Hybrid engines powered by electricity and petrol have been around for years. But it took a kick from rocketing gasoline prices to encourage large numbers of Americans to see their fuel-efficient appeal.

Toyota began selling the Prius in North America in 2000. It is now the best-selling hybrid in the United States, helped in no small part by the sight of Hollywood stars such as Brad Pitt and Cameron Diaz behind the wheel of one.

Up to the end of November, Toyota said it had sold 99,000 Prius cars this year compared to 47,700 over the same period of 2004.

According to research firm Global Insight, total US sales of hybrids are set to more than double to 200,000 this year and mushroom to 500,000 a year by

Toyota has led the way with the Prius and the four-by-four Highlander, designed to appeal to Americans' taste for sport utility vehicles (SUVs). Its Japanese rival Honda has three hybrid models and lies second in sales.

US giants General Motors and Ford are now ramping up their own hybrid production but came late to the game. The Ford Escape Hybrid made its debut in mid-2004 as the first US-made example of the genre.

GM and Ford remained wedded for too long to petrol SUVs and pick-up trucks, whose sales have slumped this year as Americans shun gas-guzzlers.

An average SUV consumes about 20 litres (five gallons) of petrol over a 100 kilometre (62 mile) trip, compared to a hybrid which will sip just four to five litres (1.1 to 1.3 gallons) of gasoline.

Ford's chairman blames Japanese government intervention for the sales advantage enjoyed by Toyota and Honda.

"Nearly a decade ago, the government offered subsidies to their domestic auto suppliers to build hybrid batteries, which are one of the most expensive components of today's hybrid vehicles," Bill Ford said in late November.

"That gave them a head-start."

Arguing that hybrids will help the United States lessen its reliance on Middle East oil, the Ford chairman has been pushing the government to adopt tax breaks for buyers and fiscal incentives for manufacturers.

Federal and state incentives are in the offing. But hybrids will remain comparatively expensive. Rebecca Lindland at Global Insight said an hybrid is on average 3,500 dollars dearer than its equivalent petrol-powered model.

The electric batteries that lie at the heart of a hybrid engine are made primarily of nickel, whose price has rocketed this year in line with the hybrid boom.

"And the owner experience is not as positive as the media may show it to be," Lindland said.

"If you are commuting on a highway you're not getting the good gas mileage. The hybrid is not the saviour of the automotive industry," she added.

Among alternatives to gasoline, diesel engines have not gained widespread acceptance in the United States because of tight emission standards and consumer perceptions that they are smelly, loud and slow.

Fuel-cell vehicles, which are powered by hydrogen and emit nothing but water from the exhaust pipe, are still years away from being commercially viable.

Japan's second-largest automaker, Nissan Motor, has remained wary of jumping on the hybrid bandwagon. But most analysts see the boom continuing.

Sales of hybrids and clean diesel autos are on course to take 11 percent of total US sales by 2012, from 4.8 percent this year, according to JD Power-LMC Automotive Forecasting Services.

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