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What’s the solution to the PHEV conundrum?

Real world emissions of plug-in hybrids depend entirely on how they are driven, but that’s not represented in the official figures. By Megan Lampinen

Plug-in hybrid electric vehicles (PHEVs) have been marketed as the best of both worlds: offering zero-emission electric propulsion for short journeys but with the comfort of knowing an internal combustion engine (ICE) is there to provide additional range if needed. On paper, their credentials are impressive, with most boasting official CO2 emissions figures of less than 50g/km. However, the real-world emissions of any PHEV will depend heavily on how it is driven. An owner that sticks to 5km a day and charges regularly can indeed realise the promised eco credentials, but another owner that uses it for a long-distance commute and doesn’t bother to plug-in will produce very different results.

“PHEVs could be great,” says Nick Molden, Founder and Chief Executive Emissions Analytics, speaking in a recent Mobex webinar. “The small size of their battery is a real advantage in terms of both CO2 and materials shortages. But without significant changes, PHEVs are too high a risk to allow into the market in substantial volumes.”

A high-risk segment?

The risk that concerns Molden and others is that PHEVs, which could arrive in large numbers as automakers look to meet CO2 targets, are used inappropriately, i.e. for long journeys that rely on the ICE. In this scenario, an automaker could point to PHEV sales figures and conclude that it was meeting CO2 targets. In reality those CO2 emissions could far exceed official limits.

As its name suggests, Emissions Analytics specialises in analysing vehicle emissions, and uses a portable emissions measurement system (PEMS) in real world driving tests. It evaluated 37 PHEVs and compared the findings to the vehicles’ official CO2 figures. The study found that the vehicles had virtually zero emissions when running on short journeys and charging regularly, but that operating in engine-only mode resulted in two- to three-times the certification values. Some of these real-world CO2 emissions were as high as 299g/km.

There are scenarios where frugal modern diesel vehicles, even non-hybrid ones, could have lower CO2 emissions than PHEVs

Similar results were published last year by the International Council on Clean Transportation (ICCT), which in September 2020 released analysis of its study into the fuel use by PHEVs in real-world operation. This much larger study, which drew on data from 100,000 PHEVs in China, Europe, and North America, concluded that fuel consumption and tailpipe CO2 emissions in real-world driving averaged two to four times higher than type-approval values.

In such a scenario of masses of PHEVs on the road emitting higher than advertised CO2, Molden suggests the industry would be better off with non-plug-in full hybrids or even modern diesels. “There are scenarios where frugal modern diesel vehicles, even non-hybrid ones, could have lower CO2 emissions than PHEVs,” he tells Automotive World.

Lifecycle emissions tell a similar story. Although the forecast emissions will vary depending on how clean the electricity grid is, modelling by Emissions Analytics leads Molden to conclude that “PHEVs are worse than ICEs if they are never charged up but better than full battery electric vehicles if always charged up.”

Schroedinger’s car

Molden describes the PHEV emissions situation as a conundrum and likens it to the famous ‘Schroedinger’s cat’ thought experiment, in which the fate of a cat trapped in a box with a 50% chance of being poisoned is not settled until that box is opened: it underlines the radical implications of quantum theories such as superposition, in which subatomic particles exist in all possible states till observed. Likewise, until Schroedinger’s box is opened, the cat is both dead and alive.

Emissions Analytics
PHEVs can be worse than BEVs and better than ICEs in some cases

With a slight adjustment, Molden presents his own thought experiment as ‘Schroedinger’s car’: “We can argue forever whether PHEVs are inherently good or bad. The answer is that they are both, until they are driven. The driving is the opening of the box, and it is only at that point that we know the verdict. We need to rip the lid off the box and see if the cat is alive or dead and then move on.”


The good news is that there are clear steps that can be taken to address this conundrum. To begin with, PHEVs could come with a label that shows both the electric-only range and CO2 emissions levels when the battery is empty. Governments could introduce incentives for owners to drive on battery power and penalties for driving on the engine. However, this approach would likely require data that could be regarded as a breach of privacy and Molden admits he knows of no government that is considering this seriously at the moment.

One of the most important steps he would like to see is a calculation of fleet average CO2 based on the actual share of driving on the battery. “This would encourage automakers to collaborate with governments to incentivise charging and not to market these vehicles at the wrong use cases,” says Molden. “It would align the industry more closely with regulators.”

We can argue forever whether PHEVs are inherently good or bad. The answer is that they are both, until they are driven

Ultimately, Molden’s message is for government and regulators more than automakers and consumers. “It’s on government to decide policies, and based on these policies manufacturers act accordingly. Manufacturers are private companies and have a fiduciary duty to their shareholders to optimise the business within the rules they are given. The danger is in setting political aspirations and rules that sound great but are undercut secretly behind the scenes, which is what happened with diesel.”

While the industry touted the fuel efficiency benefits of diesel and governments subsidised its purchase, many automakers were exploiting testing loopholes that resulted in much higher NOx emissions than thought. The dieselgate scandal rapidly put the fuel out of favour. “The danger is the same at the moment,” adds Molden. “PHEVS sound great but we may wake up in a few years and realise nobody charged them and we are stuck with CO2 emissions that are much higher than we thought. Unless the risk element is addressed and we move away from the idea that one number can encapsulate a technology, we set ourselves up for another emissions crisis a few years on.”


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