Lesson: How Hybrids Work

Hybrid electric vehicles combine a gasoline engine with one or more electric motors powered by batteries. ^{[DOE AFDC]} In this lesson you’ll learn the three technologies that save fuel, the main hybrid designs, and what’s different about caring for one.

Technology 1: Regenerative braking

Regenerative braking recaptures energy normally lost during coasting or braking. The forward motion of the wheels turns the motor, generating electricity and helping slow the vehicle. ^{[FuelEconomy.gov]} Because a regular hybrid can’t plug in, this and the engine are how its battery gets charged. ^{[DOE AFDC]}

Technology 2: Electric motor assist

The electric motor helps the engine when accelerating, passing or climbing hills, which allows a smaller, more efficient engine. In some hybrids the motor alone moves the car at low speeds, where gasoline engines are least efficient. ^{[FuelEconomy.gov]}

Technology 3: Automatic stop/start

The engine shuts off when the car stops and restarts when you press the accelerator, cutting wasted energy from idling. ^{[FuelEconomy.gov]} The battery can also power accessories while stopped, reducing idling further. ^{[DOE AFDC]}

Hybrid designs compared

Mild (or micro) hybrids use a battery and motor to help power the car and can shut the engine off at stops, but cannot drive on electricity alone; they cost less and save less. Full hybrids have bigger batteries and motors that can power the vehicle for short distances at low speeds; they cost more and save more. ^{[DOE AFDC]} Parallel hybrids, the most common design, connect both the engine and motor to the wheels. Series hybrids use only the motor to drive the wheels and are more common in plug-in hybrids. ^{[DOE AFDC]}

Caring for a hybrid

Because hybrids have engines, their maintenance is similar to conventional vehicles. The electrical system (battery, motor and electronics) typically needs minimal scheduled maintenance, and brakes generally last longer thanks to regenerative braking. ^{[DOE AFDC]} Some battery systems use liquid coolant that may need regular checks; the owner’s manual or dealer can tell you. ^{[DOE AFDC]} Hybrids use high-voltage systems typically ranging from 400 to 1,000 volts, with high-voltage cables usually marked orange; that’s work for trained technicians. ^{[DOE AFDC]} NHTSA also advises hybrid owners to keep gasoline in the tank to support the engine. ^[NHTSA]

How weather changes hybrid mileage

Cold hits hybrids harder than regular cars. FuelEconomy.gov reports that at 20°F in city driving, a conventional car’s mileage is about 15% lower than at 77°F, while a hybrid’s can drop about 30% to 34%. ^{[FuelEconomy.gov]} One reason: cold reduces battery performance, which also affects regenerative braking. ^{[FuelEconomy.gov]} Heat matters too. A/C is the main cause of reduced fuel economy in hot weather, and its effect on hybrids can be even larger on a percentage basis. ^{[FuelEconomy.gov]} In a Huntington Beach summer, parking in the shade and airing out a hot cabin before turning on the A/C are easy wins. ^{[FuelEconomy.gov]}

Is a hybrid as safe as a regular car?

Yes, by the same rules. Commercially available hybrids and EVs must meet the Federal Motor Vehicle Safety Standards and go through the same safety testing as conventional vehicles sold in the U.S. ^{[DOE AFDC]} Their battery packs are sealed and tested against conditions such as overcharge, vibration, extreme temperatures, short circuit, humidity, fire, collision and water immersion. ^{[DOE AFDC]} Light-duty vehicles are also required to have safety features that can deactivate or isolate the electrical system when they detect a collision or short circuit. ^{[DOE AFDC]} For a nervous new owner, that’s a reassuring set of facts: the high-voltage parts are designed to stay out of your way, and your job is simply to leave them to trained hands.

Is a hybrid right for you?

Hybrids often cost more than similar conventional cars, though some of that can be recovered through fuel savings or state incentives. ^{[DOE AFDC]} FuelEconomy.gov’s “Can a Hybrid Save Me Money?” tool compares a hybrid with a comparably equipped non-hybrid from the same maker and estimates the fuel savings, which is a calm, numbers-first way to decide. ^{[DOE AFDC]}

Hybrid driving habits that stretch every gallon

Because hybrids recapture energy when slowing, smooth driving pays off. FuelEconomy.gov says aggressive driving (speeding, rapid acceleration and braking) can lower gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic. ^{[FuelEconomy.gov]} An extra 100 pounds in the vehicle can reduce mpg by about 1%. ^{[FuelEconomy.gov]}

Calm next steps

1. Find your hybrid’s maintenance schedule in the owner’s manual.
2. Note your typical mpg in mild weather so you can spot changes.
3. Keep some gasoline in the tank, as NHTSA advises for hybrids.
4. Leave orange high-voltage cables to trained technicians.
5. Ask your shop whether your hybrid’s battery uses liquid coolant that needs checks.

Frequently asked questions

Do hybrids need oil changes?

Yes. Hybrids have gasoline engines, so their maintenance is similar to conventional vehicles. ^{[DOE AFDC]}

Can I plug in a regular hybrid?

No. A standard hybrid can’t plug in; it charges its battery through regenerative braking and the engine. ^{[DOE AFDC]}

Why is my hybrid's mileage worse in winter?

Cold weather hits hybrids harder: fuel economy can drop about 30% to 34% in short-trip city driving at 20°F, versus about 15% for a conventional car. ^{[FuelEconomy.gov]}

What do the orange cables mean?

High-voltage cables in hybrids and EVs are usually marked orange; these systems typically run 400 to 1,000 volts. ^{[DOE AFDC]}

Related reading

• Lesson: How Air Bags Work
• Lesson: How TPMS Sensors Work
• Lesson: Reading Tire Grades (UTQG)
• Lesson: Why Cars Use More Fuel in Cold Weather
• Ask a question

Sources

1. https://www.fueleconomy.gov/feg/hybridtech.shtml
2. https://afdc.energy.gov/vehicles/electric-basics-hev
3. https://afdc.energy.gov/vehicles/electric-maintenance
4. https://www.fueleconomy.gov/feg/coldweather.shtml
5. https://www.fueleconomy.gov/feg/hotweather.shtml
6. https://www.nhtsa.gov/summer-driving-tips
7. https://www.fueleconomy.gov/feg/driveHabits.jsp
8. https://www.bar.ca.gov/consumer/smog-check-program

General education, not a diagnosis. See our disclaimer.