Another point of confusion: because of the two modes of PHEVs, gas and electric, they’re often referred to loosely as “hybrids”: But there’s a crucial difference between a PHEV and an HEV, the latter ultimately relying on having gas in the tank.

The conventional hybrid technology of an HEV does allow for a modest amount of energy to be generated and stored as one drives the car, for instance when braking. Drawing on that stored power boosts an HEV’s overall gas mileage. However, a trip of any distance in an HEV pretty much requires gasoline.

In contrast, one can do nearly all in-town driving with a PHEV in electric-only mode and conceivably avoid a trip to a gas station for months on end.

A grafting of technologies

PHEVs debuted in the U.S. marketplace just over a decade ago, when many motorists were becoming interested in electric-car options but were consumed with “range anxiety” – the fear of stalling out in the middle of the freeway or the middle of nowhere. (The first version of the Leaf, introduced in 2010, had an EPA-rated range of just 73 miles.)

The idea of a mass-market plug-in hybrid, explored in prototypes since the 1990s, began to gain momentum around 2006 as niche firms started taking conventional Priuses and making them plug-in-friendly. “There were a couple of conversion companies that were taking your spare tire out and dropping a battery there,” Tal said.

The hugely influential California Air Resources Board, CARB, had launched its zero-emission vehicle program in 1990. This action led to a boom in so-called “compliance vehicles” – early BEVs such as the ill-fated General Motors EV1, eulogized in the documentary “Who Killed the Electric Car?” Those vehicles were designed to meet the California mandate but were poorly promoted overall and limited in range.

It wasn’t until 2008 that CARB began allowing manufacturers to produce a contingent of PHEVs to help meet the state’s zero-emission requirements. Federal tax credits, also instituted in 2008 and still in place for many models, boosted the sales of both PHEVs and BEVs, as did a scattering of state tax credits.

GM began selling the first commercial-scale PHEV, the Volt, in 2010. By the mid-2010s, about half of all plug-in electric vehicles in the U.S. were PHEVs. Several models attained a geek-cult status, with online forums filled with tips on driving the cars most efficiently – a task not always well explained by dealers or manuals.

Pros and cons of PHEVs

PHEVs aim to relieve range anxiety by pairing a regular-sized gas tank with a large enough battery to allow anywhere from about 15 to 50 miles of all-electric driving per charge. With some models, drivers can switch between electric and gas mode as desired, or they can allow the car to decide when to switch modes.

At least in theory, many motorists can get the bulk of EV benefits with a PHEV. Commuting makes up about 30 percent of vehicle-miles traveled, so PHEVs could handle most or all of a daily commute in electric mode after an overnight charge. The gas engine is there for major road trips or other occasions when charging is impractical or charging stations unavailable.

The classic tradeoff in a PHEV is between battery size and gas mileage. The bigger the battery, the longer the electric-only range, but hauling that extra weight (typically hundreds of pounds) trims the gas mileage. Size matters, too. The roomy Chrysler Pacifica minivan gets 32 miles in electric-only mode and 30 mpg in gas-only mode, while the compact Toyota Prius Prime notches just 25 miles in electric-only mode but 54 mpg in gas-only mode. (The BMW i3 is an extreme outlier, with a 126-mile electric range and a 2.4-gallon “range extender” gas tank.)

The complexities of PHEV technology and the presence of two engines add to the cost of the vehicle. Tax credits can whittle away some or all of that difference. Still, a typical PHEV will cost a few thousand dollars more than a comparable HEV or ICE model. Most drivers can eventually recoup some or all of that expense in reduced fuel and repair costs (though the unexpectedly low cost of gas during most of the past decade may have slowed the growth of both the EV and PHEV markets).

Another catch: many PHEVs effectively force the driver to use the gas engine at various points. For example, some models automatically turn on the ICE for at least a few minutes during cold weather so the cabin can quickly warm up without draining electricity. Some PHEVs also automatically switch to the ICE if a driver rapidly accelerates. And of course, a driver may simply neglect to plug in overnight, fully aware that the car can get from point A to point B whether it’s charged or not.

A conscientious PHEV driver can minimize gas use by plugging in regularly, avoiding jackrabbit starts, letting the cabin stay cool, and so forth. But how many consumers are this motivated?

