The nuts and bolts of APUs

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

In its Recommended Practice 439, "Minimum Performance Criteria for Auxiliary Power Units and Gensets," the Technology & Maintenance Council (TMC) - an organization dedicated to improve transport equipment, its maintenance and maintenance management - advocates these additional factors when evaluating APUs:

• Safety - Is there a device/switch to prevent the device from being turned on while it is being serviced?

• Training - Service and safety training should be available, including authorized installation training and technician certification and driver training or training materials..

• Service interval - At minimum, the interval should match the tractor service interval.

• High-altitude operation - Fuel-fired heaters are affected by altitudes above 5,000 feet. Users should consult with the heater manufacturer if they intend to operate their fleet at higher altitudes. (This issue primarily relates to fuel/air mixture.)

• Extreme cold ambient operation - Consideration should be given to idling the tractor's main engine when ambient temperature conditions drop to 20ºF or below as fuel gelling may occur at or below this temperature.

• Fuel requirements - Will the APU engine operate on the fuel that is to be used?

• Emission compliance - Does the APU meet or exceed current/anticipated emission requirements where it will be operated?

• Shore power - Is the unit's AC power system compatible with shore power?

• Tax exemptions - Can fuel usage for the APU be calculated for a tax exemption filing? Is the APU exempt from Federal Excise Tax (FET)?

In a session at TMC's Annual Fall Meeting, Kirk Altrichter of truckload carrier Gordon Trucking shared his experiences with APUs and provided a list of key APU purchasing considerations. These included:
• Climate for use. Are you buying a year-round system?
• Hours of daily use.
• Does the system comply with the regulations for the areas of operation?
• Will there be an inverter?
• How many batteries?
• What is the size of the truck's alternator?
• What is the size of the APU's alternator?
• How much hotel load will the system allow?
• Maintenance requirements.
• Maintenance costs.
• Will a DPF (diesel particulate filter) and DEF (diesel exhaust fluid) be required?
• Ease of use.
• Fuel usage.

In an effort to avoid problems with APUs, Altrichter said Gordon Trucking has built in some protection safeguards, including a maintenance inspection of the APU at every vehicle PM. In addition, the fleet has installed a battery conditioner, low-voltage disconnect (a device that disconnects a load from the batteries to prevent over discharge of a battery - one of the most common ways of killing a battery), battery disconnect, stand-alone starting batteries and a high-amperage alternator on the APU and on the truck engine.

Cummins demonstrates solid oxide fuel cell auxiliary power system

Earlier this year, Cummins successfully demonstrated a tubular solid oxide fuel cell (SOFC) in a hybrid-configured APU constructed to power a Class 7/8 truck's hotel loads while running on commercial ultra-low sulfur diesel (ULSD) fuel.

The use of fuel cell APUs is compelling because it not only reduces energy and environmental impacts, but has the potential to reduce costs and improve driver safety by reducing noise and vibration for sleeping drivers.

SOFCs are highly efficient, making electricity from fuel and oxidant gases in an electrochemical process that takes place across an ion-conducting, ceramic membrane.

There are two principal SOFC configurations: tubular and planar. Tubular designs bundle tube-shaped cells in parallel. One end of the tubes is closed, and the other hooks to a common air supply manifold. Oxygen ions pass through the electrolyte and react with fuel flowing over the outside of the tubes, creating an electric current.

Planar types place individual cells in a multi-layered "stack," with cells arranged in series or parallel to give the required voltage and current.

The Cummins APU incorporated hybrid power electronics, system controls and balance-of-plant components combined with SOFC modules supplied by Protonex Technology Corporation, a leading provider of advanced fuel cell power solutions for portable, remote and mobile applications.

The SOFC power unit was integrated with a modified power electronics section derived from Cummins Power Generation's Hybrid Quiet Diesel (HQD) recreational vehicle power system. The hybrid power electronics provide dynamic load sharing between the SOFC and a battery pack.

The testing replicated the 10-hour rest period that is part of the daily routine of a typical over-the-road truck.

The hybrid-configured APU can combine power from an engine-driven generator or fuel cell and a battery pack to supply both AC and DC power to the truck's hotel loads.

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