**2. Kinetic Models and Methods**

The IC engine module in the CHEMKIN-PRO software is used to simulate a lowspeed two-stroke diesel engine HCCI combustion. The selected model is the B&W 6S70MC engine produced by MAN. This research combines the parameters of this engine model with closed internal combustion on engine simulator and applies the HCCI combustion method to a marine diesel engine to study the effects of n-butanol on diesel surrogate fuel HCCI combustion and emission characteristics. The fuel is completely homogeneously mixed with the air during the intake and compression strokes to form a lean, homogeneous mixture. When the piston is compressed to the vicinity of the TDC, the combustion of the mixed gas in the cylinder is realized by the self-ignition of the air-fuel mixture. Table 1 shows the main technical parameters of the diesel engine [38].


**Table 1.** Test engine parameters and specifications.

The diesel surrogate fuel (77% n-dodecane and 23% m-xylene) reaction mechanism model came from the Lawrence Livermore National Laboratory [13], which is defined as DX in this study. The n-butanol combustion reaction mechanism came from the National University of Ireland [39]. The NOx combustion reaction mechanism came from the C0-C1 NOx mechanism of the National University of Ireland [40]. These mechanisms were combined through ANSYS Workbench to eliminate duplicate reactions and redundant species. Then, a detailed mixed fuel combustion reaction kinetics mechanism was constructed to study the HCCI combustion and emission characteristics of two-stroke diesel engines. Table 2 shows the comparison of the properties of n-butanol, DX [41] and actual diesel [41]. The cetane number of DX is much higher than that of actual diesel, while the cetane number of n-butanol is lower. Therefore, the cetane number of mixed fuel can be reduced to close to the cetane number of actual diesel by mixing with n-butanol.

**Table 2.** Properties of fuels.


The actual equivalence ratio of the low-speed two-stroke diesel engine is generally 0.45 to 0.5. In this study, the equivalence ratio is 0.5. Four different initial intake pressures of 0.8, 1.0, 1.2, and 1.4 atm are selected as the pressure boundary conditions of the two-stroke diesel engine HCCI combustion. Four different initial intake temperatures of 380, 400, 420, and 440 K are selected as the temperature boundary conditions for DX HCCI combustion. At a certain total mole fraction, DX and n-butanol are mixed in different proportions, and the mixing ratio of n-butanol increases gradually. Table 3 shows the mole fraction and cetane number of mixed fuel and air. Argon is used as the filling gas and does not participate in the actual reaction.

**Table 3.** Species composition ratio and cetane number of DX-NB mixtures.


#### **3. Results and Discussion**

This article first verifies the reliability of the skeleton mechanism by comparing the ignition delay with the experiment. Then, the effects of different initial conditions and n-butanol blending ratio on two-stroke diesel engine DX HCCI combustion are researched in the study.
