The strict regulations related to exhaust gas emissions from internal combustion engines in the transport sector, created by governments around the world, forces manufacturers to improve existing combustion techniques, develop new ones and apply alternative, environmentally more acceptable fuels. In this context, natural gas is recognized as an alternative fuel which can be used both in spark ignition and compression ignition engines with a number of benefits. In recent years, special attention has been given to the research of diesel-natural gas dual-fuel combustion, which has potential to reduce greenhouse gas emissions, as well as harmful exhaust gas emissions. Given that this combustion technique is not fully investigated, but can be easily used in existing diesel engines, there is a need for further improvement, which is the main objective of this work. The purpose of the research is to experimentally examine possibilities of using compressed natural gas in conventional dual-fuel internal combustion engine, with goals that include achieving maximum efficiency and maintaining exhaust gas emissions on permitted levels. Special attention was given to the improvement of characteristics at low and high loads, where dual-fuel shows some disadvantages compared to the normal diesel operation. By using the fully flexible fuel injection system and exhaust gas recirculation, there were achieved different combustion techniques including: diffusion flame combustion, premixed flame propagation and premixed chemically controlled combustion of homogeneous or stratified mixture. It is shown that adequate injection strategies in combination with other operating parameters can significantly improve exhaust gas emissions from dual-fuel engine. Finally, recommendations for selecting operating parameters at low and high loads are given.