pump, and oil pump, while auxiliary loads are accessories used in a vehicle’s operation, such as the power steering, air compressor, cooling fan, and air-conditioning compressor.

The energy losses for a Class 8 tractor with a 53-ft van trailer, fully loaded to 80,000 lb gross vehicle weight (GVW) and operating on a level road at 65 mph, for one hour, are shown in Table 5-1. The energy losses for a Class 3 to 6 medium duty truck, loaded to 26,000 lb GVW and operating on a level road at 40 mph for 1 hour, are shown in Table 5-2. (Note that this steady-state operating point is not typical of the duty cycle for a Class 3 to 6 medium-duty truck.) The energy losses for a 40-ft transit bus with one-half seated load (32,000 lb) and the air conditioning on, operating over the central business district cycle, for 1 hour, are shown in Table 5-3. Note the high percentage of energy devoted to auxiliary loads.

Vehicle energy balances such as those described in Tables 5-1 through 5-3 identify the energy required to propel a vehicle down the road at a specific speed and with a specific load. The following sections break down these areas of losses as follows: aerodynamics, auxiliary loads, rolling resistance, vehicle mass (weight), and idle reduction.

AERODYNAMICS

Truck Aerodynamics

Anyone comparing the commanding size of a tractor-van trailer combination to a small sedan or even a full-size sport utility vehicle (SUV) understands that the aerodynamic drag1 of these large vehicles exceeds that of any light-duty vehicle. Some quantitative comparisons of those differences will be made later. For now, consider again the energy summary for the tractor-van trailer given in Table 5-1 and Table 5-4. Clearly, for this class of truck, aerodynamic load reduction is a key for successful fuel consumption reduction.

Early Studies

K.R. Cooper of the Canadian National Research Council summarized some of the earliest heavy truck wind tunnel testing performed in 1953 at the University of Maryland (Cooper, 2004). Many of the aerodynamic design solutions now available or being developed for Class 8 tractors and box van trailers were evaluated in that 1953 study. Those devices were shown to reduce aerodynamic drag by about 50 percent as compared to the predominant truck configurations of the 1950s (the cab-over-engine tractor). The “near-practical” streamlined result is shown in Figure 5-2.

Airshield introduced a commercial cab roof-top air deflector in about 1965. This device received some trucking company interest, especially after the 1973 petroleum crisis.

TABLE 5-1 Energy Balance for a Fully Loaded Class 8 Vehicle Operating on a Level Road at 65 mph for One Hour

Energy Sources

Baseline (kWh)

Baseline (hph)

Engine losses per hour

240

321.8

Auxiliary loads

15

20.1

Drivetrain energy

9

12.1

Aerodynamic energy

85

114.0

Rolling resistance energy

51

68.4

Total energy used per hour

400

536.4

NOTE: hph, horsepower-hour.

SOURCE: TIAX (2009), p. 2-3.

TABLE 5-2 Energy Balance for a Fully Loaded Class 3 to 6 Medium-Duty Truck (26,000 lb) Operating on a Level Road at 40 mph for One Hour

Energy Sources

Baseline (kWh)

Baseline (hph)

Engine losses per hour

73.1

98.0

Auxiliary loads

1.5

2.0

Drivetrain energy

3.3

4.4

Aerodynamic energy

18.9

25.3

Rolling Resistance energy

23.0

30.8

Total energy used per hour

119.8

160.6

NOTE: hph, horsepower-hour.

SOURCE: TIAX (2009), p. 2-5.

TABLE 5-3 Energy Balance for a 40-ft Transit Bus Operating over the Central Business District Cycle for One Hour

Energy Sources

Baseline (kWh)

Baseline (hph)

Engine losses per hour

86.8

116.4

Auxiliary loads

36.4

48.8

Drivetrain energy

13.4

18.0

Aerodynamic energy

1.3

1.7

Rolling resistance energy

7.2

9.7

Total energy used per hour

145.1

194.6

NOTE: Transit bus with one-half seated load (32,000 lb) and air conditioning on. [Baseline] hph, horsepower-hour.

SOURCE: TIAX (2009), p. 2-6.

TABLE 5-4 Operational Losses from Class 8 Tractor with Sleeper Cab-Van Trailer at 65 mph and GVW of 80,000 lb

Operating Load

Power Consumed (hp)

Power Consumed (%)

Aerodynamic

114

53

Rolling resistance

68

32

Auxiliaries

20

9

Drivetrain

12

6

Braking

0

0

Total

214

100

SOURCE: DOE (2008).

1

Aerodynamic drag refers to forces that oppose the motion of a vehicle through air.



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