Auxiliary Power Units (APUs) are essential components in modern aircraft and some vehicles, designed to provide power independently of the main engines. These small, self-contained engines generate electricity and supply hydraulic and pneumatic power, which is crucial for starting the main engines and powering systems while on the ground. APUs typically run on diesel and are used to operate essential systems like air conditioning, lights, and avionics when the vehicle is not in transit. By reducing the need to run the main engines for these purposes, APUs help conserve fuel, minimize engine wear, and provide a more efficient and quieter ground experience.