Have you pulled your car up to the gas pump lately and been
shocked by the high price of
gasoline? As the pump clicked past $20 or $30, maybe you
thought about trading in that SUV for something that gets
better mileage. Or maybe you are worried that your car is
contributing to the greenhouse effect. Or maybe you just want
to have the coolest car on the block.
The 2000 Honda Insight
have the technology that might answer all of these needs. It's
the hybrid car, and both manufacturers have begun
selling their versions in the United States. You're probably
aware of hybrid cars because they've been in the news a lot.
In fact, most automobile manufacturers have announced plans to
manufacture their own versions.
How does a hybrid car work? What goes on under the hood to
give you 20 or 30 more miles per gallon than the standard
automobile? And does it pollute less just because it gets
better gas mileage? In this edition of HowStuffWorks,
we'll help you to understand how this amazing technology
works. We'll show you what is going on in the Toyota and Honda
hybrids, and even give you some advice about how to drive one
for maximum efficiency!
What Makes it a "Hybrid"? Any vehicle is
hybrid when it combines two or more sources of power. In fact,
many people have probably owned a hybrid vehicle at some
point. For example, a mo-ped (a motorized pedal bike)
is a type of hybrid because it combines the power of a gasoline
engine with the pedal power of its rider.
Hybrid vehicles are all around us. Most of the locomotives
we see pulling trains are diesel-electric hybrids.
Cities like Seattle have diesel-electric buses -- these
can draw electric power from overhead wires or run on diesel
when they are away from the wires. Giant mining trucks
are often diesel-electric hybrids. Submarines
are also hybrid vehicles -- some are nuclear-electric
and some are diesel-electric. Any vehicle that combines
two or more sources of power that can directly or indirectly
provide propulsion power is a hybrid.
The gasoline-electric hybrid car is just that -- a
cross between a gasoline-powered car and an electric car.
Let's start with a few diagrams to explain the differences.
Figure 1 shows a gas-powered car. It has a fuel
tank, which supplies gasoline
to the engine. The engine then turns a transmission,
which turns the wheels.
Figure 1. Gasoline-powered car Move your mouse
over the parts for a 3-D view.
Figure 2 below shows an electric car, which has a
set of batteries
that provides electricity to an electric
motor. The motor turns a transmission, and the
transmission turns the wheels.
Figure 2. Electric car Move your mouse over the
parts for a 3-D view.
To learn about the structure of a parallel hybrid
car and a series hybrid car, go on to the next
Hybrid Structure You can combine the two
power sources found in a hybrid car in different ways. One
way, known as a parallel hybrid, has a fuel tank, which
supplies gasoline to the engine. But it also has a set of
batteries that supplies power to an electric motor. Both the
engine and the electric
motor can turn the transmission at the same time, and the
transmission then turns the wheels.
Figure 3 shows a typical parallel hybrid. You'll
notice that the fuel tank and gas engine connect to the
transmission. The batteries and electric motor also connect to
the transmission independently. As a result, in a parallel
hybrid, both the electric motor and the gas engine can provide
Figure 3. Parallel hybrid car Move your mouse
over the parts for a 3-D view.
By contrast, in a series hybrid (Figure 4
below) the gasoline engine turns a generator, and the
generator can either charge the batteries or power an electric
motor that drives the transmission. Thus, the gasoline engine
never directly powers the vehicle.
Take a look at the diagram of the series hybrid, starting
with the fuel tank, and you'll see that all of the components
form a line that eventually connects with the transmission.
Figure 4. Series hybrid car Move your mouse over
the parts for a 3-D view.
Now let's get into the individual components of a hybrid
Hybrid Components Hybrid cars contain the
Gasoline engine - The hybrid car has a gasoline
engine much like the one you will find on most cars.
However, the engine on a hybrid will be smaller and will use
advanced technologies to reduce emissions and increase
Fuel tank - The fuel
tank in a hybrid is the energy storage device for the
gasoline engine. Gasoline has a much higher energy density
than batteries do. For example, it takes about 1,000 pounds
of batteries to store as much energy as 1 gallon (7 pounds)
Electric motor - The electric
motor on a hybrid car is very sophisticated. Advanced
electronics allow it to act as a motor as well as a
generator. For example, when it needs to, it can draw energy
from the batteries to accelerate the car. But acting as a
generator, it can slow the car down and return energy to the
Generator - The generator
is similar to an electric motor, but it acts only to produce
electrical power. It is used mostly on series hybrids.
