Diesel APUs
Diesel APUs can provide cooling, heating, and electrical power to the sleeper cab while the truck’s engine is off. Some diesel APUs on the market today provide heating, bunk cooling and AC power for hotel loads, while others provide bunk cooling and AC power only and then also use a fuel-operated air heater for cab heat. All diesel APUs also charge truck batteries and can provide electrical power for a block heater if the engine is so equipped.
They generally burn between 0.1 and 0.5 gallons of fuel per hour depending on their design, the ambient temperature, the sleeper’s insulation, and the resulting HVAC load, along with the AC power being generated.
Benefits
Complete Solutions
Provide cab cooling, heating, AC power for hotel loads, and battery charging.
Long Operating Time
Can operate as long as the truck has fuel. This provides nearly an unlimited amount of time for cooling, heating, and AC power without the need to restart the truck engine. Battery HVAC systems are limited in the hours of cooling they can provide before truck engine restart and battery recharging is required. Also, their hotel load AC power capabilities usually are less than that of their diesel-powered counterparts.
Suited for Temperature Extremes
High Btu capacity allows operation in extreme temperatures. Colder weather reduces battery capacity thus negatively affecting hotel load AC power capabilities of battery HVAC systems, while diesel APUs can avoid those issues.
Challenges
Purchase Price
Expensive to purchase, install and maintain compared to other idle-reduction technologies. Compared to battery HVACs, for example, diesel APUs require periodic oil and filter changes and have more parts and components to be maintained. A typical service interval for a diesel APU is every 6 to 12 months. With a battery HVAC system, batteries need to be changed approximately every 2 years unless and engine start/stop system is employed to keep the batteries from being discharged too often. It is likely that while diesel APUs need maintenance on a more frequent basis, the cost of that maintenance will be equal to or less than the cost of replacing batteries on battery HVAC systems.
Installation Issues
Due to their mounting location, diesel APUs are exposed to road debris, salt and spray, and need adequate frame space to be installed. Additional work is required at installation for an open-loop system.
Consume Fuel
Need diesel fuel to operate and generate emissions and noise, while reducing fuel savings less than other idle-reduction technologies.
Don’t Meet Zoning Restrictions
Urban municipalities have zoning restrictions prohibiting the use of diesel APU. In California, for example, a fleet may need to add an expensive DPF to make a diesel APU compliant with that state’s specific emissions regulations. With the upcoming Greenhouse Gas Phase 2 regulations, the emissions regulations on diesel APUs are likely going to get tighter.
Add Weight to Vehicles
Diesel APUs weigh between 450 and 550 lbs. when installed. In addition, you have to add another 20 pounds for fluids. However the Energy Policy Act of 2005 allowed for a national 400-pound exemption for the additional weight of idle-reduction technologies on heavy-duty vehicles. In 2012 the Moving Ahead for Progress in the 21st Century Act increased the weight allowance to 550 lbs. However, each state can adopt the exemption at its own discretion and not all states have done so. The Office of Energy Efficiency & Renewable Energy has a map showing states that recognize the APU weight exemption.