On the Relevance of Octane Sensitivity in Heavily Downsized Spark-Ignited Engines

Over the years, spark-ignition engine operation has changed significantly, driven by many factors including changes in operating conditions. The variation in operating conditions impacts the state of the end-gas, and therefore, its auto-ignition. This can be quantified in terms of K-factor, which weighs the relative contribution of Research Octane Number (RON) and Motor Octane Number (MON) to knocking tendency at any operating condition. The current study investigates the fuel requirements when operating an engine at increasing intake air pressures. A model engine was operated at varying intake air pressure in GT-Power software, from naturally aspirated intake air to heavily boosted intake air pressure of 4 bar absolute. The pressure-temperature information from the GT-Power model was used to calculate ignition delay times of the unburnt end-gas composed of a sensitive and a non-sensitive fuel in ChemKin software. The results show that high octane sensitivity is desired at negative K values (operating at high intake air pressures). In contrast, zero octane sensitivity fuel performed best at low load operation (positive K). Interestingly, the maximum benefit for using a sensitive fuel was achieved at an intake air pressure of 1.75 bar with diminishing returns at higher intake air pressure for 1000 rpm and at lower intake pressures, as engine speed increased. The pressure effect on auto-ignition tendency was also investigated over existing HCCI data. The auto-ignition tendency was found to be sensitive to octane index in a region of low K value (K~0). This region lies in the negative temperature coefficient (NTC) region, where Primary Reference Fuels (PRFs) shown an increased sensitivity to pressure variation.