It’s pretty amazing that it has taken over 20 years for hybrid electric vehicles to generate truly significant interest. Yet, that’s the story today as many who are interested in electrification have decided to try a gas-electric hybrid first to sate their appetite for an electrified vehicle. It’s an easy choice since there is no real downside to a hybrid – great fuel efficiency, no range anxiety, and a more affordable price of entry compared to a fully electric vehicle. But how do they work? This article, which ran in Green Car Journal a dozen years ago, explained hybridization in an easy-to-understand way that still resonates today. We’re sharing it here just as it originally ran in Green Car Journal’s Summer 2012 issue.
Excerpted from Summer 2012 Issue: The term ‘hybrid vehicle’ covers a lot of territory. Motivated by two or more different power sources, a hybrid electric vehicle (HEV) uses an internal combustion engine (ICE) and one or more electric motors with batteries that store electrical energy. The ICE is usually a gasoline engine, but diesel engines can be used.
Hydrogen Hybrid Electric Vehicles
In the future, we will see hydrogen fuel cell hybrids where a fuel cell replaces the ICE. Then, there are hydraulic hybrids, now found in large trucks and buses. Here, energy in the form of high pressure hydraulic fluid is stored in accumulators and reservoirs rather than batteries, and hydraulic pressure rather electric motors drive the wheels.
There are both series hybrids and parallel hybrids, with the latter configuration currently far more popular in automotive applications. Cars like the Chevrolet Volt and Fisker Karma are series hybrids. Here, the ICE’s sole or primary job is to drive a generator that supplies electric energy to the battery or directly to an electric motor, or motors, that power the wheels. The engine in a series hybrid can operate at an optimum speed for best fuel economy since its focus is generating electricity rather than providing mechanical power to the wheels.
Parallel Hybrids
In a parallel hybrid, both the ICE and electric motor(s) can power the wheels together or individually. The ICE can also keep the battery charged. The ICE in parallel hybrids can be smaller and more fuel efficient since their electric motors can supply supplemental power for peak loads.
Then there are mild hybrids and full hybrids. In a mild hybrid, the ICE and motor/generator operate in parallel, with the motor/generator used for regenerative braking, stop-start capability, and battery charging. While the ICE provides most of the propulsion power, the electric motor can supply additional power, such as during acceleration and hill climbing. A mild hybrid cannot travel solely on its electric motor. The Chevrolet Malibu Eco, Buick eAssist, and BMW ActiveHybrids are examples of mild hybrids.
Series-Parallel Hybrid
A full hybrid adds the ability to operate on electric power alone, at least for short distances. Sometimes a full hybrid is called a series-parallel hybrid since, like a series hybrid, its ICE and motor/generator can charge the battery that in turn powers the wheels. Examples include Toyota, Lexus, and Nissan hybrids, including the Prius with its Hybrid Synergy Drive (HSD) and Ford’s Fusion and C-Max hybrids.
Microhybrids are not really hybrids according to the above definition since they save fuel simply by shutting off the engine when a vehicles stops, such as at traffic lights. Their advantage is that microhybrids can deliver a 5 to 10 percent improvement in fuel economy with only minor modifications to a powertrain, while adding only a small amount to a vehicle’s cost. They do require more robust and powerful starters to handle the greater number of starts, plus more capable batteries to keep the air conditioning, radio, and other electronics running during the stop-and-start process when the engine is shut down. . As expected, maximum fuel economy comes in stop-and-go urban driving with no savings achieved during long-distance highway drives.
Start-Stop Adds Efficiency
Often, stop-start is combined with regenerative braking for further fuel savings. This adds complexity since the braking system must have the ability to recoup braking energy and convert it to electricity that’s used to keep batteries charged. Virtually every mild and full hybrid features stop-start and regenerative braking. In fact, these two systems are what help hybrids achieve greater EPA estimated fuel economy in city driving compared to driving on the highway, where steady speeds have traditionally resulted in much better mpg than when driving in stop-and-go traffic.
As the name implies, the plug-in hybrid electric vehicle (PHEV) operates as a conventional hybrid but can also be plugged into the electric grid to recharge its batteries. This is in contrast to conventional hybrids that recharge only by their onboard generator and regenerative braking. PHEVs, which have a larger battery pack than standard hybrids so they can be driven longer on battery power alone, may never need a drop of gasoline if driven relatively short distances. Longer drives use a combination of battery and internal combustion engine power. Examples include the Toyota Prius Plug-In, Ford Fusion Energi, and C-Max Energi hybrids.
Extended Range Electric Vehicle
An Extended Range Electric Vehicle (EREV), sometimes called a Range-Extended Electric Vehicle (REEV), is designed for battery electric driving. It creates its own on-board electricity when batteries are depleted to extend all-electric driving range. EREVs can have either series or parallel hybrid configurations. The series hybrid Chevrolet Volt and Fisker Karma are high-profile examples that travel 25 to 50 miles on battery power and then hundreds of miles more with on-board generated electricity. Other similarly-powered extended range electric vehicles are on their way. The upcoming BMW i3, for example, will have a REx option with a small ICE that extends its nominal 100 mile all-electric range.