What makes a hybrid a hybrid?
Is it alternative fuels, alternative powertrains, or
any alternative besides an internal combustion
engine? The definition of a hybrid vehicle today
seems to mean different things to many different
The most common hybrid vehicle on the road is the
hybrid electric vehicle (HEV), which combines an
energy storage unit, such as a high-voltage battery,
and a power unit, usually a gasoline or diesel
engine, with a propulsion system that receives input
both systems to propel the vehicle.
But there is much more to it than that, and new
systems are being introduced by many manufacturers.
To help sort things out, following is a mini hybrid
glossary of common hybrid systems, components and
other alternative fuel terms.
Electric Vehicle (HEV) – Combines an energy
storage system (commonly batteries), a power unit
(such as an internal
combustion engine or fuel cell), and a vehicle
Hybrid propulsion system – Joins two types
of propulsion mechanisms so that the advantages of
each can be exploited. It generally consists of two
energy storage elements (such as a fuel tank and an
electric battery) and two energy conversion
elements (such as an engine and an electric motor).
Series Hybrid – Propulsion power flows
through a single path from the engine to the
generator, to the battery, to the electric motor,
and to the drive wheels. The engine never directly
powers the vehicle; only the electric motor can
apply torque to the wheels. Used in large vehicles
such diesel-electric locomotives and non-nuclear
Parallel Hybrid One-Mode – Propulsion
power includes a mechanical connection between the
power unit — such as a gasoline or diesel engine —
and the vehicle’s wheels as well as an electric
motor/generator that drives the wheels. The electric
motor and the engine can apply torque to the wheels
either simultaneously or individually through the
one-mode continuously-variable transmission. The
power created from the engine is used for highway
driving and the power from the electric motor
provides acceleration. Used in the Toyota Prius and
Hybrid Two-Mode – Features a two-mode, multiple
planetary gear set, electronically variable
transmission. The two-mode design delivers a low
speed range and a high speed/high load range. With
two modes, the performance expectations of the
vehicle can be met while allowing the hybrid
electrical components to be smaller than that of the
one-mode design. Used in the Chevrolet Tahoe and GMC
Parallel Hybrid Belted Alternator/Starter
(BAS) – Has an engine-mounted electric
motor/generator that is driven by an
accessory belt used to auto-start the engine after
an auto-stop (during which the engine stops) and to
charge the battery pack. Only the engine drives the
wheels. Used in the Saturn VUE and Chevrolet Malibu.
Auto-Start – The hybrid’s electric
motor/generator, which is more powerful than a
traditional engine-mounted starter, is capable of
cranking the engine to its typical idle speed in
less than 300 milliseconds. This is used to start
the engine without
extra fuel after an auto-stop.
Regenerative braking – The process of
recovering some of a vehicle’s kinetic energy by
allowing the wheels to drive a
traction motor as a generator, thereby producing
electric power that is stored for later use. When
the driver brakes, the motor becomes a generator and
uses the kinetic energy of the vehicle to generate
electricity that can be stored in the battery pack.
Direct Injection (DI) – Fuel is injected
directly into the cylinder, as is typical in a
diesel engine. Most modern internal combustion
engines use port fuel injection (in which the fuel
is injected just in front of the cylinder intake
E85 – A mixture of 85% denatured ethanol
and 15% gasoline, by volume; an alternative engine
Fuel flexible (or Flex Fuel) – Ability of
a vehicle to operate on a wide range of fuel blends
(e.g., blends of gasoline and E85).
Biodiesel – A renewable diesel fuel
substitute that can be made by chemically combining
a natural oil or fat with an alcohol.
Fuel cell – An electromechanical power
unit (no moving parts) that converts the chemical
energy of hydrogen and oxygen into electricity
without combustion; the only by-product is water.
The electricity is then used to power the vehicle.
For more information, visit the U.S. Department of
Energy’s Energy Efficiency and Renewable Energy