Not an APU at all, Temp-A-Start is an electronic system that starts and stops the tractor’s main engine as needed to keep the sleeper’s temperature comfortable and batteries charged.
The system monitors temperatures of the engine’s oil and the sleeper area (the latter controlled by a thermostat), and when either drops to preset levels, it cranks over the engine to warm it. This makes the engine easy to restart later and produces hot coolant to supply heat to the truck’s heater cores, or intermittently runs the truck’s air conditioning.
Meanwhile, Temp-A-Start monitors the batteries and starts the engine so its alternator can recharge them. Temp-A-Start can also be linked to a diesel-fired bunk heater and an electrically powered A/C unit. These keep the sleeper warm or cool so the engine can run less often.
The system can be programmed to run the engine within limits set by anti-idling laws; usually this is five minutes per hour, and that’s enough to recharge batteries, says the device’s maker, TAS Distributing at http://www.tempastart.com.
The most common APU for a commercial truck is a small diesel engine with its own cooling system, heating system, generator or alternator system with or without inverter, and air conditioning compressor, housed in an enclosure and mounted to one of the frame rails of a semi-truck. Other designs fully integrate the auxiliary cooling, heating, and electrical components throughout the chassis of the truck. These units are used to provide climate control and electrical power for the truck’s sleeper cab and engine block heater during downtime on the road.
In the United States, federal Department of Transportation regulations require 10 hours of rest for every 11 hours of driving. During these times, truck drivers often idle their engines to provide heat, light, and power for various comfort items. Although diesel engines are very efficient when idling, it is still financially and environmentally costly to idle them like this, from a fuel consumption and an engine wear perspective. The APU is designed to eliminate these long idles. Since the generator engine is a fraction of the main engine’s displacement, it uses a fraction of the fuel; some models can run for eight hours on a US gallon (≈ 4 litres) of diesel. The generator also powers the main engine’s block and fuel system heaters, so the main engine can be started easily right before departure if the APU is allowed to run for a period beforehand. An APU can save up to 20 gallons (Cat 600 - 10 hours downtime @ 2 gallons per hour idling) (≈ 76 litres) of fuel a day, and can extend the useful life of the main engine by around 100,000 miles (≈ 160,000 kilometres), by reducing non-productive run time.
Some vehicle APUs can also use an external shore power connection for their heating and cooling functions, thus eliminating fuel consumption during rest periods altogether. Many truck stops already provide shore power connections in their parking areas.