3.4.5. Effect of n-Butanol Blending Ratio on CO2 Emissions

Figure 16 shows the effect of DX and n-butanol mixing ratio on CO2 emissions at an equivalence ratio of 0.5. It can be seen that the CO2 emissions decrease significantly with the increase of n-butanol blending ratio. The reason for the decrease in CO2 production is that the C/H ratio of n-butanol is lower than that of DX. The C/H ratio of the mixed fuel decreases with the increase of n-butanol. Therefore, it can be known that CO2 emissions in two-stroke diesel engine HCCI combustion reduce significantly with an increase of n-butanol mixing ratio.

**Figure 16.** Effect of DX and n-butanol blending ratio on CO2 emissions.

Figure 17 shows the effect of n-butanol blending ratio on the C/H mass ratio and the final CO2 emissions of the mixed fuel. It can be clearly seen that the C/H mass ratio of the mixed fuel decreases significantly with the increase of n-butanol mixing ratio. The decrease of the C/H mass ratio leads to the reduction of the final CO2 emissions. The final CO2 emission decreases by about 23.9% when the blending ratio of n-butanol reaches 40%. Therefore, C/H mass ratio and CO2 emissions of the two-stroke diesel engine HCCI combustion can be reduced effectively by increasing the n-butanol mixing ratio in a suitable range.

**Figure 17.** Effect of n-butanol percentage on C-H mass ratio and final CO2 emissions.

## **4. Conclusions**

A mixture of n-dodecane and m-xylene was used as an alternative fuel for diesel. The complete combustion reaction mechanism of diesel (DX), n-butanol and NOx was established for two-stroke diesel engine HCCI combustion. Through the sensitivity analysis, ignition delay time, combustion temperature, in-cylinder pressure, NOx and CO2 emissions were obtained for different n-butanol mixture ratios during the diesel alternative fuel DX HCCI combustion. Through the analysis of reaction paths, the main formation path and chemical reaction rate of NOx were observed accurately. The main findings from this research are as follows:

The peak combustion pressure increased with an increase of initial intake pressure from 0.8 to 1.4 atm on low-speed two-stroke diesel engines HCCI combustion. The combustion pressure of the model used at 1.4 atm in the cylinder was consistent with the actual combustion pressure. The final NO emissions decreased slightly with the increase of initial intake pressure.

The ignition time advances with the increase of initial intake temperature from 380 to 440 K. The combustion pressure of the model used at initial temperature of 420 K was consistent with the actual combustion pressure. The final NO emissions increased slightly with the increase of initial intake temperature.

Different DX and n-butanol mixing ratios were used at a constant total mole fraction. The ignition delay time increased with an increase of n-butanol ratio. The combustion temperature decreased rapidly with the increase of the n-butanol blending ratio after the TDC. The reaction rate and emissions of NOx reduced significantly with the increase of n-butanol mixing ratio. When the blending ratio of n-butanol reaches 40%, the production rate of NO and NO2 could be reduced by more than 40%. The NOx reaction rates decreased rapidly with the increase of n-butanol mixing ratio at the TDC. At the same time, C/H mass ratio and the CO2 emissions decreased significantly with the increase of n-butanol percentage. Therefore, the combustion and emission of DX HCCI combustion can be improved by increasing the n-butanol mixing ratio at the same engine load.

**Author Contributions:** Writing—original draft preparation, S.W.; project administration, J.Z.; writing review and editing, L.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by Research on Intelligent Ship Testing and Verification ([2018]473). The Major Research plan of the National Natural Science Foundation of China (Grant No. 91441132).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the first author.

**Acknowledgments:** All authors are very grateful to the editors and anonymous reviewers for their great contributions on the manuscript reviewing and constructive comments.

**Conflicts of Interest:** The authors declare no conflict of interest.
