The recuperated split cycle engine is a fundamentally new class of internal combustion engine that offers a step change is thermal efficiency over conventional Otto and Diesel cycle engines – 50% brake thermal efficiency in its simplest form, approaching 60% with intensively cooled compression. The technology targets the heavy duty, long-haul sector where electrification is most challenging. In a split cycle engine, the compression and expansion strokes are performed in different cylinders. Waste exhaust heat is recovered between the compression and combustion cylinders via a recuperator which gives precise control of the charge temperature. Recent experimental and analytical research has shown the split cycle combustion system also has the potential to achieve ultra-low emissions. Experimental studies on a one litre single cylinder research engine, representative of a typical medium duty truck engine have showed engine out NOx emissions of less than 110ppm at mid speed – mid load, typical of a motorway cruise condition. Rigorous analysis of the impact of applying an SCR based aftertreatment system showed tailpipe NOx emissions of less than 5ppm are readily achievable. The implications of these results are that a recuperated split cycle engine has not only the potential of delivering world leading efficiency, but also ultra-low emissions - within the California SULEV limit initially, but with potential for zero-impact levels placing the ICE on a par with battery-electric and fuel cell propulsion. In this paper, results from an experimental programme on single cylinder engine, representing the combustor cylinder of a split cycle engine are reported. Response to a range of parameters such as valve timing, injection timing and charge air pressure, temperature and oxygen concentration are reported. A number of combustion modes were observed at engine out NOx levels ranging from those typical of a conventional diesel engine to very low levels, indicative of a highly pre-mixed ‘cool’ combustion regime. The reason for the observed changes in the in-cylinder processes are discussed in the context of the unusual air-fuel mixing process in a split cycle engine. The impact on the overall cycle, including final tailpipe emissions with an SCR based aftertreatment strategy are presented to assess the in vehicle performance of a split cycle engine powered commercial vehicle.