Parallel
hybrids are powered by an electric motor, an internal combustion engine,
or both at the same time. General Motors used a parallel powertrain
in the discontinued ‘mild’ hybrid GMT
800 Chevrolet Silverado.
The truck shut its gas V8 engine off at full stops, running internal
systems off a small electric motor, until the truck started moving again
solely on V8 power. Toyota uses a parallel system too, for its Hybrid
Synergy Drive cars, like the Prius and Camry. It's a sophisticated
version of parallel technology, where the electric motor or conventional
engine solely powers or moves the vehicle, or both motors work together,
depending on engine load conditions. But parallel hybrids only swap or
pair engines in low-speed driving conditions, like city traffic, so they
are also called single-mode hybrids.
Two-mode
hybrids use an advanced automatic transmission that determines whether
to use an electric motor or the conventional engine, or both at the
same time during low-speed and highway driving - which is why it's
called two-mode. This allows the vehicle to behave like it had a continuously
variable transmission (CVT) so it can achieve optimal fuel economy
at all times while driving. GM’s new GMT 900 Chevrolet
Tahoe SUV uses a two-mode hybrid powertrain because the system doesn't
compromise much of the vehicle's all-around capabilities. It can
still tow up to 6,000-pounds, only 1,000-pounds less than a conventional
Tahoe. We'll see a new Chevrolet Silverado two-mode hybrid pickup
by December 2008.
Serial hybrids
use an electric motor to drive the wheels, while a conventional (e.g.
diesel) or non-conventional (e.g. fuel cell) engine powers a generator.
The generator either powers the electric motor directly, like a locomotive,
or is used to charge a battery pack, which powers the electric motor.
Using a battery in a serial hybrid does three things. Batteries can
be plugged into the energy grid for recharging, making the vehicle
a 'plug-in' series
hybrid, like GM's Chevrolet Volt concept. Batteries enable driving
for some distance without burning fuel, and they let the generator engine
run only at its most efficient point, and any time the generator is running
it’s charging the battery. This is unlike an idling locomotive,
stopped at a train station and burning fuel without purpose.
The advantage
of a serial hybrid over the parallel and two-mode versions is that, within
the battery’s charge range, the vehicle is emitting
zero exhaust and not using any oil. It also has fewer moving parts, even
counting the generator, which means less vehicle maintenance and lower
lifetime operating costs.
Mr. Wright
favors the plug-in serial hybrid approach for heavy duty pickups.
"The
Volt is where it's all going to go, eventually. It's
the right development path. Of course, it doesn't have enough power
(for a truck). It would be great if it had enough scale for a pickup
truck," says Mr. Wright.
It's
one thing to power a small family sedan, like the Chevy Volt, off a serial
hybrid powertrain. But the large size and demanding power requirements
of heavy duty pickups make it a more challenging proposition for use
in a truck application.