Batteries - The batteries
in a hybrid car are the energy storage device for the
electric motor. Unlike the gasoline in the fuel tank, which
can only power the gasoline engine, the electric motor on a
hybrid car can put energy into the batteries as well as draw
energy from them.
Transmission - The transmission
on a hybrid car performs the same basic function as the
transmission on a conventional car. Some hybrids, like the
Honda Insight, have conventional transmissions. Others, like
the Toyota Prius, have radically different ones, which we'll
talk about later.
Why Build Such a Complex
Car? You might wonder why anyone would build such a
complicated machine when most people are perfectly happy with
their gasoline-powered cars. The reason is twofold: to
reduce tailpipe emissions and to improve
mileage. These goals are actually tightly interwoven.
emissions standards dictate how much of each type of
pollution a car is allowed to emit in California. The amount
is usually specified in grams per mile (g/mi). For example,
the low emissions vehicle (LEV) standard allows 3.4
g/mi of carbon monoxide.
The key thing here is that the amount of pollution allowed
does not depend on the mileage your car gets. But a car that
burns twice as much gas to go a mile will generate
approximately twice as much pollution. That pollution will
have to be removed by the emissions control equipment on the
car. So decreasing the fuel consumption of the car is one of
the surest ways to decrease emissions.
Carbon dioxide (CO2) is another type
of pollution a car produces. The U.S. government does not
regulate it, but scientists suspect that it contributes to
global warming. Since it is not regulated, a car has no
devices for removing CO2 from the
exhaust, so a car that burns twice as much gas adds twice as
much CO2 to the atmosphere.
Automakers in the U.S. have another strong incentive to
improve mileage. They are required by law to meet Corporate
Average Fuel Economy (CAFE) standards. The current
standards require that the average mileage of all the new cars
sold by an automaker should be 27.5 mpg (8.55 liters per 100
km). This means that if an automaker sells one hybrid car that
gets 60 mpg (3.92 liters per 100 km), it can then sell four
big, expensive luxury cars that only get 20 mpg (11.76 liters
per 100 km)!
Evolution of the Hybrid The hybrid is a
compromise. It attempts to significantly increase the mileage
and reduce the emissions of a gas-powered car while overcoming
the shortcomings of an electric car.
The Problem with Gas-powered
Cars To be useful to you or me, a car must meet
certain minimum requirements. The car should be able to:
Drive at least 300 miles (482 km) between re-fueling
Be refueled quickly and easily
Keep up with the other traffic on the road
A gasoline car meets these requirements but produces a
relatively large amount of pollution and generally gets poor
gas mileage. An electric car, on the other hand, produces
almost no pollution, but it can only go 50 to 100 miles (80 to
161 km) between charges. And the problem has been that it is
very slow and inconvenient to recharge.
A driver's desire for quick acceleration causes our
cars to be much less efficient than they could be. You may
have noticed that a car with a less powerful engine gets
better gas mileage than an identical car with a more powerful
engine. Just look at the window stickers on new cars at a
dealership for a city and highway mpg comparison.
The amazing thing is that most of what we require a car to
do uses only a small percentage of its horsepower!
When you are driving along the freeway at 60 mph (96.6 kph),
your car engine has to provide the power to do three things:
Overcome the aerodynamic drag caused by pushing the car
through the air
For most cars, doing all this requires less than 20 horsepower!
So, why do you need a car with 200 horsepower? So you can
"floor it," which is the only time you use all that power. The
rest of the time, you use considerably less power than you
Smaller Engines are More
Efficient Most cars require a relatively big engine
to produce enough power to accelerate the car quickly. In a
small engine, however, the efficiency can be improved by using
smaller, lighter parts, by reducing the number of cylinders
and by operating the engine closer to its maximum load.
There are several reasons why smaller engine are more
efficient than big ones:
The big engine is heavier than the small engine, so the
car uses extra energy every time it accelerates or drives up
The pistons and other internal components are heavier,
requiring more energy each time they go up and down in the
The displacement of the cylinders is larger, so more
fuel is required by each cylinder.
Bigger engines usually have more cylinders, and each
cylinder uses fuel every time the engine fires, even if the
car isn't moving.
This explains why two of the same
model cars with different engines can get different mileage.
If both cars are driving along the freeway at the same speed,
the one with the smaller engine uses less energy. Both engines
have to output the same amount of power to drive the car, but
the small engine uses less power to drive itself.
