# Thermodynamic Analysis of In-Cylinder Steam Assist Technology within an Internal Combustion Engine
## 4 Conclusions  
In this study, the thermodynamic feasibility of an in-cylinder steam-assisted cycle is verified in a spark ignition engine. Some critical parameters, such as injection temperature, steam injection pressure, and intake pressure, are discussed for various steam injection masses. The thermal efficiency boundaries of this technology are also analysed at different compression ratios to investigate its maximum potential thermal efficiency, which provides considerable guidance for the future experimental and numerical studies of in-cylinder steam assist technology into modern engines. The following conclusions can be drawn from this study.  
1. Theoretical calculations based on the developed thermodynamic model show that both steam injection temperature and injection mass improve thermal efficiency. Without steam injection, the base thermal efficiency of the prototype is 46.34%. This gradually increases to 51.11% with increases in steam injection mass and to 61.25% at a steam injection temperature of 600 K. Increases in steam injection pressure contribute little to thermal efficiency.
2. The thermal efficiency of the developed cycle increases with the injected steam enthalpy, which is determined by the exhaust gas energy. However, excessive steam decreases the exhaust temperature, so there is a trade-off between them. According to the assumption of instant in-cylinder steam evaporation, the optimum thermal efficiency of the in-cylinder steam-assisted cycle is 59.71% at an injection temperature of 500 K and a steam injection mass of 0.2 g/cycle.
3. The maximum gain in thermal efficiency achieved with the cycle is 14.5% at a compression ratio of 10. From a thermodynamic perspective, the reason is that the specific heat ratio rises from 1.27 to 1.333, which improves the thermal-heat conversion efficiency. Similarly, increases in compression ratio also benefit the optimum thermal efficiency of the in-cylinder steam-assisted cycle. The optimum thermal efficiency can be increased from 54.0% to 59.71% by increasing the compression ratio from 10 to 16.
4. As this work conducts theoretical investigation regarding in-cylinder steam assist, the real in-cylinder combustion process, intake and exhaust gas exchange process, non-ideal gas effect and other realistic process are simplified, the results of this work only provide theoretical results and guidance for future in-cylinder steam assist application within modern internal combustion engine. Future work will be focused on enhancing the precision of the theoretical model by establishing 1-D or 3-D models.  
Conceptualization, J.W. and Z.K.; Formal analysis, Z.K.; Funding acquisition, Z.K. and Z.W.; Investigation, J.W.; Methodology, J.W. and Z.K.; Project administration, Z.K. and Z.W.; Resources, Z.K. and Z.W.; Software, J.W.; Supervision, Z.W.; Validation, J.W.; Writing--original draft, J.W.; Writing--review & editing, Z.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (NSFC) (Grant Nos. 52002043 and U1832179), China Postdoctoral Science Foundation (Grant No.2021M700581), Chongqing Technology Innovation and Application Development Project (Grant No. cstc2020)sox-msxmX0170) and the Fundamental Research Funds for the Central Universities (Grant No. 2022 CDJX7-006).  
Not applicable.  
The authors declare no conflict of interest.