A newly developed heavy duty diesel engine in dual fuel mode of operation has been studied in detail. The main fuel would be natural gas presented by Methane and diesel oil as pilot injection. The importance and effects of mixture preparation and formation through ports, valves and in cylinder flow field with different swirl ratio and tumble on diesel combustion phenomena is an accepted feature which has been studied using a developed CFD model together with a KIVA3-V2 code. This analysis is capable to investigate engine geometry, valves lift, and valves timing turbo charging, and its effects on dynamic flow field with variable dual fuel ratio on power and emission levels output. This complete open cycle study of a dual fuel engine has been carried out originally and for the first time. The validation is carried out using developed codes for same engine with experimental data available from test rig later on.