely with the primary tasks of a cylinder unloader. For example, enhancing energy efficiency involves optimizing performance under load rather than during startup. Similarly, controlling refrigerant flow is typically managed by expansion devices and valves, rather than the unloader function itself. Checking system pressure is usually part of routine maintenance and monitoring, rather than a direct role of the unloader. Therefore, the unloader's capability to assist with the compressor's startup stands out as the correct choice.