An investigation on the feasibility of converting a two-stroke Detroit Diesel - Allison Model 12V-149T Diesel Engine to burn natural gas with pilot injection of diesel fuel for ignition has been completed. It was found that, by mixing the natural gas with the intake air, it was impossible to achieve 70% or higher diesel fuel substitution and 1,000 BHP (746 KW) at the same time due to either flash back or engine knock. Flash back occurred whenever the natural gas concentration was set in the 5-6% range. At 1,900 rpm and maximum power, engine knock occurred with less than 65% diesel fuel substitution. With 80-85% diesel fuel substitution, and very low diesel fuel injection rates, the engine developed less than 85% of its power at 1,900 rpm.