According to the US government, the automobile makers have to meet the strict standards for 54.5 gallons per mile. in this case, they have to innovate a new technology for saving fuel.

so they found a new technology called the clean diesel. “Ultra low sulphur diesel fuel has been refined so that its sulphur content is 15 parts per million (ppm) or less. This is 97 percent cleaner than the standard highway-use diesel fuel sold in the US, which contains an average of 500 ppm of sulphur.”

there is another technology named direct injection. it can also improves fuel efficiency and gain more power within the same displacement.

Sources:

http://auto.howstuffworks.com/fuel-efficiency/alternative-fuels/how-clean-diesel-fuel-works1.htm

http://www.hotrod.com/techarticles/engine/hrdp_0909_direct_injection_guide/