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

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
efficient operating point, but suffers losses due to inefficiencies in the generator and
motor(s). Battery packs are generally larger for series hybrids than other configurations,
resulting in relatively higher costs, although they are most effective in slow speed, stopand-go
driving conditions. Accordingly, series configurations are most common in heavyduty
applications (Friedman, 2003).
In a parallel hybrid, power can flow directly to the wheels from the generator, engine, or
both as needed. The Honda Civic hybrid utilizes a parallel configuration. This type of
design requires the engine to run over a wider and less efficient operating range, but
reduces generation losses by having a direct link from engine to the wheels. Parallel
hybrid vehicles operate more efficiently at highway speeds than series configurations,
although low-speed, urban type operation is not as efficient (Friedman, 2003).
A series/parallel, or “power-split” design attempts to capture the benefits of both of the
above systems by allowing the motor and generator both to directly drive the wheels, but
decoupling them so the engine can operate more efficiently as in a series hybrid. The
Toyota Prius utilizes this type of system. Costs are generally higher for power-split
designs compared to parallel configurations due to the larger battery pack and increased
complexity of the system (Friedman, 2003). While current hybrid vehicles primarily
utilize either parallel or power-split designs, it is likely that the power-split design will
become more common over time for conventional HEVs.
Diesel powered HEVs may provide additional fuel economy improvements and GHG
emission reductions over and above gasoline HEVs. Diesel engines themselves are
inherently more efficient than comparable gasoline engines, as discussed in Section 3.4. In
addition, the higher NOx and PM emission rates associated with the diesel cycle may be
controlled more easily in a hybrid configuration, which minimizes transient engine
operation (i.e., rapid changes in engine RPM and load) (EPA, 2005). However, at this
time, there are no diesel HEV models offered for sale in the United States, although a
limited number are available in the European market. Diesel hybrid configurations
currently are being demonstrated for the heavy-duty market, as discussed in Section 3.3.2.
The following analysis evaluates the fuel economy benefits of gasoline hybrids, although
it should be noted that diesel hybrids may provide about 5 percent additional fuel
economy improvement (Bandivadekar et al., 2008).
As discussed in Section 3.3.1, advanced conventional gasoline technology packages are
expected to involve electrification of some vehicle components and functions, such as an
integrated starter/generator, which allows for turning the engine off at idle, or possibly
offering a limited amount of regenerative braking. Such packages are sometimes referred
to as “mild” hybrids, as opposed to “full” hybrids which are capable of full electrical
operation under certain engine loads and speeds (Friedman, 2003). This section focuses
on full hybrid vehicles, and their benefits and costs are assessed relative to comparable
fleet average baseline gasoline vehicles for the given time period under evaluation.
3-41