LONDON • Cars are not going away any time soon.

So, the ultimate green move will be combining two emerging technologies: autonomous driving and fully electric cars.

Autonomy brings efficiency in driving and battery use, while electric cars drastically cut emissions and fuel costs and require less maintenance.

Even so, a rift has developed within the car industry about the best way to move forward. Sceptics believe self-driving technology will have a serious impact on an electric car's range and degrade batteries; optimists think improvements in technology will likely mitigate much of that trade-off.

The split has manifested in established automakers choosing different paths to pursue.

Ford Motor is planning to launch self-driving tech in 2022, but in hybrid cars first. The company's testing found computers and sensors powering self-driving could cut the electric car's range by more than 50 per cent.

Meanwhile, the only autonomous cars Volkswagen plans to build are fully electric vehicles. The company will begin field tests that same year.

A new study in journal Nature Energy may lay the debate to rest. Dr Venkat Viswanathan, an associate professor of mechanical engineering at Carnegie Mellon University, finds that the range trade-off is likely to be less than 15 per cent for city driving and as low as 5 per cent for suburban driving.

"We're getting to a point where we won't need to choose between autonomous driving and electric cars," said Dr Viswanathan.

At the heart of the challenge of self-driving cars is that it is hard to replicate human understanding of sight and sound.

Even with the latest image-recognition technology, computers struggle to do what humans instinctively manage: understand the small but myriad changes happening on a road, in real time. Luckily, computers can use other inputs, such as radio waves (through a radar) or lasers (through a lidar).

Lidars, which you may have seen on prototype self-driving cars, are generally considered crucial. They need to protrude outwards to project lasers beyond the car body and thus "sense" surrounding obstacles even as the car moves at great speeds.

But the necessary bulge limits the aerodynamic shape of the car, increasing the amount of energy needed to drive and thus lowering how far an electric car can travel on a full charge.

Moreover, on-board computers need to analyse thousands of images every second that lidars capture to help make driving decisions.

Even with modern computing technology, that takes up a lot of energy. The combination of sensors and computers drains an electric car's battery more quickly than if a human driver were at the wheel.

Nevertheless, Dr Viswanathan found that lidars and computers are both getting better at their tasks, while consuming less and less energy.

In the case of computers, custom-built chips that can perform the specific image analysis needed for autonomous driving are able to lower energy consumption by 50 per cent or more compared with traditional graphics processing chips.

These are likely to keep getting better, which means Dr Viswanathan's estimates may be out of date soon.

Better still, companies like China's Contemporary Amperex Technology, the world's largest battery-maker, are developing batteries that can last 2 million km or 16 years.

That is a vast improvement on current batteries that have a warranty for 241,400km or eight years. If widely adopted, these newer batteries could mean car owners will not have to worry much about battery degradation.

And even if none of these technology trends plays out as expected, Dr Viswanathan found that computer-driven cars end up with energy savings of as much as 10 per cent simply by making better decisions about braking, accelerating and turning than human drivers.

The extra energy the computers and sensors consume could be balanced out by better driving - the direct result of the automation it enabled.

Regulators might not give automakers an option.

"Many of the tier-one cities where we expect to see self-driving cars are incentivising or requiring fleet cars to be zero emissions," said Mr Nick Albanese, head of intelligent mobility at BloombergNEF.

"Non-electric autonomous vehicles will likely be short-lived."

BLOOMBERG