Effect of Atkinson cycle coupling compression ratio on the combustion characteristics of natural gas engines

Optimizing the combustion system is crucial for improving the efficiency of equivalent combustion natural gas (NG) engines owing to its high thermal load and decreased break thermal efficiency (BTE). Decoupling the effective compression ratio (CR) and expansion ratio of the engine by adjusting the intake valve timing and geometric CR emerges as a pivotal approach for BTE improvement. This study, leveraging bench tests and GT-power simulation, delves into the impact of the late intake valve closing (LIVC) coupling CR strategy on the combustion performance of NG engines. The findings indicated that, under high load conditions, the temperature in the cylinder during the compression stage experiences relatively minor variations concerning the intake valve closing time. Conversely, under low load conditions, the combustion duration is less influenced by the late closure angle, primarily affected by variations in geometric CR. By increasing the CR and delaying the late closing angle of the intake valve, a substantial enhancement in the net indicated thermal efficiency (ITEn) is achievable. Despite a comparable effective CR, the ITEn of CR12.5 & LIVC45 demonstrates a mean increase of 2.04 % under high load conditions and 1.69 % under low load conditions when compared with the CR11.5 & LIVC30 strategy.