‘We used to manage MPG, but now we also manage idle time.”

A large US fleet

Executive Summary

Diesel APUs allow fleets to meet the temperature and electrical power needs of their drivers. They provide a complete solution to the reasons many fleets idle, but are more expensive to install and maintain than other systems and they consume diesel fuel.

With diesel APUs, an auxiliary power unit powered by the diesel fuel already on the tractor can provide power for HVAC, hotel, or other electrical loads while the vehicle is parked. Some units provide all three needs while some provide cooling and AC power but require adding a fuel-fired heart for cab heat. Diesel APUs also charge the vehicle’s batteries. They burn between 0.1 and 0.5 gallons of fuel per hour. Diesel APUs provide AC power either through a generator or using an inverter.

Introduction to Idle Reduction
Common Fleet Strategies

Diesel APUs are one of the possible anchor technologies for an overall idle-reduction strategy:

  • The installation of additional insulation will greatly enhance the effectiveness of a diesel APU.
  • Adding an inverter to power the HVAC system lowers the amount of fuel used by the diesel APU.
  • Installing an inverter for HVAC and a fuel-operated air heater for bunk heat along with the diesel APU would use the least amount of fuel.
  • The most common practice is for fleets using diesel APUs as their anchor technology to install extra insulation and a fuel-operated air heater, while still using the diesel APU to meet their HVAC needs.


What Others Are Saying
Decision-Making Tools

There are already a wide variety of payback calculators for any given type of idle-reduction system. In an effort to meet the industry’s need for more information, we have also developed a high-level payback equation to aid fleets in choosing the right combination of technologies.

Decision-Making Tools


Only one truck manufacturer offers a diesel APU as a factory-installed option, but others have an option that provides a mounting location and some of the connections to enable faster and easier installation of a diesel APU. Manufacturers agree that a diesel APU should be ordered with an inverter. 


Diesel APUs address many of the reasons fleets cite for idling including heating, cooling and AC power. They can operate in extreme temperature conditions and have almost limitless hours of operation — they’ll run as long as there is diesel fuel available. However, since they run off diesel fuel they generate emissions and noise, and they require maintenance on a regular basis.