Detail the specific functions and operational scenarios where an Auxiliary Power Unit (APU) is critical for a locomotive, differentiating its role from the main prime mover, particularly concerning hotel power, climate control, and main engine cold start support.

An Auxiliary Power Unit (APU) in a locomotive is a small, independent internal combustion engine coupled with a generator, entirely separate from the locomotive's main prime mover. The main prime mover is the large diesel engine designed to generate massive power for the locomotive's traction motors, which propel the train. The APU's criticality stems from its ability to provide essential power for auxiliary systems without requiring the operation of the large, fuel-intensive main engine, thereby optimizing fuel efficiency, reducing wear, and enabling crucial functions in specific scenarios.

Regarding hotel power, the APU is critical for supplying electrical power to all non-traction systems onboard the locomotive when the main prime mover is shut down or idling. Hotel power encompasses crucial functions such as lighting within the cab and car body, communication systems like radios and data links, onboard computers, safety equipment like event recorders and Positive Train Control (PTC) systems, and recharging the locomotive's batteries. This is especially critical when a locomotive is parked in a yard, waiting for crew changes, or during extended periods of inactivity on a siding. Running the main prime mover solely to provide these relatively low electrical loads would be extremely inefficient, consuming substantially more fuel and generating excessive emissions compared to the small, efficient APU.

For climate control, the APU is indispensable for powering the locomotive's heating, ventilation, and air conditioning (HVAC) systems. This ensures the comfort and safety of the crew, which is vital for long shifts and maintaining alertness, particularly during prolonged periods of inactivity in extreme hot or cold weather. Unlike the main prime mover, which would burn a significant amount of fuel and produce substantial noise and exhaust while idling to power the HVAC, the APU provides the necessary electricity much more quietly and with far greater fuel economy, making it the critical power source for cab climate control during stationary or low-power operational states.

Concerning main engine cold start support, the APU plays a vital and critical role, particularly in cold environments. A large diesel prime mover is very difficult and potentially damaging to start when it is cold. The APU provides several key functions: it ensures the locomotive's main batteries are fully charged, delivering the robust electrical current required by the main engine's starter motors to crank the large engine. Furthermore, many APUs are equipped to pre-heat the main engine's coolant or lubricating oil. This pre-heating reduces the viscosity, or thickness, of the oil and fuel, allowing components to move more freely, preventing wear and tear during starting, and ensuring a successful ignition. Without this critical pre-heating, the main engine might not start at all, or a cold start could cause significant damage due to increased friction and stress on components. The main prime mover is incapable of performing these self-heating functions from a cold, shut-down state, thus making the APU an essential component for reliable and safe main engine operation in low temperatures.