Comparison of pilot fuel ignited premixed ammonia versus methane dual-fuel combustion

Abstract

In view of reducing greenhouse gas emissions the transition from fossil 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. The 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. Thermodynamic heat release analysis in terms of ignition and combustion characteristics has been established. Simultaneously applied high speed Schlieren/OH* chemiluminescence measurements allow the examination of the combustion process. Premixed ammonia dual-fuel combustion has been compared to methane combustion process. Ignition delay, combustion transition, and turbulent flame propagation as well as heat release characteristics have been investigated within variation of air-fuel equivalence ratio, start of pilot fuel injection, and other operation conditions. Different gas properties (lower heating value, air-fuel ratio) illustrate ammonia lower reactivity affecting heat release and flame propagation. Moreover, 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 is present. The optical investigations confirm the thermodynamic analysis and promote assessment of pilot fuel evaporation, ignition, combustion transition, and flame propagation. Conclusions give insight into the thermo-chemical processes of ammonia pilot fuel ignited dual-fuel combustion.