Liquefied natural gas (LNG) 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 LNG 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 results showed that the CFD modeling approach could predict the knock phenomenon in LNG engines reasonably well under different thermodynamic and flow field conditions. The predicted threshold between “no knock” and “knock” conditions was found to be in good agreement with experimental results, which means it provides a valid way to estimate the capability of knock suppression and knock-limited thermal efficiency for the design and optimization of LNG combustion system. Based on the validated CFD model, the effects of combustion chamber structures on turbulent flow and combustion process were discussed. The results showed that lower mean flow velocity in the spark plug region and higher turbulent kinetic energy in the center of the combustion chamber and near the spark plug can be obtained with a shallow re-entrant chamber geometry at the time of ignition and during the early combustion stage, which could effectively promote the initial flame propagation. However, the knock propensity is also higher compared to other combustion chamber geometries, mainly due to the preheating of the flame front in the squish crevices and the suppression of flame propagation to the bottom of the combustion chamber, which limits the thermal efficiency improvement. In addition, it’s found that the thermal efficiency of the current LNG engine with aluminum piston is restricted by both the knock and durable 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 LNG engine.