Performance of a novel liquid nitrogen power system

Highlights: • Experimental investigation of an LN₂ system with a novel integrated HEF subsystem. • The system achieved up to 41% thermal efficiency and 172 kJ/kg-LN₂ specific energy. • The novel in-cylinder HEF supply notably improved the system thermal efficiency. • The use of HEF improves the compression ratio and the in-cylinder heat transfer. • A thermodynamic model verifies that the engine efficiency benefits from the HEF. In this article, we examine the hybridisation of refrigerated commercial vehicles through replacing the traditionally used auxiliary diesel engine with a non-polluting, non-electric unit as an effective emissions reduction alternative. The zero-emission hybrid solution presented in this article is a Liquid Nitrogen (LN₂) engine system, featured with a novel integrated Heat Exchange Fluid (HEF) subsystem, that can provide simultaneous cooling and auxiliary power in, for example, refrigerated trucks. Evaporation of LN₂ provides the cooling/refrigeration power. The resulting high pressure gaseous N₂ then expands in the engine, producing shaft power. A major contribution of this research is the use of a novel direct in-cylinder HEF supply technology which we show experimentally that it leads to reliable and significantly enhanced engine performance. Specifically, a detailed experimental investigation into the effects of HEF temperature and flow rate at different inlet N₂ conditions and engine speeds on engine performance is presented. Results from a thermodynamic analysis, based on an idealised cycle, are also presented to better understand the engine performance and assess the potential of the proposed engine architecture. The results show up to 41% brake thermal efficiency and up to 172 kJ/kg-LN₂ specific work from the engine system, which are significantly higher figures when compared to previously reported maximum values in the literature (i.e. 9.2% and 40 kJ/kg-LN₂, respectively). It is also shown that the thermodynamic model can predict with good accuracy the upper and lower limits of the measured indicated power and efficiency.