# Thermodynamic Analysis of In-Cylinder Steam Assist Technology within an Internal Combustion Engine
###### Abstract
[25; 26; 27; 28; 29] conducted theoretical and experimental studies on the internal combustion Rankine cycle to achieve high thermal efficiency and a low emission powertrain. The results show that the thermal efficiency of a spark ignition engine was improved by 26.1% with the help of a 160 \({}^{\circ}\)C water injection. [PERSON] et al. [30] evaluated steam injection at the power stroke in a natural gas engine. Advancing steam injection timing reduced the brake-specific fuel consumption, with reductions of 3.9-5.2% obtained over different speeds at 50\({}^{\circ}\) CA ATDC. The concept of a six-stroke internal combustion engine cycle was proposed by [PERSON] and [PERSON] [31]. This concept mainly modifies the current four-stroke engine cycle, but with two water injection and expansion strokes added to improve fuel economy and power output. The experiment conducted by [PERSON] et al. [32] shows that the specific fuel consumption reduced by 9%, and brake power improved by 10% at full load (2750 r/min) with stoichiometric AFR. [PERSON] et al. [33] coupled the conventional Otto or diesel cycle with a steam-expansion cycle in an ICE using an open steam power cycle. In this cycle, three cylinders are used for ignition, and a fourth is used for steam expansion. Calculations indicate that thermal efficiency is enhanced by 6.3% at the maximum engine speed (6000 r/min). [PERSON] et al. [34; 35] combined steam injection with turbo-compounding, demonstrating fuel economy gains of 6.0-11.2% in a turbocharged diesel engine at different speeds. [PERSON] et al. [36; 37] also reported that the matching attribute of a turbocharging system with an engine could also be enhanced by pre-turbine steam injection, which improves fuel economy by about 5.9% at full load when combined with the Miller cycle. Despite the research reviewed above, it should be noted that the mechanical structure of a six-stroke engine is complex to implementation and maintenance, and the oxy-fuel combustion mode of ICRC limits its application and industrialization. Although intake port and pre-turbine steam injection enhanced thermal efficiency, they decreased engine charge efficiency, resulting in a limitation of efficiency enhancement ability. The steam assist technology shows potential for ICE implementation because of its high thermal efficiency, low fuel consumption, compact size and flexible features. However, few studies apply steam assist technology in modern gasoline engines. Stimulated by its application prospects, this work provides a thermodynamic study on the feasibility of an in-cylinder steam-assisted cycle in a spark ignition engine, which provides the theoretical basis and boundaries for forthcoming experimental study. In the following sections, a theoretical model is established for an in-cylinder steam-assisted cycle. Some critical parameters, such as injection temperature, injection pressure and intake pressure, are discussed for various injection masses. The thermal efficiency boundaries of this technology are also analyzed at different compression ratios to investigate its maximum potential thermal efficiency. The results provide comprehensive information for the industrial application of the in-cylinder steam assist technology.