ABCs of APUs

The unit has a main engine and generator (left) and a climate control unit (right), as well as a condenser unit that has already been unpacked.

Overnight idling may be part and parcel of the trucker’s life, but that is changing fast. Aside from the environmental issues, your truck’s engine is optimized for the hard work it does hour after hour out on the road, but at idle it takes in as much as 10 times the air needed for combustion. This extra air chills the engine parts and oil, creating unnecessary wear. It also steals most of the heat and power created.

Using a small engine to do the same work of cooling the cab, charging the batteries and, in most cases, providing heat in winter means far less fuel consumption than idling the main engine. The small, two-cylinder engine typically used will be running hot enough to maintain efficient and complete combustion, and its tiny pistons and crankshaft will produce only minimal friction while giving you more than enough heat, electricity and air-conditioning. An auxiliary power unit or genset will use about 0.2 gallons per hour vs. a full gallon an hour for the diesel.

Because of all the fuel consumed, and because injection pressures and air motion and pressure within the cylinder of any diesel drop way down at idle, all but the latest truck engines will crank out high emissions when sitting there idling. Even 2007 engines will be far dirtier than an emissions-certified APU or genset. That’s why idling the main engine is prohibited in more and more areas.

Every high-quality APU or genset arrangement on the market will cool and heat the cab, charge the batteries, give you what you need for running your personal appliances and keep the engine warm, depending upon your exact needs. While doing all that, it will greatly extend the life of your main diesel engine and significantly extend its oil change intervals.

Choosing your APU
There is an amazing variety of APU and genset design. Gensets normally have a large AC generator that produces 110-120-volt, 60-cycle house current and use that current to operate everything connected to the unit. APUs often have a large AC generator and may also have a 12-volt alternator. Some drive an electrical, “hermetic” A/C compressor, but many drive an automotive-style A/C compressor directly off the APU engine’s flywheel. So, picking an APU is a matter of individual philosophy and choice for the same reason that each manufacturer of heavy-duty diesels has its devoted customers. Each type of design has its advantages.

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APUs are integrated into the truck in various ways. For example, many interconnect with the truck engine’s cooling system. That way, the APU engine’s heat, which is normally just thrown away, can be used to keep the diesel warm overnight. When integrated in this way, the truck diesel will also keep the APU engine hot so that, when you stop, you’ll be able to start it up easily even if running in Minnesota in January.

Nearly all APUs and gensets use the truck batteries for starting, which not only provides excess cranking power but an easy path for battery charging current coming back from the APU. They also use one of the truck’s fuel tanks for their fuel.

For example, Thermo King’s TriPac combines a small Thermo King diesel, inter-connected with the truck cooling system, with a belt-driven A/C compressor, a 65-amp, 12-volt DC alternator, and a fuel-fired heater that heats the air in the cab and sleeper. If the user wants power for in-cab appliances, an optional 12-volt DC to 120-volt AC inverter is available. Thermo King says the fuel-fired heater produces fewer emissions and uses less fuel to heat the cab than any other setup. The Tri-Pac’s A/C evaporator and blower are contained in a compact unit normally mounted under the bunk.

Such a unit could be left shut down for most of the night in winter and then started in the morning an hour before departure to charge the batteries and warm the main engine.

Carrier’s ComfortPro unit has both a 4,000-watt, 120-volt AC generator and a 60-amp DC alternator. The main generator provides electric resistance heat to the cab in winter, while using the waste heat generated to warm the diesel via the inter-connected APU and engine cooling systems. Like all high-quality arrangements, it has its own radiator and thermostat because the amount of heat its engine produces is normally more than the truck’s cooling system can throw off without the fan and water pump operating. (An exception to this rule is the unusual Willis APU, which drives the truck engine fan with its own electric motor. It thus uses the truck’s radiator to cool the APU engine and the truck’s A/C condenser to throw off the heat generated by its own A/C compressor.)

