Are Plug-in Hybrid Cars Worse For Environment Than Factory Tests Suggest?

Are plug-in hybrid cars worse for environment than factory tests suggest? It depends how you drive them

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Currently accounting for 3% of new car sales, plug-in hybrid electric vehicles are sold as low-carbon alternatives to fossil fuel and conventional hybrid cars. But a new report threatens to shatter their green credentials.

Research from the pressure groups Transport and Environment and Greenpeace has claimed that CO₂ emissions from plug-in hybrid cars are “two and a half times” higher than tests by manufacturers suggest. While these official figures place the average emissions from plug-in hybrid vehicles at 44g of CO₂ per kilometre, the new report argues that it’s more like 120g on roads.

So why is there such a large discrepancy, and which one is correct? Are customers being misled about the environmental impact of the cars they drive?

How carmakers measure emissions

A plug-in hybrid electric vehicle has both a petrol engine and battery, either of which can power the wheels. Unlike a normal hybrid vehicle, where all the energy comes from the fuel, a plug-in hybrid has a larger battery and can be plugged into the mains to charge. Because a plug-in hybrid has two energy sources, petrol and electric, its emissions will vary widely depending on how much time is spent in full electric or petrol mode.

To find out how far a plug-in hybrid car can travel without switching the engine on (known as “the all-electric range”), car manufacturers in the UK and EU use a test called the Worldwide harmonised Light-duty vehicles Test Procedure (WLTP). For a plug-in hybrid car, the battery is fully charged and then driven on a repeated 30-minute cycle under controlled conditions on a dynamometer, which is a bit like a treadmill for cars.

During each cycle, the battery is slowly discharged as the car uses energy from both the battery and the engine. Eventually it reaches a point when the battery charge at the beginning and end of each 30-minute cycle is the same. This is referred to as charge-sustain mode and is the point at which all the energy comes from the petrol engine and the battery is essentially empty. Manufacturers calculate their official emissions figures based partly on emissions during the time when the car is using petrol and electricity, and partly when the car is running on petrol only.

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This article is republished from The Conversation under a Creative Commons license.

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