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" Merging with northbound traffic on Interstate 75 just outside Auburn Hills, Mich., I punch the accelerator, quickly swing left into the passing lane and pull forcefully ahead of the cars around me. In any other ride, on any other gray morning, it&amp;#039;d be just another Interstate moment. But this rush hour, I&amp;#039;m behind the wheel of a preproduction 2009 Volkswagen Jetta, which is powered by a 2.0-liter turbo-charged, direct-injected diesel engine that, even as I leave the speed limit in tatters, is averaging nearly 50 mpg. Equally important, what&amp;#039;s coming out of the tailpipe is no dirtier than the emissions from the 35-mpg econoboxes I can now see in my rearview mirror. Speed, fuel efficiency and minimal emissions? These aren&amp;#039;t characteristics usually associated with diesel-powered vehicles. But they will be. Most Americans have a bad impression of diesel cars. We think of them as loud, hard to start and foul-smelling. We sneer at them for lacking the get-up-and-go of their gasoline-powered cousins. And we dislike them for their perceived environmental sins, chiefly the polluting brew of sulfur and nitrogen compounds that they emit into the atmosphere. All those complaints were fair a generation ago, when the twin energy crises of the 1970s propelled diesels into national popularity and kept them there for a decade. Back then, many drivers ignored diesel&amp;#039;s faults, or were unaware of them, because diesel cars ran 30 percent farther on a gallon of fuel than similar gasoline-powered cars. It felt savvy to buy a diesel, even daring. Then fuel prices dropped in the mid-1980s, and drivers abandoned their clattering, odoriferous fuel sippers. They went back to gasoline. Today, diesel powertrains are on the map again, for both car manufacturers and efficiency-minded drivers. The technology could be here to stay, even if fuel prices (improbably) decline. The new cars run as well as their gasoline-powered competitors. And as for the emissions problems of the past--well, the dirty bird of fossil fuels isn&amp;#039;t so dirty anymore. The Coming Diesel Invasion  style=&amp;quot;background-color:#cccccc; border:1px solid #000000;&amp;quot; rules=&amp;quot;all&amp;quot;&amp;amp;gt;AUDI A4 SEDAN  %&amp;quot; align=&amp;quot;center&amp;quot;&amp;amp;gt;BMW 3 SERIES SEDAN  %&amp;quot; align=&amp;quot;center&amp;quot;&amp;amp;gt;CADILLAC CTS SEDAN  Engine: 3.0-liter V6 Target Release Date: 2008 This 240-hp turbocharged direct-injected engine will also be available in the full-size Audi Q7 sport utility. %&amp;quot; style=&amp;quot;padding:2px;&amp;quot;&amp;amp;gt;Engine: 3.0-liter twin-turbo inline Six Target Release Date: 2008 BMW plans to release this 3.0-liter in 5 and 6 Series models in 2008 as well. It reportedly gets more than 35 mpg %&amp;quot; style=&amp;quot;padding:2px;&amp;quot;&amp;amp;gt;Engine: 2.9-liter TDI V6 Target Release Date: 2009 Look for clean diesel also in the Buick Enclave, GMC Acadia and GM light-duty pickups, around 2010.FORD F-150 PICKUP  %&amp;quot; align=&amp;quot;center&amp;quot;&amp;amp;gt;HONDA ACCORD SEDAN  %&amp;quot; align=&amp;quot;center&amp;quot;&amp;amp;gt;SATURN AURA SEDAN  Engine: 4.4-liter V8 Target Release Date: 2009 Derived from the Euro-spec 3.6-liter currently used in Land Rovers, which get around 31 mpg on the highway %&amp;quot; style=&amp;quot;padding:2px;&amp;quot;&amp;amp;gt;Engine: 2.2-liter i-DTEC Target Release Date: 2009 The British version gets 51.4 mpg in combined city/highway driving. The hybrid Civic gets 50 mpg. %&amp;quot; style=&amp;quot;padding:2px;&amp;quot;&amp;amp;gt;Engine: To be determined Target Release Date: 2010 The Aura&amp;#039;s Euro twin, the Opel Vectra, gets more TDI options in &amp;#039;09, which will transfer to the model coming here. The Sulfur Solution &amp;quot;If you told me 10 years ago that I&amp;#039;d be putting `clean&amp;#039; and `diesel&amp;#039; in the same sentence, I&amp;#039;d have said you were out of your mind,&amp;quot; says Margo Oge, director of the Office of Transportation and Air Quality at the Environmental Protection Agency. However, in response to EPA mandates that went into effect in late 2006, oil refineries