Who will find the winning formula as car companies race to produce a range of greener vehicles?
The major car companies have been lining up in recent months to show off their green credentials. At motor shows across the globe they have been making headlines proclaiming their ambitions to move away from the standard internal-combustion engine.
This is no surprise. It's not just that global warming is putting the spotlight on car emissions. The multinational companies are also waking up to the fact that regulators ranging from the Governor of California to the Mayor of London to the European Union in Brussels are seeking to clamp down on gas guzzlers.
In Europe, car makers are facing a deadline of 2012 to reduce emissions of the average new car from 160g/km (the 2006 figure) to 130g/km. Higher-emission vehicles are also being penalised with bigger road tax and congestion charges in the UK. Even US consumers are reacting to unprecedented oil price rises by seeking alternatives to big petrol vehicles (although not diesel engines, which appear to be anathema to American consumers).
These pressures have contributed to two key technological trends in the motor industry. One is a push for small, highly-efficient conventional petrol cars. US giant Ford, for example, recently announced that it was bringing its mini brand, the Fiesta, back to America; while Fiat of Italy has revamped its tiny 500 model.
The other key trend is the rise of the 'hybrid' vehicle. Last month's Geneva Motor Show, the biggest in Europe, was full of announcements about plans and aspirations to develop hybrid technologies, which involve using a combination of petrol and battery power. As well as the hybrid model, we also have the purely electric vehicles and even ones apparently running on air.
There is also hydrogen fuel cell technology, which turns hydrogen into electricity; and biofuel vehicles that run on ethanol produced from plants. Not forgetting that petrol cars have long been able to be converted to run on liquefied petroleum gas (LPG) which, while still a fossil fuel, is cheaper than petrol and emits less CO2.
The electric-powered vehicle was regarded just a few years ago as unlikely to become the frontrunner because of the problems of battery size and weight, power drainage, and low speed. Now, however, car battery technology has moved to the fore with the development of lithium-ion batteries.
To date, probably the most famous electric-petrol hybrid car has been Toyota's Prius, which has come to symbolise green motoring in parts of the world. But the Prius's nickel-metal hydride batteries have been overtaken by the recently improved lithium-ion version. Most of the major car companies are turning to lithium-ion as the next-generation battery.
Lithium-ion batteries are claimed to provide twice the power, energy density and lifecycle of nickel-metal hydride ones, but have less than half the weight and size, and half the cost. The attraction of the 'plug-in' hybrid petrol-electric car for the ordinary motorist is it can be recharged to power shorter journeys, while longer trips can draw on the petrol tank.
"[Lithium-ion batteries] are starting to turn the corner," says Prabhakar Patiel, head of Compact Power, which is helping to develop a lithium-ion battery for US group General Motors (GM). "For the first time with a lithium-ion, we see a battery that can make the technology work."
German car group Daimler recently announced that it would be the first company to put a lithium-ion battery powered car on the market. Next year, it will launch an S-Class Mercedes with the battery, which has been developed by auto technology group Continental. BMW, another German giant, recently unveiled its 'Vision EfficientDynamics' concept vehicle, a so-called 'mild' hybrid that combines a diesel engine with an electric motor, lithium-ion battery and roof-mounted solar panel.
Meanwhile, GM says it will produce 100,000 cars a year powered by lithium-ion batteries, starting in 2010. The batteries will be made by Hitachi.
Toyota has also indicated that it plans to begin mass production of lithium-ion batteries for cars by 2010, with batteries supplied by Matsushita. Japanese rival Nissan already has a joint venture with technology group NEC to produce lithium-ion batteries, while Honda plans to launch a new hybrid car next year. Honda says hybrids could account for 10 per cent of its global sales in 2010.
One key problem that the lithium-ion battery makers and car companies have overcome is the heat of the batteries. Daimler says it has integrated the lithium-ion battery into a vehicle's climate control system, ensuring the battery works at optimum system temperatures of between 15°C and 35°C.
Industrial conglomerate GE announced recently that it would invest $20m in US lithium-ion battery maker A123Systems, which will supply batteries to Norwegian electric car maker Think Globally. Johnson Controls of the US and French battery producer Saft are opening a new factory in France dedicated to producing lithium-ion batteries. Customers include GM, Mercedes and Chrysler.
"The market for lithium-ion batteries has probably one of the biggest growth areas in the future," says Thomas Weber, Daimler's head of technology.
But lithium-ion isn't the only key technology that auto companies are looking at. Honda is among those also developing hydrogen fuel cell technology, which converts hydrogen to electricity. The Japanese company is launching the FCX Clarity in the US later this year.
Daimler and Ford last year set up a joint venture to develop hydrogen fuel cells, through the shared acquisition of troubled Canadian fuel cell business Ballard Power Systems, which had been struggling commercially. BMW is also pursuing fuel cell technology.
Some analysts argue that hydrogen fuel cell technology is still far too expensive, with some estimates putting it at 100 times the cost of an internal-combustion engine. There are also doubts about the viability of purely electric cars, which may suit inner-city driving needs (short distances and moderate speeds), but not much else.
At the Geneva Motor Show, Think Globally unveiled the Ox concept electric car which, it says, will have a range of 125 miles on a single charge and a top speed of 85mph. US electric car maker ZAP says it is developing "a new generation of vehicles using advanced nanotech batteries".
If you go small enough - 350kg per car, to be precise - you'll soon be able to buy a city vehicle that runs on air. The OneCAT, developed by French group MDI and backed by Tata Motors of India, uses compressed air to power a piston engine. It could be launched in India by next year.
It seems that, despite the current high profile of lithium-ion hybrid systems, it is still unclear which single technology will take off as the key driver of green vehicle development. Perhaps it will be horses for courses - a variety of technologies, depending on type of car usage.
What isn't in doubt is that green driving is here to stay.