The nuts and bolts of APUs

With the proliferation of anti-idling regulations, the need to reduce fuel costs and emissions and the urgency to keep drivers, fleets are turning to auxiliary power units (APUs) to provide creature comforts without the need for running a truck's engine. However, if not properly selected, operated and maintained, APUs can create service and repair issues.

In simplest terms, a truck APU is a device that provides climate control and converts DC to AC power for driver creature comforts without the need to idle the engine.

Idling the engine maintains a comfortable environment for drivers but it wastes energy. An hour of idling time consumes about one gallon of diesel fuel, whereas APUs consume far less. Fuel is one of a fleet's largest large expenses.

Even though diesel engines are efficient when idling, constantly running high horsepower engines at low rpm combusts fuel incompletely. What's more, continual operation of an engine at low speed causes additional wear on internal parts compared with road speed rpm.

Along with consuming significant amounts of diesel fuel and emitting large amounts of pollution, long periods of idling heavy duty truck engines accelerates engine wear and tear and causes noise and vibrations.

The matter of selecting the appropriate APU is complicated because of the wide variety of units to choose from. Configurations include diesel-powered engine-based, shore power, thermal storage systems, hybrid systems and pure electrified system. As would be expected, each type of design has its advantages.

APU manufacturers advise that the expectations of the unit and its operational requirements must be considered to ensure that the APU chosen will perform as desired. Other important elements to take into account are systems placement and installation, as reliability problems tend to be in large part due to improper installation.

When purchasing APUs, Dean J. Lande, manager of business development for Carrier Transicold says there are seven key criteria fleets should consider:

Performance (diesel vs. battery) - "Diesel APUs tend to provide more consistent performance over longer periods of time and are not dependent on batteries that need to be recharged every four to eight hours," he says. "Especially for fleets that travel through the hotter region of the southern tier states, it is very important to ask whether the APU will provide at least eight hours of continuous cooling. Battery-powered systems tend to under-perform diesel APUs in this region."

Lifecycle cost - Lifecycle cost includes initial purchase price plus both operating and maintenance costs. Diesel APU buyers must budget for oil and filter changes, so extended service intervals are important. Buyers of battery systems must budget for potential battery replacement.

Warranty - It is necessary to understand what components are covered under the manufacturer's standard warranty and for how long, recommends Lande. Look for standard warranty packages that cover big-ticket major components such as engines, compressors, generators, evaporators and condensers. Determine if the manufacturer provides an extended warranty option. "It is important to compare among manufacturers, not just cost, but what specific components are covered," he says. "Look for the lowest cost coverage that covers the most major components."

• Service network - For fleets that travel long distances, it's best to go with manufacturers that provide nationwide service support. "If an APU problem arises on the road, you don't want your drivers to find themselves in a situation where no one knows how to service it."

• Installation time - Shorter install times typically reduce overall acquisition cost. Truck OEMs are beginning to offer preconfigured cabs to accommodate an APU, which "drastically cuts down on installation time, saving money," notes Lande.

• Product features --Always compare product features among various APUs, he stresses. For example, if routes cover the Northeast, Midwest or Canada, in the winter it helps if the APU has a truck-engine heating option in order to avoid cold starts. Or, if drivers watch TV when resting or like to store fresh food in an onboard refrigerator, be sure to spec an APU that provides enough power to the cab to run these devices.

• Ease of use - APUs should be simple for drivers to operate. Fleets need to be aware that in California, diesel-powered APUs used on Class 8 tractors equipped with model year 2007 or newer engines require an additional emissions reduction device, such as a diesel particulate filter (DPF), he says.

The average over-the-road tractor idles approximately 2,400 hours per year, according to Dwayne Cowan, APU product manager for Thermo King, a business of Ingersoll Rand. A fleet can save on average 0.8 gallons per hour idling with an APU vs. tractor engine. If fuel is $3 per gallon,

When deciding between a battery-based or engine-based APU system, battery-based have reduced install time and less overall maintenance, Cowan says. Engine-based APUs have an advantage in cooling and hotel loads, plus provide tractor engine warming and charge tractor batteries.

