Effect of Varying Compression Ratio on a Natural Gas SI Engine Performance in the Presence of EGR

The use of an exhaust gas recirculation (EGR) strategy is economically capable of satisfying the increasingly restricted emission standards. However, the use of EGR in natural gas spark ignition (SI) engines has not been fully optimized yet. In this paper, the effects of change of compression ratio (r c) on the performance and NO emissions of a natural gas SI engine were experimentally investigated for different EGR dilution conditions. It was found that the use of EGR dilution with a stoichiometric air−fuel mixture suppressed both surface ignition and engine knock and improved engine stability at higher compression ratios. Also, the increase of EGR dilution led to a significant reduction in NO emissions. NO emission decreased by about 70% when EGR dilution increased from 0 to 10% at r c = 10. In addition, engine performance was significantly improved when the EGR dilution strategy was employed at higher compression ratios. The increase of r c from 8 to 12 at an EGR dilution of 10% increased engine brake power by about 11% and decreased engine fuel consumption by about 10%.