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

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
separate power flow paths, each with a clutch, that alternate with each shift. Shifts can
take place more quickly as one clutch can be released as the other is engaging.
Another option being investigated for LDVs involves higher voltage DC systems. In
conventional vehicles, mechanical power is taken from the engine to run accessory
functions such as power steering, oil pumps, and water pumps. These accessories tend to
be overdriven at higher engine speeds. Improvements can be made allowing engine
accessories to run off of electrical power using a higher voltage system, running them only
when needed and at their most efficient speeds. Adopting a higher voltage electrical
system also would enhance the ability to offer automatic start/stop operation similar to
that of a hybrid vehicle. By having more battery power available, a vehicle’s engine could
be shut down during braking and idling to save fuel, then instantly restart when the
driver is ready to accelerate.
In an internal combustion engine fuel energy is transformed into rotational power and
heat that enters the vehicle’s cooling system and exits with the engine exhaust. Nearly
70 percent of the fuel energy that enters a conventional engine is lost as waste heat, but
research is underway to find means of recapturing this lost energy. A leading option in
this area is a thermoelectric generator which can create electrical power flow in the
presence of a temperature difference. If efficient enough, a thermoelectric device in the
exhaust stream of an engine could allow the elimination of the alternator and its
associated mechanical power consumption. This technology could be employed with
hybrid vehicles or with conventional vehicles that have electrically powered accessories.
In addition to vehicle powertrain and engine improvements, other strategies such as
reduced vehicle weight, lower rolling resistance tires, improved aerodynamics, and
improved lubricants also can reduce energy consumption. These technologies generally
improve the state of conventional technology gradually and offer relatively small fuel
efficiency benefits with each succeeding model year, instead of providing a step change
upon market penetration as expected with many of the technologies listed above.
Vehicle weight reduction strategies include material substitution, vehicle redesign, and
market shifts to smaller and lighter vehicles. Weight reduction offers particular promise
as a GHG reduction strategy, due to positive synergies with other LDV design
considerations. For example, reductions in weight allow for secondary downsizing of
components that no longer need to be as large in order to perform their functions. It is
estimated that secondary reductions can total 50 percent of the primary reduction
(Bandivadekar et al., 2008).
Material substitution in LDVs has been taking place over the last 20 years, primarily
through the increased use of aluminum and high strength steel (HSS), used to replace
mild steel and iron. In addition, the use of various materials such as plastic, magnesium,
and carbon- and glass- reinforced composites may become more widespread in the future
to further reduce vehicle weight.
It also is possible to reduce vehicle weight by changing the design of vehicles. An
example of this type of change was the movement from body-on-frame cars to unibody
construction which reduces weight without requiring higher-cost materials. Designs that
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