What is the primary purpose of the drain box?

The primary purpose of the drain box is to capture fuel during engine shutdown. This mechanism is essential to ensure that any excess fuel or fuel that may have accumulated in the fuel system is properly contained rather than leaking into the environment or causing safety hazards. The drain box serves as a safeguard, preventing potential fuel spills that could occur when the engine is shut down and fuel pressure changes, which might lead to fuel being forced out of the system. This functionality is crucial for both environmental safety and ensuring that fuel system components are free from unnecessary clutter or pressure buildup when the engine is not operational.

In terms of the other options, while storing excess fuel, maintaining fuel temperature, and filtering impurities are important aspects of fuel management, they do not describe the primary purpose of the drain box. Each of these functions serves a different role in the operation and management of fuel systems, but they do not pertain directly to the primary role of capturing fuel during engine shutdown.