Numerical study of autoignition of fuel-air mixtures at elevated temperatures and pressures

Abstract

In a conventional spark-ignition (SI) engine the fuel-air mixture is injected into the cylinder where it is mixed with the residual gasses and compressed. Under normal operation, combustion takes place at the end of the compression stroke, the mixture is ignited by a spark, flame kernel grows, turbulent flame develops and propagates to the walls where it is quenched. As we strive for better thermal efficiency, the compression ratio is the ideal parameter to increase. However, a phenomenon called knock occurs at elevated temperatures and pressures and impedes the improvements to thermal efficiency by simply increasing the compression ratio. Knock occurs because of auto-ignition initiated locally in hot spots in the unburnt fuel-air mixture ahead of the advancing flame front. It deteriorates the working characteristics of the engine and drastically reduces its durability. Previously, most of the research into knock was carried out experimentally. The recent developments in computer hardware, software, and dedicated chemical kinetic mechanisms that can accurately model the behaviour of gasoline have made it possible to calculate the autoignition of fuels with acceptable accuracy. The main objective of this project is to study the autoignition behaviour of various fuel-air mixtures at elevated temperatures and pressures using the aforementioned chemical kinetic mechanisms. The autoignition of commercially available fuels is the focus of this thesis. The chemical kinetic mechanisms chosen are from RWTH Aachen University, King Abdullah University of Science, and Technology and Lawrence Livermore National Laboratory. Using the software CHEMKIN-Pro, these mechanisms are adopted to simulate the ignition delay times of iso-octane, n-heptane, and primary reference fuel blended with ethanol (PRF-E) at various temperatures, pressures, and equivalence ratios (fuel-air ratio). The calculated values of ignition delay times are compared with the experimental data available for the same input parameters and the chemical mechanisms from Aachen university is chosen based on the accuracy of the results and the speed of computations. Using the selected mechanism, ignition delay times for E10 (fuel of RON95 with 10% ethanol by volume), 95 unleaded (RON95), and 98 unleaded (RON98) are calculated. The knock prediction model by Kalghatgi et al., 2017 is used as the criterion for suggesting the best operating range temperature range of the fuels to avoid knock in an engine. Using MATLAB and Excel, graphs for the calculated ignition delay times vs temperature are plotted to study the dependence of various input parameters (temperature, pressure, equivalence ratio, and fuel composition). The following results are observed: • The ignition delay times reduce with an increase in the input pressure at a particular temperature. • The ignition delay times decrease with an increase in equivalence ratio (ϕ). • The knock resistance of a fuel increases with an increase in the RON of a fuel. • The ignition delay times reduce with an increase in temperature at a particular pressure.