Because fleet suitability is a major factor in APU selection, he says careful thought needs to be given to the following:
• Battery-based system alternator size requirements
• What are the tractor operating days?
• What is the desired runtime of the HVAC system?
• What is the ambient temperature and the desired temperature in the bunk?
• Is the layover mostly during the day or night? Daytime layover will reduce the runtime of a battery-based system due to the increased solar load.
• What is the cab insulation and age of tractor?
• Are the sleeper and window curtains closed?
• What are the expected hotel loads?
• Does the tractor have multiple stops during a route? If so, how many stops and duration of each stop? Does the driver run the auxiliary HVAC during each stop? If this is the case, what is the typical time between stops for recharging the batteries?
• If there are not multiple stops, what is the typical drive time between rest periods? The longer drive time between stops will provide additional time to recharge batteries for battery based system.

The benefits of adding a power inverter to an APU

Truck-approved power inverters convert the direct current (DC) electrical power coming from the truck's batteries to ordinary 120-volt alternating current (AC) to power creature comforts in sleeper cab tractors such as microwave ovens, coffee makers, computers, entertainment centers, etc.

Often, inverters are perceived as competing with APUs, which are essentially a source of air conditioning and heating, observes Francois Prevost, president of Tundra International, a Canadian company specializing in the developing and manufacturing of truck power inverters since 1995. Rather, "power inverters are the perfect complement to an APU and should be part of the overall driver comfort solution."

The diesel-powered APU without an integrated AC power source is the most common type of APU used these days, he says. For these, a power inverter is essential.

Other diesel-powered APUs have a built-in AC generator that provides AC power, but only if running the APU. Here again, says Prevost, the appropriate power inverter is equally important.

"A driver who appreciates a cool or warm cabin for an extended period would clearly benefit from using the APU instead of the truck's engine," he says. "On the other hand, for the driver who only wants to use the microwave or to have a pot of coffee, it would certainly be more reasonable and prudent to use a power inverter rather than starting and running the APU engine for such a short period."

Unlike a truck engine, an APU engine needs to run for a minimum period of time (± 20 to 30 minutes) to reach the appropriate temperature for a high efficiency, says Prevost. An intermittent usage of short periods is conducive to the formation of carbon deposits inside the engine and the exhaust filters. In addition to poor lubrication, the intermittent usage will contribute to the deterioration of the starter and charging system - all of which "will dramatically increase the maintenance cost and decrease the intended profitability of the APU."

The installation of a good quality, high-efficiency, truck-approved power inverter with an APU unit "will provide the best of both worlds," maintains Prevost. "The inverter will provide a suitable usage of the APU's benefits while minimizing its maintenance costs."

In its Recommended Practice 439, "Minimum Performance Criteria for Auxiliary Power Units and Gensets," the Technology & Maintenance Council (TMC) - an organization dedicated to improve transport equipment, its maintenance and maintenance management - advocates these additional factors when evaluating APUs:

• Safety - Is there a device/switch to prevent the device from being turned on while it is being serviced?

• Training - Service and safety training should be available, including authorized installation training and technician certification and driver training or training materials..

• Service interval - At minimum, the interval should match the tractor service interval.

• High-altitude operation - Fuel-fired heaters are affected by altitudes above 5,000 feet. Users should consult with the heater manufacturer if they intend to operate their fleet at higher altitudes. (This issue primarily relates to fuel/air mixture.)

• Extreme cold ambient operation - Consideration should be given to idling the tractor's main engine when ambient temperature conditions drop to 20ºF or below as fuel gelling may occur at or below this temperature.

• Fuel requirements - Will the APU engine operate on the fuel that is to be used?

• Emission compliance - Does the APU meet or exceed current/anticipated emission requirements where it will be operated?

• Shore power - Is the unit's AC power system compatible with shore power?

• Tax exemptions - Can fuel usage for the APU be calculated for a tax exemption filing? Is the APU exempt from Federal Excise Tax (FET)?

