With a constant threat of gas prices rising and individuals looking to be more “green,” car shoppers are putting fuel efficiency higher on their list of considerations.

That’s good news for automakers that have invested across the board in developing fuel-efficient models, including a fast-growing number of gasoline-electric hybrids.

A new generation of the Toyota Prius went on sale in the spring. It’s still king of the hill in U.S. hybrid sales, but it has new competition in an all-new Honda Insight hybrid. Luxury brands, including Mercedes-Benz and Lexus, have also said they are adding more hybrids to their mix of vehicles.

Hybrids today have a conventional, internal-combustion engine and an electric motor that runs on battery power.

Hybrids can drive on battery power, gasoline-engine power or both. When the battery needs recharging, the conventional motor acts like a generator to recharge the battery. Some energy to recharge the battery is also recovered during braking. When a hybrid’s special “regenerative” brakes are applied, they, in effect, spin a small generator.

The energy needed to drive today’s hybrids, including the charge in the battery, ultimately comes from gasoline and the on-board gasoline engine.

In the works now are so-called “plug-in” hybrids that will have the capability of recharging from an on-board conventional engine or from an ordinary household current, according to General Motors. A household current is a much cheaper way to buy energy than recharging from a gasoline engine that’s acting as a generator. The Chevrolet Volt, which goes on sale in late 2010 as a 2011 model, is expected to be the first mass-market, plug-in hybrid, according to GM.

Hybrids rely more heavily on battery power at start-up and at low speeds, so unlike conventional cars, most hybrids get better mileage in city driving than in highway driving, according to EPA estimates.

In addition to gasoline-electric hybrids, other fuel-saving technologies on the market today include improved diesel-engine cars. Before last year, diesels couldn’t be sold in California or other states with tougher emissions regulations, including the Northeast’s large markets.

That meant it wasn’t a paying proposition for European — read, German — automakers to offer diesels in the U.S., even though diesel engines regularly get about 25 percent to 30 percent more miles per gallon than gasoline engines, according to German carmaker Volkswagen AG.

According to a 2002 statement from John Dingell of the House Energy and Commerce Committee, those same clean-burning diesels are estimated to account for 75 percent of light-duty vehicles sold in Europe in 2010.

A couple of additional factors have kept clean-diesel technology from catching on in the U.S. before now. First, fuel prices are several times higher in Europe than they are here, which makes Europeans much more concerned about fuel economy than Americans. Second, government regulations in Europe in 2005 mandated cleaner diesel fuel, with less sulfur content than the diesel fuel sold in the U.S. In contrast, California led the way here, targeting toxic nitrogen oxide and so-called particulates, which contribute to smog but were not covered by Europe’s standards.

Meanwhile, business conditions in the U.S. market have the potential to become more diesel-friendly. U.S. fuel prices spiked last fall, making U.S. consumers more conscious of fuel economy. In addition, the price of diesel fuel relative to regular gasoline has come down in the last year. A year ago, diesel was not only much pricier, it was more expensive per gallon than premium gasoline. Today, diesel is about the same as regular gas.

The U.S. government joined Europe in adopting cleaner, low-sulfur diesel fuel standards in 2006. Also, German automakers came up with better diesel emissions controls, which started reaching the U.S. market last year. So today, diesels can be sold in all 50 states. If diesel catches on in the U.S., that could be an advantage for German brands such as Volkswagen over those from Japan, as diesel isn’t popular in Japan.

At the same time that Americans are demanding better fuel efficiency, the U.S. government is hiking Corporate Average Fuel Economy, or CAFE, standards to 35 miles per gallon by 2020, an increase of 40 percent from the current level.

Those forces have the car companies pursuing fuel efficiency on many fronts, even if the gains in fuel efficiency aren’t as dramatic as hybrids. For instance, direct-injection gasoline engines get more power out of a given amount of fuel, and Ford and GM say they are both rolling out direct-injection gasoline engines in pursuit of better gas mileage and performance.

Direct injection forces a computer-controlled squirt of fuel under extremely high pressure into the engine’s combustion chamber. The fuel is atomized into a precisely shaped burst of fuel-air mixture that explodes more powerfully for the amount of fuel than a conventional gasoline engine. That allows automakers to get more power out of the same size engine, or substitute a smaller, more fuel-efficient engine with no sacrifice in power.

For instance, Ford calls its direct-injection gasoline engines “EcoBoost.” EcoBoost engines are also turbocharged for still more power. The net effect is a V-6 engine with the power of a V-8. Ford had no EcoBoost engines in 2008. It expects to offer EcoBoost engines in almost 25 percent of its lineup in 2010 and 90 percent of its lineup in 2013, according to Ford.

Other incremental improvements include six-speed automatic transmissions, which allow the engine to work at lower rpms at highway speeds, and electrically powered power steering, which uses less energy than hydraulic power steering.

Another positive change is the use of lighter materials to save weight, better aerodynamics and low-rolling resistance tires. Tires heat up when rolling down the highway, which represents energy that becomes “wasted” heat instead of power to drive the car, according to Michelin North America. The chemical composition of low-rolling resistance tires makes them less susceptible to producng that heat, and instead can improve mileage by about 1 mpg.

The following is a list of greener cars you can expect to see in 2010. It’s a diverse list, that includes mass marketers and luxury brands, and cars and trucks. You’ll also find diesel, gasoline, gasoline-electric and battery-powered hybrids.

