## 4 Conclusions  
The research was performed with a four-stroke diesel engine at a speed of 1400 rpm, boost pressure values from 0.15 to 0.45 MPa, and a constant CR injection pressure value of 300 MPa. The value of the injection fuel was kept constant at 60 mg per cycle, and the diameter of the combustion chamber increased from 60 to 100 mm on the condition that the compression ratio was unchanged. Based on the results received, we have the following conclusions:  
* The boost pressure has a great influence on the fuel injection characteristics. As the boost pressure increases, the penetration and the spray tip velocity decrease, but the spray cone angle of the fuel spray increases.
* The increase in the boost pressure has a positive effect on the mixing rate due to an increase in the value of the pressure and the temperature at the end of the compression stroke. Therefore, at the value of \(D_{b}\)/\(T\) = 10, the boost pressure variation from 0.15 to 0.45 MPa leads to a decrease of 38% in \((dp/d\varphi)_{\rm max}\) and of 9% in \(T_{\rm max}\).
* When the diameter values of the combustion chamber increase, the penetration and the value of the spray tip velocity also increase, but it slightly affects the spray cone angle. Additionally, the variation in the diameter of the combustion chamber has an influence on the fluctuation of the spray tip velocity and penetration.
* With an increase in the \(D_{b}\)/\(T\) ratio, the time of ignition delay decreases and the amount of heat transfer to the wall of the combustion chamber increases. Additionally, the combustion process is faster and produces more effective work by the expansion process.
* The maximum values of the indicated power and the ISFC depend mainly on the \(D_{b}\)/\(T\) ratio. The excess air ratio does not depend on the \(D_{b}\)/\(T\) ratio, but it has a weak effect on \(p_{\rm max}\) at the \(\varepsilon\) value unchanged condition.
* At a high boost pressure, the geometry of the combustion chamber has a slight effect on the production of NO\({}_  
* As the boost pressure goes up, carbon monoxide (CO), hydrocarbons (HC), and soot increase.  
Conceptualization, A.Y.D.; methodology, A.Y.D.; software, T.Q.N.; validation, T.Q.N. and A.Y.D.; formal analysis, A.Y.D.; investigation, T.Q.N.; resources, T.Q.N.; data curation, T.Q.N.; writing--original draft preparation, T.Q.N.; writing--review and editing, A.Y.D.; visualization, A.Y.D.; supervision, A.Y.D.; project administration, T.Q.N.; funding acquisition, T.Q.N. All authors have read and agreed to the published version of the manuscript.  
This research is funded by University of Transport and Communications (UTC) under grant number T2023-CK-004 TD.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
Authors also sincerely thank the teachers and colleagues of Faculty of Heat Engineering and Automotive Engines in Moscow Automobile and Road Construction State Technical University (MADI) who supported the research.  
The authors declare no conflicts of interest.  
The reaction mechanism model of soot formation is applied in the model:  
\[\text{C}_{\text{n}}\text{H}_{\text{m}}+\frac{\text{n}}{2}\text{O}_{2}\to \text{nCO}+\frac{\text{m}}{2}\text{H}_{2}\] (A1)  
\[\text{H}_{2}+\text{H}_{2}+\text{O}_{2}\to\text{H}_{2}\text{O}+\text{H}_{2} \text{O}\] (A2)  
\[\text{CO}+\text{CO}+\text{O}_{2}\to\text{CO}_{2}+\text{CO}_{2}\] (A3)  
\[\text{CO}+\text{H}_{2}\text{O}\to\text{CO}_{2}+\text{H}_{2}\] (A4)  
\[\text{C}_{\text{n}}\text{H}_{\text{m}}+\text{C}_{\text{n}}\text{H}_{\text{m}} \to 2\text{nC}+\text{mH}_{2}\] (A5)  
\[\text{C}+\text{C}+\text{O}_{2}\to\text{CO}+\text{CO}\] (A6)  
\[\text{C}+\text{H}_{2}\text{O}\to\text{CO}+\text{H}_{2}\] (A7)