Plug-in hybrid vehicles (PHEVs) are frequently billed as the tree-hugging environment-friendly technology of the future, at least until fully-electric vehicles become more widely available. However, a recent study by Fraunhofer ISI institute and the International Council on Clean Transportation (ICCT) shows that they might not be as green as we thought they were.

The study, which examined usage data from more than 100,000 PHEVs across more than 66 different models sold in Europe, the US, and China, found that at best, a PHEV produces average CO2 emissions that are twice as high as the carmaker’s claim, regardless of the test cycle it was measured in (NEDC or WLTP).

However, the real stinger is that at worst, they produce up to four times as much CO2 as stated on the spec sheet. That means that a PHEV with a “certified” 50 g/km CO2 emissions figure actually outputs 100 to 200 g/km of carbon dioxide into the environment during regular operation.

For context, a modern engine in a regular sedan vehicle like the Mercedes-Benz C200 only emits 146 – 156 g/km of CO2 during operation. And the figure goes even lower on a diesel-powered vehicle.

So, what gives? The biggest culprit to the higher-than-claimed emissions is the limited fully-electric range of these PHEVs, as well as only being recharged every other day on average. When PHEVs are driving around without extra electric propulsion due to the batteries being depleted, the internal combustion engines (ICE) will have to work extra hard to lug around the battery packs which are essentially dead weight.

Company cars are the number one contributor to the problem, as they are regularly driven distances that exceed the fully-electric range, and have less incentives to recharge the batteries as the employers are the ones who pay for fuel.

“If we want to see lower fuel consumption and CO2 emissions in real life, the engine’s horsepower needs to be reduced and the electric range increased,” said the study’s leading author, Patrick Plotz.

Plotz estimates that for a PHEV to replace regular ICE cars (in Germany at least) for environment-friendly reasons, they will need an all-electric range of 80-90 km instead of the 30-60 km as seen on most of these vehicles on sale today.

The study, being described as the first systematic analysis into the actual benefits of plug-in hybrids, essentially labels the current PHEVs as nothing more than “compliance cars” – which means they are fuel-efficient on paper but emit just as much CO2 as conventional models.

The findings are made even more controversial, as Plotz has previously published a study titled “Plug-in hybrid vehicles are better than their reputation”, which was cited by carmakers – in this case Volkswagen – as recent as this September.

However, Plotz claims that these carmakers fail to mention that the study was not designed to provide a comprehensive and definitive conclusion. “At the time, we looked solely at the number of kilometres driven electrically,” Plotz told Automotive News Europe. “We did not examine the deviation from the test cycle, and that is where these unflattering results can be found.”

The study calls for regulators to reduce the excessive incentives for PHEVs, especially for company cars – by only issuing the incentives to organizations that provide a sufficient workplace charging infrastructure or support employees’ efforts keep their cars charged.