Abstract
In the 3D-CFD practice, actual gasoline fuels are usually replaced by surrogate blends composed of Iso-Octane, n-Heptane and Toluene (Toluene Reference Fuels, TRFs). In this work, the impact of surrogate formulation on the probability of end-gas auto-ignition is investigated in a single cylinder engine. CFD simulations are run on equal charge stratification to discern the effect of fuel reactivity from that of evaporation and mixing. Blends are formulated using an internal methodology, coupled with a proprietary method to predict knock statistical occurrence within a RANS framework. Chemical kinetics calculations of Ignition delay times are performed in a 0D constant pressure reactor using a mechanism for gasoline surrogates, proposed by the Clean Combustion Research Center of King Abdullah University of Science and Technology (KAUST), consisting of 2406 species and 9633 reactions. Surrogates mimic a commercial European gasoline (ULG95). Five different formulations are presented. Three are characterised by equal RON (95) with progressively decreasing Octane Sensitivity S. The fourth and the fifth have a sensitivity of 10 but with lower RON (92.5 and 90). The combinations allow the reader to separate the effects of octane sensitivity from those of RON quality of the tested fuels. Applying the different surrogates, changes in each of autoignition phasing, magnitude and statistical probability are investigated. Results confirm the dependency of knock occurrence on the Octane Sensitivity, as well as the need to include engine-specific and operation-specific characteristics in the analysis of knock. The Octane Index (OI) formulation developed by Kalghatgi is discussed.
Highlights
Knock is a complex phenomenon which depends on fuel chemistry, combustion chamber design and engine operating conditions
Attention was paid to the definition and characterization of representative gasoline surrogates using simplified TRF blends
A chemical kinetics-driven analysis of mixture reactivity was carried out using constant pressure reactors to build up Look-Up tables of ignition delay times
Summary
Knock is a complex phenomenon which depends on fuel chemistry, combustion chamber design and engine operating conditions. Nowadays, it represents a major barrier on the performance of spark-ignition engines [1]. The anti-knock quality of the fuel affects engine performance. Knock intensity depends on fuel composition (e.g aromatic and oxygenates contents), as well as on engine speed and, broadly speaking, on combustion characteristics. These must be thoroughly understood in order to face economical, technological, and societal challenges [2]. An original statical knock model is here adopted and it is briefly recalled
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