What's the easiest way to get a kid to eat broccoli? Just tell them they can't have any. It's one of the tried and true tricks of parenting, because wanting what we can't have is one of those primal human instincts. But desiring something that's been forbidden to us is made all that much worse when it's something good, like cake and ice cream. In this case, feel free to decide for yourself whether the following list of cars are green veggies or dessert, but know that you can't buy any of them. These are real cars, ones that get amazing fuel economy, but for a variety of reasons none are available for purchase in the United States – at least not yet. Volkswagen Polo BlueMotion

VW's Polo was named World Car of The Year for 2010 and its BlueMotion diesel models were named this year's World Green Car. The accolades are unsurprising, given that the Polo BlueMotion is capable of over 73 miles per gallon and can go 846 miles on a single tank of diesel fuel.

This high-mileage version of the five-passenger subcompact is powered by a turbocharged, direct-injection, 1.2-liter three-cylinder engine making just 74 horsepower. To conserve fuel, the engine shuts off when the vehicle comes to a stop, like in a hybrid. Other fuel-saving features include low rolling resistance tires and a rear spoiler to improve aerodynamics. The Polo BlueMotion also has a limited type of regenerative braking system that uses the alternator to eke out further economies.

The Polo is available as a three-door or five-door hatchback in Europe, where it's a popular seller, and a new sedan version was recently introduced in Russia. So far VW has been coy about its plans to sell any model of Polo here in the U.S. We think that if the German brain trust is to be convinced that we'd like a nattily attired subcompact stateside, pressure may come in the form of the recently-reintroduced-to-America Ford Fiesta, one of the Polo's chief overseas rivals.

The chances of VW offering the BlueMotion diesel seem slim, however, as it's a costly model. In the U.K., for example, it sells for over eight percent more than the regular diesel Polo, and only the Polo GTI sits atop it in the model range. Americans are not usually too keen on buying expensive small cars, so if we get a Polo, it's much more likely that it will be one of the gasoline-engine versions, which would be a shame.

Mini E

BMW just wrapped up a yearlong trial that saw 450 of these electric Mini Coopers zipping around New York, New Jersey and Los Angeles. While many were leased to private individuals, and by all accounts the Mini E is ready for prime time, BMW does not plan to make it available anytime soon. It's a shame, really, as the 35 kWh lithium-ion battery pack gave the Mini E a reported range of 70-100 miles per charge, according to results of a survey of drivers conducted by the University of California Davis.

"What they shared with us is that, for the most part, the MINI E suits their daily driving needs and that they really enjoy driving it. This makes us optimistic that electric vehicles have a role in the future of mobility in America by being a part of the overall vehicle mix," said a statement by Rich Steinberg, Manager – EV Operations and Strategy for BMW of North America.

The Mini E uses the standard production vehicle as its basis, but its internal combustion engine is replaced by a 201-hp electric motor. BMW installed 220-volt charging systems in the garages of its trial participants, which allowed for a full charge in less than three hours. The downside to the Mini E is that the rear seat had to be jettisoned to make way for the battery pack.

BMW has said it will offer some Mini E drivers lease extensions, but the company's electric vehicle plans do not seem to include Mini, at least not in the near future. BMW will hold another, larger EV trial in 2011, but this new model will wear a BMW badge. The ActiveE is based on the 1-Series, and represents the next step in BMW's EV strategy, which is directed at producing a city vehicle. Set to launch in 2013, we bet it won't be nearly as charming as the Mini E.

Audi A1 e-tron

With the A1 going on sale this summer in Europe, it will no doubt attract plenty of attention from those fans of Minis and Fiestas who wish there were more subcompacts in the U.S. market. Based on VW's Polo, the A1 is a lot like the Audi A3 writ even smaller, which means it's a premium small car, the likes of which Americans have shown little appetite for. So Audi has no plans to bring it to the U.S. anytime soon, and while that's unfortunate, even more galling is that Audi seems just as unlikely to ever produce the A1 e-tron concept it showed this year's Geneva Motor Show.

