Thermal efficiency boundary analysis of an internal combustion Rankine cycle engine

Abstract

This paper discusses a novel oxy-fuel combustion method named ICRC (internal combustion Rankine cycle) used in reciprocating engines. Pure oxygen replaces air as oxidant for NOx emission avoidance and CO2 recovery. Water is heated up through heat exchanger by exhaust gas, and then injected into the cylinder near top dead center to control the combustion temperature, meanwhile increases the mass of working fluid and therefore enhances the thermo efficiency of the cycle. An ideal engine thermodynamic model combined with a heat exchange model was developed to investigate the thermal efficiency upper boundary of this cycle. The results indicate that the added water increases the thermal efficiency significantly considering the heat exchange between water and exhaust gas, and thermal efficiency increase from 33% (without water injection) to 56% when engine speed is 2000 rpm and engine compression ratio is 9.2. Lower engine speed, intake pressure and higher compression ratio are propitious to higher thermal efficiency. The best thermal efficiency of the whole ICRC system can reach to 58% when engine compression ratio is 14. Thus this concept has the potential for high thermal efficiency and low emission.