Hybrid Performance The key to a hybrid car
is that the gasoline engine can be much smaller than the one
in a conventional car and therefore more efficient. But how
can this smaller engine provide the power your car needs to
keep up with the more powerful cars on the road?
Let's compare a car like the Chevy Camaro, with its big V-8
engine, to our hybrid car with its small gas engine and
electric motor. The engine in the Camaro has more than enough
power to handle any driving situation. The engine in the
hybrid car is powerful enough to move the car along on the
freeway, but when it needs to get the car moving in a hurry,
or go up a steep hill, it needs help. That "help" comes from
the electric motor and battery -- this system steps in to
provide the necessary extra power.
The gas engine on a conventional car is sized for the peak
power requirement (those few times when you floor the
accelerator pedal). In fact, most drivers use the peak power
of their engines less than one percent of the time. The hybrid
car uses a much smaller engine, one that is sized closer to
the average power requirement than to the peak power.
Hybrid Efficiency Besides a smaller, more
efficient engine, today's hybrids use many other tricks to
increase fuel efficiency. Some of those tricks will help any
type of car get better mileage, and some only apply to a
hybrid. To squeeze every last mile out of a gallon of
gasoline, a hybrid car can:
Recover energy and store it in the battery -
Whenever you step on the brake
pedal in your car, you are removing energy from
the car. The faster a car is going, the more kinetic
energy it has. The brakes of a car remove this energy and
dissipate it in the form of heat. A hybrid car can capture
some of this energy and store it in the battery to use
later. It does this by using "regenerative braking." That
is, instead of just using the brakes to stop the car, the
electric motor that drives the hybrid can also slow the car.
In this mode, the electric motor acts as a generator and
charges the batteries while the car is slowing down.
Sometimes shut off the engine - A hybrid car does
not need to rely on the gasoline engine all of the time
because it has an alternate power source -- the electric
motor and batteries. So the hybrid car can sometimes turn
off the gasoline engine, for example when the vehicle is
stopped at a red light.
Figure 5. The frontal area profile of a
small and large
Use advanced aerodynamics to reduce drag - When
you are driving on the freeway, most of the work your engine
does goes into pushing the car through the air. This force
is known as aerodynamic drag. This drag force can be
reduced in a variety of ways. One sure way is to reduce the
frontal area of the car (Figure 5). Think of how a
big SUV has to push a much greater area through the air than
a tiny sports car.
Reducing disturbances around objects that stick out from
the car or eliminating them altogether can also help to
improve the aerodynamics. For example, covers over the wheel
housings smooth the airflow and reduce drag. And sometimes,
mirrors are replaced with small cameras.
Use low-rolling resistance tires - The tires on
most cars are optimized to give a smooth ride, minimize
noise, and provide good traction in a variety of weather
conditions. But they are rarely optimized for efficiency. In
fact, the tires cause
a surprising amount of drag while you are driving. Hybrid
cars use special tires that are both stiffer and inflated to
a higher pressure than conventional tires. The result is
that they cause about half the drag of regular tires.
Use lightweight materials - Reducing the overall
weight of a car is one easy way to increase the mileage. A
lighter vehicle uses less energy each time you accelerate or
drive up a hill. Composite materials like carbon fiber or
lightweight metals like aluminum and magnesium can be used
to reduce weight.
What's Available Now? Three hybrid cars are
now available in the United States -- the Honda
Civic Hybrid, the Honda
Insight and the Toyota
Prius. We will be discussing the latter two, and although
both of these cars are hybrids, they are actually quite
different in character.
The Honda is about $18,000, and the Toyota about $20,000.
Both cars have a gasoline engine, an electric motor and
batteries, but that is where the similarities end.
The Honda Insight, which was introduced in early
2000 in the United States, is designed to get the best
possible mileage. Honda used every trick in the book to
make the car as efficient as it can be. The Insight is a
small, lightweight two-seater with a tiny, high-efficiency gas
The Toyota Prius, which came out in Japan at the end
of 1997, is designed to reduce emissions in urban
areas. It meets California's super ultra low emissions vehicle
(SULEV) standard. It is a four-door sedan that seats five, and
the powertrain is capable of accelerating the vehicle to
speeds up to 15 mph (24 kph) on electric power alone.
Now let's look at how each of these cars works.
The Honda Insight Figure 6 shows the
layout of the Honda
Insight, which is a simplified parallel hybrid. It
has an electric motor coupled to the engine at the spot where
the flywheel usually goes. Honda calls this system "Integrated
Motor Assist." The Insight also has a conventional five-speed
Figure 6. Layout of the Honda Insight Move your
mouse over the parts for a 3-D view.
