Evaluation of a passive pre-chamber design to enable turbulent jet ignition in a retrofitted CNG SI engine: A CFD-based analysis with comparison against the conventional spark plug mode

Abstract

Compressed natural gas is a promising alternative fuel for spark-ignition engines due to its availability and potential for reduced CO2 emissions. However, its low laminar flame speed leads to longer combustion duration, especially under low-load conditions, adversely affecting efficiency and stability. This study investigates a passive pre-chamber system to enable turbulent jet ignition and improve combustion in a retrofitted CNG SI engine (converted from a diesel engine) operating at low load. CFD simulations were used to optimize the pre-chamber design, and the best-performing configuration is presented. A comparison with a conventional spark plug setup evaluates differences in scavenging, turbulence, combustion, heat transfer, efficiency, and emissions. Results show that the pre-chamber region had a 2.17% higher residual gas fraction at spark timing, with gases located away from the spark plug. The pre-chamber significantly reduced the main combustion duration (MFB 10–90%) from 49 to 26 crank angle degrees due to enhanced turbulence and multi-point ignition from turbulent jets. However, this configuration increased heat transfer losses by 31%, mainly due to flame proximity to piston walls and added surface area. Consequently, in-cylinder pressure and temperature during late expansion decreased, leading to a 0.94% drop in efficiency. Lower temperatures also increased unburned hydrocarbon emissions, while faster combustion raised peak temperatures, increasing NOx but reducing CO emissions. Overall, the study highlights both benefits and trade-offs of pre-chamber systems and provides insight into their impact on in-cylinder processes and engine performance.