What Is a Hybrid Car? Learn How Hybrid Vehicles Work

The gasoline-electric hybrid car is just what it sounds like — a cross between a gasoline-powered car and an electric car. Let's start with a few differences between a gasoline-powered car and a typical electric car.

A gas-powered car has a fuel tank supplies gasoline to the internal combustion engine. The engine then turns a transmission, which turns the wheels.

An electric car, on the other hand, has a set of batteries that provides electricity to an electric motor. The motor turns a transmission, and the transmission turns the wheels.

The hybrid is a compromise. It attempts to significantly increase the mileage and reduce the emissions of a gas-powered car while providing the convenience of refueling at the gas station.

Gasoline-electric hybrid cars contain the following parts:

Parallel Hybrids

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 that supplies gasoline to the engine and a set of batteries that supplies power to the electric motor.

Both the engine and the electric motor can turn the transmission at the same time, and the transmission then turns the wheels.

Series Hybrids

In a series hybrid, 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.

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. 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.

Why Are Smaller Engines More Efficient?

There are several reasons why smaller engines are more efficient than bigger ones.

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.

Getting Enough Power

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 — and 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 1 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.

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 improved fuel economy, and some only apply to a hybrid.

1. Recovering Energy and Storing 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 a "regenerative braking" system. 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.

2. Sometimes Shutting 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.

3. Using 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. 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.

4. Using 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 less drag than regular tires.

5. Using 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.

The power split device is the heart of the Toyota Prius, the most famous hybrid vehicle. 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.

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. 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 speed.

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.

Once you reach about 40 mph (64 km/h), 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 the generator.

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 km/h), your car engine has to provide the power to do three things:

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 have available.

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.)

How to Get the Best Mileage

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 car.