Auto-ignition in Diesel engines, occurring essentially under non-premixed and partially premixed conditions, is considerably different to homogeneous ignition. In order to study the relevant chemistry--mixing interactions, it is assumed that the ignition of Diesel fuel can be described by using the single component model fuel n-heptane.
Starting from a detailed chemical reaction scheme with about 1000 elementary reactions among 168 chemical components, a skeletal mechanism consisting of 98 reactions and 40 components is derived, which is still capable of describing the auto-ignition process under Diesel engine conditions and concentrations of NO, relevant intermediate components. Introducing steady state assumptions for intermediate species which are consumed rapidly leads to a reduced 14-step mechanism. The mechanism is validated with auto-ignition delay times from shock tube experiments by Adomeit for different temperatures, pressures, and equivalence ratios.
Applying the flamelet model the influence of the turbulent flow field can be described by the scalar dissipation rate, which is a global quantity that can easily be obtained from CFD calculations. One-dimensional flamelet calculations for the auto-ignition process are performed for engine relevant pressures and temperatures. The influence of the scalar dissipation rate on the ignition delay times is discussed and an approximation for the ignition delay times as a function of the scalar dissipation rate is derived. The formula can be used to estimate the departure of the ignition in Diesel engines from the homogeneous ignition delay time.