Clean diesel engines - better late than never

by Aaron Turpen

The term "clean diesel" refers to Ultra-low Sulfur Diesel (ULSD) being used in vehicles using near-zero emissions diesel burning engine systems. Most notably the German-engineered turbo-charged direct injection (TDI) system which both Audi and Volkswagen have used to win the Green Car of the Year Award for the past two years.

Diesel engines have several advantages over gasoline and diesel fuel also has distinct advantages as an energy source. Today's modern engines and fuels combine to make diesel one of the cleanest options for current combustion engine technologies.

How It Works

Clean diesel is a blend of two technologies working together: ULSD and high-efficiency engines utilizing specialized "scrubbing" technology to clean exhaust.

With the removal of most of the sulfur from diesel fuel (ULSD), engine designers were free to utilize more comprehensive exhaust cleaning technologies. Aftertreatment devices, especially advanced catalytic converters, can now be used to remove pollutants from the exhaust and recycle them into the engine to be burned down and released as CO2 rather than particulates or nitrogen oxides (NOx).

photo from Audi

Direct injection and better fuel control have also meant that engines are now much more precise and efficient in their fuel use.


Clean diesel options have lower emissions of most gases and particulates than most gasoline-burning engines and are near-zero in most respects. While CO2 is not lowered significantly, ozone-creating NOx and polluting particulate matter (PM) are lowered to nearly zero in most clean diesel applications.

Fuel Blends

Clean diesel engines are generally capable of burning B5 (5% biodiesel mix) and some are rated up to B20. Clean diesel engines are also often mixed with methane (natural gas as CNG or LPG) for even greater efficiency.


In most developed nations, including the U.S., Canada and most of Europe, diesel is a well-established fuel with wide availability and ULSD is the only fuel available for most applications.


The greatest advantage of clean diesel engines is the greater fuel efficiency they can achieve over their gasoline counterparts. The Volkswagen Jetta clean diesel car can get 58mpg, the Audi A3 gets 42mpg, and the BMW 335d sedan gets 35mpg (all highway mileage). Most diesel engines are also capable of service lives in excess of 300,000 miles, making them much longer-lived than most gasoline engines.

Another advantage is the overall petroleum use for diesel. While diesel fuel requires more petroleum to make it, clean diesels use about 30% less petroleum overall than their gasoline counterparts because diesel requires less refinement to make and the vehicles using it are generally much more efficient in fuel use compared to a similar gasoline vehicle.


The major road block to diesel engines in the United States is public perception. Most people associate diesel engines with heavy-duty highway trucks, trains, and the dirty diesel cars of the 1970s and 80s. Diesel vehicles are also generally more expensive (about 7% more so, on average) than comparable gasoline versions, especially in North America. This is generally counterbalanced by longer life and better economy.

The Future

Clean diesel will likely become more and more popular and will be seen more often in hybrid and range-extended hybrid power trains, replacing the gasoline version. Being cleaner, more efficient, and in many ways better suited to the job of electrical generation, it's an obvious choice. Some American carmakers, such as Ford, are putting more development into their diesel engines for market.

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