Clean diesel in the USA – part of the air pollution solution?

Friend or foe? Problem or solution? Diesel has been cast into these and other roles both in Europe and the U.S. for over a decade. Loved for its fuel efficiency or the scapegoat for all air pollution and health problems, any discussion about diesel is bound to bring out strongly held views, including this one: there is no question that diesel is a part of the air quality challenges we all face and there is also no question that it is part of the solution.

As parts of Europe emphasized low-carbon dioxide (CO2) emissions and efficiency, car buyers were propelled toward diesels by both policy and technology. In the U.S., emissions control was the order of the day through a fuel and technology neutral approach. And whether in London or Los Angeles, growing economies and populations have led to major traffic congestion, with vehicles of all types and fuels often stuck idling or running in less efficient modes.

In the U.S., pollution concerns of the diesel were driven by visible smoke emissions from commercial trucks and buses painting a very visible and negative diesel image. It also created a regulatory emphasis toward eliminating particulate emissions, which met their fate in 2007 with the advent of the diesel particulate filter (DPF) that removed over 90% of the solid particles from the exhaust. Today DPFs are standard fare in diesels large and small. Some gasoline engines emit more fine particles today than a modern diesel.

Nitrogen oxides

A focus on NOx emissions (including nitrogen dioxide or NO2) reduction followed and was until recently a major challenge. Inside the diesel engine, strategies to reduce formation of particulates tended to increase the formation of nitrogen oxides and reduce fuel efficiency, and vice-versa. Today’s two-pronged solution minimises NOx formation through precision control of the combustion process, and then uses selective catalytic reduction (SCR) technology to scrub out the remainder in the exhaust. SCR is near standard fare for all on-highway trucks and most all passenger cars as well. Today in the U.S., heavy duty diesel trucks achieve near zero emissions performance (0.20 g/BHP-hr) for nitrogen oxides, compared to 6.0 g/BHP-hr some eight years ago. They are also achieving gains in fuel efficiency of 3-6%.

What we call ‘clean diesel’ in the U.S. is new technology that has reduced NOx and PM by 98% and ultra-low sulfur diesel that has removed 97% of the sulfur from diesel fuel.

Complex air

In the U.S. some science questions further reductions in NOx emissions because of a counter-intuitive finding – that lowering NOx emissions can actually increase smog formation, at least in some cities. It is clear that air is a complex mixture of things including emissions from many sources and that air emissions chemistry and local atmospheric and environmental conditions can play a big role.

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