Design and Validation of a GT Power Model of the CFR Engine towards the Development of a Boosted Octane Number 2018-01-0214
Developments in modern spark ignition (SI) engines such as intake boosting, direct-injection, and engine downsizing techniques have demonstrated improved performance and thermal efficiency, however, these strategies induce significant deviation in end-gas pressure/temperature histories from those of the traditional Research and Motor Octane Number (RON and MON) standards. Attempting to extrapolate the anti-knock performance of fuels tested under the traditional RON/MON conditions to boosted operation has yielded mixed results in both SI and advanced compression ignition (ACI) engines. This consideration motivates the present work with seeks to establish a pathway towards the development of the test conditions of a boosted octane number, which would better correlate to fuel performance at high intake pressure conditions.
In this work, a one dimensional engine model was developed in GT-Power of the cooperative fuel research (CFR) engine following the standard conditions of the RON and MON methods (ASTM 2699 and ASTM 2700 respectively), and validated with experimental data sourced from the relevant literature. The full-flow model utilizes the built-in predictive SI flame propagation model with coefficients tuned to match experimental data, including a new laminar flame speed correlation based on current literature data for blends of n-heptane and iso-octane (Primary Reference fuels, PRFs), and toluene (TPRFs). Knock predictions were performed using a chemical mechanism with detailed toluene and skeletal PRF kinetics, and a standard knock intensity criteria was defined in the model as the critical mass fraction burned (MFB) for each PRF fuel. Toluene Standardized Fuels (TSFs) were used to examine the validity of this methodology according to the ASTM 2699 and 2700 procedures. The results of the developed model of the CFR engine based on standard RON/MON conditions were in good agreement with those from the experimental data for PRF fuels, and for a range of TSFs with octane numbers between 90 and 100.
The GT model was then used to outline the operating conditions that would define a boosted octane number test, including elevated intake pressure, decreased intake temperature, and increased engine speed. Under boosted conditions, high sensitivity (RON-MON) fuels with RON greater than 96 exhibited increased knock resistance compared to PRF fuels, but were not consistent with Octane Index (OI = RON-KS) predictions.