In a session at TMC's Annual Fall Meeting, Kirk Altrichter of truckload carrier Gordon Trucking shared his experiences with APUs and provided a list of key APU purchasing considerations. These included:
• Climate for use. Are you buying a year-round system?
• Hours of daily use.
• Does the system comply with the regulations for the areas of operation?
• Will there be an inverter?
• How many batteries?
• What is the size of the truck's alternator?
• What is the size of the APU's alternator?
• How much hotel load will the system allow?
• Maintenance requirements.
• Maintenance costs.
• Will a DPF (diesel particulate filter) and DEF (diesel exhaust fluid) be required?
• Ease of use.
• Fuel usage.

In an effort to avoid problems with APUs, Altrichter said Gordon Trucking has built in some protection safeguards, including a maintenance inspection of the APU at every vehicle PM. In addition, the fleet has installed a battery conditioner, low-voltage disconnect (a device that disconnects a load from the batteries to prevent over discharge of a battery - one of the most common ways of killing a battery), battery disconnect, stand-alone starting batteries and a high-amperage alternator on the APU and on the truck engine.

Cummins demonstrates solid oxide fuel cell auxiliary power system

Earlier this year, Cummins successfully demonstrated a tubular solid oxide fuel cell (SOFC) in a hybrid-configured APU constructed to power a Class 7/8 truck's hotel loads while running on commercial ultra-low sulfur diesel (ULSD) fuel.

The use of fuel cell APUs is compelling because it not only reduces energy and environmental impacts, but has the potential to reduce costs and improve driver safety by reducing noise and vibration for sleeping drivers.

SOFCs are highly efficient, making electricity from fuel and oxidant gases in an electrochemical process that takes place across an ion-conducting, ceramic membrane.

There are two principal SOFC configurations: tubular and planar. Tubular designs bundle tube-shaped cells in parallel. One end of the tubes is closed, and the other hooks to a common air supply manifold. Oxygen ions pass through the electrolyte and react with fuel flowing over the outside of the tubes, creating an electric current.

Planar types place individual cells in a multi-layered "stack," with cells arranged in series or parallel to give the required voltage and current.

The Cummins APU incorporated hybrid power electronics, system controls and balance-of-plant components combined with SOFC modules supplied by Protonex Technology Corporation, a leading provider of advanced fuel cell power solutions for portable, remote and mobile applications.

The SOFC power unit was integrated with a modified power electronics section derived from Cummins Power Generation's Hybrid Quiet Diesel (HQD) recreational vehicle power system. The hybrid power electronics provide dynamic load sharing between the SOFC and a battery pack.

The testing replicated the 10-hour rest period that is part of the daily routine of a typical over-the-road truck.

The hybrid-configured APU can combine power from an engine-driven generator or fuel cell and a battery pack to supply both AC and DC power to the truck's hotel loads.

An often overlooked factor in selecting APUs is hotel loads, also known as creature comforts, said Bruce Purkey, president of Purkey's Electrical Consulting, in his presentation to the TMC Fall Meeting. These are energy loads to operate such things as televisions, microwaves, coffee makers, cell phone chargers, electric blankets, computers, hair dryers, DVD players and so forth. Hotel loads create the non-start issues and short battery load.

Wattage requirements for hotel loads can be significant, according to the TMC. Some typical current draws, not counting surge requirements, are: 13-inch color TV, 70 watts; 19-inch color TV, 300 watts; electric blanket, 150 watts; brewing coffee maker, 1,250 watts; and microwave oven, 600 to 1,100 watts.

A fleet needs to be cognizant not only of amperage load but run times, which can be very short or very long, Purkey pointed out. The hotel load is not the one load that a driver puts on, it's the cumulative load.

Typically, over-the-road tractors have a 160-amp alternator. When running down the road, there is an 80-amp load and an added 15-amp load for a driver's hotel loads, leaving enough load to charge the batteries, explained Purkey. But when the engine is off, the alternator input is zero, so every amp coming out of the electrical system comes from the battery.

Hotel loads impact the starter because lower voltage means less cranking speed, so more current and torque is required to start the engine, he added. This damages the starter because the starter needs to start quickly, not grind away. The alternator must also work longer and harder, and that takes more horsepower.

