Defining the Boundary Conditions of the CFR Engine under MON Conditions, and Evaluating Chemical Kinetic Predictions at RON and MON for PRFs 2021-01-0469

Expanding upon the authors’ previous work which utilized a GT-Power model of the Cooperative Fuels Research (CFR) engine under Research Octane Number (RON) conditions, this work defines the boundary conditions of the CFR engine under Motored Octane Number (MON) test conditions. The GT-Power model was validated against experimental CFR engine data for primary reference fuel (PRF) blends between 60 and 100 under standard MON conditions, defining the full range of interest of MON for gasoline-type fuels. The CFR engine model utilizes a predictive turbulent flame propagation sub-model, and a chemical kinetic solver for the end-gas chemistry. The validation was performed simultaneously for thermodynamic and chemical kinetic parameters to match in-cylinder pressure conditions, burn rate, and knock point prediction with experimental data, requiring only minor modifications to the flame propagation model from previous model iterations.

A recently published chemical kinetic mechanism was implemented in GT-Power to match experimental knock point values. The model yielded excellent agreement in terms of the knock point phasing, burn rate, and the thermodynamic conditions compared to the experiments. The developed model provides the initial/boundary conditions of the CFR engine under MON conditions, including IVC temperature and pressure, Mass Fraction Burned (MFB) profile, residual fraction and composition. These conditions were then correlated with polynomial fits as a function of CFR engine compression ratio, analogous to the previous work with RON.

Following this, the boundary conditions from the authors’ previous work and the current work were then used as inputs into a 0-D SI engine model in Chemkin Pro to evaluate chemical kinetic mechanism auto-ignition predictions under RON and MON conditions for PRF blends between 80 and 100. The tested mechanisms represent the state of the art in gasoline kinetic modeling. Compression ratio was swept for each PRF blend, and the predicted knock point crank angle was compared to the experimental baseline to establish a predicted octane number (ON) based on mechanism performance. While qualitatively, the mechanisms exhibited the expected trends in auto-ignition tendencies for the PRFs, all mechanisms exhibited significant discrepancies between predicted and expected octane number for the full range of tested PRFs under RON, MON, or both conditions, up to 10-20 ONs for some mechanisms.