Plug-in Hybrids: Are They Really A Solution To Reducing Emissions?

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It’s immediately clear how fully-electric battery electric vehicles (BEVs) can help reduce emissions; eliminating gasoline and tailpipes in favor of increasingly clean electricity helps limit both climate change and air pollution. Plug-in hybrids are a bit more complicated. A plug-in hybrid electric vehicle (or PHEV) has both a gasoline engine and one or more battery-powered electric motors. The battery in a PHEV can be charged using grid electricity just like a fully electric vehicle but a PHEV can continue driving when the battery is low on charge by switching to the gasoline engine (either to generate electricity for the motors or to directly drive the wheels).

BEV or PHEV? Yes!

Recently General Motors made headlines in saying they are looking at restarting production of PHEVs after moving to only BEVs in 2019. Does this mean they are moving backwards on electrification? Not necessarily. To reduce climate emissions, we need to move from gasoline to electricity as soon as possible, and in the short term, PHEVs can be part of that shift. The important thing will be for automakers to not move into PHEVs at the expense of progress on fully-electric BEVs. Plug-in hybrids need to displace high emission gasoline-only vehicles, not replace lower-emission battery-electric cars and trucks.

Plug-in hybrids in theory are an attractive solution to electrify transportation

The ability of PHEV to switch between grid electricity and gasoline means that even though PHEVs often only have 20-50 miles of electric range, they can still complete longer trips without stopping to recharge. And since drivers on average go less than 40 miles a day, a PHEV that is recharged every night could drive the majority of its miles on grid electricity despite having a battery 5 to 10 times smaller than a fully electric vehicle. So in theory, PHEVs offer the ability to switch a significant number of miles from gasoline to electricity with smaller batteries that cost less and require fewer raw materials.

Unlike fully electric vehicles, emissions depend on owners’ behavior

However, one downside of PHEVs is that their emissions depend in part on the behavior of drivers. If a PHEV is plugged in at every opportunity (like both at home and work), there can be very low gasoline use. And that is what was seen with many buyers of the now-discontinued Chevy Volt. Even with an electric range that was no more than 53 miles, these drivers were able to get 60 to 70 percent of their miles on electricity.

But on the other hand, a PHEV can be driven without plugging in at all. In a study by UC Davis, 17 percent of Plug-in Prius drivers and 12 percent of Ford Fusion Energi drivers studied chose not to plug their PHEVs in at all. Another look at PHEVs in the European Union found that PHEVs had much higher emissions in real-world driving compared to predicted emissions assuming charging every day.  This was particularly true for company cars, where drivers were often given a company fuel card but would have to pay for electricity themselves.

Some automakers are making very inefficient PHEVs

In addition to the potential for increased emissions from not plugging in, some newer PHEV models are very inefficient, both when using electricity and gasoline, making the total emissions from driving higher. For example, the 2023 Jeep Wrangler 4xe PHEV requires 68 kWh of electricity to go 100 miles. That’s over 2½ times more energy required per mile than the Tesla Model 3 (25 kWh/100miles) and significantly more than some all-electric SUVs like the Rivian R1S (as low as 43 KWh/100 miles). And when the Jeep switches to its gasoline engine, it’s also inefficient at 20 miles per gallon.

Because the Jeep Wrangler PHEV is so inefficient on both electricity and gasoline, the emissions for this SUV are very similar to a conventional gasoline vehicle. Using UCS’s EV emissions calculator, driving the Wrangler on California’s relatively clean electricity grid would result in total emissions (from electricity generation and gasoline use) equal to a 28 mpg gasoline-only car. Even an inefficient all-electric vehicle, like the Ford F-150 Lightning pickup is much cleaner, at 75 mpg equivalent on California’s grid.

The Jeep Wrangler PHEV is one of the least efficient EVs available, but it’s not alone. According to the US Department of Energy, 14 current PHEV models use more electricity per mile than fully-electric pickup trucks (more than 50 kWh per 100 miles).

Regulations need to be based on real world results

It’s important to credit PHEV vehicles for the emissions benefits that they actually provide, rather than a hypothetical savings based on optimistic charging behavior.

The federal greenhouse gas emissions standards currently use a formula to estimate the fraction of PHEV driving that occurs using the electric motor that is based on the range of the battery and historical driving behavior. This calculation assumes that the vehicle is always at full charge at the start of the day, which isn’t always the case in real-world operation. Data from the International Council on Clean Transportation show that the actual fraction of electric-only driving (and therefore tailpipe emissions reductions) can be less than half of the predicted value. PHEVs can be part of the climate solution, but automakers need to make them more efficient, and regulators like the EPA need to correctly value their contributions.

While PHEVs can be part of the transition to a cleaner transportation system, in the long-term we need to get rid of tailpipes, even if they are only used part of the time.

By David Reichmuth. Courtesy of Union of Concerned Scientists, The Equation.


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