Cutaway of an Espar coolant heater. Airflow into the burner and exhaust flow-out are shown at the bottom, while the coolant flows are shown at top right.
The majority of fleets – an estimated 75 percent – are looking aggressively at anti-idling technology for a variety of reasons, according to Brian Dennehy, vice president of marketing and communications for Espar, a provider of such systems. Dennehy’s not alone in his assessment.
“The interest is huge,” says Amy Egerter, RigMaster’s marketing and communications manager. “With fuel being 20 to 40 percent of costs for some small fleets, they would be foolish not to be looking at what’s available” – especially when many jurisdictions are levying fines running to the thousands of dollars for idling beyond five minutes, she says.
Due to increasing interest, Technology and Maintenance Council Study Group S4 – chaired by Will Watson, vice president of sales and marketing of Auxiliary Power Systems – has created a task force that is developing a recommended practice for APUs. In the meantime, fleet managers wanting to eliminate idling now must choose among many approaches.
One approach is simply to equip the vehicle with a direct fuel-fired coolant and/or cab heater; in this case, the cost and weight of the installation will be reduced when compared with those of an APU. This may be a solution to the idling problem in Northern climates or with day cabs – wherever overnight air conditioning may not be needed.
Direct-fired cab heaters draw only about 1 amp per hour, meaning only about 8 amps overnight. Coolant heaters draw a little more than twice that – 2.3 amps – because they need to operate an integral water pump, as well as the motor the heater uses to drive fuel and air into the burner. With a reasonable battery capacity, supplying the heaters with electric power overnight would not be a challenge. Espar’s units have a number of sensors, including an integral low voltage cutoff set at 10.5 amps, Dennehy says.
Using a direct-fired cab heater can be a way to reduce the weight and size of an APU by taking a good deal of the electrical load off the generator or thermal load off the engine. It can save fuel, too. The Airtronic D2 bunk heater is an integral part of Thermo King’s Tri Pac, an APU the company brought to market a few years ago. Both systems are engineered into a seamless unit; the driver only has to operate one thermostat.
Running a direct-fired heater of either variety provides a savings over idling the truck’s engine. That’s because the engine pulls a tremendous amount of air through its cylinders at idle – more than is needed to combust the tiny shot of fuel injected each power cycle. Between the excess airflow through the engine and what may be lost through the block, oil pan and radiator, more fuel is needed to supply a given amount of heat for the cab and sleeper.
Are APUs for you?
Dennehy admits that an APU – which has the advantage of providing air conditioning, heat and power in a single unit – can be more costly. So the deciding factor normally would be the critical need for overnight air conditioning. If so, the next choice to make is whether you should have an APU or its close cousin – a genset. APUs are integrated into the truck’s cooling and air-conditioning systems, while a genset is not.
An APU is likely to be more energy-efficient because the APU engine’s waste heat is used instead of being thrown away. For example, the Willis APU’s engine has no cooling system, but is interconnected directly with the truck engine cooling system. It generates enough BTUs to warm the cab and sleeper to 68 degrees – even when it’s -20 outside. The unit’s three-cylinder Kubota diesel normally operates at only 1,100 rpm, which minimizes noise and wear, but rpm can be brought up to 2,400 when necessary.
In the winter, the driver can use the standard cab and sleeper blowers, vents and controls to supply heat to his living environment. The APU’s 120-amp DC alternator will replace the battery power used for this purpose. A separate DC motor and clutch are integrated into the fan drive. When the engine is off, the fan clutch is disengaged, allowing it to freewheel. The APU engine’s waste heat warms the engine until the controls start the electric motor and then cycle the fan on and off to maintain temperature.
In the summer, the APU uses the truck air-conditioning system’s condenser and its own direct-driven compressor. It runs at 2,400 rpm and can supply 33,000 BTUs of cooling. Because the integration includes the fan controls and separate motor, the fan can run and cool both the engine and air-conditioning condenser, reacting to both cooling system temperature and condenser pressure. Because the Willis APU uses its own engine waste heat and air conditioner compressor, it needs no large AC generator.
Egerter describes RigMaster’s genset system as “completely independent from the truck.” It offers a Perkins or Caterpillar diesel; a 6 KW AC 120-volt generator; and a 60-amp alternator. A separate heat and air-conditioning unit that is run automotively and does not draw power from the generator is installed in the sleeper. If the truck’s air conditioner were to fail, the driver could run the APU and its air-conditioning system while running down the road.
Eduardo Andrade, Carrier Transicold’s business manager for special products, says his company’s ComfortPro unit provides sleeper air conditioning electrically, using its 4-kilowatt, 110-volt AC generator to provide power for a separate air-conditioning unit in the sleeper. This allows use of a hermetically sealed compressor, eliminating drive belts, the clutch and the shaft seal, all high-maintenance parts, Andrade says.
Many APUs are based on motor-home gensets and air-conditioning systems, says Watson, who claims Willis incorporates only heavy-duty parts. Egerter points out that RigMaster’s Caterpillar engine runs at only 2,700 rpm while design rpm is 3,600, minimizing stresses. Carrier’s Andrade says fleet managers should consider how well the cab and sleeper are insulated in determining BTU requirements for heat and air conditioning.
APUs use only .1 to .3 gallons per hour vs. a gallon an hour or more for the main engine, Egerter says, so today’s fuel prices accelerate payback. You’ll also benefit because you won’t be subjecting the main engine to the cold running conditions and poor combustion of idle, the effects of which shorten both engine and lube oil life.