What Is a Plug-In Hybrid? How Does It Work?

2023 toyota prius prime charging
What's a Plug-In Hybrid and How Does It Work?Toyota

Compared with gasoline cars, conventional hybrids, or battery-electric cars, a plug-in hybrid may be the hardest to understand. It can run like a regular hybrid, e.g. a Toyota Prius, but there's another aspect: It's also an electric car that plugs in to recharge a battery pack that lets it drive on electric power some of the time—but usually only for 20 to 60 miles.

This can be confusing to car shoppers, to say the least. One automotive wag suggested plug-in hybrids are an engineer's answer to a bureaucrat's question that no actual car shopper has ever asked: How do I cover the majority of my predictable daily miles on electricity, without the range anxiety of a battery-electric car?

The idea is that drivers who can plug in at home overnight get 20 to 50 miles of EV range, which is enough to cover daily commutes and shopping trips. According to the U.S. Department of Transportation, the average driver covers 37 miles per day. But for unexpected needs, longer trips, or if they can't plug in somewhere, they still have a fuel-efficient hybrid. For owners whose vehicles only do short trips, the engine may not switch on for weeks at a time, with gasoline purchases only coming every other month or so. If they're plugged in regularly, that is.

Power-Split vs. Series

There are basically two types of plug-in hybrids: those that blend power from both the battery pack and the gasoline engine (known as "power-split hybrids") and the much rarer kind where the gasoline engine acts solely as a generator to recharge the battery, but can't mechanically power the wheels (known as "series hybrids"). The BMW i3 is one of the few examples.

Most power-split hybrids blend torque from the engine and the motor to power the same axle. This is the setup used by Toyota, Ford, and others. Many are front-wheel-drive vehicles (like the Toyota Prius), but some also provide all-wheel drive. This can come either through driveshafts that power the rear wheels mechanically (e.g. the 2005–2012 Ford Escape Hybrid) or an electric motor that powers the rears (e.g. the current Toyota Prius or RAV4 Hybrid).

Or "Through the Road"

A few use the gasoline engine primarily to power one axle but add an electric motor to propel the other end of the vehicle. The hybrid aspect comes when the powertrain control system decides either gasoline power or electric power must be added to whichever is already moving the car. This type is known as a "through-the-road hybrid," since the engine and motor power combine only through the four wheels, not within the powertrain itself. Although there aren't any vehicles that use only a gasoline engine to power one axle and only electric to power the other, one close example is the new Dodge Hornet R/T (and the similar Alfa Romeo Tonale). Both of these have a powerful electric motor that powers the rear wheels (and are thus rear-wheel drive in electric mode), but they also have a small electric motor that assists the gas engine up front.

None of these distinctions should matter a lot if the hybrid control system can shuffle power demands smoothly and seamlessly among two very different power sources. It's abundantly clear that there's a learning curve for automakers; first-generation hybrids and plug-in hybrids are far less smooth than later generations, as their makers successively refine the algorithms that blend power to be gentle and smooth in the transitions.

One Motor or Two

Drivers will notice, however, a difference in driving feel between plug-in hybrids with a two-motor system that combines through a planetary gearset to also function as a transmission (the Toyota and Ford approach), and those that use a single electric motor between the engine and a conventional automatic transmission (some newer Fords, all Hyundai and Kia models, Mazda, etc.).

Using two motors and a planetary gearset allows for infinite ratio adjustment. By contrast, an electric motor between an engine and gearbox still drives through that gearbox, meaning you get standard shifting patterns even when it operates as an EV. And those shifts tend to be less smooth than those from a typical internal-combustion-engine powertrain.

A more difficult challenge with single-motor systems comes because they operate in one of four modes: engine power alone; electric power alone; engine plus motor power together; or regenerative braking, where the motor acts like a generator to recharge the battery. Shifting among those modes may occur dozens of times each mile in variable traffic, and drivers often feel the transitions—especially if they're sudden. Single-motor hybrids are almost always "lumpier" and less smooth than dual-motor systems. But, on the other hand, the two-motor systems have the CVT effect and can hang the engine up at a high and droning speed during aggressive acceleration maneuvers.

E-Power vs. Total Power

The other difference drivers will notice is that PHEVs tend to be substantially less powerful operating on electricity alone than on gasoline alone (or gasoline plus electric). Take the 2024 Mazda CX-90 PHEV: its electric motor is rated at 173 horsepower, but the combined gasoline-electric hybrid system overall puts out 323 horsepower. That means acceleration to 60 mph slows from a swift 5.9 seconds when operating as a hybrid to above 11 seconds in EV mode, making it difficult to keep up with traffic or accelerate onto an interstate without waking the gas engine.

The two generations of Chevrolet Volt (2011–2015 and 2016–2018) were unique among plug-in hybrids in offering nearly identical performance, regardless of whether the car was running on battery power alone or in gasoline hybrid mode.

Every time a driver needs more power than the e-motor provides, and the gasoline engine flips on, it runs for up to two minutes to warm up the catalytic converter so it cleans exhaust gases properly before they exit the tailpipe. If it stays warm, that only happens once—but infrequent use of the gasoline engine, enough for the catalyst to cool down, can mean multiple two-minute engine runs even if the power it provided was only needed for 10 or 15 seconds.

Do They Get Plugged In?

The final challenge for plug-in hybrids—not for owners, but for regulators—is a big question: Are they ever plugged in? If they're not, they're just slightly better hybrids that are heavier, cost more, and often get generous purchase incentives even if they never once connect to the electric grid.

During the 2010s, the two largest sellers of plug-in hybrids (GM and Ford) released aggregated data on owners' plugging-in behavior to both reporters and regulators. A 2013 U.S. Department of Energy report, for example, said 75 percent of Chevy Volt miles were covered on grid power.

Regrettably, neither of the two largest sellers of PHEVs in the early 2020s (Toyota and Jeep) is willing to provide data on whether and how often those cars are plugged in. And, quite frankly, it doesn't matter to them. They get regulatory credit for selling the car regardless of what the owner does with it, and plug-in hybrids often come with incentives salespeople can use to reduce the purchase price to one comparable to a regular hybrid—even though the plug-in hybrid often has a nicer trim spec to further justify its higher sticker price.

For that reason and the difficulty in explaining PHEVs to new-car shoppers (some dealers don't even try), many EV advocates and auto-regulation wonks suggest that until proven otherwise, plug-in hybrids are "compliance cars"—those sold not because consumers demand them or automakers want to sell them, but purely to tick the box on regulatory goals. One case study may be Stellantis, which earlier this year made plug-in hybrids the default option for states with tougher emission rules but relegated them to special-order status in the rest of the country.

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