Newer clean diesel engines have reduced particulate matter and NOx emissions by up to 98 percent compared to older diesel engines, according to the Diesel Technology Forum.

"Earth Day is an opportunity to reflect on our commitments to protect and enhance our environment and natural resources, and use energy resources efficiently," said Allen Schaeffer, the forum's executive director. "There is no better example than diesel as a technology and an industry that has undergone a complete transformation in so little time that is enhancing our environment and quality of life around the globe.

After billions of dollars to research and development, truck, car, bus and off-road manufacturers are now producing clean diesel engines and equipment that have near zero emissions, Schaeffer added.

"The United States has experienced a truly remarkable transition with the development of new clean diesel technology," said Schaeffer. "The new diesel truck, car and off-road engines have reduced particulate matter and nitrogen oxide emissions by more than 90 percent in the past two decade."

Schaeffer added, "we're proud to note that this all has been accomplished by an industry that has worked cooperatively with policymakers and environmental stakeholders from California to Washington D.C. and around the world to dramatically reduce emissions while also increasing clean diesel's efficiency and productivity."