New research suggest that CO2 emissions from plug-in hybrid cars are as much as two-and-a-half times higher than official tests suggest.
Plug-in hybrid vehicles are powered by an electric motor using a battery that is recharged by being plugged in or via an on-board petrol or diesel engine.
They account for approximately 3% of new car sales.
But recent analysis from pressure groups Transport and Environment and Greenpeace suggest their emissions are different from official testing:
They suggest they emit an average of 120g of CO2 per km.That compares with the 44g per km in official “lab” tests
Plug-in hybrid electric vehicles (PHEVs) are sold as a low-carbon alternative to traditional vehicles and conventional hybrids – which cannot be recharged from an external source – and are proving increasingly popular.
The new research is published at a time when EU legislation considers whether to bring forward a proposed sales bans of new petrol, diesel and conventional hybrid cars from 2035 to 2030.
Official versus real world
One suggestion is that plug-in hybrids should be given a stay of execution, with new sales allowed to continue until 2035. That’s because they can offer a 20- to 40-mile range as a purely electric vehicle and are therefore potentially significantly less polluting than other vehicles.
But the new analysis from Greenpeace suggests they don’t offer anything like the carbon dioxide savings claimed for them by manufacturers. The pressure groups have analysed what they say is “real-world” data on fuel efficiency collected from some 20,000 plug-in hybrid drivers around Europe.
These are drivers who have chosen to record their mileage and fuel consumption for surveys or who drive company or leased vehicles whose fuel efficiency is recorded.
According to this data-set the lifetime emissions of a plug-in hybrid average around 28 tonnes of CO2.
By comparison, the average petrol or diesel car is estimated to emit between 39 and 41 tonnes of CO2 from fuel during its lifetime, a conventional hybrid would typically emit more like 33 tonnes.
According to these figures a plug-in hybrid would only deliver an emissions reduction of about a third on a typical petrol or diesel car – far less than the official estimates.
The motor industry acknowledges that lab tests don’t always reflect real-world use but criticised the report, saying it uses emissions data from a test that is two years old.
What about driver behaviour?
Common analysis suggests a key problem with plug-in hybrids is that many owners rarely actually charge their cars, meaning they rely on the petrol or diesel engine. Charging cables are often returned unopened after a lease.
Another is that many plug-in hybrid models include design features that automatically turn on the petrol/diesel engine at start-up on a cold day, or will kick in that engine if driver accelerates hard.
The latter mode means that the car’s emissions will depend a lot on the driver’s behaviour.
Equally, more aggressive drivers will impact fleet cost through increased tyre and brake wear for example.
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A quick guide to the pros & cons of different vehicle powertrains
|Great around town centres||Not so economical on motorway|
|No need to plug in to charge||No longer exempt from road tax|
|Better economy than regular cars||Mild hybrids aren’t full hybrids|
|No ‘range anxiety’|
Owners need charging facilities
|Suits an urban lifestyle||Extra weight affects drive|
Engine’s economy not great
|Very low running costs|
Need to be plugged in to charge
|Zero tailpipe emissions|
Charging can take a long time. But improving all the time
|Relaxing to drive||Possible range anxiety|