Kinetic Modeling Study of the Ignition Process of Homogeneous Charge Compression Ignition Engine Fueled with Three-Component Diesel Surrogate

An improved surrogate
diesel fuel composition has been proposed
to simulate the autoignition time of diesel fuel under homogeneous
charge compression ignition (HCCI) engine conditions. The surrogate
fuel is modeled as a blend of n-heptane, toluene,
and cyclohexane. Detailed mechanisms consisting of 1140 species and
4590 reactions were constructed by merging well-developed available
chemical kinetics substructures for each chemical species. The optimal
ratio of the selected diesel surrogate fuel components, n-heptane/toluene/cyclohexane = 8:1:1, was determined using trial-and-error
blend methods. Numerically, the modeled heat-release rate obtained
from a zero-dimensional single-zone code for the proposed new model
was intensively validated against detailed single- and two-component
kinetic models together with the referenced experimental engine data.
The obtained results show that the new model provides a remarkable
agreement with the obtained experimental data and can capture the
autoignition angle and the whole combustion process effectively. Sensitivity
analysis and flux analysis were further conducted to understand the
roles of the different hydrocarbon classes in diesel fuels, and the
key elementary reactions involved in ignition inhibition upon the
addition of cyclohexane were identified. It is demonstrated that cyclohexane
is a highly reactive species and can be used as a “tuning knob”
to control the delay between the two stages of combustion.