The Ultra Low Emissions Potential of the Recuperated Split Cycle Combustion System

The recuperated split cycle engine is a fundamentally new class of internal combustion engine that offers a step change in thermal efficiency over conventional Otto and Diesel cycle engines. In a split cycle engine, the compression and combustion strokes are performed in different cylinders. Intensive cooling of the compression stroke by the injection of liquid nitrogen directly into the chamber enables the recovery of waste heat from the exhaust between the compression and combustion cylinders. Brake efficiencies of over 50% have been reported without compression cooling, rising to 60% where the compression stroke is cooled by the injection of liquid nitrogen. The technology targets the heavy duty, long-haul sector where electrification is ineffective. In this paper, results from an experimental program conducted on a single cylinder research engine, representing the combustor cylinder of a recuperated split cycle engine are reported. The effect of fuel injection timing, valve timing and injection pressure were studied at 1200rpm at a range of loads. Experiments using oxygen depleted air to represent the effect of the injection of liquid nitrogen in the compression cylinder are also reported. Engine out NOx emissions of less than 110ppm at mid speed and loads typical of motorway cruise conditions for a truck engine were observed. The low NOx emissions suggest a significant contribution of cool, pre-mixed combustion. A conceptual modeling is proposed that describes the mixing process inside the combustion chamber to explain the low NOx emissions. It is proposed an air jet is formed during the induction of charge air which interacts with the liquid fuel jet, enhancing mixing. This together with the lower charge temperatures used account for the low NOx emissions. The impact of fitting an SCR based aftertreatment catalyst on final vehicle emissions is presented, demonstrating the potential of achieving very low levels of NOx emissions. With aftertreatment, NOx emissions below 5ppm were predicted, meaning the recuperated split cycle engine has the potential of meeting the SULEV NOx standard.