Theoretical investigation of indicated thermal efficiency variation within in-cylinder steam assist cycle under different working fluids

Abstract

Improving efficiency and reducing emissions are essential to achieving carbon peaking and carbon neutrality in transportation sector. As a carbon neutral fuel, hydrogen is widely considered the most promising fuel for future zero-carbon internal combustion power, while iso-octane, represented by gasoline, symbolizes the current hydrocarbon fuel utilized in modern internal combustion engines. In this study, the maximum indicated thermal efficiency of both hydrogen and iso-octane utilizing air (blending O2 and N2), oxy-fuel (blending O2 and CO2) and argon power cycle (blending O2 and Ar) were investigated theoretically based on in-cylinder steam assist cycle. A theoretical thermodynamic model combining in-cylinder steam assist and exhaust gases waste heat recovery was established to explore the thermodynamic process of in-cylinder steam assist and systematically analyze the effects of different working fluids. Results show that the utilization of in-cylinder steam assist improves thermal efficiency under O2, N2, and CO2 working fluids, but limits the improvement of thermal efficiency under Ar. In addition, the maximum achievable steam temperature and mass are limited by the exhaust gases temperature and heat exchanger efficiency. When running with H2 under 400 K steam temperature, the optimized water-to-gas ratios were 1.58, 1.13, 1.62, and 1.13 for O2, Ar, N2, and CO2, respectively, the maximum thermal efficiencies increased by 35.80 %, 0.28 %, 28.21 %, and 62.79 % compared with baseline condition. Meanwhile, the competition of overall specific heat ratio and working fluid mass under different working fluids leads to different thermal efficiency enhancement, such as Ar, the decrement of overall specific heat ratio under limited steam mass leads to thermal efficiency deterioration, which can be further improved as steam mass increased.