With rocketing fuel prices and fears that oil supplies will dry up within 50 years, petrol-powered cars are starting to lose their lustre. So, what will the cars of the future run on?
It was during the last oil crisis in the 1970s that motor manufacturers seriously began to consider alternative fuels. The electric car enjoyed a brief vogue, but development of the idea was beset with problems - the cars lacked range, and batteries were both heavy and costly.
As oil prices head up to record highs once again, the range of options for alternatively-powered cars is far wider.
Conceived of in the 1840s, the fuel cell still looms on the horizon as the automotive El Dorado. Not only do they mean quieter engines - there is no combustion - but, if powered by hydrogen, they emit nothing more harmful than water vapour.
Even with the fuel cell, there's no guarantee all cars will look like this
Yet despite the millions invested in research and development, serious obstacles remain. Liquid hydrogen does not store easily. The cells are slow to warm up, and performance still lags behind that of petrol engines.
In addition, the process of extracting hydrogen from water uses huge amounts of (fossil fuel) energy and is polluting.
Auto industry expert Professor Garel Rhys says we are at least 10 years from a significant breakthrough. "General Motors has spent a billion pounds on fuel cell technology but the cost needs to be reduced by 80% if they are to rival petrol engines."
If Hollywood leads by example then hybrid cars are set for a glittering future. Stars such as Billy Crystal, Harrison Ford and Susan Sarandon have been seen riding in the Toyota Prius, which harnesses power from its petrol engine to feed an on-board electric motor. The result is greater fuel efficiency.
But there's a long way to go before hybrids match ordinary cars in price, says Prof Rhys. "In the world of Newtonian physics you get nothing for nothing, so for a Fiesta-sized car you pay a Focus price. It's the cost of the extra engine."
Additionally, hybrids are only about as efficient as diesel cars - appealing in the US where diesel is rare, but less so in the UK.
More popular in the UK are hybrids that combine petrol with low-emission gas fuels such as liquid petroleum gas.
In the 1950s Devon farmer Harold Bate developed a "digester" which turned decomposing livestock droppings into methane on which he ran his car and truck for next to nothing. While Bate's idea never caught on, many believe it highlights the potential nature has to provide us with cheap, clean and plentiful alternatives to petrol.
That's not all it's good for
Scientists have begun a £100,000 study looking at making bio-ethanol from Scottish heather. Bio-ethanol is already mixed with petrol in France. And in Brazil, where maize husks are used, some cars run on little else.
In the UK, innovative motorists who started using cooking oil, another bio-fuel, were arrested for evading tax. But a number of firms are providing legal versions of the fuel, which can entirely replace diesel and is readily available from restaurant kitchens.
"The diesel engine was originally designed to run on peanut oil anyway," says Martin Brook of Biofuel.org.uk. "There's a smell to bio-diesel a bit like a kitchen fryer, but you would only notice it if you put your nose to the exhaust."
It sounds like a joke, but in recent years vehicles that run on air have been appearing in cities around the world. In fact they use compressed air as a fuel and are described as non-polluting by their inventor, French engineer Guy Negre.
The compressed air car
The air is stored in tanks which, when released, drives pistons which propel the vehicles at up to 75mph for up to 120 miles. While the idea has helped cut pollution in cities such as Mexico City and Cape Town, critics point out that electricity is still needed to compress the air.
Another apparently free alternative, solar power, has also failed to catch on. Enthusiasm for the idea remains strong at research level, with solar-powered car races frequently attracting competing teams of scientists.
But in the UK, the lack of sunlight makes it a non-start, says Jacinta McDermot, of the Centre for Alternative Technology.
Should a time come when just about all fuel reserves dry up, drivers may find some solace in the works of Leonardo da Vinci.
What a wind-up: Da Vinci's clockwork car
Among the celebrated genius' ground-breaking inventions was a drawing of what would have been the world's first self-propelled vehicle - a "clockwork car".
Da Vinci's wooden cart would have been powered by springs wound in the opposite direction to the one in which the driver planned to travel. A rudimentary braking system was also incorporated.
Had Leonardo also invented the first assembly line and car showroom in the 15th Century, the history of travel may have been quite different.
Professor Paolo Galluzzi, who headed a team which recently built the vehicle from da Vinci's blueprint, said: "It is a very powerful machine. It could run into something and do serious damage."