How green are plug-in hybrids?

Plug-in hybrid vehicles tend to have lower CO2 emissions due to their improved fuel economy and their ability to part-operate in electric-only mode. Other emissions are also reduced as the engine loading is better managed than is the case in conventional petrol and diesel cars – or are zero at-point-of-use when in electric mode.

Climate change

When in hybrid mode, most plug-in hybrids reduce life cycle green-house gas emissions (such as CO2) by 15%-30% as compared to an equivalent non-hybrid model. This is principally due to the increase in fuel efficiency which results from hybridising the drive-train.

When in electric-only mode, plug-in hybrid cars charged using average UK 'mains' electricity show a significant reduction in CO2 – the figures suggest a reduction of around 40% compared to an small petrol car (tailpipe 130 gCO2/km). Renewable electricity reduces fuel life cycle emissions to almost zero.

Although highly dependent on the driving and fuelling routines of each owner, initial experience suggests that plug-in hybrid cars tend to be powered on electricity for around half of miles driven. The official test, however, assumes that the vehicle is operating on electric for around 80% of the time. Either way, the fact that plug-in hybrids are effectively electric cars for some of the time, and low emission hybrids for the remainder, means that overall, emissions are significantly reduced.

For example, the new Toyota Plug-in Prius and Vauxhall Ampera RE-EV achieves CO2 emissions of under 75 g/km – well below the emissions for an average conventional car (of around 140 g/km) and officially classing them as 'ultra-low carbon vehicles'. The CO2 benefits are particularly apparent in congested driving conditions where the electric mode is more often used, and the hybrid mode is able to utilise electric drive and regenerative braking to recoup some of the energy that would otherwise be lost on start-stop driving.

Local air quality

When in hybrid mode, all regulated emissions are significantly reduced for a plug-in hybrid passenger cars as compared to a conventional petrol and diesel models. These include reductions in carbon monoxide (CO), nitrogen oxides (NOx) and particulates (PMs).

The lower regulated emissions (when in hybrid mode) are a result of the hybridised drive-train which is able to operate the main combustion engine closer to its maximum efficiency for longer than is possible in conventional cars. The effect is to lower the engine loadings, which in turn reduces time on peak power when the majority of NOx and particulates are produced.

When in electric-only mode, plug-in hybrid cars charged using average UK 'mains' electricity can result in an increase in life cycle emissions of nitrogen oxides (NOx) and particulates (PMs). However, as these are emitted from power-stations which are well away from urban areas, their overall impact tends to be much less than when emitted from the exhausts of conventional cars. If renewable electricity is used, fuel life cycle emissions to almost zero.

Peter Thomas

Author:Peter Thomas
Date Updated:10th Mar 2015

Latest plug-in hybrid news