Hybrid
Electric Vehicle
A
hybrid electric vehicle (HEV) typically combines the internal
combustion engine of a conventional vehicle with the battery
and electric motor of an electric vehicle. The combination
offers low emissions, with the power, range, and convenient
fueling of conventional (gasoline and diesel fuel) vehicles
- and HEVs never need to be plugged in.
Hybrid electric vehicles of the future could use alternative
fuels such as biodiesel, natural gas, or ethanol. The flexibility of HEVs makes them well suited
for fleet and personal transportation.
How hybrid electric vehicles work
Hybrid electric vehicles are powered by two energy sources
- an energy conversion unit (such as an internal combustion
engine or fuel cell) and an energy storage device (such as
batteries or ultracapacitors). The energy conversion unit
can be powered by gasoline, diesel, compressed natural gas, hydrogen, or other
fuels.
Hybrid electric vehicles have the potential to be two to three
times more fuel-efficient than conventional vehicles. HEVs
can have a parallel design, a series design, or a combination
of the two.
Hybrid electric vehicle components
A hybrid electric vehicle (HEV) employs an optimized mix of
various components. These include:
- Electric traction motor and controller
- Energy storage systems, including batteries and ultracapacitors
- Power units and transmissions, including spark ignition
engines, compression-ignition and direct-injection (diesel)
engines, gas turbines, and fuel cells
- Energy management and systems control
Hybrid electric vehicle motors and controllers
Motors are the workhorses of HEV drive systems. In an HEV,
an electric traction motor converts electrical energy from
the energy storage unit to mechanical energy that drives the
wheels of the vehicle. Unlike a traditional vehicle, where
the engine must "ramp up" before full torque can be provided,
an electric motor provides full torque at low speeds. This
characteristic gives the vehicle excellent "off the line"
acceleration.
Important characteristics of an HEV motor include good drive
control and fault tolerance, as well as low noise and high
efficiency. Other characteristics include flexibility in relation
to voltage fluctuations and acceptable mass production costs.
Front-running motor technologies for HEV applications include
permanent magnet, AC induction, and switched reluctance motors.
Hybrid electric vehicle energy storage systems
An energy storage system is an essential component in HEVs.
Batteries used in HEVs should have high power (with high-peak
and pulse-specific power), high specific energy at pulse power,
high charge acceptance to maximize regenerative braking utilization,
and long calendar and cycle life. See below to learn about
HEV battery options, ultracapacitors (another energy storage
device), and battery thermal management strategies.
Lead-acid batteries
Nickel-metal hydride batteries
Lithium ion batteries
Lithium polymer batteries
Ultracapacitors
Battery thermal management
Hybrid electric vehicle power units and transmissions
Several types of engines can be used to power an HEV. There
is also another power unit - fuel cells - that could eventually
be used to power HEVs. Learn more about the following hybrid
electric vehicle components:
Spark-ignition engines
Compress-ignition direct-injection engines
Gas turbine engines
Fuel cells
Transmissions
Hybrid electric vehicle energy management and systems control
A hybrid electric vehicle has two or more sources of onboard
power. In current production vehicles, these power sources
are an electric motor and a gasoline or diesel internal combustion
engine (ICE). The integration of these power-producing components
allows for many different types of HEV designs. A strategy
is needed to control the flow of power and to maintain adequate
reserves of energy in the storage devices. Although this is
an added complexity not found in conventional vehicles, it
allows the components to work together in an optimal manner
to achieve multiple design objectives, such as high fuel economy
and low emissions.
The biggest distinction between hybrid designs is whether
the electric and power producing components operate in parallel,
series, or a combination of the two. In a parallel design,
the auxiliary power unit or APU (a diesel or gasoline ICE
in current production hybrid vehicles) can mechanically drive
the wheels. In a series design, the APU generates electricity
and does not directly drive the wheels. A third type combines
the best aspects of both and is sometimes called a combined
or series/parallel design. A combined design allows the APU
to directly drive the wheels but also has the ability to charge
the energy storage device through a generator. The combined
hybrid is a subset of the parallel design because it can directly
drive the wheels from the APU. The way the hardware components
are connected (parallel, series, or combination) is referred
to here as the "hardware configuration," and the management
of the power flow among the components is referred to as the
"control strategy" or more generally "energy management."
Hybrid Electric Vehicle Parallel Design
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Hybrid Electric Vehicle Series Design |
A secondary distinction between hybrids is charge-sustaining
versus charge-depleting hardware configurations and control
strategies. Charge-depleting vehicles allow their batteries
to become depleted and cannot recharge them at the same rate
they are being discharged. The common charge-depleting vehicle
is often referred to as a "range-extender," because it provides
enough energy to extend the driving range of the vehicle but
is not recharged quickly enough to power the vehicle completely,
unless the APU is larger than the average power load of the
vehicle. A charge-sustaining HEV has an APU that is adequately
sized to meet the average power load and, if operated under
the expected conditions, will be able to maintain adequate
electrical energy storage reserves indefinitely.
The flexibility in HEV design comes from the ability of the
control strategy to manage how much power is flowing to or
from each component. This way, the components can be integrated
with a control strategy to achieve the optimal design for
a given set of design constraints. There are many (often conflicting)
objectives desirable for HEVs. The primary ones are to:
Maximize fuel economy
Minimize emissions
Minimize propulsion system cost to keep the vehicles affordable
to the consumer market
Do all of the above while maintaining or improving on acceptable
performance (acceleration, range, handling, noise, etc.) To
achieve these objectives, the hardware configuration and the
power control strategy are designed together. The hardware
configuration dictates to some extent what control strategies
make sense.
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