There's been a lot of discussion on the use of LP-Gas (bottled gas) for farm tractors. A few farmers are using it. This article reports the results of some research on its use made of Iowa State College.
"LP-Gas for Tractors,"
Iowa Farm Science: Vol. 6
, Article 4.
Available at: https://lib.dr.iastate.edu/farmscience/vol6/iss5/4