The nuts and bolts of APUs

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

• 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."

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