are now producing what&amp;#039;s called ultra-low-sulfur diesel (ULSD). By definition, this &amp;quot;clean diesel&amp;quot; has sulfur concentrations of no more than 15 parts per million (ppm). That&amp;#039;s 98.5 percent cleaner than the sludge that coursed through the fuel delivery systems in those disco-era rides, and 97 percent less sulfur than was allowed under a 500-ppm stand\u00ADard instituted in 1993. The cut in sulfur means that less sulfur dioxide goes into the atmosphere, where it can combine with water to produce sulfuric acid--and thus, acid rain. There are further beneficial effects of the sulfur-light fuel, ones that could make the advent of clean diesel as environmentally momentous as the introduction of unleaded gasoline in 1974. &amp;quot;Sulfur clogs emission-control devices in diesel-powered cars the same way lead impeded catalytic converters in gasoline systems,&amp;quot; Oge says. &amp;quot;Removing the lead from gasoline enabled engineers to develop a new generation of emission-control technologies that helped reduce noxious exhaust emissions by 98 to 99 percent.&amp;quot; Carmakers have already started building exhaust-scrubbing systems for engines that burn ULSD--so that not just sulfur but a rogue&amp;#039;s gallery of other pollutants are kept out of the air. Though they differ in design, the systems share some basic components. In a Mercedes-Benz BlueTec system, for example, exhaust from the engine is first filtered through a device that lowers carbon monoxide and hydrocarbon levels. Then it runs through an apparatus that removes soot and other particulates. Finally, the remaining exhaust gas is sprayed with a urea-based substance that helps convert harmful nitrogen oxides--NOx is the shorthand for this group of compounds--into harmless nitrogen gas and water vapor. According to Mercedes-Benz, the system reduces the total output of harmful emissions by 80 to 90 percent. The process even eliminates diesel&amp;#039;s bad odor. What&amp;#039;s the Difference Between Diesel and Gasoline Engines? &amp;amp;gt;  frameborder=&amp;quot;0&amp;quot; scrolling=&amp;quot;no&amp;quot;&amp;amp;gt; Both are internal combustion engines designed to convert the chemical energy in fuel into mechanical energy. However, diesel and gasoline engines differ in the way combustion is accomplished. A gas engine takes in a mixture of gasoline and air, compresses it and ignites the mixture with a spark plug. A diesel engine takes in air, compresses it--raising the temperature past 800 F--and then injects fuel into the cylinder. When the fuel contacts the hot air, it ignites. This process is possible because diesel engines produce much greater compression ratios and diesel fuel is more stable than gasoline. Typically, a gasoline engine compresses at a ratio no higher than 12:1 to avoid preignition, which can damage vital components. A diesel engine com\u00ADpresses at ratios of 14:1 to as high as 25:1. How They Work: Emission Reduction Systems &amp;amp;gt;  Exhaust leaves the engine and passes through a diesel oxidation catalyst (DOC), which minimizes carbon monoxide and hydro\u00ADcarbons. It then passes through a self-cleaning particulate filter that traps and stores soot. A urea-based solution, commercially known as AdBlue, is then sprayed into the exhaust flow. Heat from the exhaust transforms the urea into ammonia, which reacts with nitrogen oxides (NOx) in the SCR, where the mix is converted into harmless nitrogen gas and water vapor.  Exhaust from the engine flows through a combination DOC (diminishing the amount of carbon monoxide and hydrocarbons) and a self-cleaning particulate filter, which removes soot. From there, it enters a new type of catalytic converter that absorbs a portion of the NOx from the exhaust gas and converts it into ammonia without the need for an organic catalyst like AdBlue. The system recombines that ammonia with the remaining NOx, converting it into harmless nitrogen and water vapor, just like the SCR system.  