Prediction of Diesel Combustion and Emission Characteristics in CI Engine Using Computational Fluid Dynamics Simulations

Abstract

Computational Fluid Dynamics (CFD) study of compression ignition (CI) engines provides invaluable insights into in-cylinder conditions and processes, which greatly expands on the very limited detail provided by engine output measurements, fuel consumption measurements, and engine-out measurements of exhaust emissions. CFD modeling and simulation has therefore become an attractive alternative for engine analysis in place of full experimental testbed study in recent years. In this research work, the performance of a single cylinder four stroke diesel engine was investigated. Commercial simulation software ANSYS Forte was used to study the combustion and emission characteristics of a diesel engine, in order to establish strategies for improvement of in-cylinder combustion and emission control. Normal heptane (n-heptane) was used as the surrogate fuel to represent diesel. Simulation results are compared against data from experimental testbed studies in terms of in-cylinder pressure profiles, heat release rate and exhaust emission of oxides of nitrogen (NOx), soot and unburned hydrocarbon (UHC) levels. The pressure trace from the simulations is found to be within a reasonable error limit of 10%. The combustion process is simulated with special focus on exhaust emissions of soot, NOx and unburned hydrocarbon. Graphical plots for mass fraction of soot, NOx and UHC are presented and discussed to elucidate the formation of these emissions. Graphics contours of temperature, NO mass fraction and oxygen concentration within the combustion chamber are also presented and discussed. The effects of injection timing on engine in-cylinder pressure, heat release rate and exhaust emissions are also studied by varying the injection timing and maintaining constant injection duration. Results are compared for the three different injection timings investigated, namely start of injection (SOI) 18° bTDC, 15° bTDC and 12° bTDC. Emissions of soot and NOx are found to decrease with retarded injection timing. However, the peak in-cylinder pressure is greatly reduced and hence the output power is low. Injection timing is found to have no significant effect on emissions of UHC. The optimum injection timing that gives high output power and relatively low emission is 15° bTDC.