 Solution

A plug-in hybrid electric vehicle can move short journeys to electricity while retaining liquid-fuel range for longer ones. It can reduce oil dependence and some local tailpipe emissions, but the carbon benefit depends on charging, trip length, electricity supply, vehicle manufacture and the fuel used when the engine runs.

For illustration, electricity consumption $0.20\mathrm{kWh}\mathrm{k}{\mathrm{m}}^{-1}$ at grid intensity $500\mathrm{g}\mathrm{C}{\mathrm{O}}_2\mathrm{kW}{\mathrm{h}}^{-1}$ gives $100\mathrm{g}\mathrm{C}{\mathrm{O}}_2\mathrm{k}{\mathrm{m}}^{-1}$ in use. A petrol vehicle using $0.07$ litres per km emits about $158\mathrm{g}\mathrm{C}{\mathrm{O}}_2\mathrm{k}{\mathrm{m}}^{-1}$ from combustion under the paper's octane-density assumptions. Ignoring other life-cycle differences, their use-phase break-even electricity intensity is therefore roughly $790\mathrm{g}\mathrm{C}{\mathrm{O}}_2\mathrm{kW}{\mathrm{h}}^{-1}$. Cleaner electricity improves the electric case; sufficiently carbon-intensive electricity can reverse it. These are conditional comparisons, not measured fleet averages.

A plug-in hybrid that is rarely charged may obtain little benefit while carrying both propulsion systems. Battery and fuel production also belong in a life-cycle assessment. Fully electric vehicles, efficient conventional vehicles, public transport, walking, cycling and reduced travel demand provide alternative or complementary routes. Plug-in hybrids can be a useful transition technology, but no physical argument makes them an essential component of every successful climate strategy.