Modelling of Turbulent Combustion of Liquid Ammonia in Dual Fuel CI Engine

Combustion of fossil fuel results in CO2 emission which is considered to be a leading cause of global warming. Alternative fuels with little or no carbon foot is being actively considered. Direct combustion of ammonia is being actively investigated as it is considered to be a carbon neutral fuel that does not emit CO 2 . It is one of the largest industrially produced chemicals and is easy to store and transport with a well-established supply-chain system. Compared to conventional fuels, ammonia has inferior combustion properties like high ignition delay, low flame speed and a high propensity to produce NOx due to the availability of fuel bound nitrogen. This study uses numerical simulations and a chemical kinetic model to examine the combustion characteristics of ammonia and diesel blend in a dual fuel compression ignition IC engine. Both liquid ammonia and diesel are directly injected into the cylinder, with ammonia as the primary fuel and diesel as a pilot to initiate combustion. An in-house 242 species and 1769 reaction chemical kinetic mechanism is employed to simulate the combustion kinetics, incorporating a semi-detailed reaction scheme that accounts for NOₓ formation and soot sub-mechanism. Turbulent combustion is modelled using an unsteady flamelet approach. N-dodecane is used as a diesel surrogate fuel. Simulations are performed over the closed engine cycle; i.e. between crank angles of intake valve closure (IVC) and exhaust valve opening (EVO). The study focuses on an ammonia energy share of 50%. It investigates the effects of varying ammonia injection timing from -90° to -9° before the top dead center (BTDC) and ammonia injection pressure ranging from 80-100 bars, while maintaining a fixed diesel pilot injection timing at -9° BTDC. Key combustion parameters, such as in-cylinder pressure, peak cylinder pressure, heat release rate, ignition delay, and combustion phasing, are analyzed under different operating conditions. The study aims to evaluate the influence of ammonia start-of-injection (SOI) on effective work done, NOₓ emissions, and overall combustion performance in a dual-fuel ammonia–diesel engine configuration.