The 250,000-Mile Engine ULSD fuel and the new exhaust systems receive most of diesel&amp;#039;s accolades, but they aren&amp;#039;t the only advances to set today&amp;#039;s diesels apart from their predecessors. Advanced electronic engine control modules regulate functions, from fuel injection to timing, more precisely than old mechanical control systems, further minimizing emissions and maximizing fuel economy. Turbochargers pump more air into the engines, helping to overcome diesel&amp;#039;s former sluggishness at higher rpm. And improved engine isolation strategies, such as the incorporation of active mounts, has quieted the knocking noises typically associated with diesel-powered vehicles. Meanwhile, diesel&amp;#039;s core virtues remain unchanged. The fuel contains more energy per unit volume than gasoline, and diesel engines operate at higher compression ratios than gasoline engines--typically 14:1 to 25:1, compared to 8:1 to 12:1. (The compression ratio is the relationship between the volume of the cylinder when the piston is at the bottom of its stroke and the volume when it&amp;#039;s at the top.) The higher the compression ratio, the more mechanical energy an engine can squeeze from its fuel/air mixture. So each time the mixture in a diesel engine&amp;#039;s cylinder ignites, the car gets a slightly bigger push than it would in a gasoline engine. That means it takes less fuel to move the car down the road. It also means that the engine generates a lot of power even when it isn&amp;#039;t cycling fast--and that&amp;#039;s the source of the beefy low-end torque these vehicles are famous for. The high compression ratios also explain why diesel engines tend to last so long. &amp;quot;Diesel engines need to be built stronger,&amp;quot; explains Tony Molla, author of Chilton&amp;#039;s Diesel Engine Service Manual. &amp;quot;The crankshaft and connecting rods are quite a bit heavier than those in gasoline engines.&amp;quot; And because diesel has a low coefficient of friction, it also happens to be a good lubricant that provides protection to the cylinder walls. The result? It&amp;#039;s not unusual to see diesel engines still chugging along at 250,000 miles.  The European Model Fans of diesel technology often look longingly at Europe to see how their favorite automotive fuel is faring in a friendlier environment--namely, one where gasoline costs $7 per gallon. Many European countries provide tax breaks on diesel that make it consistently cheaper than gas. More than half of the vehicles sold on the continent are diesel powered, and the technology&amp;#039;s proponents like to point out that Europe&amp;#039;s average fuel consumption is 36 mpg, compared to just 22 mpg in the United States. (To be fair, that fuel economy number is partly due to the fact that Europeans drive smaller cars, regardless of their powertrains.) Could diesel gain a similar level of acceptance here? Perhaps. A recent study by J.D. Power and Associates found that in 2007, 23 percent of U.S. car buyers were considering a clean diesel for their next purchase, up from 12 percent the year before.  Ultra-low-sulfur fuel is the key to clean diesel. Shell fuel scientist Nigel Dickens displays the fruits of the lab. (Photograph by Getty Images) In the short term, those numbers won&amp;#039;t be reflected in new-car purchases, simply because there aren&amp;#039;t many choices for American consumers. Only one clean-diesel sedan is on sale in the United States today, the Mercedes-Benz E320 BlueTec--and it costs around $50,000. Drivers may also be discouraged by the fact that only about 45 percent of the nation&amp;#039;s 170,000 filling stations carry diesel fuel. On the other hand, the 75,600 stations that do carry it are spread conveniently along the nation&amp;#039;s highways. &amp;quot;Yes, you might have to change filling stations, but most people won&amp;#039;t even notice,&amp;quot; says Tom Fulks, the West Coast representative for the Diesel Technology Forum, a trade group composed of automakers, parts suppliers and petrochemical companies. Further down the line, diesel could lose some of its allure if the price jumps. Diesel and gasoline prices have been neck and neck in recent years, normally staying within about 20 cents of each other. However, &amp;quot;Diesel use has been growing for several years, and there&amp;#039;s no guarantee it couldn&amp;#039;t become more expensive than gasoline,&amp;quot; says Jeff Hazle, technical director of the National Petrochemical &amp;amp;amp; Refiners Association. Federal renewable-fuel policy is one factor that seems destined to push diesel prices higher. If so, it will be an unintended consequence of legislation that aims to raise production of ethanol from 4.7 billion gallons in 2007 to 7.5 billion in 2012. That&amp;#039;s because the machinery required to grow and harvest the corn that&amp;#039;s made into ethanol runs on ... you guessed it, diesel. &amp;quot;I wouldn&amp;#039;t go so far as to predict shortages,&amp;quot; Hazle says. &amp;quot;But I can&amp;#039;t say it&amp;#039;s going to be a cheap fuel.