The Diesel Dilemma

The report compares conventional, advanced, and hybrid-electric diesel and gasoline vehicles across five passenger vehicle classes. It finds that while diesel engines offer higher inherent fuel economy, they are less cost-effective than improved gasoline vehicles for reducing oil use and heat-trapping gases due to higher up-front costs and the higher petroleum intensity of diesel fuel. The report also highlights significant public health concerns, noting that diesel vehicles historically emit substantially more toxic soot and nitrogen oxides than gasoline vehicles. While new federal Tier 2 standards aim to close this gap, the report argues that diesel's future depends on overcoming technical challenges in emissions control and ensuring real-world performance matches certification. It recommends maintaining strict emissions standards, modifying CAFE to use energy-equivalent measures, and expanding inspection and maintenance programs to include diesel vehicles.

• Improved gasoline vehicles are more cost-effective than diesel for reducing oil use and global warming pollution. To achieve a 20 to 30 percent reduction in oil demand, the average diesel vehicle adds 2.5 to over 4 times more in up-front costs ($1,700 to $1,800) than an equivalent gasoline vehicle.
• Diesel's fuel economy advantages are partially offset by the fuel's higher petroleum and carbon intensity. Low-sulfur diesel requires 25 percent more oil per gallon and results in 17 percent more heat-trapping gas emissions per gallon than reformulated gasoline.
• Diesel vehicles pose greater public health risks than gasoline vehicles due to higher emissions of toxic soot and smog-forming nitrogen oxides. The average diesel car releases ten to several hundred times more soot and about twice as much smog-forming pollution as a properly maintained gasoline car.
• While new federal Tier 2 standards (fully implemented by 2009) require diesel and gasoline vehicles to meet the same standards, the 'fleet averaging' structure allows some vehicles to release two times more soot and nearly three times more nitrogen oxides than the average vehicle.
• Diesel particulate filters (traps) can reduce soot by 90 percent or more and significantly reduce toxicity, but nitrogen oxide (NOx) controls remain a significant technical challenge.
• Current Corporate Average Fuel Economy (CAFE) standards are inequitable because they credit vehicles based on fuel economy rather than oil use, which could potentially increase U.S. oil dependence if diesel adoption grows.
• Diesel vehicles maintain a performance advantage in extreme towing applications for SUVs and pickups due to higher low-end torque.

APA

Monahan, P., & Friedman, D. (2004). The Diesel Dilemma. Union of Concerned Scientists. https://www.ucs.org/sites/default/files/2019-09/dieseldilemma_fullreport.pdf

Chicago

Monahan, Patricia, and David Friedman. The Diesel Dilemma. Union of Concerned Scientists, 2004. https://www.ucs.org/sites/default/files/2019-09/dieseldilemma_fullreport.pdf.

Wikipedia

{{cite report |last1=Monahan |first1=Patricia |last2=Friedman |first2=David |title=The Diesel Dilemma |publisher=Union of Concerned Scientists |date=January 2004 |url=https://www.ucs.org/sites/default/files/2019-09/dieseldilemma_fullreport.pdf |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{monahan2004diesel, author = {Monahan, Patricia and Friedman, David}, title = {{The Diesel Dilemma}}, institution = {Union of Concerned Scientists}, year = {2004}, month = jan, url = {https://www.ucs.org/sites/default/files/2019-09/dieseldilemma_fullreport.pdf}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }