Optical investigation and thermodynamic analysis of premixed ammonia dual-fuel combustion initiated by dodecane pilot fuel

In view of reducing greenhouse gas emissions the transition from fossils fuels to sustainable energy carriers is a prerequisite to keep global warming within tolerable limits. Since IC engines will continue to play a role in global energy strategies during a transitional phase, especially for large engine applications difficult to electrify, the use of ammonia as substitute fuel may be an approach for decarbonization. However, its utilization needs research since ignition concepts and combustion properties still pose considerable challenges in view of reliable and efficient operation. A new "optical engine" test facility ("Flex-OeCoS") has been successfully adapted enabling dodecane pilot fuel ignited premixed ammonia dual-fuel combustion investigations. It features IC engine relevant operation conditions such as pressures, temperatures, and flow (turbulence) conditions as well as adjustable mixture charge composition and pilot fuel injection settings. In parallel, thermodynamic heat release analysis in terms of ignition and combustion characteristics was performed. Simultaneously applied high-speed Schlieren/OH* chemiluminescence measurements supported the examination of the combustion process. Initially premixed ammonia dual fuel combustion has been compared to a representative methane combustion process in terms of different gas properties (lower heating value, air-fuel ratio) which illustrates its lower reactivity affecting heat release and flame propagation. Moreover, ignition delay, combustion transition, and turbulent flame propagation as well as heat release characteristics have been investigated for premixed ammonia dual-fuel combustion within variation of air-fuel equivalence ratio, start of pilot fuel injection, and pressure/temperature operation conditions. The results illustrate strong dependency on air-fuel equivalence ratio (energy content) and temperature conditions in terms of ignition delay, dual-fuel combustion transition, and corresponding heat release. The optical investigations confirm the thermodynamic analysis and promote assessment of pilot fuel evaporation, ignition, combustion transition, and flame propagation. Conclusions give extended insight into the thermo-chemical processes of ammonia pilot fuel ignited dual-fuel combustion. The acquired data may also support further development of numerical CRFD methods.