Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
Finally, fuel prices—whether affected through policies or market forces—will play an
important role in affecting truck purchasers’ decisions regarding fuel efficiency savings, as
well as manufacturers’ decisions to invest in more efficient technology. The expectation of
sustained higher fuel prices would likely encourage more manufacturers and operators to
adopt fuel efficiency improvements.
Heavy-Duty Engine/Powertrain Measures and Integrated Strategies
Overview
Current heavy-duty diesel engines operate at approximately 40 percent thermal efficiency
under optimal operating conditions. Taken with auxiliary loads and drivetrain losses,
66 percent of fuel energy is not converted to motive power (ORNL, 2000). Technologies
can be implemented to reduce this energy loss and make heavy-duty trucks more efficient.
There are four primary ways in which heavy-duty powertrains can be modified to reduce
GHG emissions:
1. In-cylinder/combustion improvements;
2. Accessory load reduction;
3. Frictional loss reduction;
4. Hybridization strategies; and
5. NOx aftertreatment strategies.
The first three of these technologies will reduce the waste heat lost from drivetrain
components. HDV hybridization strategies share many characteristics with LDV hybrids,
described in the previous section. Unless otherwise noted, the specific technologies under
consideration for HDVs are just now entering the marketplace, or are expected to become
available in the next few years (U.S. DOE, 2009b). In-cylinder and combustion
improvements modify the way that air and fuel are brought into the engine and how the
fuel is burned to convert its chemical energy to mechanical power. Accessory load
reduction decreases the power needs for pumps, air compressors, electrical power
generation, cooling fans, and air conditioning. Frictional losses generate waste heat in the
engine and transmission, decreasing the power that could otherwise be used to drive the
wheels. Hybridization approaches utilize two power delivery sources and recover
braking energy to improve overall efficiency. NOx aftertreatment strategies reduce the
amount of energy dedicated to lowering NOx emissions through control efficiency
improvements, thereby reducing fuel consumption.
Today’s diesel engines typically use turbochargers to compress the intake air, leading to
greater efficiency and power output. Technologies aimed to improve turbocharger
operation can improve efficiency, drivability, and emissions. One modification that recently
entered the market is the variable geometry turbo (VGT). This type of turbo involves
moveable surfaces in the exhaust turbine that allows control over the speed of the
compressor. With the use of exhaust gas recirculation (EGR), the VGT has become
universal. Ironically, the VGT system is generally used to reduce the efficiency of the
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