Ammonia–air laminar flame speeds from ambient to IC engine conditions: A review

Ammonia as a renewable fuel has potential to replace hydrocarbons in internal combustion engines, as carbon emissions are absent from the exhaust products. Before it can be considered a viable alternative, its combustion characteristics under Internal Combustion (IC) engine conditions must be thoroughly understood. One of the critical characteristics of combustion for the development of an IC engine is the laminar flame speed (LFS). To date, several studies have been carried out on ammonia–air to measure laminar flame speeds under ambient conditions (herein described as low temperature & pressure) but these conditions are very different to those during IC engine operation. Some studies of the laminar flame speed at elevated pressures & temperatures, including close to IC engine operating conditions, have been published but this information is incomplete and is found in scattered sources. A single reliable source is in demand and one focus of this paper is to consolidate the published information for ammonia–air laminar flame speed characteristics to create interest in ammonia–air based IC engine development. The Cantera software package was used to study various ammonia combustion reaction mechanisms and compare their flame speed predictions with available experimental data. The relevant unburnt gas IC engine conditions were identified and used as initial conditions for the simulations conducted in this study. Our study concludes that experimental validation is required to prove the accuracy of the simulations at engine conditions. In addition, we examine discrepancies that still exist between modeling and experiment LFS under initial conditions that have been studied extensively in the past such as ambient pressures and temperatures.