Numerical assessment of methane number and critical compression ratio of gaseous alternative fuels: CFR engine quasi dimensional simulation approach

Abstract

The development of a quasi-dimensional engine model and its adaptation for CFR configuration for Methane Number assessment of gaseous fuel mixtures is reported. The stochastic model based on eddy entrainment and laminar burn-up concept for turbulent combustion adopts full reaction kinetics with detailed reaction mechanism in the unburned mixture region for auto-ignition. Adaptive time step solver is invoked for the solution of the stiff system of equations. Modular validation of the developed engine simulation model is performed by independently validating an iso-choric system and an HCCI engine module followed by complete engine simulation model validation based on experimental results over a range of operating conditions and different engine frames. The model is extended to replicate a CFR engine as per ASTM D2700-16a/ASTMWK54121 and validated based on literature reported data. The use of flame quench limit for knock recognition in lieu of equivalence ratio independence, as adapted in the ASTM standards, is a particular case in point example.The developed model is used for knock rating assessment of eight different syngas compositions. Preliminary analysis suggests the model to over-predict the critical CR to the tune of 1 ± 0.5 with the over-prediction attributed to aspects like turbulence, heat transfer, fame propagation etc. Relaxing the flame out limit by 2° crank angle improves the critical CR prediction to within 0.75 ± 0.25. The estimated Methane Number remains within 5 units of the experimental results establishing the capability of the proposed model for assessment of gaseous fuel knock rating. The proposed approach can be extended to evaluate other fuels for knock quality assessment by incorporating appropriate fuel specific features.