Plug-in hybrid vehicles are designed to combine the benefits of electric driving with the convenience of a gasoline engine for longer journeys.
They can travel short distances using only battery power and automatically switch to the engine when needed.
However, new research from Switzerland shows that many plug-in hybrids use much more fuel in real life than their official ratings suggest, mainly because of how they are driven and charged.
The studies, carried out by researchers at Empa and funded by the Swiss Federal Office for the Environment, found that charging habits play the biggest role in determining how efficient these vehicles really are.
If drivers rarely plug in their cars, the battery cannot do much of the work. Instead, the gasoline engine ends up powering a vehicle that is heavier because it also carries a battery and an electric motor. In some cases, this can result in even higher fuel use than a similar gasoline-only car.
The researchers also found that vehicle design makes a difference. Factors such as the car’s weight, the size of the battery and the way the hybrid system is designed all affect fuel use and overall efficiency.
Lighter vehicles with moderately sized engines and well-balanced batteries generally performed better than heavier models.
To better understand why real-world fuel use is often higher than expected, the team tested 12 modern plug-in hybrid cars in a laboratory.
They measured electric driving range, fuel consumption, carbon dioxide emissions and other pollutants under different conditions. The tests included normal temperatures of 23°C, cold weather at -7°C, and cold weather with the cabin heater turned on.
They also examined the effects of more aggressive driving.
The results showed that plug-in hybrids perform well under ideal conditions. When the weather is mild, the battery is fully charged and driving is gentle, the vehicles can travel long distances using electricity alone while producing very low emissions.
The picture changes in everyday conditions. Cold weather reduces battery performance, while heating the cabin uses additional electricity. Faster acceleration and more demanding driving also drain the battery more quickly.
As a result, the gasoline engine starts earlier and runs more often, increasing fuel consumption as well as carbon dioxide and air pollutant emissions.
The researchers also looked at how plug-in hybrids are officially tested in Europe. These tests use a value called the “utility factor,” which estimates how much of a vehicle’s driving is done on electricity. The European system has recently been updated because real-world data showed that many drivers use electric mode less often than earlier estimates suggested.
Using Swiss travel data, the researchers found that the electric driving share in Switzerland may actually be higher than the European average because Swiss drivers typically travel shorter daily distances. Private plug-in hybrids are also more common than company cars, which are often charged less frequently. However, the researchers noted that their calculations assumed drivers charged their vehicles every day, so actual results may vary.
The study concludes that plug-in hybrids can still play an important role in reducing fuel use and emissions, but only when they are used as intended. Regular charging is essential to get the full benefit of electric driving.
Expanding reliable charging at homes and workplaces, encouraging regular charging habits and creating incentives that reward using electricity instead of fuel could help these vehicles deliver the environmental benefits they were designed to provide.