The ComfortPro provides air-conditioning electrically via a hermetic A/C compressor powered by the large generator on the APU. The electric resistance heaters, hermetic compressor, and high capacity blower are all contained in a unit typically mounted in one of the cab’s side storage compartments. The hermetic compressor retains the advantage of having no belt drive or shaft seal, as well as soldered connections, reducing related maintenance as well as refrigerant leak points.

The RigMaster APU (virtually a genset) minimizes integration of the APU and truck systems to help ensure reliability, according to the company’s marketing and communications manager, Amy Egerter. Evan Gaffney, who is in charge of the company’s educational development and training, says the unit’s 6,000-watt, 120-volt generator heats the engine via a block heater so there is no risk of an APU-related engine coolant leak.

The high-capacity AC generator lets the driver keep the engine warm and run his microwave oven and other such accessories at the same time. APU engine waste heat is used to heat the cab via a heater core installed in the same Rigmaster unit that contains the blower motor and A/C evaporator, thus making use of APU engine waste heat without tapping into the truck cooling system. An electronically controlled valve stops and starts water flow from the APU engine to maintain cab and sleeper temperature.

Gaffney says RigMaster chose a belt-driven, automotive-style A/C compressor for its separate cab cooling unit because these units take up much less space and weigh less than a hermetic unit. The goal, says Egerter, is to minimize the unit’s effect on available storage space in the cab.

The hermetic unit is considered supreme among A/C technicians for its reduced maintenance and long service life. However, the automotive-style compressor’s advantage is that it lends itself to simple do-it-yourself replacement if it ever fails, provided you have the refrigerant removal and charging tasks performed by a licensed technician. A hermetic unit’s lines must be soldered in a skillful manner.

You should choose the unit that you feel will be most reliable and efficient, depending on the way you use your vehicle. Integration of APU and truck systems always carries a small risk of mechanical failure, hence the decision of many APU makers to provide completely separate cab A/C systems. On the other hand, integration always saves weight, complexity and cost. Features such as APU and engine coolant interconnections that include high-quality shutoff valves may allow for integration with confidence.

“Poor installation has a large effect on the operation of the unit,” says RigMaster’s Egerter. It’s clear that 95 percent or more of APU reliability problems are due in large part to poor installation. Performance can also be affected, a theme echoed by every manufacturer. In fact, every single manufacturer we spoke to trains its technicians and certifies them prior to allowing them to do an install.

The problem, according to technicians Mark Loring and Will Reynolds of Barr International, in Salisbury, Md., is that the job is extremely complex. Installations go much better after you have had experience and learned from your mis-steps. Also, since every truck is different, every installation is different. Much of the time and effort involve carefully planning the locations of the various components and the routing of ducting and wiring.

A ComfortPro installation Loring and Reynolds did recently took almost two full days with both highly experienced techs working full-time. They had had Carrier training and this was the second one they had performed.

Unfortunately, few manufacturers will allow you to install your own unit without voiding the warranty. They have learned from experience that if you don’t have a great deal of training and experience, you’ll fail to get everything right, resulting in problems not covered under the unit’s warranty.

Rich Barr, marketing manager at Barr International, says that, in spite of the work involved, the installation involves only about 15 percent of the total cost of the unit. He priced the installation mentioned above based on the belief that his technicians would be able to do the job faster and faster as they gain experience. Thermo King’s Tom Kampf (product manager, APU business) says the critical issue is certification. Thermo King will not put a warranty on a unit unless they have certified the installer. Kampf did allow, however, for the possibility of a one- or two-truck operator becoming certified.

But Kampf says a TriPac takes about a full day to install, depending on the truck’s design. Getting certified would at least double that time. You might well be able to make more money by operating for those two days than you’d save by doing the install yourself, anyway. You also need to realize that the refrigeration part of the air-conditioning system needs to be evacuated (to remove moisture) and charged with refrigerant by a licensed technician using specialized tools. Unless you have these tools and are certified, you need to get this part of the job done by your seller’s technicians anyway.

