Efficiency of a new Triangle Cycle with flash evaporation in a piston engine

•The technical realization of a new Triangle Cycle with flash evaporation in a piston engine is described.•A thermodynamic model of the isentropic efficiency with different impacts is presented.•Dead volume, residual mass, injection valve performance and wall heat transfer have been taken into account.•The efficiency of the expansion unit significantly depends on temperature, stroke volume and engine speed.•The Triangle Cycle has a significantly higher efficiency than the Organic Rankine Cycle for heat sources below 450K.

A Triangle Cycle with a piston engine expansion unit is used to convert low temperature heat into electrical energy. In this process, the isentropic efficiency of the expansion unit is considered to be unknown, and a theoretical approach for the calculation of isentropic efficiency is presented. A number of influences are taken into account – dead volume, residual mass, liquid injection performance and wall heat transfer. Various working fluids are investigated in a wide range of temperatures (333K–573K), engine speeds (5 Hz–30 Hz) and stroke volumes (0.1 L–50 L). The isentropic efficiency of water as working fluid is in the range of 0.75–0.88 and drops significantly for high stroke volumes and engine speeds. In general, injection mass has the most impact on isentropic efficiency because it influences dead volume and injection performance. The injection mass increases with vapor density and therefore is significantly influenced by working fluid and temperatures. The Triangle Cycle is compared with Organic Rankine Cycles by using determined isentropic efficiency. The exergetic efficiency of the Triangle Cycle using water is up to 35–70% higher than that of supercritical Organic Rankine Cycles in situations with a heat source temperature of up to 450K.