Emission testing crisis shifts from diesels to plug-in hybrids

Powertrain electrification has been hailed as a likely solution to the transport industry's environmental challenges, but plug-in hybrid electric vehicles (PHEVs) are now facing scrutiny. Recent research casts doubt on the environmental benefits of these models, which combine an electric and a conventional combustion engine drivetrain. PHEVs can potentially tackle global greenhouse gas (GHG) emissions and local air pollution, provided they operate primarily on electricity. But there's limited evidence into just how much driving they do  in this mode. So what does that mean for their overall emissions?