What is an APU on a Truck?

As the demand for more fuel-efficient and environmentally friendly practices in the trucking industry has grown, so too has the use of Auxiliary Power Units (APUs) in over-the-road trucking. When you pull into a truck stop, you can easily spot a truck equipped with an APU by the condenser mounted on the back of the cab. But what exactly is an APU and what functions does it serve?

What is an Auxiliary Power Unit (APU)?

Think of an APU as a generator specifically designed for the sleeper cab of a semi-truck. Its main purpose is to produce or store power for use when the main engine of the truck is turned off. APUs come in two main types: diesel-powered and battery-powered. These units offer various functions that enhance the driver’s comfort while parked, eliminating the need to idle the truck throughout the night to maintain a comfortable environment in the cab.

What does an APU do on a truck?

The primary function of an APU is to cool the sleeper cab while the driver rests. However, many APUs also provide additional features such as in-cab heat, engine pre-heat, and 120V power. As the market has expanded, the functionality of APU systems has evolved, offering a wider range of options to meet the driver’s needs.

Types of APUs for trucks:

In general, there are two basic types of APUs: diesel-powered and electric-powered. Diesel APUs typically have a small diesel engine mounted on the truck’s frame rail, behind the fuel tank and in front of the first axle. These engines run off the truck’s fuel supply and power a compressor, alternator, or generator. Electric APUs, on the other hand, utilize a battery bank that may be mounted to the frame rail or integrated into the truck’s existing 12V battery system. Each type of APU has its own strengths and weaknesses and should be matched to the truck’s typical operation.

Diesel APUs

Diesel APUs are frame-mounted and utilize the truck’s fuel supply. When the driver parks and shuts off their truck, they can start the APU by simply pushing a button on the control panel inside the cab. The APU will then start up and begin cooling the truck’s cab. Popular diesel APU systems include the Carrier ComfortPro APU and the ThermoKing TriPac APU. Most diesel systems can cool the truck cab, preheat the truck’s engine, and charge the truck’s 12V battery system, but there may be some differences in operation among manufacturers.

The Carrier APU, for example, powers a 5.3kW generator that supplies 120V power to the cab. It features a fully sealed compressor powered by the 120V system, operating similarly to a household refrigeration system. The APU also includes electric heat, providing 10,000 BTU/hr of instant heat to the cab. Additionally, the Carrier APU has a battery monitor as a standard feature, which gauges the truck’s battery and starts up automatically if the voltage falls below a pre-determined level. This feature is especially beneficial for drivers who may leave their truck unattended for a period of time.

Electric APUs

Electric APUs can come in various configurations. Some have a frame-mounted battery bank that resembles the diesel APU setup, while others are integrated into the truck’s existing 12V battery system. One way to identify if a truck has an electric APU is to look for the condenser, a small rectangular box mounted outside the rear of the truck cab. The condenser functions like a mini radiator, helping to remove heat from the inside of the cab.

Many new trucks now come installed with electric APUs from the factory. These units integrate into the truck’s 12V system to provide in-cab cooling. Most systems also include a low-battery cutoff or auto-start feature, which starts the truck’s engine when the battery falls below a set threshold. While electric APUs offer nearly emission-free and maintenance-free operation, they do have limitations. Electric APUs need to be charged between uses, meaning the truck engine must run for 6 or more hours to charge the system. In-cab heat and engine pre-heat are usually handled by fuel-fired bunk heaters and coolant heaters, respectively, which can add to the cost of an electric APU.

What APU is right for me?

Choosing the right APU for your truck depends on your specific needs, the existing equipment on your tractor, and your budget. Trucks have unique dimensions, such as frame length, fairings, and optional accessories. At Crosspoint Power and Refrigeration, our APU experts can provide a free assessment of your truck to determine the best placement option. It’s important to note that just because there may not be clear frame rail space doesn’t mean an APU won’t fit. We have successfully mounted APUs in toolboxes, under fairings, and even under steps. We also take into consideration vent placement, control panel location, and 120V receptacle position to ensure a customized installation. Our professional installation crew aims to complete most installations in two days or less, minimizing downtime for your operation.

If you have any questions or need further assistance, feel free to contact us at Crosspoint Power and Refrigeration. We are here to help you find the right APU solution for your unique operation and truck.