The nuts and bolts of APUs

How to select and maintain the appropriate auxiliary power unit (APU)

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."

We Recommend