A DNS Study of the Effects of CH2O and H2O2 Addition on the Ignition of a Lean PRF/air Mixture under HCCI Conditions

Abstract

The effects of dual additives including formaldehyde (CH2O) and hydrogen peroxide (H2O2) on the ignition characteristics of a lean primary reference fuel (PRF)/air mixture under homogeneous-charge compression ignition (HCCI) conditions are investigated using direct numerical simulations (DNSs) with a 118-species PRF/air reduced mechanism. 2-D DNSs are performed by varying two key parameters: 1) amounts, and 2) injection timing of additives. It is found that the overall HCCI combustion with dual additives injection occurs at appropriate time and its heat release rate (HRR) is more distributed over time than that without additives due to the distinct effects of CH2O and H2O2 on low- and intermediatetemperature chemistry (LTC and ITC). The combustion mode analysis reveals that H2O2 injection induces fast combustion process with an excessive HRR due to the dominance of the spontaneousignition, while CH2O injection leads to a longer combustion duration because of the mixed mode of spontaneous-ignition and deflagration with lower peak HRR. Through chemical explosive mode analysis (CEMA), the direct injection of dual additives affects LTC and ITC reaction pathways. In the hightemperature chemistry (HTC) regime, the difference of HCCI combustion between with and without additive injection becomes marginal because the ignition of the PRF/air mixture is more sensitive to temperature fluctuation than additive reactivity. These results suggest that the combined effects of CH2O and H2O2 which induce relatively gradual ignition process for smooth operation of HCCI engines occur at appropriate ignition timing which should be considered in preventing an overly rapid increase in pressure in HCCI combustion.