A 2020 study from the UC-Davis center, led by Debrapriya Charkraborty, analyzed the charging habits of more than 5,000 PHEV owners in California over a month’s time. Up to 17% of owners of some models did not charge their vehicles even once during that month. Among the detriments to charging were a lack of overnight chargers at multifamily housing, low potential cost savings from using electricity versus gasoline, and a low electric-only driving range. 

“Many of the reasons why consumers do not plug-in seem to be avoidable or solvable through effective policy making,” authors of the study concluded.

Meeting increasingly ambitious emission goals in the E.U. and U.S.

In 2020, almost half of the 1.05 million plug-in vehicles sold in the E.U. were PHEVs. However, PHEVs appear to be running afoul of the EU’s stringent efforts to improve gas mileage and reduce emissions.

A 2020 analysis by a European NGO, Transport & Environment, spotlighted the various Achilles’ heels of PHEV design and execution.  The report includes the results of commissioned testing of three popular PHEV models in a range of real-world settings. The testing found that the cars’ emissions were 28% to 89% higher than advertised. Those bigger-than-projected emissions can be traced in large part to overly optimistic estimates of utility factors, the percentage of all miles driven that is purely electric.

“The climate credentials of plug-in hybrids are directly proportional to how much they drive on their electric motor,” Julia Poliscanova, director of clean vehicles and e-mobility for Transport & Environment, told Automotive News Europe.

A separate 2020 study by the International Council on Clean Transportation found that roughly half of plug-in vehicles sold in 21 European countries were registered by companies and not by individuals. These company cars (about half of them PHEVs) tend to be driven for longer distances between charges than individually owned plug-in vehicles, and there can be perverse incentives – such as employer-provided fuel cards without parallel subsidies for the electricity used in recharging.

An uncertain and challenging PHEV future lies ahead

In the end, PHEV company cars were responsible for three to four times more emissions than assumed by EU rules. Even personally owned PHEVs spewed two to three times more than the approved New European Drive Cycle values.

The days of PHEVs in the EU are indeed numbered if a comprehensive climate proposal unveiled in August becomes law. That plan, which is called “Fit for 55” and nicknamed the European Green Deal, would ban the sale of new PHEVs and ICE vehicles within the next 15 years. It includes a mandate to cut emissions from new vehicles 55% by 2030 and 100% by 2035 compared to 2021 levels.

The trade journal Automotive Logistics recently projected that PHEV sales in the EU could enjoy a brief boom before their ultimate demise. Between 2020 and 2030, the journal’s analysis projects that the share of EVs and PHEVs among all cars sold in the EU will jump from about 6% and 4%, respectively, to about 33% and 17%. However, by 2035, PHEVs would virtually disappear from the new-car scene.

An executive order issued in 2020 by California governor Gavin Newsom calls for all new vehicles sold in the state to be “zero-emission” starting in 2035. It hasn’t yet been determined whether PHEVs would be included in the implied phase-out of ICEs.

For the United States as a whole, an executive order issued by President Joe Biden on August 5 sets a more modest yet still ambitious goal: half of all new passenger vehicles and light trucks sold in 2030 should be “zero-emission vehicles, including battery electric, plug-in hybrid electric, or fuel cell electric vehicles.”

The UC-Davis center is encouraging the federal government to require that new PHEVs include enough range for a typical day’s driving to take place entirely in electric mode. “If you drive your car 40 or 50 or 60 miles a day, you could start and finish without the engine starting,” said Tal. “Our studies show that if you finish a day without an engine start, you are more likely to keep plugging your car in every day. If your engine starts, you’re more likely to move toward driving it as a hybrid only.”

Another priority involves urging carmakers to adopt technology that keeps the ICE turned off at all times unless the user actively engages it. “We believe that this is a key factor in the success and survival of PHEVs,” Tal said.

“Future PHEVs will need both the range and the electric power to drive most of the miles in pure EV mode and not on blended mode.”

It’s possible that the gateway aspect of PHEVs will become more appealing, at least for a few years, to a broader segment of motorists who want or need to keep one foot in the century-old world of ICEs while putting the other in an electrified future. Tal envisions many two-car households, for example, with one EV and one PHEV.

“I don’t want PHEVs to cannibalize the full electric,” Tal said. “I want them to fill in the gaps where full electrics are not the right solution yet.” Rural America could be one such venue for PHEV growth, according to Tal.

Ultimately, though, instead of representing the brave new motoring world they did in the 2010s, PHEVs just might become the closing act of the internal combustion engine.

“Could the last gas cars be PHEVs? I think that could be the case,” Tal said.