A Numerical Investigation on Ignition Characteristics of n-Heptane/Methane in Homogenous Systems and HCCI

A promising solution for improving combustion performance and reducing emissions is to control the ignition behavior, such as ignition delay in internal combustion engines. The purpose of this study is to numerically investigate the effect of natural gas on the ignition characteristics of diesel fuel in a homogeneous system and an HCCI engine. Methane is considered as the main component of natural gas and n-heptane is considered as the surrogate for diesel fuel. Simulations are performed using Chemkin software for a constant-pressure reactor, a constant-volume reactor and an HCCI engine. Parameters considered in this study include n-heptane/methane blend composition, initial temperature and equivalence ratio for homogenous systems with constant pressure or constant volume. In addition, effects of the start of injection, equivalence ratio and blend composition in an HCCI engine are examined. The three different mechanisms used are the CRECK mechanism (435 species and 13532 reactions) developed at Polytechnic University of Milan, LLNL mechanism (874 species and 3796 reactions) developed at Lawrence Livermore National Laboratory and Chalmers mechanism (73 species and 417 reactions) developed at the Chalmers University of Technology in Sweden. These mechanisms are validated using ignition delay data for n-heptane and methane. Numerical results are found to be in good agreement with experiments. CRECK and LLNL mechanisms predict a continuous increase in ignition delay as the amount of methane is increased in the blend. However, Chalmers mechanism shows that the ignition delay first decreases and then increases. In order to identify the important reactions, a sensitivity analysis is performed using the CRECK and Chalmers mechanisms. Simulations are also performed using the engine simulator of Chemkin and 3D CFD software Converge. An additional comparison using homogenous constant volume reactor and closed internal combustion engine simulator has been performed under comparable conditions. The effect of blend composition on two-stage ignition process has been analyzed. As more methane is added, the first stage ignition delay remains nearly constant, while the second stage ignition delay increases dramatically. It is shown that the first- and second-stages ignition delays for HCCI engine and constant-volume system show quantitatively good agreement with each other. Finally, the effect of adding methane on BMEP and the production of important species, NOx and PAH are discussed.