FIGURE 3-1 Energy audit of a typical Class 8 tractor-trailer combination on a level road at a constant speed of 65 mph and a GVW of 80,000 lb. SOURCE: DOE, 2006, p. 7.

TABLE 3-1 Baseline and 21CTP Target Values from the Energy Audit Shown in Figure 3-1




Percentage Reduction

Total Energy Consumption

380 kW

225 kW


Engine Power Required

160 kW

112.8 kW


Thermal Efficiency



Auxiliary Loads

15 kW

7.5 kW



9 kW

6.3 kW


Rolling Resistance

51 kW

30.6 kW


Aerodynamic Losses

85 kW

68 kW


and 21CTP target values from the energy audit shown in Figure 3-1 are also listed in Table 3-1.

The following observations can be derived from this energy audit:

  1. Improvements in engine efficiency offer the largest potential reductions in fuel usage. Reductions in rolling resistance and aerodynamic losses offer lesser reductions in fuel usage.

  2. The engine power output of 160 kilowatts (kW), which is required by the vehicle at 65 mph, is about 42 percent of the total fuel energy consumption rate of 380 kW (which equates to 6.8 mpg). Therefore, the thermal efficiency is 42 percent, which is representative of today’s typical diesel engine thermal efficiency at the 65 mph road load operating condition.

  3. A 20 percent improvement in engine thermal efficiency from the current baseline of 42 percent will yield the 50 percent thermal efficiency objective.

Increases in fuel economy, expressed in miles per gallon (mpg), will be directly proportional to improvements in thermal efficiency. However, fuel usage in gallons is inversely proportional to miles per gallon. Therefore, a 20 percent improvement in thermal efficiency will result in only a 16.7 percent reduction in fuel usage, as shown below.

Thermal efficiency = (work output)/(fuel energy input) ~ miles/gal ~ mpg

Fuel usage ~ 1/mpg

Percentage change in fuel usage = (1/mpgimproved − 1/mpgbase) × 100

Percentage change in fuel usage = (1/1.2 − 1/1.0) × 100 = −16.7 percent

This result illustrates an inconsistency in a presentation to the committee,2 which erroneously suggested that a 20 percent improvement in engine thermal efficiency would yield a 20 percent reduction in fuel usage.

For consistency with DOE, the following terminology is used in this report:

  • Individual vehicle fuel consumption is expressed as gallons per mile (gpm). (Note: Alternative units such


Vinod K. Duggal, Cummins Engine Company, Inc., “Diesel Engine R&D and Integration,” Presentation to the committee, Washington, D.C. February 9, 2007, Slide 11.

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