Control mechanism of negative-temperature-coefficient phenomenon based on the first temperature jump in two-stage auto-ignition

Two-stage auto-ignition is often found in homogeneous charge compression ignition (HCCI) engines, which simultaneously achieve high efficiency and low emissions. Two-stage auto-ignition in the intermediate-temperature range causes a negative-temperature-coefficient (NTC) phenomenon, and this work focuses on the relationship between NTC and the first temperature jump in two-stage auto-ignition. Homogeneous models with and without droplet evaporation are applied under different pressures (5–20 bar), temperatures (600–900 K), diameters (0–100 μm), n-heptane molar fractions (0∼100%) and equivalence ratios (0.25–1.0). The results show that the mechanism of NTC control is different for the two models. For the model without evaporation, the NTC phenomenon is closely related to the linear slope of the first temperature jump to the initial temperature. A slope map is divided into NTC, zero-temperature coefficient (ZTC) and non-NTC regions when the slope is less than, equal to or more than the critical slope. For the model with evaporation, the ZTC phenomenon is found when two-stage auto-ignition translates to single-stage auto-ignition. Two- and single-stage auto-ignition are distinguished based on whether the first temperature jump is higher than a critical value. The NTC control helps to improve the heat-release rate because violent heat release limits the application of HCCI engines at high load.