Feasibility study on power generation from waste heat recovery of a passenger car exhaust gas by using a steam engine

A steam Rankine cycle has been studied for its applicability to a passenger car to produce power from waste heat recovery of exhaust gas. Main components of the steam Rankine cycle are super-heater, boiler, steam engine, condenser, and feed pump. Super-heater and boiler are used to recover waste heat of the exhaust gas, and steam engine is used to convert recovered waste heat to shaft power. For steam engine, swash plate type expander was designed and its performance was numerically estimated. At the vehicle speed of 120 km/hr, the shaft output of a five-cylinder expander was calculated to be 1.973 kW under the design operating conditions of inlet and outlet pressures of 25 bar and 2.5 bar, respectively, and with inlet temperature of 300°C. The expander output was equivalent to about 7.4% of the driving power. The expander and Rankine cycle efficiencies were 67.5% and 12.1%, respectively. At lower vehicle speed, the relative expander output becomes lowered mainly because of reduced volumetric efficiency: it reduced to 3.5% at 60 km/hr.