Performance Prediction of a Practical Low-Pressure-Ratio Highly Efficient Split-Cycle Recuperated Engine

Split-cycle recuperated engines are promising candidates to compete with hydrogen-based fuel cells for high-duty cycles. They can potentially achieve similar, or even higher, efficiencies at the cost of historically cheap piston engines. However, existing approaches are either limited in efficiency or difficult to develop, mainly because of the challenges around the high-temperature expansion piston. This article presents a practical architecture of a low-pressure-ratio, recuperated split-cycle engine with a contact-free expansion piston using labyrinth seals supported by thermodynamics and numerical modeling. The engine operates under a regenerative dual Brayton cycle to combine the benefits of constant pressure heat recuperation and near-constant volume combustion. Thermodynamics results reveal pre-compressing the residual mass in the expansion cylinder before intake is crucial. A 0D transient model integrating main losses is implemented to explore the design space and maximize efficiency through a numerical design of experiments. The blowby in the expansion cylinder is the main loss but remains acceptable for relatively tight clearances. An indicated efficiency of 60% is predicted for a cycle pressure of 20 bar and an expansion piston exhaust temperature of 1250 K. The predicted indicated power density of 6.5 kW/L is relatively low but in the range of micro-combined-heat-power diesel engines.