The aim of the work is a comparison of two combustion systems of fuels with different reactivity. The first is combustion of the fuel mixture and the second is combustion in a dual-fuel engine. Diesel fuel was burned with pure ethanol. Both methods of co-firing fuels have both advantages and disadvantages. Attention was paid to the combustion stability aspect determined by COV IMEP as well as the probability density function of IMEP. It was analyzed also the spread of the maximum pressure value, the angle of the position of maximum pressure. The influence of ethanol on ignition delay time spread and end of combustion process was evaluated. The experimental investigation was conducted on 1-cylinder air cooled compression ignition engine. The test engine operated with constant rpm equal to 1500 rpm and constant angle of start of diesel fuel injection. The engine was operated with ethanol up to 50% of its energy fraction.Processes 2019, 7, 946 2 of 10 opportunities to use alcohol-based fuels in compression ignition engines. In the works devoted to the combustion of fuels in the form of blends or dual fuel mode, reference is made to operating parameters of engine and emission standards fulfillment. Lee et al. presented results of investigation of a heavy-duty single-cylinder diesel engine with two direct injectors. Engine speed was fixed at 1000 rpm and the load condition varied for an indicated mean effective pressure ranging from 0.2 to 0.8 MPa. The ratio of ethanol to the total input energy was controlled from zero to nearly 50% of the input energy. The NO x and PM emissions decreased with increasing ethanol substitution and the mean size of the PM emissions decreased. For the mid-load condition, the substitution was increased to 63%; however, for low and high loads, higher ethanol fractions could not be used because of insufficient ignition energy at low loads and high values of pressure rise under high loads [7]. Similar results were obtained by other authors; all of them showed that the share of ethanol has a positive effect on soot emissions, while there are different opinions regarding NO x emissions [8][9][10][11]. On the one hand, ethanol reduces the temperature on the compressed load due to the high heat of vaporization. On the other hand, it increases the value of the maximum pressure and, consequently, the combustion temperature, contains additional oxygen in the structure, it promotes the formation of NO x [12][13][14]. Ethanol greatly affects the combustion stage. This is confirmed by the result of basic research carried out using specialized combustion chambers [15][16][17][18]. The authors of research papers using piston engines came to similar conclusions with basic research [9,11,19]. In the paper of Paul et al., regarding the investigation of blend combustion, authors presented results of investigation of diesel-ethanol combustion process in diesel engine. Ethanol percentage was increased from 5% to 20% with intervals of 5%, thus reducing the diesel participation. Paul et al. stated that a blend w...