Like the forthcoming Chevy Volt, the A1 e-tron is an electric vehicle that also carries a gasoline-powered internal combustion engine aboard, to recharge the batteries once their range is exhausted. The A1 e-tron is novel among this growing field of plug-in, series-hybrid vehicles in that it employs a Wankel engine as its "range extender." The Wankel is a rotary engine design popularized in the 1980's by Mazda in its RX-7 sports car. While the current Mazda RX-8 also uses a rotary engine, the technology has never really caught on in the mass market, despite packaging advantages including the engine assembly's low weight and compact size.

Those qualities would seem to make the Wankel a good fit for a subcompact hybrid like the A1 e-tron. In the concept, the 61-hp electric drive motor was fitted under the hood in place of the gasoline engine, while the Wankel engine was mounted in the rear, behind the under-seat battery pack. Audi says the battery pack's 12 kWh capacity would be good for a 30-mile range before the gas engine kicks in, extending the range another 100 miles.

Fiat Panda Natural Power

Earlier this year, when the German Automobilclub (ADAC) set out to determine the most economical car to own, it emerged with might seem an unlikely victor: The Fiat Panda Natural Power, a bi-fuel car that runs on either gasoline or compressed natural gas (CNG). The test was about as simple as it gets: How far can you drive on 30 Euros of fuel (about $37)? The club tested 241 cars, including both high-mileage stalwarts like the Toyota Prius, and gas-guzzling performance machines like the Chevy Corvette. The Panda Natural Power managed 450 miles on its allotment of CNG, handily beating the Prius' 290 miles and going nearly four times as far as the Corvette, which managed just 112 miles.

If you're scratching your head trying to picture the Panda, understand that this is one of those European vehicles the likes of which we just don't see in the U.S. market, sort of a subcompact crossover like the Suzuki SX4. But the Panda is highly successful overseas, winning European Car Of The Year in 2004. Because of its high roof and considerable cargo capacity for its size, the Panda is often employed as a commercial vehicle, including use by police and postal services. Fiat sells a four-wheel-drive version of the Panda, and it's that chassis that's used for the Natural Power model, with the natural gas tanks taking up residence in the rear, where the extra drivetrain pieces would normally reside.

As to the Fiat Panda Natural Power's prospects for finding a path to the U.S., don't hold your breath. Fiat is highly unlikely to market this car here, regardless of plans to sell its European cars through the Chrysler dealer network. While there was a big push to deploy CNG fleet vehicles by the federal government in the 1990s, there's only one CNG car left for sale in the U.S. market today (the Honda Civic GX), meaning the technology is basically a nonstarter here. This can be partly explained by our gasoline prices, which are low when compared to Europe, where gasoline routinely sells for over $6 a gallon -- one of the factors that helped the Panda Natural Power achieve its victory in the ADAC test.

Porsche 918 Spyder

Let's start by saying that Porsche is claiming 78 miles per gallon from this 718-horsepower beast, a number that seems to defy the laws of physics. But the hybrid 918 Spyder is still a concept car, so perhaps we need to take the engineers in Stuttgart at their word. Given that the car would easily be the most expensive hybrid ever created, perhaps the 918 Spyder will answer the "What if money were no object?" question that many green car fans, no doubt, think about as they drift off to sleep each night.

Porsche showed the 918 Spyder at this year's Geneva show, teasing gearheads with some impressive claims: 3.2-seconds from 0-62 mph and a top speed of 198 mph. Even more tantalizing is that Porsche claims this hybrid racer can lap the Nurburgring racecourse faster than its Carrera GT supercar ever did. Motive power is provided by a mid-mounted V8 and two electric motors, one for each axle. Porsche claims when driven in electric mode the 918 Spyder will have a range of 16 miles, in addition to three distinct hybrid modes, including a race mode that uses the lithium-ion battery pack to provide an immediate power surge that's activated by a "push to pass" button on the steering wheel.

Porsche has said it will only build the 918 Spyder if it can get 1,000 takers, and recent reports say it's close. At the risk of sounding like a public radio station during its summer pledge drive, we would urge those who can afford it to please call now.

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