The electric motor on the Insight helps in several ways. It
Assist the gasoline engine, providing extra power
while the car is accelerating or climbing a hill
Provide some regenerative braking to capture
energy during braking
Start the engine, eliminating the need for a
However, the motor cannot power the car by itself; the gas
engine must be running for the car to move.
Efficiency Because the Insight was designed to get
the best mileage possible, Honda used all of the tricks
discussed in the previous section. But the Insight relies
mainly on three areas:
It reduces the weight - Already a small car, the
Insight uses a lightweight aluminum body and structure to
further reduce weight. By making the car lightweight, Honda
was able to use a smaller, lighter engine that could still
maintain the performance level we have come to expect from
our cars. The Insight weighs less than 1,900 pounds (862
kg), which is 500 pounds (227 kg) less than the lightest
It uses a small, efficient engine - The engine in
the Insight, shown in Figure 7, weighs only 124
pounds (56 kg) and is a tiny, 1.0-liter three-cylinder that
produces 67 horsepower at 5,700 rpm. It incorporates Honda's
VTEC system and uses lean
burn technology to maximize efficiency. The Insight
achieves an EPA mileage rating of 61 mpg/city and 70
mpg/highway. Also, with the additional power provided by the
small electric motor, this system is able to accelerate the
Insight from 0 to 60 mph in about 11 seconds.
Figure 7. Insight
electric motor running, the Insight produces 73 horsepower
at 5,700 rpm. If you compare that to the engine horsepower
alone, it looks like the electric motor only adds 6
horsepower. But the real effectiveness of the electric motor
occurs at lower engine speeds. The electric motor on
the Insight is rated at 10 kilowatts (about 13 horsepower)
at 3,000 rpm.
It's the peak torque
numbers that really tell the story. Without the electric
motor, the Insight makes its peak torque of 66 pound-feet at
4,800 rpm. With the electric motor, it makes 91 pound-feet
at 2,000 rpm. So the motor adds a lot of torque to the low
end of the speed range, where the engine is weaker. This is
a nice compromise that allows Honda to give a very small
engine the feel of a much larger one.
It uses advanced aerodynamics - The Honda Insight
is designed using the classical teardrop shape: The
back of the car is narrower than the front. (Note that real
teardrops do not behave this way aerodynamically -- click here
for an interesting article on the aerodynamics of falling
water droplets.) The rear wheels are partially covered by
bodywork to provide a smoother shape, and some parts of the
underside of the car are enclosed with plastic panels. These
tricks result in a drag
coefficient of 0.25, which makes it one of the most
aerodynamic cars on the market (click
here for a comparison table of drag coefficients for
lots of car models).
Driving the Insight The
Insight is actually not very different from a conventional car
once you get behind the wheel. When you accelerate, the gas
engine does most of the work. If you accelerate quickly, the
electric motor kicks in to provide a little extra power.
When you are cruising along the freeway, the gas engine is
doing all of the work. When you slow down by hitting the
brakes or letting off the gas, the electric motor kicks in to
generate a little electricity to charge the batteries. You
never have to plug the Insight into an electrical outlet; the
motor generates all of the power needed to charge the battery.
One interesting thing to note is that in the Insight, the
manual transmission is separated from the engine and motor by
This means that if you are the type of driver who likes to put
the clutch in or put the car in neutral when you slow down to
a stop, you are not going to get any regenerative braking. In
order to recover energy when you slow down, the car has to be
The Toyota Prius One of the main goals of
Prius is to improve emissions in urban driving. To
accomplish this, Toyota has designed a parallel hybrid
powertrain, called the Toyota Hybrid System (THS), that
adds some of the benefits of a series hybrid.
Unlike Honda, Toyota has focused primarily on the
powertrain to achieve its emissions and mileage goals. The
Prius weighs 2,765 pounds (1,255 kg) and has as much interior
space and trunk space as a Toyota Corolla. Figure 8
provides a layout of all the pieces.
Figure 8. Prius layout Move your mouse over the
parts for a 3-D view.
How Does the Prius Get Good Mileage
and Lower Emissions? The Prius mainly relies on two
features to optimize efficiency and reduce emissions:
Its engine only runs at an efficient speed and
load - In order to reduce emissions, the Prius can
accelerate to a speed of about 15 mph (24 kph) before
switching on the gasoline engine. The engine only starts
once the vehicle has passed a certain speed. And once the
engine starts, it operates in a narrow speed band.
