- Ray, J.D.; Bishop, G.; Schuchmann, B.G.; Frey, C.; Sandhu. G; Graver, B. Yellowstone Over-snow Vehicle Emission Tests – 2012: Summary Vehicle Data and Fleet Estimates for Modeling. Natural Resource Technical Report NPS/NRSS/ARD/NRTR—2013/661; National Park Service, U.S. Department of the Interior in Denver, Colorado, January 2013. Link.
- Frey, H.C.; Sandhu G.S. Comparison of Energy Use and Emissions for Three Wheel-Loaders Operated on Ultra Low Sulfur Diesel, B20 Biodiesel, and B35 Biodiesel. Prepared by North Carolina State University for Greener by Design in New Brunswick, New Jersey, September 2012.
- Frey, H.C.; Sandhu, G.S.; Graver, B.M.; Hu, J. Measurement of Fuel Use and Emissions of Over-Snow Vehicles at Yellowstone National Park. Prepared by North Carolina State University for Louis Berger Group in Denver, Colorado, September 2012.
- Frey, H.C.; Sandhu, G.S. Comparison of Fuel Use and Emissions of B-20 Fueled Combination Trucks with versus without a Fuel Additive. Prepared by North Carolina State University for Hydrotex Partners Inc. in Farmers Branch, Texas, October 2011.
- Frey, H.C.; Sandhu, G.S. Comparison of Fuel Use and Emissions of B-20 Fueled Combination Trucks with versus without a Fuel Additive. Prepared by North Carolina State University for Nanostar Advanced Technologies in Burlington, North Carolina, September 2009.
- Frey, H.C.; Choi, H-W.; Graver, B.M.; Sandhu, G.S. Energy Use and Emissions of a Locomotive Engine during Dynamometer Testing. Prepared by North Carolina State University for North Carolina Department of Transportation in Raleigh, North Carolina, July 2009.