RigMaster’s Gaffney opened up a third possibility. Work with your dealer and get their guidance and advice. Do much of the work yourself, but bring the unit to the dealer for evacuation and recharge of the refrigerant system. Then pay a certified technician for the time necessary to inspect and approve the installation. If you feel doing an installation yourself could actually be economical, we suggest you discuss such an approach prior to purchasing a unit to see whether or not your dealer and the manufacturer will work with you on this basis.

Even if you don’t install the unit yourself, you should be closely involved with the process. As the RigMaster installation manual says, “Poor placement of the APU will have a negative impact on the performance and accessibility of the unit. Remember that the best location involves practicality, serviceability and aesthetics.”

It goes on to say, “Remember when choosing a location that the harder it is to access the unit, the more difficult it will be to service.” If the unit’s hard to get at, service won’t be done as often, especially when hired drivers are involved. Be at the shop during the early stages of installation so the location of the engine and generator unit will be the one you prefer for easy access where you park. You will also want to influence the location of the unit that carries the A/C evaporator, heating element, and blower – you don’t want them taking your most valued storage space.

Witnessing an Installation
Barr International’s Rich Barr invited us to witness the installation of a ComfortPro unit in their shop as technicians Reynolds and Loring worked. The ComfortPro includes three major components: the engine and generator and their housing; the climate control unit, including the evaporator, fan, and resistance heating mechanism, and its housing; and the condensing unit. They installed the ComfortPro on an International 9500i tractor with sleeper.

Here are just a few highlights of the two-day installation process:
Fig. 1. The unit has a main engine and generator (left) and a climate control unit (right), as well as a condenser unit that has already been unpacked.

Fig. 2. The first step was preparation of the vehicle. The truck had had a fuel-fired heater that became unnecessary once the Carrier unit’s electric resistance heaters were installed. This was removed and Reynolds repaired the hole in one panel by pop riveting in a new piece of sheet metal, shown here being cut to size. He also replaced and painted an entire new panel to get rid of a second hole.

Fig. 3. Careful planning led to a decision to mount the main unit behind the left-hand fuel tank and the climate control unit in the storage compartment nearby. A template was used to drill holes for mounting the climate control unit on the floor of the storage compartment. Insulation was then cut so the mounting screws could be installed.

Fig. 4. Here, the climate control unit is being put into position.

Fig. 5. Drilling holes for the flexible ducts to pass through requires the use of a saw of the right diameter.

Fig. 6. The left-side cab mount huck bolts, whose threads stuck out of the frame, had to be drilled out and replaced with normal bolts turned in the other direction so just the bolt heads would be showing. This was done for clearance. The area was painted once the new bolts were in place.

Fig. 7. The main unit was lifted with a ceiling hoist onto a floor jack for transport to where it would be mounted on the frame rail.

Fig. 8. The unit was suspended by the floor jack as self-locking nuts and bolts were used to attach it. This was done with special slotted fittings that slide over the top and bottom horizontal sections of the frame rail. The nuts were then torqued to 100 lb.-ft.

Fig. 9. The three ducts that carry conditioned air into the sleeper were routed under the bunk. Note that ties are used to fasten them securely in place. The technicians used hundreds of them to secure all wiring and hoses to prevent chafing.

Fig. 10. The condenser unit was mounted on back of the rear sleeper wall, not far from the climate control unit. It has slotted mounting holes so it can be tilted slightly in either direction for leveling. After drilling mounting holes using it as a template and installing the mounting bolts loosely, its position was adjusted with a level and the mounting bolts finally tightened.

Fig. 11. A high quality installation demands purchase and assembly of tees with effective shutoff valves so the APU can be teed into the engine cooling system at the heater hoses yet isolated in case it develops a leak.

Once the unit is installed, says Thermo King’s Kampf, get into the habit of using it every time you’re shut down. “Old idling habits die hard,” he reminds us. He also recommends opting for such devices as the auto start switch offered with the TriPac. Such effective controls help you to get the most for your money by maximizing use of the unit.

For more information:
Carrier ComfortPro

Rigmaster Power Corp.

Thermo King TriPac

Willis APU

Barr International

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