By Paul Hudson for CNN
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LONDON, England (CNN) -- The major theme of this year's early motor shows in Detroit and Los Angeles confirmed that "green" cars are finally credible in the U.S., in the wake of increased awareness of climate change and continued high fuel prices.
While development of the hydrogen fuel-cell as the probable motive force of the future, the technology of the moment is the petrol-electric hybrid, in which a petrol engine and electric motors work in tandem.
The benefits are zero emissions when the car operates in electric-only mode, while the petrol engine can be smaller due to its electrical assistance. A smaller capacity engine also means less carbon dioxide emissions, while battery charging is taken care of by the petrol engine.
Drawbacks include greater complexity and weight, then there's the issue of making and disposing of the batteries, not to mention limited range in electric-only mode.
But industry analysts say that hybrids are just token models. The reason for their increasing popularity in the U.S. is due to the Corporate Average Fuel Economy (CAFE) regulations -- the average economy of a car maker's range has to be 27.5 miles per gallon. This means that if a manufacturer sells a hybrid capable of 60 miles per gallon, it is allowed to sell less efficient cars -- typically SUVs and trucks -- that only manage 20 miles per gallon.
California is the undisputed hybrid capital of the world, due to a commitment to low-emissions regulations. The Los Angeles show featured offerings from the world's big four manufacturers of hybrid cars; Ford, General Motors, Honda and Toyota. Nissan also joined the party, showing a hybrid version of the Altima saloon.
Ford showed its Escape Hybrid, America's first hybrid-electric vehicle and the most-fuel efficient SUV on sale, delivering 36 miles per gallon in city driving.
General Motors introduced two of the four hybrid models it is launching this year, the Saturn Aura Green Line Hybrid and the 2008 Yukon Hybrid. The latter is only the second GM vehicle to be marketed with GM's new two-mode full hybrid system, which uses an electrically variable transmission with two hybrid modes of operation that optimize power and torque for various driving conditions. The first mode is for low speeds and light loads, while the second is primarily for highway speeds.
This system is also the starting point for a collaboration between GM, DaimlerChrysler and BMW.
At Detroit, GM launched its Chevrolet Volt concept car, the first mass market prototype designed to operate purely using an electric motor.
The vehicle will be powered by new-style lithium batteries, charged via a domestic electricity socket.
It could also feature a range of supplementary power sources including an ethanol-based petrol engine and, eventually, hydrogen fuel cells.
The Volt can run in electric-only mode for up to 40 miles. In a daily commute of 30 miles it would deliver an equivalent fuel consumption of about 150 miles per gallon using ethanol.
Honda, which reached a milestone of 100,000 hybrid sales in August 2005, launched an all-new version of the popular Civic Hybrid capable of 49 miles per gallon in urban driving.
The Civic, and the larger Accord Hybrid, are powered by Honda's Integrated Motor Assist (IMA) hybrid technology which uses an ultra-compact electric motor/generator to provide supplemental power to the engine using energy captured during deceleration or braking.
Toyota is a sales leader with its Prius, which combines a 1.5-liter petrol engine and an electric motor to give economy in the region of 60 miles per gallon in urban driving. The larger Camry hybrid, with similar Hybrid Synergy Drive, delivers an EPA-estimated 40 miles per gallon in the city.
Its Lexus luxury division is offering the LS600h, the world's first full V8 hybrid. Although it's a large, luxury car, thanks to the assistance of an electric motor the 600h is claimed to deliver the performance of a V12 petrol engine with emissions 70 per cent lower than equivalent V8-powered saloons.
The all-new Nissan Altima Hybrid, Nissan's first entry into the hybrid petrol-electric vehicle segment, made its debut in Los Angeles. It features a 2.5-liter, four-cylinder engine and electronic Continuously Variable Transmission (eCVT) with an electric motor and generator that combines greater economy (an estimated 41 miles per gallon around town) with low emissions.
At Detroit, Mazda unveiled a hybrid version of its Tribute SUV, its first production hybrid vehicle, which goes on sale in the U.S. later this year. With a combined power output from its petrol engine and electric motor of 155bhp, the Tribute HEV meets both the strict Super Ultra Low Emissions Vehicle (SULEVII) and the Advanced Technology Partial Zero Emissions Vehicle (AT-PZEV) standards in California -- the world's strictest emission regulations for a petrol-engined vehicle.
Hybrid taxis are unveiled in New York City. At Detroit's Auto Show earlier this year, U.S. automakers highlighted the latest technology in hybrid models