What Are the Different Types of Hybrid Cars?

In a parallel hybrid system, both an engine and an electric motor can power the wheels, together or separately. Toyota’s system, first introduced in a Japan-market Prius in 1997, uses two motors between the engine and front wheels, powered by a small battery pack (0.8 to 1.4 kilowatt-hours).

The “power split” part of the description means both sources come together to provide a single torque output to the wheels. Under light loads, at lower speeds, one or both motors can power the vehicle while the engine is off. The engine will kick on when more power is needed; it may power the car by itself, or power may be drawn from a combination of the engine and the motors.

The key to the system is that regenerative braking—using a motor as a generator when the car slows to recharge the battery—recaptures energy that would otherwise be wasted as brake heat. Under ideal circumstances, as much as 30 percent of the energy expended to power the car can be recaptured—leading to real-world fuel economy of 40 to 55 miles per gallon in the Toyota Prius or its Corolla Hybrid sibling (pictured at top). Efficiency is highest in stop-and-go traffic that allows the most electric-only running.

The two electric motors and engine are connected through a planetary gear set, which replaces a conventional transmission. This allows continuously adjustable gear ratios; many times each second, the vehicle’s control algorithms vary the power drawn from the battery and from the engine to maximize the use of the electrical power and minimize fuel burned.

Under some circumstances, one motor may power the car while the other uses engine power or regenerative braking to recharge the battery. With close to three decades of experience under its belt, Toyota has software algorithms that make all these transitions entirely seamless—and these days, drivers and passengers may not even notice the engine switching on and off.

A series hybrid is much simpler in concept. Whenever more power is needed than the battery can supply, a gasoline engine switches on to power a generator that recharges the pack. The key here is that the engine torque only powers the generator. It’s not mechanically connected to the wheels, which are powered only by electric motors.

This is the setup used by diesel-electric railroad locomotives. It’s best at steady vehicle speeds, where the engine can operate most efficiently at preset engine speeds. But the range of power needs of a passenger road vehicle spans more than an order of magnitude from steady-speed cruising to maximum acceleration, and that can be hard on batteries—meaning series hybrids usually require higher battery capacity than conventional hybrids.

Few series hybrid passenger vehicles have been sold in the States: the BMW i3 REx, which added a range-extending two-cylinder engine to the i3 battery-electric model; the Fisker Karma, as well as its successors the Karma Revero and GS-6 (all sleek four-door luxury sedans); and soon, the Ram 1500 Ramcharger full-size pickup truck.

A plug-in hybrid can be a hard concept to explain. In the simplest terms, it’s a regular hybrid with a much larger battery that can be plugged in to recharge from the grid. That lets it operate as an electric vehicle for some distance. (For the 2024 model year, the EPA range estimates for PHEVs go from as few as 7 miles to as many as 51 miles.)

Chevrolet arguably popularized the plug-in hybrid concept with its 2011–2018 Volt. Many drivers found the Volt could provide daily driving solely on electricity, with the engine often only switching on when driving to farther-away destinations.

The Volt, unique among PHEVs, offered nearly identical performance regardless of whether it was running on battery alone or acting as a regular hybrid after the battery charge was depleted. Other plug-in hybrids have more powerful engines than electric motors. When more power is needed, the engine has to switch on. The frequency of engine use falls as battery size (and hence electric range) rises; higher battery capacity is needed for 40 miles of electric range versus, say, 10 miles.

All plug-in hybrids today are adapted from power-split hybrids, including Toyota’s Prius Prime and RAV4 Prime and a variety of PHEV versions of Hyundai and Kia hybrids. Stellantis, however, offers no conventional hybrids (without a plug); its Chrysler Pacifica Hybrid and a growing lineup of Jeep 4xe hybrids all have plugs.

Carmakers see plug-in hybrids as a way to meet rising zero-emission-vehicle requirements without the expense of engineering entirely new battery-electric cars. Those EVs require battery packs of 60.0 to 100.0 kWh, versus 10.0 to 25.0 kWh in a PHEV. The challenge is that the manufacturers get credit for producing emission-free vehicles even if the PHEV is never plugged in, meaning it operates only as a heavier, pricier conventional hybrid.

Finally, mild hybrids have electric motors that aren’t powerful enough to propel the vehicle alone. They may sit between the engine and transmission, or be part of an integrated starter-generator system in which a beefier electric motor replaces both those components. Fuel economy benefits here are far less dramatic, and they may not even be marketed under the name “hybrid.”

In this case, too, a small battery pack is required to store energy recaptured from excess engine power and/or regenerative braking. The motor effectively adds some extra boost to the engine when needed, meaning potential fuel savings by avoiding a downshift.

Recent mild-hybrid systems often run at 48 volts. They deliver more power than the vehicle’s 12-volt system but are far less expensive than full hybrid systems that operate at 280 to 400 volts.

John Voelcker edited Green Car Reports for nine years, publishing more than 12,000 articles on hybrids, electric cars, and other low- and zero-emission vehicles and the energy ecosystem around them. He now covers advanced auto technologies and energy policy as a reporter and analyst. His work has appeared in print, online, and radio outlets that include Wired, Popular Science, Tech Review, IEEE Spectrum, and NPR’s “All Things Considered.” He splits his time between the Catskill Mountains and New York City and still has hopes of one day becoming an international man of mystery.