The Tour engine is a novel split-cycle internal combustion engine (ICE) that divides the four-stroke Otto cycle of a conventional ICE between two separate cylinders, an intake and compression cylinder and a second expansion and exhaust cylinder, interconnected by an innovative charge transfer mechanism. The engine working fluid, air and fuel, is inducted into the engine and compressed by a dedicated compression cylinder, transferred with minimal pressure loss via an input port to a specifically designed combined spool shuttle transfer mechanism and combustion chamber. It is then ignited and then transferred from the combustion chamber via an exit port to a separate expansion cylinder where it is expanded and exhausted from the engine. The primary advantage of the Tour engine is that it provides the engineering freedom to independently design, control and optimize the compression, combustion, and expansion processes within a slider-crank piston engine. By decoupling the compression ratio from the expansion ratio and by allowing better combustion phasing, the Tour engine can be optimized to operate on any gaseous or liquid fuel with improved power output, exhaust emissions, and fuel efficiency. Tour Engine Inc. has undertaken the development of this engine technology since 2005, with support from private funding and major grants, from early functional prototypes to the current advanced clean sheet design 5-kW engine, specifically developed for both high efficiency and ultra-low nitrogen oxides (NOx) emissions. The current evolution of the Tour engine, a 5-kW natural gas-fueled spark-ignited engine, has been extensively tested for over 1000 hours of operation without any major failures. This paper describes an overview of the Tour engine architecture as well as the 5-kW prototype engine’s performance, efficiency, and exhaust emissions characteristics. This alternative fuel engine has demonstrated ultra-low engine-out NOx emissions with state-of-the-art brake thermal efficiency (BTE) for an engine in this power range.