&amp;quot; Hybrid Versus Diesel: Which Powertrain Produces the Fewest Greenhouse Gases?  style=&amp;quot;background-color:#cccccc;&amp;quot;&amp;amp;gt; To see how hybrids compare to their diesel-powered cousins, we pit Europe&amp;#039;s version of the Toyota Prius against the efficient Volkswagen Polo BlueMotion 1, currently only available overseas, in a war of numbers. The Polo is lighter (it weighs almost 500 pounds less) and somewhat smaller, so we expected it to be more fuel efficient. However, we didn&amp;#039;t think it would go 38 percent farther on a gallon of fuel or emit 5 percent fewer greenhouse gases per mile. (We didn&amp;#039;t look at particulates or NOx emissions.) While hybrids might be the green champions, clean diesels can be just as friendly to the environment--possibly even friendlier.   Base Price: $20,950 Engine: 1.5-liter, four-cylinder gas/electric hybrid MPG: 54 CO2 Emissions: 0.369 lb./mile Weight: 2866 lb.   valign=&amp;quot;top&amp;quot;&amp;amp;gt;  Base Price: $23,315 Engine: 1.4-liter, three-cylinder turbodiesel MPG: 74.3 CO2 Emissions: 0.351 lb./mile Weight: 2390 lb.  The 1.5 Million Barrel Wild Card For now, at least, diesel vehicles are an intriguing option if only because they promise a viable alternative to hybrid vehicles--and drivers like to have choices. Especially choices that offer the bold, torque-heavy performance of some diesels. Nearly all the major carmakers say they&amp;#039;ll deliver clean-diesel models to the U.S. market within two years. German carmakers may have a head start. Audi, BMW and Volkswagen all plan to offer multiple models within two years. Honda is preparing a diesel Accord to be sold first in the U.K., and all three major American car companies plan to introduce diesel cars by 2010. But they won&amp;#039;t be cheap. The manufacturers will begin by introducing the technology in V6 and V8 engines, rather than in four-cylinder versions. And while little pricing information is available, Volkswagen says its clean-diesel Jetta will carry a premium of about $2000 over its gas-powered sibling. Sadly, the real fuel misers of the diesel world--inexpensive three- and four-cylinder wonders like the Volkswagen Polo on our cover, which gets 74.3 mpg--probably aren&amp;#039;t coming to the U.S. anytime soon. Right now, even the best of these European diesels don&amp;#039;t meet the emissions standards in the five states that follow California clean-air regulations. So far, manufacturers have been mum about whether they will modify these vehicles to take advantage of ULSD-ready emissions controls. But diesel has outsize potential even without these models. According to the EPA, if 33 percent of U.S. drivers switched to diesel vehicles, the country would reduce its oil consumption by about 1.5 million barrels a day, cutting oil imports by more than 10 percent. The agency&amp;#039;s Oge, for one, is optimistic that the technology will leave behind its dirty old image and win a whole new generation of fans. &amp;quot;This is going to be remembered as the decade when we transformed diesel.&amp;quot;  &amp;amp;gt; &amp;amp;gt; Turning to diesel power in the late &amp;#039;70s and early &amp;#039;80s was easy. This 1977 VW Rabbit engine could travel over 50 miles on a gallon of fuel. But it was loud, didn&amp;#039;t offer much performance and belched smoke.  &amp;amp;gt; Today&amp;#039;s clean diesels, such as this engine from a Euro-spec 2007 VW Polo, can offer better fuel economy (74.3 mpg) and produce fewer greenhouse gases than some gas/electric hybrids. "^^<http://www.w3.org/2001/XMLSchema#string>
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