How Hybrid Vehicles Work: Combining Electric Motors and Gasoline Engines

A hybrid electric vehicle (HEV) combines a spark-ignited gasoline engine with one or more electric motors that draw power from a battery pack, according to the U.S. Department of Energy's Alternative Fuels Data Center (AFDC). Unlike plug-in hybrids, a conventional hybrid's battery cannot be charged from an electrical outlet; it is topped up only through the gasoline engine and through regenerative braking.

The main components

AFDC lists the core parts of an HEV powertrain: an internal combustion engine, an electric traction motor, an electric generator, a traction battery pack, a power electronics controller that regulates current and motor speed, a DC/DC converter that steps down voltage for onboard accessories, and a transmission that sends mechanical power from both the engine and the motor to the wheels.

How the two power sources share the work

According to fueleconomy.gov, the gasoline engine and electric motor are managed automatically by the vehicle's control system, which continuously decides how much each should contribute. At low speeds and light acceleration, many hybrids can run on the electric motor alone. At cruising speed, the combustion engine typically does most of the work. Under hard acceleration or on hills, the engine and motor can combine their output, and when the car is stopped the engine shuts off so no fuel is wasted idling.

Toyota's official description of its Hybrid Synergy Drive system, used across its hybrid range, gives a concrete example of this logic: at idle or standstill, Toyota says 'both engines are automatically switched off so no energy is wasted,' while on hills or when overtaking the company states that 'both engines power the vehicle to deliver maximum performance.' Toyota also says its system lets drivers accelerate up to 50 km/h on electric power alone in EV mode, without producing tailpipe emissions during that phase.

Regenerative braking: recovering energy that would otherwise be lost

Regenerative braking is the technology that lets a hybrid recharge its battery without plugging in. Fueleconomy.gov explains that the system 'uses the forward motion of the wheels to turn the motor,' which generates electricity while also helping to slow the car down, instead of dissipating that energy purely as heat through conventional friction brakes. AFDC confirms that an electric generator captures energy from the braking wheels and redirects it to the traction battery.

Series, parallel and plug-in configurations

Plug-in hybrids are not all wired the same way. Fueleconomy.gov's description of plug-in hybrid electric vehicles (PHEVs) distinguishes parallel, or 'blended,' PHEVs, in which both the engine and the electric motor are mechanically connected to the wheels and can drive them directly, from series PHEVs, in which only the electric motor turns the wheels while the gasoline engine's sole job is to generate electricity for that motor. Fueleconomy.gov notes that series PHEVs are also known as extended-range electric vehicles (EREVs).

Plug-in hybrid electric vehicles (PHEVs) add a larger, externally rechargeable battery to the same basic concept. Fueleconomy.gov states that PHEVs 'can store enough electricity to significantly reduce their petroleum use under typical driving conditions,' and that, depending on the model, they use roughly 30% to 60% less petroleum than conventional vehicles. The U.S. Environmental Protection Agency (EPA) states that PHEVs carry both a battery and electric motor alongside a gasoline tank and internal-combustion engine, and that they 'produce tailpipe emissions when gasoline is being used as a fuel source,' making them cleaner than conventional gasoline cars but not emissions-free like battery electric vehicles.

Why combining the two sources saves fuel

AFDC explains that the dual powertrain lets automakers fit a smaller gasoline engine than a comparable conventional car would need, because the electric motor can supply extra torque when it is needed, such as during acceleration or hill climbing. AFDC states that this 'can potentially allow for a smaller engine,' and that the combination of regenerative charging, motor assistance and reduced idling results in better fuel economy without sacrificing performance. Toyota's own figures illustrate the effect at the model level: the company lists its Auris Hybrid Synergy Drive model's fuel consumption at 3.8 litres per 100 km in normal driving, with CO2 emissions of 87 grams per km.