What is a plug-in hybrid and how does it work?
Compared to gasoline-powered cars, conventional hybrids, or battery electric cars, a plug-in hybrid may be the trickiest to understand. It can work like a regular hybrid car, for example a Toyota Prius, but there is a flip side: it is also an electric car that plugs in to recharge the battery pack that allows it to drive on electric power some of the time – but usually only for 20 to 60 minutes. Miles.
This can be confusing for car shoppers, to say the least. One auto expert suggested that hybrids are an engineer’s answer to a bureaucratic question no actual car shopper has ever asked: How can I cover the majority of my predictable daily miles on electricity, without worrying about the range of a battery-powered electric car?
The idea is that drivers who can plug in at home overnight get a range of 20 to 50 miles, enough to cover daily commutes and shopping trips. According to the US Department of Transportation, the average driver covers 37 miles per day. But for unexpected needs, long trips, or if they can’t deliver somewhere, they still have a fuel-efficient hybrid car. For owners whose cars only make short trips, the engine may not be run for weeks at a time, with gasoline only purchased every two months or so. If it’s delivered regularly, that’s the case.
Power division vs series
There are two basic types of plug-in hybrids: those that mix power from both the battery pack and the gasoline engine (known as “split-energy hybrids”) and the more rare type where the gasoline engine acts only as a generator to recharge the battery, but cannot power the wheels mechanically (known as (known as the “Hybrid Series”). The BMW i3 is one of the few examples.
Most split-power hybrids mix torque from the engine and engine to drive the same axle. This is the setup used by Toyota, Ford, and others. Many are front-wheel drive vehicles (such as the Toyota Prius), but some also offer all-wheel drive. This can come either through drive shafts that mechanically power the rear wheels (e.g. 2005-2012 Ford Escape Hybrid) or an electric motor powering the rear (e.g. Toyota Prius or current RAV4 Hybrid).
Some use a gasoline engine primarily to power one axle but add an electric motor to drive the other end of the vehicle. The hybrid aspect comes about when the powertrain control system decides that gasoline or electric power should be added to whichever is already moving the car. This type is known as a “road hybrid,” since the engine and powertrain are only met through all four wheels, rather than within the drivetrain itself. Although there are no vehicles that use only a gasoline engine to power one axle and only an electric motor to power the other, one close example is the new Dodge Hornet R/T (and the similar Alfa Romeo Tonale). They both have a powerful electric motor that powers the rear wheels (and are therefore rear-wheel drive in electric mode), but they also have a small electric motor that assists the gasoline engine in the front.
None of these differences should matter much if the hybrid control system is able to smoothly and seamlessly mix power requirements between two very different power sources. It’s quite clear that there is a learning curve for automakers; First-generation hybrids and plug-in hybrids are much less smooth than later generations, as their makers respectively improve the algorithms that blend power to be gentle and smooth in transitions.
One or two engines
However, drivers will notice a difference in driving feel between hybrids with a two-motor system that is combined through a planetary gear train to also act as a transmission (Toyota and Ford’s approach), and those that use a single electric motor between the engine and a conventional automatic transmission (some newer Fords , and all models of Hyundai, Kia, Mazda, etc.).
The use of two motors and a planetary gear set allows for infinite ratio adjustment. By contrast, the electric motor between the engine and gearbox still passes through the gearbox, meaning you get standard shifting modes even when operating as an electric vehicle. These shifts tend to be less smooth than those that occur in a typical internal combustion engine powertrain.
The most difficult challenge comes with single-engine systems because they operate in one of four modes: engine power alone; Electrical energy alone; Engine plus engine power together; Or regenerative braking, where the engine acts as a generator to recharge the battery. Switching between these modes may occur dozens of times per mile in shifting traffic, and drivers often feel these transitions — especially if they are sudden. Single-motor hybrids are often “lumpier” and less smooth than dual-motor systems. But, on the other hand, two-motor systems have a CVT effect and can stall the engine at high speed during strong acceleration maneuvers.
Electronic energy versus total energy
Another difference drivers will notice is that plug-in hybrid electric vehicles (PHEV) tend to be much less powerful when powered by electricity alone than when powered by gasoline alone (or gasoline plus electricity). Take the 2024 Mazda CX-90 PHEV: Its electric motor is rated at 173 horsepower, but the gasoline-electric hybrid system generates 323 horsepower overall. This means that acceleration to 60 mph slows from a quick 5.9 seconds when operating as a hybrid to more than 11 seconds in EV mode, making it difficult to keep up with traffic or accelerate on the freeway without waking up the gas engine.
The two generation Chevrolet Volts (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-powered hybrid mode.
Every time the driver needs more power than the e-motor can provide, and the gasoline engine is started, it works for up to two minutes to warm up the catalytic converter so that it properly cleans the exhaust gases before they exit the tailpipe. If the catalyst stays warm, this only happens once, but infrequent use of a gasoline engine, long enough for the catalyst to cool, can mean running the engine multiple times for two minutes even if the power it supplies is needed for only 10 or 15 seconds.
Are they delivered?
The final challenge facing hybrid cars — not for owners, but for regulators — is a big question: Are they ever plugged in at all? If not, they’re just hybrids that are slightly better, heavier, more expensive, and often get generous purchase incentives even if they’re never connected to the electric grid.
During the 2000s, the two largest sellers of hybrid vehicles (General Motors and Ford) released aggregate data on owners’ plugging behavior to both reporters and regulators. For example, a 2013 U.S. Department of Energy report stated that 75 percent of the Chevy Volt’s miles were covered by grid power.
Unfortunately, none of the biggest vendors of plug-in hybrid electric vehicles (PHEV) in early 2020 (Toyota and Jeep) were willing to provide data on whether and how often these vehicles plugged in. Quite frankly, they don’t care. They receive a regulatory credit for selling the car regardless of what the owner does with it, and plug-in hybrids often come with incentives that salespeople can use to lower the purchase price to a price similar to a regular hybrid – although plug-in hybrids often have specification Nice trim to justify the high sticker price.
For this reason, and because plug-in hybrid electric vehicles (PHEVs) are difficult to explain to new car shoppers (some dealers don’t even try), many EV advocates and auto regulation specialists point out that, until proven otherwise, plug-in hybrids are “cars.” Compliant” – those that are sold not because consumers demand them or because automakers want to sell them, but just to check the box for regulatory targets. One case study might be Stellantis, which earlier this year made hybrids the default for states with stricter emissions rules, but moved them to special order status in the rest of the country.
Edited by John Voelker Green car reports For nine years, he published more than 12,000 articles on hybrid cars, electric cars, other low- and zero-emission vehicles, and the energy ecosystem surrounding them. He now covers advanced automotive technologies and energy policy as a reporter and analyst. His work has appeared in print, electronic and radio media including Wired, Popular Science, Technical Review, IEEE Spectrum, and “All Things Considered” on NPR. He divides his time between the Catskill Mountains and New York City and still hopes to one day become a global man of mystery.