They don’t call it a “sweet spot” for nothing. That’s the range of engine speed where your truck’s diesel engine is producing the ideal mix of fuel economy and pulling power. Because your truck’s engine is designed to, well, truck, that sweet spot shows itself as you’re cruising down the highway.
But it’s nowhere to be seen when you’re idling.
The U.S. Environmental Protection Agency says the average big truck engine consumes 0.8 gallons of fuel an hour to idle – more if you’re running accessories like air conditioning. If you’re spending $3 a gallon for diesel fuel, and you idle eight hours a day, you’re paying almost $20 a day to idle the engine.
Spread over a working year of 330 days, your annual idling cost is $6,600. And that’s for fuel only. It doesn’t include engine wear and tear, fines for violating anti-idling laws, or the cost of out-of-route miles spent looking for an inconspicuous place to bunk down for the night.
With fuel costs so high, more owner-operators are reserving the big engine under the hood for pulling the load and using a smaller motor to power everything else when the truck isn’t moving.
Auxiliary power units and generators – referred to as APUs and gen sets – are a cost-effective alternative to idling. Mounted on the truck, these efficient diesel motors burn between a pint and a quart of fuel per hour, depending on the engine’s displacement, the number of cylinders and typical operating range of rpm.
While the terms are often used interchangeably, there are important differences between APUs and gen sets.
A gen set has a diesel engine and a generator that produces 120-volt AC to power appliances in the sleeper such as fridges, microwaves, small electric heaters and air conditioners.
An APU, on the other hand, uses a diesel engine to power DC devices and climate-control systems built into the truck. This level of integration gives an APU added capabilities. It can route warm coolant to the main engine for easier, more dependable cold-weather starts. It uses the truck’s climate control system to provide comfort in both the bunk and cab.
An APU with an inverter can use battery power to run the truck’s electrical devices. Then the APU can automatically recharge the batteries when they run low while still powering the truck.
Making the choice
The good news for truckers is that there are a lot of gen sets and APUs to choose from. What you need to be concerned about, according to Will Watson of Auxiliary Power Dynamics, is that many are not designed for the rigors of a heavy-truck environment.
Watson is vice president of sales and marketing for Auxiliary Power Dynamics, which manufacturers the Willis APU.
“Many of today’s designs originated from recreational vehicles,” Watson said. “What works in a state park environment and 11,000 miles a year may not hold up in a heavy-truck environment and 130,000 miles a year.”
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’s diesel engine. Plus, they can act as backup for their counterparts in the truck engine.
Another point of comparison is the heating and cooling capacity. An APU with a 30,000-Btu capacity will heat or cool much better and faster than one with 15,000 Btu or less. A system with higher Btu can more easily overcome the effects of solar heating.
“If it’s 90 degrees outside with the sun beating down, the temperature inside the sleeper and cab can reach as high as 140 degrees,” Watson said.
“The APU must blow air out of the vents that’s 40 to 45 degrees in order for the ambient temperature to reach a comfortable 70 to 75 degrees. An APU with a low Btu rating just isn’t going to get the job done, particularly if you have a large sleeper unit.”
Truckers should also consider that an APU can heat and cool the cab and sleeper more efficiently if it uses the truck’s existing ductwork for heating, ventilation and air conditioning. Those that use their own vents to provide heating and cooling only heat or cool the sleeper berth.
Other important points to consider include:
An APU should heat the engine through block heaters or truck engine coolant circulation. By keeping engines warm at all times, operators can avoid cold starts.
APUs require 24 to 35 inches of clear rail space for installation. Some units offer alternatives if the truck’s frame doesn’t offer enough free rail space.
An APU engine with more cylinders will run quieter, perform better and run longer without major issues. That’s because it runs at a lower RPM and the cylinders fire more evenly.
Warranties typically cover parts and labor for one year, with extended warranties available. Two-year warranties will cover the APU engine and other major components.
Some APU manufacturers offer fixed distribution and service centers. Others offer service at several locations that also provide service on Class 8 trucks.
Truckers should understand how to change the APU’s belts and how to access its air, oil and fuel filters. The design should provide easy access to critical components for servicing. Some units require an oil change and a fuel, oil and air filter change every 500 hours.
When an APU is fitted with an optional air compressor, it can fill tires and overcome leaks, keeping air brakes and air suspension system lines charged. That can help truck operators avoid costly downtime.
Some manufacturers can offer financing, directly or through third-party lenders, to help qualified truckers get the device they need, particularly if they don’t have the cash.
A smart buy?
For those still wondering if the investment will be worth it, Watson said there’s no mystery to it.
“With the savings an operator can get by using an APU instead of idling his truck eight hours per day and 330 days per year, he will realize a payback in less than 16 months,” Watson said. “The monthly lease payment can be less than what a trucker saves in fuel costs. That means the APU will actually make money through fuel savings.”