1. Audi Q7 TDI

Starting price: $51,725

On sale: Now

Green technology: Clean diesel

EPA-estimated mpg: 17 city/25 highway

Audi and its mass-market sister Volkswagen are counting on a U.S. comeback for diesels. The clean-diesel technology in the Audi Q7 TDI provides 30 percent better fuel economy and 50 percent more torque than the same-size gasoline engine, Audi says. For the Audi Q7, that’s an EPA-estimated 25 mpg highway and 406 foot-pounds of torque. Torque is the twisting force used to launch a car from a standing start or to accelerate on the highway.

2. BMW X6 ActiveHybrid

Starting price: Not yet released

On sale: November 2009

Green technology: Gasoline-electric hybrid

Estimated fuel savings: Estimated at about 20 percent better mileage than a gasoline model with the same size standard engine.

BMW adds its first-ever hybrid this fall, the BMW X6 ActiveHybrid. Two factors make it an “active” hybrid, BMW says. First, the hybrid system provides a boost of electric power at higher speeds than other hybrids. Second, a complex transmission gives a sportier ride, more like a conventional, gasoline-powered BMW.

3. Buick LaCrosse Ecotec

Starting price: Not yet released

On sale: Late fall 2009

Green technology: Direct-injection gasoline engines

EPA-estimated mpg: 20 city/30 highway

The all-new Buick LaCrosse is on sale now. Customers can order a LaCrosse with two V-6 engines. Both are fuel-efficient, direct-injection gasoline engines. In the fourth quarter, Buick will add a direct-injection, four-cylinder engine that’s expected to get 30 mpg on the highway. Direct injection forces gasoline into the engine’s combustion chambers at extremely high pressure to create a precise fuel-air mixture that gets more power out of a given amount of fuel.

4. Ford Fusion/Mercury Milan

Starting price: $19,999

On sale: Now

Green technology: Gasoline-electric hybrid

EPA-estimated mpg: 41 city/36 highway

The Ford Fusion and its twin, the Mercury Milan, join a growing number of hybrids with a dashboard display that tells the driver how well they’re doing at achieving the best possible mileage. If the driver keeps the (virtual) needles in the “green” — mostly by staying off the accelerator — the display “rewards” the driver by adding green leaves to a picture of a plant.

5. Honda Insight

Starting price: $20,510

On sale: Now

Green technology: Gasoline-electric hybrid

EPA-estimated mpg: 40 city/43 highway

Ads touting the Honda Insight as the hybrid for everyone are a swipe at the more expensive Toyota Prius. However, the Honda Insight’s exterior styling is similar to the Prius. Also, like the Prius, the Honda Insight is a hybrid-only model. One obvious Honda Insight innovation is that the driver can select an “Econ” mode to optimize gas mileage.

6. Lexus HS 250h

Starting price: $35,075

On sale: Late August 2009

Green technology: Gasoline-electric hybrid

EPA-estimated mpg: 35 city/34 highway

The Lexus HS 250h is the first dedicated hybrid car from a luxury make. That is, there’s no conventionally powered twin. The driver can select four modes: power, normal, eco and EV, taking the driver from the least to most fuel-efficient driving. In EV mode, the car can be driven up to about 25 mph on battery power, but only for a couple of miles.

7. Mercedes-Benz ML450 Hybrid

Starting price: Not yet released

On sale: December 2009

Green technology: Gasoline-electric hybrid

EPA-estimated mpg: 21 city/24 highway

Like German rivals BMW and Porsche, the first Mercedes-Benz hybrid is a SUV, the Mercedes-Benz ML450 Hybrid, due later this year. It gets 30 percent better fuel economy than a conventional model. That’s still only 24 mpg highway, but that’s good for a fairly large SUV, with a gasoline-electric powertrain that delivers 335 hp. The gasoline engine is a 3.5-liter V-6.

8. Tesla Roadster

Starting price: $109,000

On sale: Now

Green technology: Exclusively battery powered

Estimated miles per charge: 244

The Tesla Roadster, the industry’s only battery-powered car, is for well-heeled early adopters. The six-figure sticker price reflects small sales volumes and high development costs. Tesla claims the roadster gets up to 244 miles on a full charge, which takes about 3.5 hours. The good news is blazing acceleration, 3.9 seconds from zero to 60 mph, plus exclusivity and a “green” aura.

9. Toyota Prius

Starting price: $21,750

On sale: Now

Green technology: Gasoline-electric hybrid

EPA-estimated mpg: 51 city/48 highway

The all-new 2010 Toyota Prius is Toyota’s third-generation hybrid, while most other automakers are still on their first. Each generation has gotten better gas mileage than the previous one, even though the Toyota Prius has gradually gotten bigger and included more creature comforts. The newest Toyota Prius is the first car to achieve an EPA-estimated 50 mpg. The Lexus HS 250h uses similar technology.

10. Volkswagen Jetta SportWagen TDI

Starting price: $23,870

On sale: Now

Green technology: Clean diesel

EPA-estimated mpg: 30 city/41 highway

Volkswagen says that with gas prices rising and diesel now the same price as regular gasoline, the diesel-powered VW Jetta SportWagen TDI is in short supply. The diesel version accounted for 81 percent of Jetta SportWagen sales in June, the company says. Diesel also accounted for 40 percent of sales for the Jetta sedan.

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