It uses a unique power split device - Gasoline
engines can be tuned to run most efficiently in certain
speed and load ranges. The power split device on the
Prius, which we'll talk about in a minute, allows the engine
to stay in its most efficient load and speed range most of
Toyota designed the 1.5-liter engine in the Prius to run
at a maximum speed of only 4,500 rpm, where it makes 70
horsepower. Keeping the maximum speed of the engine low
allows for the use of lighter components that improve
The electric motor on the Prius is rated at 44 horsepower
from 1,040-5,600 rpm. It produces 258 pound-feet of torque
from 0 to 400 rpm, which is more than enough to get the car
going without the aid of the gasoline engine.
The "Power Split
Device" The power split device is the heart
of the Toyota Prius. This is a clever gearbox that hooks the
gasoline engine, generator and electric motor together. It
allows the car to operate like a parallel hybrid -- the
electric motor can power the car by itself, the gas engine can
power the car by itself or they can power the car together.
Figure 9. The Prius planetary gear
The power split device also allows the car to operate like
a series hybrid -- the gasoline engine can operate
independently of the vehicle speed, charging the batteries or
providing power to the wheels as needed. It also acts as a
continuously variable transmission (CVT), eliminating
the need for a manual or automatic
transmission. Finally, because the power split device
allows the generator to start the engine, the car does not
need a starter.
The power split device is a planetary
gear set (Figure 9). The electric motor is
connected to the ring gear of the gear set. It is also
directly connected to the differential,
which drives the wheels. So, whatever speed the electric motor
and ring gear spin at determines the speed of the car.
The generator is connected to the sun gear of the gear set,
and the engine is connected to the planet carrier. The speed
of the ring gear depends on all three components, so they all
have to work together at all times to control the output
Driving the Prius When
you accelerate, initially the electric motor and batteries
provide all of the power. The ring gear of the power split
device is connected to the electric motor, so it starts to
spin with the motor. The planet carrier, which is connected to
the engine, is stationary because the engine is not running.
Since the ring gear is spinning, the planets have to spin,
which causes the sun gear and generator to spin. As the car
accelerates, the generator spins at whatever speed it needs to
in order for the engine to remain off. You can see all of this
in Figure 10.
Figure 10. Watch the Prius' power split
device as the car accelerates from 0 to 30
Once you reach about 15 mph (24 kph), the gasoline engine
will turn on. The generator suddenly changes speed, causing
the planet carrier to turn and start the engine. Once the
engine is running, it settles into a constant speed while the
generator varies its speed to match the output speed with the
electric motor. If you are really accelerating hard, the motor
will draw extra power from the batteries. Once you are up to
freeway speed, the car will move under a combination of gas
and electric power, with all of the electricity coming from
Like the Insight, the Prius never needs to be recharged;
the onboard generator automatically maintains the proper level
of charge in the batteries.
Hybrid Maintenance Both the Honda and the
Toyota have long warranties on the hybrid systems. The
Insight has an eight-year/80,000-mile warranty on most of the
powertrain, including batteries, and a three-year/36,000-mile
warranty on the rest of the car. The Prius has an
eight-year/100,000-mile warranty on the battery and hybrid
systems and a three-year/36,000-mile warranty on everything
The motors and batteries in these cars don't require any
maintenance over the life of the vehicle. And the
engine doesn't require any more maintenance than the one in
any other car. And because both hybrids have regenerative
braking, the brake pads may even last a little longer than
those in most cars.
However, if you do have to replace the batteries after the
warranty expires, it will most likely cost you several
Hybrid Mileage Tips You can get the best
mileage from a hybrid car by using the same kind of driving
habits that give you better mileage in your gasoline-engine
Drive slower - The aerodynamic drag on the car
increases dramatically the faster you drive. For example,
the drag force at 70 mph (113 kph) is about double that at
50 mph (81 kph). So, keeping your speed down can increase
your mileage significantly.
Maintain a constant speed - Each time you speed
up the car you use energy, some of which is wasted when you
slow the car down again. By maintaining a constant speed,
you will make the most efficient use of your fuel.
Avoid abrupt stops - When you stop your car, the
electric motor in the hybrid acts like a generator and takes
some of the energy out of the car while slowing it down. If
you give the electric motor more time to slow the vehicle,
it can recover more of the energy. If you stop quickly, the
brakes on the car will do most of the work of slowing the
car down, and that energy will be wasted.
For more information on hybrid cars and related topics,
check out the links on the next page.