Light-duty vehicles have less than an 8,500-lb gross vehicle weight rating (GVWR). They include sedans, pickup trucks, high-performance sports cars, sport-utility vehicles, and passenger vans. Gasoline-powered vehicles dominate this class in the United States. However, more light-duty diesel vehicles are becoming available as state-of-the-art "clean diesel" technologies enable them to meet the same stringent emissions regulations as gasoline vehicles. The high fuel economy of light-duty diesel vehicles makes them attractive to consumers.

Biodiesel

Although light-duty diesel vehicles are not technically "alternative fuel vehicles," they can run on biodiesel, an alternative fuel under the Energy Policy Act of 1992. Biodiesel, which is mainly used as a blend, can be used in most light-duty diesel vehicles with no engine modification. The most common biodiesel blend is B20, which is 20% biodiesel and 80% conventional diesel. B5 (5% biodiesel, 95% diesel) is also commonly used in fleets. To learn more about this fuel, go to the Biodiesel section. 

Emissions

Currently, most light-duty diesel vehicles are equipped with oxidation catalysts that reduce carbon monoxide (CO) and hydrocarbon (HC) emissions, and many have particulate matter (PM) traps that reduce PM emissions as well as CO and HC emissions. In combination, these devices can decrease emissions of CO by 80%, HC by 90%, and PM by 98%. New emissions control devices soon will be required to meet stringent emissions standards. Learn more about diesel emissions and how technologies such as selective catalytic reduction, Diesel Exhaust Fluid, and NOx adsorbers are controlling them.

Clean Diesel

Ultra-low sulfur diesel (ULSD)—which is called "clean diesel" when used in conjunction with advanced emission control devices—is available at fueling stations nationwide and can be used in any diesel vehicle. This fuel reduces the sulfur content in diesel fuel by 97%. Europe has used ULSD for several years. The United States began its changeover to ULSD in June 2006, after the U.S. Environmental Protection Agency (EPA) mandated that 80% of highway diesel fuel produced or imported contain 15 ppm or less sulfur.

The Federal Alternative Fuel Vehicle Tax Credit provision of EPAct 2005 includes a tax credit for lean-burn diesel vehicles. The credit is sometimes referred to as the Clean Diesel Tax Credit and is effective January 1, 2006; however, no 2006 or 2007 diesel vehicles met the emissions requirements for credit. No 2008 vehicles have been certified as qualifying for the credit. Diesel vehicles up to 6,000 lb GVWR that meet EPA Tier II Bin 5 emissions requirements will be eligible for the credit. Diesel vehicles of 6,001 to 8,500 lb GVWR must meet Tier II Bin 8 requirements. Manufacturers will certify with the EPA that their vehicles meet the emissions requirements. The IRS must then issue a notice that the vehicle qualifies for the tax credit before consumers or commercial businesses can claim the credit. There are other IRS requirements to claim the credit.

Volkwagen Jetta TDI – The Most Fuel Efficient Diesel Car

Advanced diesel vehicles using EPA-mandated ultra-low sulfur diesel (ULSD) fuel are among the most fuel-efficient vehicles available today. Collaborative R&D between DOE, industry and the national laboratories has resulted in improved engine efficiency and very low emissions. Collaboration with the U.S. Environmental Protection Agency, industry, and national laboratories under the DOE Diesel Emission Control Sulfur Effects (DECSE) program provided the supporting data needed to mandate 15 ppm sulfur in diesel fuel as the appropriate level to maintain effectiveness of diesel engine emission control technologies. Most diesel vehicles also can run on biodiesel blends without engine modification.

Diesel Vehicle Classes

• Light-Duty Vehicles
• Medium- and Heavy-Duty Vehicles

Diesel Emissions

Advanced diesel engine technologies combining in-cylinder combustion control for low engine-out emissions with advanced aftertreatment technologies and using ultra-low sulfur diesel (ULSD) fuel can meet EPA emissions standards. Advanced combustion strategies such as exhaust gas recirculation (EGR) lower engine-out NOx emissions. Aftertreatment devices such as NOx adsorbers and selective catalytic reduction (SCR), reduce NO_x emissions by up to 90 percent. Diesel particulate filters and oxidation catalysts lower particulate matter emissions by over 95 percent. A combination of these aftertreament technologies is used to meet stringent emissions standards.

Source: Diesel Technology Forum. See DieselNet’s Emission Standards for details. 

Diesel Selective Catalytic Reduction

Selective catalytic reduction (SCR) is an advanced emission-control technology that can help light-, medium-, and heavy-duty diesel vehicles meet stringent regulations on nitrogen oxides (NO_x) emissions. In an SCR system, a liquid reducing agent composed of urea and water—known as Diesel Exhaust Fluid (DEF)—is combined with engine exhaust in the presence of a catalyst to convert smog-forming NO_x into harmless nitrogen and water vapor. See the diagram below.

