An auxiliary power unit for trucks works by powering many of the truck's components that traditionally rely on the main engine. For most trucks, the APU is a small diesel engine that runs off the truck's main fuel supply. Unlike the truck's main diesel engine, the engine in the APU only generates enough power to run systems such as the air conditioning, in-cabin electronics and engine block warmer.
A truck's APU typically remains off when the truck's main engine is in operation, either while the truck is moving or while the main engine idles. When the driver stops for any extended period of time, she turns off the main engine and turns on the APU.
Before APUs were in wide use, the truck's internal components, such as heating, air conditioning, radio and television relied on the truck's main engine for power. Many drivers also left the main engine running during down time, as this kept the engine warm and reduced cold-start costs. However, keeping a truck's massive engine running consumes large amounts of fuel, wasting money and resources.
While an APU does not have sufficient power to operate the truck's drive system, it provides adequate power to keep the driver comfortable when off-duty. Many APUs also power block warmers, devices that reduce starting costs by preventing the truck's main engine from becoming fully cool.