Where do they perform most efficiently?

By Jack StewartFeatures correspondent

Hybrid cars are good for the environment, right? Their ability to switch to battery power means more miles to the gallon, and lower greenhouse gas emissions.

These things are true, however recent research suggests that precisely how green hybrid vehicles are may depend on traffic levels, road design and, perhaps most intriguingly, national driving styles. It also shows these vehicles provide significantly greater environmental benefits in cities in India and China, where there are hardly any hybrids, than they do in places like Tokyo and Los Angeles, where they are most common.

While sitting in a traffic jam in India, Anand Gopal and his colleagues at the US Department of Energy’s Lawrence Berkeley National Laboratory began thinking about how much driving conditions vary from country to country. They decided to investigate real-world conditions and the performance of different systems that deliver power to vehicles.

The fuel economy tests that generate the mpg or l/100km figures we see quoted are based on driving patterns associated with cities or motorways. Yet the lab tests used have been shown to favour certain types of vehicles over others, and don’t reflect real driving styles.

To get a more realistic picture, Gopal’s team began by creating computer simulations of a standard, internal combustion engine vehicle and of hybrid models that are popular in their three countries of interest – the Buick Excelle in China, the Maruti Alto in India, and the Toyota Prius for the US.

Finding real driving data for their Asian countries proved difficult but they eventually did so for Pune and New Delhi in India, and for 11 Chinese cities. 

When the computer generated vehicles were “driven” according to the real world driving data, the hybrids generated fuel savings of 48% in India and up to 55% in China, compared with around 40% in the US.

Why the discrepancy? At low speeds, such as found in many cities, the internal combustion engine is inefficient, and so in the hybrids the electric motor took over. Energy recovered through regenerative breaking – when the electric motor is allowed to run backwards as a generator when the car is slowing – was, as expected, the main reason why they hybrids were much more efficient.

The second most important factor surprised the researchers. “We forgot about the aggressiveness of the driving styles,” says Gopal. “Dense traffic and aggressive driving styles favour hybrids.”

In India and China, driving involves a lot of accelerating and braking – which can both be done more efficiently with an electric drive train versus a petrol engine.

Although a major road in Los Angeles or London may be a pain to get through at rush hour, it does not require the levels of hard, emergency, braking required in New Delhi, Gopal says. Drives that include more time in traffic jams and fewer motorways also generated greater benefits from hybrids.

Over seven million hybrid vehicles have been sold globally since they were introduced to the mass market in 2007. Toyota sells the most, and 2013 figures from the company show that Japan and the US are the big buyers, with European countries (France, UK, Netherlands, Germany) behind them. Hybrids represent just a small fraction of cars sold in most of the rest of the world.

Gopal’s results could help the Indian government in devising policies to help it achieve its goal of getting up to seven million hybrid vehicles on the road by 2020. His group hopes to publish the results of comparisons with other types of vehicles, including diesels, soon.

There are also lessons for those who do not drive a hybrid. If you want to save fuel and reduce carbon emissions avoid aggressive driving. And don’t live in a major city in China or India.

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