Environmental Engineering Reference
In-Depth Information
and starts. Each time a vehicle is accelerated to operating
speed a certain amount of energy is used. Each time it is
stopped that energy is thrown away (except in a hybrid
like the Prius). While it takes more energy to accelerate to
highway speed, the more frequent stops and starts in city
driving are the main factor resulting in a lower fuel
economy.
The Toyota Prius is an example of a hybrid that uses
two drive systems: an ordinary ICE, and an auxiliary
electric motor and battery system that recovers energy
lost in braking and uses it again in a system much more
ef
cient than an ICE working alone. The secret of the
hybrid is that electric drive is about
cient in
delivering energy from a battery to the wheels, compared
with the ICE
%ef
ciency. You do not have to recover
all of the braking loss to make a big difference. In add-
ition, the hybrids turn the engine off when idling for a
long time and reduce losses there too. My wife
%ef
'
s Prius got
about
mpg in her city driving, roughly double what an
ICE would get. It can use either or both engines at any
one time.
What I think of as the ultimate hybrid is sometimes
called a serial hybrid. Here, the ICE only drives an elec-
trical generator that either charges the battery or helps
drive the electric motor. This allows the ICE to operate
most of the time at its optimum ef
ciency, getting even
more of an improvement than systems like that used in
the Prius. General Motors
Chevy Volt, a plug-in hybrid
that will be discussed later, operates as a serial hybrid
when running on gasoline.
The new US CAFE standard for all vehicles is
'
.
mpg by
and
.
mpg by
. Technologies to