Some fleets feel that adding an inverter - a device that converts direct current into alternating current - can help with hotel loads, said Purkey. But inverters still draw power from the batteries.

He recommended that all the electrical loads need to be calculated, along with the amount of power required and for how long. This is needed to make sure the APU system will have enough power for the hotel loads and to start the truck.

Purkey concluded: "You have batteries on board, not nuclear power plants, and those batteries have a fixed amount of energy. You want to give drivers hotel loads but you still need to start the truck in the morning."

Depending on operational requirements, some fleets are opting for diesel-fired heaters instead of APUs as a solution to deal with keeping drivers warm in the winter, preventing cold weather no-starts and complying with no-idle policies, all while using only a fraction of the fuel that would be used by idling the vehicle's engine.

Diesel-fired heaters have been proven effective and are easy to maintain says Bob Giguere, clean diesel product sales manager with the Inland Power Group, a Detroit Diesel and Allison distributor with six locations in Illinois, Indiana, Wisconsin and Michigan. Typically, all that is required is an annual fuel filter change and running the heater a few times during the warm months to replace dormant fuel with fresh fuel in the lines and nozzle to prevent potential problems.

Because features of diesel-fired heating systems vary depending on the needs of the fleet, Giguere advises taking the following key purchase considerations:

Most diesel-fired coolant heaters are thermostatically controlled and either operated with an on/off toggle switch or an optional programmable timer. Some timers can be programmed for on/off times for each day of the week and can easily be preset.

Diesel-fired bunk heaters, which allow drivers to heat the sleeper compartment without idling the truck, are controlled either by a simple on-off switch or a dial. Units powered by variable heaters use less fuel and provide greater comfort than their counterparts, which are either fully on or completely off.

Look for a kit that includes all the required harnesses, brackets, connectors and controls supplied in one box rather than having to buy a stand-alone heater and have to source the other parts separately.

"There is a definite cost savings with idle reduction heaters through reduced fuel consumption, engine maintenance and labor to start cold trucks," says Giguere. "In as little as three months, depending on actual idling time, but definitely less than one winter, a bunk heater pays back on its investment. Few aftermarket products offer such rapid ROI."

More details on diesel-fired heating systems, which can also be paired with other HVAC technologies for a more complete heating and cooling package, can be found at and

Temperature control performance requirements for APUs

The Technology & Maintenance Council, in its Recommended Practice RP 439, establishes minimum performance criteria for auxiliary power units used on Class 6 to 8 day cab and sleeper cab tractors operated within an ambient temperature range from -10°F to 120°F.

In general, these criteria state that after a tractor's engine is turned off, an operating APU should maintain, at a minimum, the following HVAC performance:
• Cooling - A maximum cab or sleeper temperature of 78°F for 10 hours.
• Heating - A minimum cab or sleeper temperature of 68°F for 10 hours.
• Perform as such for at least 10 continuous hours of operation as mandated by the original 2004 Hours of Service (HOS) rules and all subsequent updates.


Truck anti-idling regulations

More and more localities, municipalities and states are enacting anti-idling laws to reduce emissions and noise levels. Often, these regulations, and their penalties, are inconsistent.

At present, 28 states have one or multiple regional anti-idling regulations. They are: AZ, CA, CO, CT, DE, D.C., FL, GA, IL, MA, MD, ME, MI, MN, MO, NC, NV, NH, NJ, NY, OH, PA, RI, SC, TX, UT, VA and WV.

The American Transportation Research Institute, a transportation-related research organization, maintains a compendium of current idling regulations by state.

It is provided for free in two different PDF formats - the original compendium listing and as a foldable cab card for quick reference.

These can be found at


Where to find funding opportunities

Various federal, state and nonprofit organizations provide funding and incentive opportunities for the reduction of emissions from long-duration truck idling.

Among the states offering such programs are California, Oregon and Pennsylvania. Federal agencies include the U.S. EPA, U.S. DOT and U.S. DOE.

Details on these and other idling reduction incentive and funding opportunities can be found within the EPA's SmartWay website: