The paper presents some results of an examination of a dual fuel compression ignition engine fuelled with natural gas and diesel oil and a new concept of a dual fuel highly efficient internal combustion engine. For the examined dual fuel engine the homogeneous air-natural gas mixture was formed outside the engine cylinder and was ignited by a dose of diesel oil directly injected to the cylinder. The analysis of the combustion of a dual fuel mixture is based on cylinder pressure diagrams. The diagrams show some problems connected with self-ignition and control of the combustion of the mixture that should be solved to obtain a better engine performance and lower emissions. Particularly important is the control of the combustion process that causes many problems such as knock combustion or engine overheating. For the above-mentioned solution it is also difficult to reduce the engine emissions of hydrocarbons and nitrogen oxides. As a result of the analysis of the theoretical engine cycle and the dual fuel engine performance as well as some phenomena connected with dual fuelling of compression ignition engines a new idea of fuelling and combustion control was created. Generally, the idea combines Otto and Sabathè theoretical processes in one engine. The Otto process, which is more efficient, would be applied for lower engine load but the Sabathè process with changing share of heat supplied at constant volume is to be applied for higher engine loads. The concept of a dual fuel highly efficient internal combustion engine foresees the application of ethanol and diesel oil and two separate direct injection systems for these fuels as well as spark ignition system which would enable the ignition of stratified ethanol-air mixture. The engine can run in two modes - spark ignition mode (the engine fuelled with ethanol) for lower loads and compression ignition mode (the engine fuelled with ethanol and diesel oil) for higher and full loads. The ethanol-air mixture is lean for both modes. It could also contribute to higher engine efficiency as well as a relatively high maximum cylinder pressure for all ranges of loads. The tests of the new concept that are to be carried out will focus on the optimization of the engine control parameters. The parameters of the injection of both fuels and amount of air (controlled by throttle) would be changed in a very high range that could give a higher efficiency and lower emissions. It is also possible to apply natural gas instead of ethanol for the new concept of fuelling of IC engine. The concept could be described as dual fuel mixed ignition (DFMI) engine.