Diesel APUs
Diesel APUs can provide the sleeper cab’s cooling, heating, and electrical power while the truck’s engine is off. Note that some diesel APUs today offer heating, bunk cooling, and AC power for hotel loads. In contrast, others supply bunk cooling and AC power only and utilize a fuel-operated air heater for cab heat. All diesel APUs also charge truck batteries and can deliver electrical power for a block heater if the engine has one. They generally consume between 0.1 and 0.5 gallons of fuel per hour, depending on their design, the ambient temperature, the sleeper’s insulation, the resulting HVAC load, and the generated AC power.
All diesel APU engines drive an air conditioning compressor to circulate refrigerant for bunk cooling. There are two basic types of diesel APUs depending on how electrical power is supplied to the cab. One type features a 4kW to 6kW generator providing AC power directly to the cab, while a 12V DC alternator delivers power to the truck’s electrical system to charge the batteries. The other type generates electrical power with a 12V DC alternator that charges the truck’s battery system, with an in-cab inverter providing 110V or 120V AC power for appliances and other 110V loads.
Benefits
Fuel Savings
A modern diesel APU burns approximately 0.25 gal per hour, roughly 75% less than the fuel consumption of an idling tractor engine. This can save up to about 2,500 gallons of diesel per truck per year, or approximately 10,000 gallons over a four-year trade cycle.
Lower Operating Costs and Longer Engine Life
Because the main engine no longer racks up idle hours, fleets cut oil changes, DPF regens, and unplanned downtime, resulting in trucks spending less time in shops.
Rapid ROI and Resale Upside
Installed prices of $8,500 to 13,000 typically pay back in 12 to 24 months for trucks idling more than 1,000 hours per year. Used tractors with APUs also sell faster and for more because of the lower odometer-equivalent engine hours.
Substantial Emissions Cuts and Noise Reduction Eliminating long-duration idling reduces CO₂, NOx, and PM, while also decreasing cab-side noise, which improves driver rest and supports corporate ESG goals.
Full Compliance with Anti‑Idling Regulations SmartWay-verified APUs are accepted nationwide (including CARB’s 50-state after-treatment variant), helping fleets avoid fines and allowing them to operate in idle-restricted zones.
Enhanced Driver Comfort and Retention
APUs provide sleeper HVAC, 120 V AC for microwaves, CPAP machines, battery charging,
and engine preheating, offering drivers the “hotel loads” they demand without sacrificing productivity.
Challenges
Up‑front Price and Payback Risk
Installed costs of $8,500 to $13,000 are substantial; savings disappear if diesel prices fall or the truck idles for less than roughly 1000 hours per year.
Weight and Space Penalty
A complete diesel APU, condenser, and bunk evaporator add 400 lbs. to 550 lbs. and take up frame rail real estate, often requiring drivers to rely on the 550 lbs. federal/state weight exemption to avoid payload losses.
Extra Maintenance
The APU’s engine needs oil, filters, and valve adjustments every 500 to 2000 operating hours, in addition to periodic fuel system and HVAC service, which adds labor and parts that the truck would otherwise avoid.
Residual Fuel Use, Emissions and Noise
Even at 0.25 gal. per hour, diesel APUs still burn fuel, emit CO₂, NOx, PM, and produce approximately 60 dBA of sound outside the cab — issues that electric or fuel cell APUs sidestep.
Regulatory Complexity
California already requires a Level 3 DPF or routing APU exhaust through the truck’s DPF. CARB is reviewing whether to further tighten hours of use and health limits, so compliance costs could increase.
Installation and Service Network Constraints
Fit up can be tricky on short wheelbase tractors or those with fairings, and fleets operating far from OEM dealers may face delays in parts or warranty service.
