Electrification of long haul freight applications offers a number of major challenges mainly the cost and weight of on-board energy storage. Efforts to reduce the cost and complexity of electrification will continue, but there will remain a long term need for a clean and efficient chemically fuelled thermal powertrain. Best in class Otto and Diesel cycles engines are now approaching the practical limits of efficiency, requiring new approaches to deliver future improvements. Harnessing waste heat through a bottoming cycle delivers limited benefit due to the narrow temperature range at which heat is recovered and rejected. Integration of heat recovery directly to the main power cycle, via a ‘split engine cycle’ offers a novel approach to achieving significant improvements in efficiency. In the split engine cycle, compression and combustion strokes are performed in separate chambers. This enables direct cooling of the compression cylinder reducing compression work, intra cycle heat recovery and low heat rejection expansion. Previously reported analysis has shown that brake efficiencies approaching 60% are attainable. However, the achievement of complete, stable, compression ignition combustion has remained elusive to date. The challenge is to induct the charge air close to top dead centre into the hot combustion cylinder and then inject and burn the fuel before the piston has travelled significantly down the expansion stroke. In this paper, we report results from testing on a single cylinder combustion research engine. The test facility will first be described, including how the high pressure and temperature intake conditions were achieved and controlled. Combustion results will be presented including heat release analysis demonstrating that the required combustion stability and burn rates can be achieved at the conditions studied. The paper concludes with cycle analysis using combustion data derived from the test results to determine the efficiency of the complete engine cycle.