Table 3 Estimated Cab/Sleeper Heating and Engine Block Heating Energy
Truck engine idling
15 (11 for heat
Auxiliary power unit
83 (64 for heat
Thermal storage (cab heat only)
Truck stop electrification
To provide 10,650 Btu/h to heat engine block and 4,100 Btu/h to heat cab/sleeper, based on medium
setting of direct-fired heater.
Defined as energy output (as heat to provide warming of engine and cab/sleeper) divided by energy input.
Engine idling provides more heat than is required to maintain acceptable low-smoke start-up
temperatures. We assume 10,650 Btu/h is required to maintain acceptable start-up temperature.
Includes energy required to supply electricity, including that to recharge partially discharged batteries.
References: EEI (1996); Espar (undated); Wang (1996); TMC (1995).
pumping the storage medium while the unit is running. Manufacturers do not provide information
on the increased fuel requirements during vehicle operation to supply additional heating or
cooling for the storage medium. However, Webasto estimates an increased load of 1 hp to cool
the medium (Koziel 1999). If the truck averages 200 hp at 55 mph and gets 6 mpg, it uses
55 gal of fuel during the 4
6 h that the medium is cooled. The marginal cooling load adds
about 0.5% (1/200) to that, or 0.19
0.28 gal. This is the fuel used for 8 h of cooling, so the fuel
use is approximately 0.023
0.034 gal/h of cooling.
One drawback of the direct-fired heater and storage options is that they need electrical
power, and so truckers are concerned that they will not have enough battery capacity to restart
the engine, especially during cold weather (Jessiman 1996). During a typical overnight heating
cycle of 8 h, at a current draw of 1.9 A, a cab/sleeper heater requires about 15 Ah of battery
capacity (Espar undated). The engine block heater requires power for the coolant circulating
pump; about 3 A is required for this unit on the medium setting (5.8 A on high), and so about
24 Ah of battery capacity is required overnight on the medium setting (46 Ah on high). A total
battery capacity of 39 Ah is therefore required on the medium setting (61 on high). The typical
truck has three 12-V batteries with a total capacity of 150 Ah, and engine start-up requires about
60 Ah. Under most conditions, sufficient battery capacity would therefore remain for start-up. In
addition, thermoelectric converters for direct-fired heaters promise to reduce electrical power