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Road Tests: Full Test
Full Test: 2001 Toyota Prius
Hybrid Highlights
By Brent Romans
Date Posted 12-12-2000
One
of the more impressive aspects of the Prius is
its hybrid powertrain. Toyota calls it the Toyota
Hybrid System (THS). The THS is comprised of both
a gasoline engine and an electric motor. Based
on driver inputs and other conditions, the THS
selects the best combination of engine power and
electric power to accelerate the Prius. Check
out the diagrams on the right to see how the THS
operates in different conditions.
The THS consists of four main components. These
are the 1.5-liter gasoline engine, the battery
pack, the hybrid transaxle and the inverter. Following
are descriptions of each, as well as an overview
of the Prius' special emissions-reducing hardware.
Engine
If you look at the power ratings, the Prius' 1NZ-FXE
engine might not seem all that great. It makes
70 horsepower at 4,500 rpm and 82 foot-pounds
of torque at 4,200 rpm, about 30 less of each
than the similar 1.5-liter engine in the Toyota
Echo. The 1NZ-FXE does have numerous changes to
enhance fuel economy and reduce emissions, however.
Here are some highlights:
- Variable
valve timing. Used to improve performance and
fuel economy.
-
Atkinson cycle. The Atkinson cycle is a refinement
of the normal intake, compression, combustion
and exhaust four-stroke engine cycle. Similar
to the Miller cycle found on Mazda's supercharged
Millenia S Sedan, the Atkinson cycle enables
the duration of the compression stroke and the
expansion stroke to be set independently of
each other. Because the Atkinson cycle does
not generate high output, there is no practical
application for this system unless it is combined
with a supercharger; however, this system offers
a high level of thermal efficiency. On Prius,
the Atkinson cycle's low output is negated by
the electric motor's additional power.
-
Lightweight engine construction. Since the engine
has been limited to 4,500 rpm, many internal
parts have been designed to be lighter and produce
less friction. The cylinder bores are offset
from the crankshaft to help reduce friction.
In addition, the crankshaft has a smaller diameter,
piston rings have lower tension and the valve
spring load is less, when compared to a standard
high-revving engine.
Hybrid
Transaxle
The hybrid transaxle is unique to the Prius. It
has a continuously variable transmission mechanism
and a power-splitting device that adjusts and blends
the amount of torque applied to the front wheels
from the engine and the electric motor. Actually,
there are two electric motors, and both are contained
within the transaxle. Both of the electric motors
are compact, lightweight and highly efficient.
Motor generator one (MG1) recharges the hybrid battery
and supplies electrical power to drive motor generator
two (MG2). In addition, by regulating the amount
of electrical power generated (thus varying the
generator's rpm), MG1 effectively controls the continuously
variable transmission function of the transaxle.
MG1 also serves as the starter to start the 1NZ-FXE
engine. MG2 provides the torque and power that motives
the Prius under light loads, as well as supplementing
the engine during normal operation. It also converts
the vehicle's kinetic energy that is generated through
the activation of the regenerative brakes into electrical
energy, which is then stored in the HV batteries.
Inverter
The inverter is an electric power converter that
converts the direct current (DC) of the hybrid battery
and the alternating current (AC) of the electric
motors. The inverter also transmits information
to the car's computer that is needed for precise
current and voltage control. Because the hybrid
battery is rated at DC 273.6 volts, the inverter
is equipped with a converter. The converter is used
for transforming some of the hybrid battery's power
into 12-volt power necessary for vehicle accessories
like the lights, audio system and the climate fan.
Hybrid Battery
The sealed nickel-metal hydride (Ni-MH) battery
pack is mounted behind the rear seats. It weighs
about 100 pounds. The battery pack consists of six
1.2-volt cells connected in series to form one module.
A total of 38 modules are connected in series, thereby
giving a total voltage rating of 273.6 volts. Toyota
has installed a cooling fan to keep the battery
pack at a constant temperature. The fan draws in
air from air vents mounted on the top of the rear
parcel shelf.
Reduced Emissions Equipment
The Prius is fitted with two three-way catalytic
converters. On models bound for the U.S. market,
an additional hydrocarbon absorber has been added
for reduced emissions. Toyota installed the hydrocarbon
absorber to reduce emissions at vehicle start up.
Cars generally produce the most emissions at start
up because the catalytic converters don't work well
when cold. When the Prius is first started, a bypass
valve activates and routes exhaust gasses into the
absorber and prevents them from leaving the tailpipe.
The gasses stay there for a short period until the
catalytic converter heats up. After the converter
has warmed up, the bypass valve opens, allowing
the gasses to be released by the absorber and cleaned
by the converter.
The Prius' vapor-reducing gas tank is another feature
designed to reduce the amount of emissions. You
know how when you open the gas cap of a normal car
and gasoline vapors rush out? That's because gas
evaporates. And the more space that is available
in the gas tank, the more vapors will be produced.
These vapors are harmful to the environment, and
they can escape even when the vehicle is parked
or while driving. The Prius' tank utilizes a plastic
liner, or bladder, inside the steel gas tank. As
fuel is burned, the bladder collapses, minimizing
the volume of the gas tank. With less volume, there
is less room in which fuel can evaporate.
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