Text By James M. Flammang
Mild versus Full Hybrid (Strong Hybrid)
Hybrid vehicles come in two distinct flavors: mild and full. The basic principle is the same with each version. A relatively small gasoline engine works in accord with an electric motor to propel the vehicle, resulting in greater fuel economy and reduced tailpipe emissions. The difference between the two hybrid types lies in the way their systems make use of each power source: whether they work jointly or separately, and the conditions that determine when the output from each source is called for.
Trio of Components
Like a full hybrid gasoline/electric vehicle (HEV), the “mild” version (MHEV) contains a gasoline internal-combustion engine (ICE), a lithium-ion traction battery, and an electric motor. The MHEV battery is smaller in size, because it’s not required to produce as much electricity. Because most batteries are 48-volt, the MHEV is sometimes referred to as a 48-volt system. Like their “full” counterparts, mild hybrids generally use a gear-free continuously variable transmission, though conventional “stepped-gear” transmission are also sometimes used.
No Electric-Only Operation
Unlike the regular (full) hybrid, that mild version with its smaller battery and electric motor cannot run on battery power alone. Because only one power source at a time is operational, it’s considered a “series” configuration, whereas full hybrids have a “parallel” layout. In a typical model, the MHEV system comes into action as the vehicle prepares to start off from a standstill. As soon as the driver releases the brake pedal and is ready to push on the accelerator, the gasoline engine starts and immediately takes over the task of setting the vehicle in motion.
Regenerative Braking
Slowing down the vehicle is similar to the action that takes place in a full hybrid. As an element of the MHEV system, regenerative braking recovers some of the kinetic energy that builds up as the vehicle loses speed. That energy is transformed into electricity, which is sent to the battery and stored for later use. Next time the vehicle comes to a halt, the MHEV system draws upon that stored energy to work in accord with the gas engine for accelerating from a standstill once again. The procedure takes place automatically, with no particular action required from the driver.
Battery Charging
Because the battery is constantly being charged by either the gas engine or the regenerative braking system, it’s unlikely to drop significantly below full charge. Even if that should happen, the gasoline engine stands ready to step in and power the vehicle.
Electric Boost
In addition to providing an energy boost to the gasoline engine when accelerating, yielding slightly swifter behavior, the electric motor can dispense power for operating air conditioning and accessories. Climate-control and other systems continue to operate when the engine shuts down.
Affordable Hybrid
Because the MHEV system is simpler, using smaller components, fuel economy improves only modestly compared to a full hybrid. However, the mild hybrid costs less to manufacture and is less expensive at the dealership. It’s considered the more affordable way to get benefits from hybrid operation. Dozens of mild-hybrid models are available in the U.S.
How They Work
Compared to mild hybrids, the full hybrid (HEV) works in tandem with the internal-combustion (gasoline) engine. Its larger electric motor can power the vehicle on its own, without aid from the gas engine, at least for short distances at lower speeds. An MHEV never operates like a fully electric vehicle (EV), although the gas engine can shut off when coasting or braking to a stop.
Pros
Considering the moderate purchase price for most models, mild-hybrid benefits are tempting:
- Modest fuel economy increase
- Slight reduction in tailpipe emissions
- Mild increase in acceleration from a standstill
- More affordable than full hybrid or EV
- Never necessary (or possible) to charge battery by plugging it into an electrical outlet; it’s done while driving
- Transparent operation–drivers might not even realize that an MHEV system is assuming part of workload, though gas/electric transition may be noticeable
History of Mild Hybrids
When Honda launched the first hybrid vehicle in the mainstream U.S. market, the 2000 Insight, it was a mild hybrid, controlled by the company’s Integrated Motor Assist (IMA) system. Its traction battery and little gasoline engine both provided energy to drive the tiny two-seater’s wheels, but it could not run on battery power alone. Honda’s next entry, the 2006 Civic Hybrid, was a four-door sedan, thus far more likely to interest families. Even though its IMA was an improvement over the Insight’s, the Civic Hybrid still could not travel as a battery-powered vehicle.
Honda Versus Toyota
By 2006, the Toyota Prius–launched soon after the original Insight–had become a phenomenon in the marketplace. And because Toyota’s hybrid system qualified as full rather than mild, it could run short distances on battery power. To many shoppers, the word “hybrid” described just about any small, fuel-efficient hybrid-powertrain passenger car.
Meanwhile, Ford reached past the sedan market with its 2005 Escape compact crossover, offered in both gas-engine and hybrid form. General Motors took an unusual stab at the market in 2012, making its eAssist mild-hybrid system standard in the Buick LaCrosse sedan. A year later, the Chevrolet Malibu got an Eco hybrid option, but then GM backed away from that end of the market. Only the newly-launched Corvette e-Ray has a hybrid powertrain, and it’s obviously in a category of its own.
Euro Brands
Today, quite a number of automakers manufacture vehicles with mild-hybrid powertrains, mainly as an option. European brands have been especially interested in the hybrid principle. Luxury-level vehicles aren’t left out, either. Audi, BMW, and Volvo offer a mild-hybrid powertrain for most of their models. BMW alone produces more than 50 hybrid versions of its car models. Volvo makes MHEV available in its S90 sedan, as well as XC crossovers. Audi markets mild-hybrid versions of its full range of sedans: compact A4 through full-luxury A8, along with “Q” SUVs. Most of the Audis employ a 48-volt system, but the A4 sedan gets a 12-volt setup.
Nearly every Land Rover model can be purchased with a mild-hybrid powertrain. The legendary British automaker also offers plug-in hybrid (PHEV) and full-electric models. Comparisons can be confusing due to differing definitions, but Mercedes-Benz might earn credit as the foremost producer of mild-hybrid models. The U.S. Department of Energy counts 36 hybrids in the M-B lineup. Only one Jaguar contains a mild-hybrid powertrain: the F-Pace compact SUV.
Affordable Hybrids
As it has for two decades, Ford continues to produce its compact Escape crossover wagon with a conventional gasoline engine, but also as either a mild or full hybrid. Even a plug-in hybrid is available, thus covering all but one of the electric-power bases.
Numerous Toyotas are available as mild hybrids, but the Prius qualifies as a full hybrid because of its ability to run on battery power alone. Nissan sells mild hybrids, whereas Hyundai and Kia gasoline/electric models rank as full hybrids. Stellantis (formerly Chrysler and DaimlerChrysler) sells a Ram 1500 MHEV pickup (only with V6 gas engine) and continues to offer a Pacifica minivan, but that has an EV-only mode. Two Jeep models can have a plug-in hybrid powertrain, and a mild e-Hybrid Compass is expected in 2026.
Is That Hybrid Mild?
Determining whether a given hybrid is mild or full, based on advertisements or a salesperson’s spiels, isn’t so easy. Definitions and claims tend to be on the hazy side. Best bet: follow the simplest is/is not rule. If the hybrid you like will move using battery power alone, it’s a full hybrid. If not, it’s mild.
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Mild Hybrid Pictures
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