How Do Hybrid Cars Work?

A hybrid car provides motive power using two forms of energy, electricity and gasoline. They are known for being quiet, efficient and relatively affordable. Commercially available hybrids come in three flavors: mild hybrid, conventional hybrid and plug-in hybrid. So, how do these hybrid cars actually work?

This will be more of an intermediate-level discussion, going over the highlights of the hybrid variations. If you need a more introductory lesson, take a look at "What Are Hybrids and How Do They Work?" For now, we’ll start with conventional hybrids since they are the most common on the market and the ones that most people will end up buying.

Conventional hybrids

This is the type that most people associate with hybrids. These have a larger battery than a mild hybrid and can drive on electric power alone for short distances, but generally only at lower speeds before the gasoline engine kicks in. 

A series hybrid only gets its motive power from its electric motor. This motor can get its power from a battery or from a generator attached to an internal combustion engine. To date, only a handful of series hybrids have been available to consumers in the U.S.: the BMW i3 Range Extender, the Fisker Karma (and by extension, the Karma Revero), and most notably the Chevrolet Volt (though technically, the Volt’s internal combustion engine will directly power the wheels at speeds over 70 mph). Ram’s forthcoming Ramcharger pickup is also a series hybrid.

A parallel hybrid differs from a series hybrid in that it can be powered either by the internal combustion engine or by its electric motors. There are a number of different versions of this tech; Toyota popularized the traditional hybrid with the Prius in the late 1990s. This kind of hybrid — also known as a power-split or series-parallel hybrid — uses an internal combustion engine paired with one or more electric motors, all of which drive either a traditional gearset or a continuously variable transmission. 

When driving, most hybrid vehicles use their computers to determine whether it's most efficient to use one electric motor, both electric motors or the gasoline engine in combination with the electric motors, effectively decoupling the driver from the means of power delivery. Also, many hybrid systems have the ability to recapture energy through regenerative braking. This uses the electric motors as generators to help recharge the batteries when the vehicle is slowing, further increasing fuel efficiency.

Examples of parallel and power-split hybrids available today include the Toyota Prius, Lexus ES Hybrid and Kia Sportage Hybrid.

Plug-in hybrids

These function like parallel hybrids in many ways but use larger batteries to provide more assistance from the electric motors. Plug-ins can also drive solely on electric power for longer distances and at much higher speeds. Newer plug-ins, like those being used by Porsche in its Cayenne E-Hybrid range, can go even farther and faster and use the electric part of their drivetrain to add more performance as well as efficiency.

Plug-in hybrids are becoming increasingly attractive to both manufacturers and consumers due to their generally lower cost relative to EVs and their lack of EV range limitations due to their internal combustion powertrains. Other benefits include reduced charging times compared to fully battery electric vehicles thanks to their smaller battery packs. These smaller battery packs also make charging at Level 1 (120-volt) chargers viable, negating the need for specialized charging equipment in the home.

Level 2 charging (220 volts) is faster, and most plug-ins will accept this level of charging. Some PHEVs can also take advantage of high-speed Level 3 DC fast charging, which allows drivers to charge in minutes rather than hours.

Other examples of plug-in hybrid vehicles available today include the Toyota RAV4 Prime, BMW XM and Volvo XC90 Recharge.

Read What Is a Plug-In Hybrid (PHEV) and How Does It Work? For more detailed information.

Mild hybrids

A mild hybrid has the smallest battery of any type of hybrid vehicle and typically uses a combination starter generator to both start the vehicle and function as an alternator. This allows the vehicle to be more efficient due to less drag on the engine from a belt-drive accessory and makes the vehicle’s auto stop-start system a lot smoother. That starter generator also fills in power dips in an internal combustion engine’s torque curve, making downshifts less frequent and letting the engine work in its most efficient range more of the time. Unlike conventional and plug-in hybrids, modern mild hybrids generally use 48-volt systems, making them much cheaper to implement than the 400-plus-volt systems found in conventional and plug-in hybrid models.

Examples of mild hybrids available today include the Ram 1500, Mazda CX-90, Land Rover Defender P400 and the Volvo XC40.

Read What Is a Mild Hybrid (MHEV) Car? for more detailed information.

Why hybrids are so efficient

A parallel hybrid gets most of its efficiency from the intelligent handoff between its electric motor(s) and combustion engine. It’s always monitoring your driving and determining when to switch from one energy source to the other. Even the internal combustion engine in parallel hybrids is typically focused first on efficiency. Many hybrid vehicle engines run on what’s called the Atkinson cycle, which changes the length of time that the intake valves are open so that they’re open longer. This sacrifices low-end power for more fuel efficiency, and that lost power is compensated for by the electric motors.

These factors — the efficient engine and the smart handover between electric and gas power — are why parallel hybrids excel in city driving scenarios. They can use their electric motors more often since average speeds are lower. This is also why parallel hybrids usually get better fuel economy in the city than on the highway, which is the opposite of standard gas-powered cars.

Plug-in hybrids are even more efficient because the larger battery means you can drive in a fully electric mode for longer periods of time. Of course, because PHEVs have large lithium-ion batteries and more robust electrical components, they’re heavier than traditional hybrids, which does eat into overall efficiency.

So, if you’re primarily a city driver looking to save money on fuel, a hybrid car is a great option.