Environmental Engineering Reference
In-Depth Information
Table 13.6 Estimated cab/sleeper heating and engine block heating energy requirements based on the need
to provide 11.2 MJ/h (10,650 Btu/h) to heat engine block and 4.3 MJ/h (4,100 Btu/h) to heat cab/sleeper.
Option
Fuel input
(MJ/h [Btu/h])
Electrical power
required or
produced (W)
Total usable
energy output
(MJ/h [Btu/h])
Net efficiency (%) a
Truck engine idling b
136 (128,500)
1,300
20.2 (19,187)
15 (11 for heat only)
Direct-fired heater b
19.5 (18,438)
52
15.6 (14,750)
80
Auxiliary power unit
24.4 (23,130)
1,300
20.2 (19,187)
83 (64 for heat only)
Thermal storage
(cab heat only) c
-
30
0.11 (102)
-
Truck stop
electrification
47.9 (45,378)
4,300
15.5 (14,676)
33
a Defined as energy output divided by energy input.
b Engine idling provides more heat than is required for low-smoke startup.
c Includes energy required to supply electricity and recharge partially discharged batteries.
Adapted from Stodolsky et al., 2000.
power units, engine idle management systems, battery-operated electric air-conditioners to
cool just the sleeper, and truck stop electrification. Storey et al. (2003) compared the emissions
and amount of fuel used by several Class 8 trucks while idling against an 11-hp diesel auxil-
iary power unit (APU) and a diesel direct-fired heater (DFH). The diesel APU showed reduc-
tions of fuel consumption between 60 and 85 percent, depending on the truck; 50 to 97 percent
reduction in nitrogen oxides, carbon monoxide and volatile organic compounds, and reduction
of particle matter of up to 97 percent. Direct-fired heaters had lower emissions and fuel con-
sumption than the diesel APU, which can be expected because DFH units provide just heating.
Truck stop electrification allows drivers to get heating, ventilation, air-conditioning, and
power for electronics while at truck stops. There are two systems in place: the single- and the
dual-system electrification. In the single-system, heating, ventilation, and air-conditioning are
provided to the truck via a hose connected to a window through an adaptor. With the
dual-system trucks are “plugged” into electrical outlets (shore-power connection) and the
electricity used to run on-board equipment. For this system to operate the truck has to
be equipped with the electrical powered equipment that can be run with 120/240 volt and the
necessary hardware to make the connection. Table 13.6 compares net efficiencies of different
alternatives to idling.
Another factor that affects fuel economy of trucks is the driver's behavior. According to
Cummins Engine Company (2007) the most efficient drivers get a 30 percent better fuel mile-
age than the least efficient ones. Aggressive driving with unnecessary accelerations and sud-
den stops reduces fuel efficiency. Fuel consumption of trucks can be improved by controlling
the speed of the truck. An increase in the speed from 60 to 65 mph produces a decrease in fuel
economy of 6.5 percent, by going from 60 to 70 mph the fuel economy decreases by 10.9
percent, and by driving at 75 instead of 60 mph, the fuel economy decreases by 17.3 percent
(Kenworth, 2008a). This is not surprising because the force of aerodynamic drag is propor-
tional to the square of the speed; so the higher the speed the more important the aerodynamic
drag quickly becomes.
Proper maintenance of the truck, such as correct tire inflation, clean filters, adequate axle
alignment, and no air leaks prevents a drop in fuel mileage. For every 10 psi a truck tires are
underinflated fuel economy decreases by 1 percent. Dirty air filters create a restriction that
make the engine starved for air and prevents optimal fuel combustion. Out of alignment axles
increase rolling resistance and take a toll on fuel efficiency, in addition to producing incorrect
tire wear. Air leaks make the compressor have to work harder, which robs energy from the
engine (Cummins Engine Company, 2007).
 
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