Understanding the antagonistic effect of methanol as a component in surrogate fuel models: A case study of methanol/n-heptane mixtures

Methanol is a widely used engine fuel, blend component, and additive. However, no systematic auto-ignition data or laminar flame speed measurements are available for kinetic studies of the effect of methanol as a blending or additive component. In this work, both ignition delay times and laminar flame speeds of pure methanol, n-heptane and their blends at various blending ratios were measured at engine-relevant conditions. Results show that increasing methanol in a blend promotes reactivity at high temperatures and inhibits it at low temperatures, with the crossover temperature occurring at approximately 970–980 K with it being almost independent of pressure. The experimental data measured in this work, together with those in the literature are used to validate NUIGMech1.1, which predicts well the experimental ignition delay times and laminar flame speeds of the pure fuels and their blends over a wide range of conditions. Furthermore, kinetic analyses were conducted to reveal the effects of methanol addition on the oxidation pathways of n-heptane and the dominant reactions determining the fuel reactivities. It is found that competition for ȮH radicals between methanol and n-heptane plays an important role in the auto-ignition of the fuel blends at low temperatures. At high temperatures, methanol produces higher concentrations of HȮ₂ radicals which produce two ȮH radicals either through the production of H₂O₂ and its subsequent decomposition or through direct reaction with Ḣ atoms. This promotes the high temperature reactivity of methanol/n-heptane mixtures for ignition delay times and laminar flame speeds, respectively.