A Multi-Dimensional CFD-Chemical Kinetics Approach in Detection and Reduction of Knocking Combustion in Diesel-Natural Gas Dual-Fuel Engines Using Local Heat Release Analysis

Abstract

Dual-fuel diesel-natural gas (NG) engine exhibits higher power density and lower specific emissions compared to dedicated diesel engines. However, high intake temperatures, high compression ratios, combined with high engine loads may lead to engine knock. This is potentially a limiting factor on engine downsizing and getting higher power. In the present study, the combustion process under knocking conditions has been investigated in a dual-fuel diesel-NG engine. A comprehensive multi-dimensional simulation framework was generated by integrating the CHEMKIN chemistry solver into the KIVA-3V code. A detailed chemical kinetics mechanism was used for n-heptane and methane as diesel and NG surrogates. Combination of detailed chemical kinetics and detailed fluid dynamics calculation enabled the model to take into account the characteristics of most pronounced knock type in dual-fuel engines, so called end-gas knock. Within the CFD computational domain, eight regions that are the representatives of the dual-fuel heat release patterns have been selected to extract local properties. Using local knock identification factors, end-gas knock was observed in abnormal combustion cases. A new Knock Intensity factor (K.I) was introduced based on local heat release rate. Using developed knock prediction method, results showed knock could be mitigated by using EGR. Moreover, effect of premixed methane equivalence ratio on knocking combustion was investigated. © 2013 SAE International.