Evaluation of Knocking Tendency and Knock-Limited Thermal Efficiency of Different Combustion Chambers in Stoichiometric Operation LNG Engine 2019-01-1137
Natural gas (NG) engine could provide both reduced operating cost and reduction of greenhouse gas (GHG) emissions. Stoichiometric operation with EGR and the three-way catalyst has become a potential approach for commercial NG engines to meet the Euro VI emissions legislation.
In the current study, numerical investigations on the knocking tendency of several combustion chambers with different geometries and corresponding performances were conducted using CONVERGE CFD code with G-equation flame propagation model coupled with a reduced natural gas chemical kinetic mechanism. The CFD modeling approach could predict the knock phenomenon in NG engines reasonably well under different thermodynamic and flow field conditions. The threshold between “no knock” and “knock” conditions was found to be in good agreement with experimental results, which therefore provides a valid way to estimate the capability of knock suppression and knock-limited thermal efficiency for the design and optimization of NG combustion system. Based on the CFD model, the effects of combustion chamber structures on turbulent flow and combustion process were discussed. The results showed that lower mean flow velocity and higher turbulent kinetic energy in the spark plug region can be obtained with a shallow re-entrant chamber geometry at the time of ignition, which could effectively promote the initial flame propagation. However, the knock propensity is also higher compared to other geometries, mainly due to the acceleration of the flame front in the squish crevices and the autoignition of the end-gas in the bottom of chamber, which limits the thermal efficiency improvement. In addition, it’s found that the thermal efficiency of the current NG engine with aluminum piston is restricted by both the knock and peak in-cylinder pressure (mechanical strength). Therefore, it’s essential to develop effective combustion and knock control strategies under higher peak in-cylinder pressure conditions (with higher CR steel piston) to further improve the thermal efficiency of stoichiometric NG engine.
Xumin Zhao, Hu Wang, Zunqing Zheng, Mingfa Yao, Li Sheng, Zan Zhu