Abnormal combustion phenomena with different fuels in a spark ignition engine with direct fuel injection

Characterizing abnormal combustion behavior of different fuels is of significant importance to explore the highest fuel economy and to improve the power density of modern spark ignited (SI) engines. Many experimental and numerical investigations have been reported in the literature for conventional pump fuels, whereas scientific findings for biofuels and gasoline-biofuel blends are less available. In the present study, special focus is set on the correlation of experimental investigations and corresponding auto-ignition theories. Therefore, the auto-ignition characteristics of biofuels and gasoline-biofuel mixtures for two different abnormal combustion phenomena (hot spot surface-ignition and gas phase auto-ignition under typical engine knock conditions) are studied using a direct injection spark ignition (DI-SI) single cylinder research engine featuring variable charge motion and a compression ratio of 11. The obtained results for all tested fuels are benchmarked against investigations using conventional E0RON95 fuel and isooctane as reference fuels. Additionally, kinetic investigations are performed to confirm the experimental results combined with auto-ignition theories. It could be shown via engine experiments with a controlled hot surface that the hot spot surface-ignition tendency is influenced by the chemical heat release rate of the fuel and the charge cooling effect due to the stoichiometric air requirement related heat of vaporization. Due to the increased enthalpy of vaporization with the applied (engine) configuration all gasoline-biofuel mixtures improve the hot spot surface-ignition propensity compared to the reference fuel EORON95 by 20-50 K. The gas phase auto-ignition propensity decreases substantially with increasing ethanol content, revealing a boost pressure increase of at least 40-60 % under conditions typically relevant for knocking combustion in the end gas. For the applied configuration and with direct fuel injection, the location of auto-ignition has the highest frequency near the spark plug for all investigated fuels. Often, the most critical point for auto-ignition is the center electrode or the gas space in its immediate proximity between the ceramic insulator and thread.