THE ABCs of APUs

Rigmaster Power APU

Consider this: If a sizeable percentage of all North American over-the-road trucks could eliminate the need to idle for driver comfort entirely, there’d be nearly 4 billion gallons of diesel fuel saved every year.

In individual terms, eliminating idle time completely could save a truck’s owner $30,000 or more over five years. Still say an APU is too expensive? Our math suggests even one of the more elaborate models could return triple what you paid for it over five years – even more if it winds up on a second truck.

To achieve year-round idle reduction capability, one needs an on-board source of power, primarily to run a cooling system. Cab and engine auxiliary heating systems have progressed to the point where they are as compact and efficient as they can be, but that leaves the summer months to consider. Battery powered A/C systems are emerging, but they’re expensive, and not quite as effective, yet, as a full air conditioning system.
Enter the genset or auxiliary power system. They aren’t the same thing, for the record, but one or the other may be a better solution for your personal needs.

Both consume a fraction of the fuel required to run the truck’s big diesel – by a factor of about 8:1, but a genset (generator) is simply a diesel-engine- powered source of 110-volt AC power – which can be used to run appliances like fridges, microwaves, etc., and remote heater and/or air conditioning systems.

An APU, on the other hand, uses a small diesel engine to produce DC power to run built-in climate control systems. Coolant from the APU engine can be used to heat the big diesel in the winter for reliable cold starts, and depending on the level of integration, some APUs have their own AC compressors, but use the truck’s AC condenser for cooling. Most of the DC systems onboard the truck can be used as usual – with an inverter – while a DC alternator on the APU engine keeps the main batteries charged.
In accordance with the EPA’s new emissions guidelines, come 2008, APUs will require diesel particulate traps, similar to the ones coming our way on 2007-model trucks. Though, according to Ed O’Malley, Blackrock’s VP of dealer development, “the smaller diesels will not come under serious environmental regulation until 2017.”
And at least one APU manufacturer has declared its product ULSD compliant. RigMaster has announced that all its current models will operate on the ultra low sulfur diesel.

Integrated or Not?
Splitting it down even farther, APUs come in two configurations: integrated – meaning they have their own AC compressor and condenser and heat exchanger, and don’t need to tie into the truck’s system; and non-integrated, meaning they use the truck’s HVAC system and circulate coolant from the APU to provide heat, while running an AC compressor, but use the truck’s condenser and air circulation fans.

The integrated APUs usually include an under-bunk unit housing the AC and the heat exchangers, as well as the fans and ductwork.

O’Malley says there can be warranty and reliability issues when breaching an OEM AC circuit, for example. “When you have multiple hoses and connectors, you increase the potential for leaks,” he says. “And when you tap into the cooling circuit, you’re adding more hose again, and a coolant leak can take whole truck down.”

Carrier Comfort Pro

Some of the non-integrated units, such as Carrier’s ComfortPro, use an 120 V AC generator to power an electric-drive AC compressor, mounted under the sleeper. These non-integrated units offer some flexibility in the mounting positions of both the external and internal components.

And we’re not done with variety just yet, The Willis APU, manufactured by Auxiliary Power Dynamics LLC, can be ordered with an integrated air compressor to maintain system air pressure on the truck, and a heavy-duty alternator capable of supplying power to the truck’s electrical system.

“APUs with heavy-duty truck components, like alternators, can better handle the high power demands and the high operating temperatures of a class 8 truck diesel engine,” says Will Watson, vice president of sales and marketing for Auxiliary Power Dynamics. “Plus, they can act as backup for their counterparts in the truck engine.”
The OEMs have also been experimenting with various technologies to provide on-board sources of heating, cooling and power.

Kenworth’s “Clean Power” system provides up to 10 hours of cooling capacity, used in conjunction with a set of deep-cycle batteries to power fans, as well as electric heat in the cooler months. Peterbilt offers a similar system, called the Comfort Class system. Both work with shorepower systems.

Freightliner’s NITE (No-Idle Thermal Environment) by Bergstrom, consists of a rechargeable battery bank that supplies power to a 3,500 BTU AC system and an auxiliary heater. It provides up to 10 hours of cooling capacity.

And International ‘s No Idle APU allows drivers to operate heating or air conditioning, as well as “hotel loads,” such as a microwave or television, without running the engine of the truck. It offers 6kW, 120-volt AC power; plus 50 Amp, 12-volt battery charging; and provides 10,000 BTUs an hour of air conditioning or heat.

System Capacity
APU makers, as you’ll see from the chart on page 24, offer a variety of system capacities. The cooling system capacities – from 10,000 to 26,000 BTU – is quite a spread, but one system may be designed to cool a large sleeper and the cab, while the other may be intended just for the sleeper. Buy as much capacity as you can without over-buying.
Same goes for AC and DC electric production. If you plan to run a TV, an air conditioner, a microwave, and a hairdryer all at the same time, you may need a larger generator or alternator. If your power needs are more modest, you may save a few dollars spec’ing a lighter unit.

At the end of the day, any auxiliary system should accomplish three things: provide a power source for driver convenience, and provide heating and cooling capabilities for driver comfort. The means varies across the brands, but the end result is the same.

Blackrock APU

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