You wouldn't know it from looking at them, or at their sales numbers in recent years, but hybrids have reason to be worried. There's a technology out there today that some are calling the new "hybrid-killer" -- six little letters that have become the greenest thing on four wheels in the last year, and it's nothing new; in fact, it's pretty darn old -- diesel.

You may not have heard, but it was a diesel that won the Super Bowl of efficient auto technology this past fall when the Volkswagen Jetta TDI was named “Green Car of the Year,” beating the popular Ford Fusion and Saturn Vue hybrids. The Jetta TDI certainly seems to be going where few diesels have gone in recent years –- toward success on American soil. As the Green Car Journal editors put it:

The 2009 Jetta TDI breaks new ground in the field of clean diesels, achieving emissions certification in all 50 states without the use of special additives or extraordinary measures. ... It also achieves estimated highway fuel economy of 41 mpg with greatly reduced greenhouse gas emissions.”

That's a pretty strong endorsement of a fuel once almost entirely written off in our hemisphere. To understand the turnaround, here's a quick look at Big D's history, and some of the other burgeoning “green car” technologies giving the “new diesel” a run for its money:

Diesel: Old-school green

Biodiesel fans love to point out that Rudolf Diesel originally ran his internal combustion engine on peanut oil back in 1898. And, at first, Americans warmed up to the powerful fuel, with mechanic Clessie Cummins building diesel up to become the standard for big rigs and fleet vehicles.

In 1936, the first diesel passenger vehicle was built by Mercedes Benz. The engine never really took off in American consumer cars until the OPEC oil embargo in 1973, and ensuing lines and shortages led General Motors to take a more serious look at diesel as an “alternative” fuel. Unfortunately, a design flaw in GM's Oldsmobile diesel V8 engines and a few other models led to a class-action lawsuit and turned diesel's name in the United States to mud virtually overnight.

Meanwhile, in Europe, diesel maintained a decent reputation. When gas prices there were double what we paid in the United States during the 1990s, European officials created tax breaks to encourage diesel ownership, and technological improvements essentially eliminated the pollution and performance issues with the fuel.

Diesel manufacturers have long had designs on the American consumer market, but when Hurricane Katrina slammed into the Gulf Coast in 2005, shutting down refineries and oil production there, the cost of stateside diesel skyrocketed, and the focus of green-conscious drivers seemed to be firmly planted on hybrids.

But with the crash of gas and diesel prices, and all the apparent advantages of biodiesel over ethanol, it makes sense that a car like the Jetta TDI is suddenly at the forefront. Even with new, tougher emissions standards for diesels coming in 2010, many major manufacturers are working on consumer diesels.

Hybrids hit back

Less than six months after the hybrid snubbing at the Green Car Awards, the 2010 Prius ups the ante with 50 miles per gallon for the same sticker price as the Jetta TDI. In Europe, many models are already achieving upwards of 70 mpg, and now that gas prices are inching up again, it's likely that the hybrid's real heyday is yet to come. And of course, hybrids offer something that pure diesels can't -- the possibility of having the best of both worlds. Just imagine running on renewable biodiesel and reaching 50-plus mpg in your new diesel hybrid.

Not just a two-horse race: CNG, electrics and more

When it comes to green fuels, it can be argued that ethanol won the race a long time ago, with a percentage of each gallon of unleaded now coming from corn. But, for those Flex-Fuel vehicles that can burn E-85 (85 percent ethanol, 15 percent petrol), the jury is still out, with many scientists finding it takes more energy to make a gallon of the stuff than you'll get out in performance.

Biodiesel's fortunes have been looking up, and its fate is inextricably linked with mass acceptance of vehicles like the Jetta TDI.

And what about oil tycoon T. Boone Pickens' plan to run all our cars of good ol' American natural gas? It has been tried in the past with limited success, due largely to bad timing: As CNG stations popped up in the '90s, gas prices soon crashed. Enough of a public relations and subsidies push could change its fortunes, but that doesn't seem to be on the horizon, not to mention that it's not sustainable.

Diesel: Could an old fuel become the new 'hybrid-killer'?
Fuel-efficient diesels are getting a closer look in the competitive green car market.