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

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
requirements while allowing for better performance over the engine’s operating range.
The primary way to decouple accessories from engine speed is by electrification, driving
each accessory with an electric motor that can run at the accessory’s optimum speed. By
using this method, only smaller engine loads associated with electrical generation are
required, resulting in reduced fuel consumption. Another option is to clutch accessories,
so that they only operate when needed. This is normally done with the cooling fan and air
conditioning compressor, but the idea can also be applied to the alternator and air
compressor as well.
Even assuming significant improvements in combustion efficiency and accessory loads,
large amounts of energy will continue to be in the form of heat energy in the exhaust
stream. Conventional turbochargers are designed to operate using some of this otherwisewasted
energy. There are three other emerging technologies that can allow even more of
this energy to be used for the necessary functions of the truck. Thermoelectric generators
can use the temperature gradient between the exhaust and ambient air to generate
electricity. These devices also are described in the LDV section, but are anticipated to be
promising for heavy-duty long-haul trucks because these trucks operate under fairly high
load the majority of the time and subsequently produce a relatively continuous supply of
high-energy exhaust. 58 These systems are still in development and are not yet
commercially viable.
Another option to recoup exhaust energy is turbocompounding. Turbocompounding
uses an exhaust turbine for shaft power, either as a stand alone system or in addition to
powering the intake compressor. The output shaft can be connected to an electric
generator or to a transmission connected to the engine’s crankshaft. Excess turbine power
can then be used to either assist the engine shaft output or to reduce the alternator’s
electrical generation load, either partially or completely.
A third option, a bottoming cycle, consists of a heat engine that uses waste heat from the
primary diesel engine to produce additional work. There are many concepts available for
use in a bottoming cycle, including refrigerant-based cycles and steam turbines. The great
advantage of a bottoming cycle is that it uses “free” energy—energy that is going to be lost
by the primary engine. The great disadvantage of a bottoming cycle is that its efficiency is
limited by the poor quality (i.e., relatively low temperature) of most waste heat from the
engine. In addition, the amount of waste heat available also varies greatly with the
engine’s operating condition. Bottoming cycles have been used for many years in
stationary power plants, but so far they have not found application in vehicles because of
cost, weight, packaging, reliability, and performance issues (NESCCAF/ICCT, 2009).
Friction reduction can contribute to overall truck fuel efficiency. Design changes to the
bearings, seals, and mating surfaces in engines, transmissions, and differentials can reduce
energy requirements. Additionally, designing these devices to take advantage of lower
viscosity lubricants can reduce energy lost to pumping fluids through the engine and
58 Diesel exhaust temperatures are reduced by lean operation, the presence of EGR, and
turbocharger use, all of which will tend to reduce the effectiveness of thermoelectrics.
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