Evaluation of a Passive Prechamber Design to Enable Turbulent Jet Ignition in a Retrofitted Compressed Natural Gas Spark Ignition Engine: A Computational Fluid Dynamics–Based Analysis With Comparison Against the Conventional Spark Plug Mode

Abstract Compressed natural gas (CNG) is a promising alternative fuel for spark ignition engines due to its widespread availability and potential to reduce CO2 emissions. However, CNG's low laminar flame speed leads to prolonged combustion duration, particularly under low-load conditions, negatively impacting thermal efficiency and combustion stability. To address these challenges, this study explored the implementation of a passive prechamber system to enable turbulent jet ignition, and thereby, improve the combustion duration of a CNG-fueled retrofitted spark ignition engine (converted from an old diesel engine) operating under low-load conditions. Computational fluid dynamics (CFD) simulations were employed to guide the passive prechamber design, and the results of the best-performing design, which demonstrated the greatest improvement in combustion duration, are presented here. A quantitative comparison with the conventional spark plug configuration is conducted to evaluate the relative advantages and limitations of the prechamber design. Key in-cylinder phenomena compared include the scavenging process, turbulence levels, combustion process, heat transfer losses, efficiency, and emission levels, and the important findings are as follows. At spark timing, the residual gas fraction in the prechamber region was 2.17% higher (on an absolute scale) than in the in-cylinder region of the conventional spark plug engine, and spatial distribution analysis showed that these gases were located slightly away from the spark plug region. Compared to conventional spark plug operation, the prechamber configuration reduced the main combustion phase duration (MFB 10–90%) from 49 crank angle degree (CAD) to 26 CAD. This improvement was due to enhanced turbulence and simultaneous ignition at multiple locations, driven by the turbulent jets from the prechamber. However, the prechamber engine experienced a 31% increase in heat transfer losses, primarily due to flame initiation near the piston walls and the additional surface area introduced by the prechamber. These higher heat transfer losses resulted in lower in-cylinder pressures and temperatures, particularly during the late expansion stroke, leading to a 0.94% (absolute) drop in efficiency. Additionally, the lower in-cylinder temperatures hindered the oxidation of charge entering from crevice regions, causing slightly higher unburned hydrocarbon (UBHC) emissions. On the other hand, the faster combustion in the prechamber engine resulted in higher peak in-cylinder temperatures, leading to increased NOx emissions and reduced CO emissions. Overall, this study highlights the advantages and limitations of a prechamber design while comprehensively analyzing the underlying factors contributing to these changes. Furthermore, the analysis presented in this study provides a comprehensive understanding of how the addition of a prechamber system to a spark plug engine instigates changes in various in-cylinder phenomena and engine outcomes.