Source: Diesel Technology Forum—Selective Catalytic Reduction

The process starts with ultra-low sulfur diesel fuel combusted in an optimized diesel engine. Hot exhaust from the engine flows through a diesel particulate filter (which removes particulate matter) toward the SCR catalyst. DEF from a storage tank is injected into the exhaust stream, and the exhaust and finely atomized DEF enter the SCR catalyst chamber together. In the presence of the SCR catalyst, the exhaust and DEF react to convert NO_x into nitrogen (N₂) and water vapor.

One important requirement of an SCR system is consistently refilling the DEF storage tank. This occurs at approximately the interval of recommended oil changes for light-duty vehicles. The interval varies based on application for medium- and heavy-duty vehicles. A DEF distribution system is being established to meet refilling needs.

Selective catalytic reduction has been used for decades in marine and large stationary diesel engines. The technology is used extensively with heavy-duty highway vehicles in Europe, where the DEF is marketed as AdBlue. The use of SCR in highway and non-road engines has been demonstrated in the United States, and several auto manufacturers are incorporating SCR into their U.S. diesel products. Tests have shown that SCR can reduce NO_x emissions by 75% to 90%.

NO_x Adsorbers

NO_x adsorbers—also known as NO_x traps or lean NO_x traps—are advanced emission-control technologies that can help diesel vehicles meet stringent nitrogen oxides (NOx) emissions standards. NO_x adsorbers trap and store NO_x present in the lean (i.e., oxygen-rich) exhaust produced by diesel engines. The stored NO_x is transformed into more environmentally benign compounds before these compounds are emitted into the atmosphere. See the diagram below.

Adapted from: APBF-DEC 2,000-Hour Performance of a NOx Adsorber Catalyst and Diesel Particle Filter System for a Medium-Duty, Pick-Up Truck Diesel Engine Platform 

The process starts with ultra-low sulfur diesel (ULSD) fuel combusted in an optimized diesel engine. (Use of ULSD minimizes potential poisoning of the NO_x adsorber by sulfur compounds.) Lean exhaust from the engine flows into the NO_x adsorber; in some configurations, the exhaust flows first through other emission control devices (as shown in the diagram above). With the aid of a noble metal catalyst, NO_x is captured and stored within the NO_x adsorber substrate.

When the NO_x adsorber nears its NO_x storage capacity, it begins "regeneration." The engine exhaust is briefly made rich (i.e., fuel rich and oxygen poor) by means such as injecting diesel fuel into the exhaust stream ("In-Exhaust Fuel Injection" in the diagram above) or late-cycle in-cylinder injection. In the presence of this fuel-rich exhaust, the stored NO_x is released then reduced to carbon dioxide (CO₂), water (H₂O), nitrogen (N₂), and other nitrogen-containing gases over a noble metal catalyst. These gases are then emitted out the tailpipe.

NOx adsorbers are not a new technology but are just beginning to be seen in vehicle applications. Tests have shown they can reduce automotive NO_x emissions by 80% to 90.

List of Top Diesel Cars:

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Biodiesel is a liquid fuel made up of fatty acid alkyl esters, fatty acid methyl esters (FAME), or long-chain mono alkyl esters. It is produced from renewable sources such as new and used vegetable oils and animal fats and is a cleaner-burning replacement for petroleum-based diesel fuel. It is nontoxic and biodegradable. Biodiesel has physical properties similar to those of petroleum diesel:

Like petroleum diesel, biodiesel is used to fuel compression-ignition (diesel) engines. Low-level blends of biodiesel with petroleum diesel also provide benefits.

Related articles:

• B20 & B100: Alternative Fuels
• Biodiesel Production
• Benefits
• Biodiesel Emissions

When biodiesel is blended with petroleum diesel, it produces a fuel that is compatible with diesel engines, displaces imported petroleum, and reduces harmful emissions. Blends like B2 (2% biodiesel and 98% diesel) and B5 (5% biodiesel and 95% diesel) are becoming increasingly common as drivers become more aware of the many benefits. Higher-level biodiesel blends, such as B20, are also becoming more widely available and can qualify for credits under the Energy Policy Act of 1992.

Benefits

As of 2006, most U.S. highway diesel fuel is limited to 15-ppm sulfur, down from a 500-ppm limit. This new ultra-low sulfur diesel fuel (ULSD) might have reduced lubricating properties before additives. Adding as little as 0.25% biodiesel, which is very low in sulfur, can significantly increase fuel lubricity. B2 and B5 are becoming increasingly popular for this reason.

Another reason for using B2 or B5 is to introduce a large quantity of renewable fuel into the diesel fuel market with no noticeable impact on users or fuel properties. Using 100 gallons of B5 brings roughly the same air-quality and alternative fuel use benefits as using 25 gallons of B20 or 5 gallons of B100. Minnesota adopted a statewide requirement for B2 use in 2005. After some problems from out-of-specification fuel the first winter, the program is working well. Louisiana, Maryland, and Washington have enacted similar programs contingent on state biodiesel production reaching certain levels.

What Types of Vehicles Can Run on Biodiesel Blends?

As long as the biodiesel used for blending meets ASTM D6751 standards, low-level biodiesel blends such as B2 and B5 can be used safely in any compression-ignition engine designed to be operated on diesel fuel. This can include light-duty and heavy-duty diesel cars and trucks, tractors, boats, and electrical generators.