Quantitative evaluation of several chemical mechanisms for gasoline surrogate-ethanol blends and study of dominant chemistry under super-knock relevant conditions

Abstract

A comprehensive database that covers wide ranges of temperature, pressure and equivalence ratio conditions was established for gasoline surrogate fuels composed of n-heptane, iso-octane, toluene with and without ethanol addition. The database includes ignition delay time measurements in shock tubes and laminar flame velocity measurements which are relevant combustion properties under super-knock relevant conditions. Due to the lack of reaction pathways for low-temperature iso-octane combustion and ethanol oxidation in Caltech mechanism, an updated mechanism has been constructed. The update has been carried out as follows: (i) changing the iso-octane sub-mechanism, (ii) adding an ethanol sub-mechanism, and (iii) modifying thermodynamic and kinetic parameters for few species and reactions for removing collision limit violation. The predictive capabilities of the modified Caltech mechanism and of three reaction models developed for n-heptane/iso-octane/toluene/ethanol based mixtures have been quantified using the model evaluation method of Olm. Significant improvement of the combustion properties predictions by the modified Caltech model has been demonstrated by the lower error value. For the prediction of ignition delay time, the updated Caltech mechanism shows better performance than the other models, especially in the high-temperature regime. Error maps were drawn over wide ranges of thermodynamic conditions to show the overall performance of all the evaluated mechanisms. For the prediction of laminar burning velocity, the modified Caltech mechanism performs better for most cases. Laminar burning velocity is always over-estimated by Livermore’s mechanism. The new mechanism was employed to determine the most important chemical pathways under super-knock relevant conditions corresponding to the following thermodynamic states: (630 K, 2.3 MPa), (750 K, 6.0 MPa), (720 K, 2.8 MPa), and (1000 K, 12.0 MPa). The modified Caltech model, or reduced versions of it, could be employed in future work to investigate super-knock for realistic gasoline surrogate.