Exhaust Gas Recirculation (EGR) system has been used to reduce NOx emissions, to improve fuel economy and to suppress knock since it offers the benefits of charge dilution. However, the occurrence of excessive cyclic variation with high EGR rates, brings about an undesirable combustion instability within the engine cylinder resulting in the deterioration of both engine performance and emissions. Therefore, in order to avoid the reduction of thermal efficiency and to improve fuel economy, the optimum EGR rate should be derived effectively as a function of the operating conditions.
An experimental study was conducted to investigate the effects of EGR variables on performance and emission characteristics in a 2-liter 4-cylinder spark-ignition LPG fuelled engine. It also studied the effects of EGR on the reduction of thermal loading at exhaust manifold because the reduced gas temperature is desirable for the reliability of an engine in light of both thermal efficiency and material issue of exhaust manifold.
The steady-state test showed that the brake thermal efficiency increased and the brake specific fuel consumption decreased with the increase of EGR rate in hot EGR and with the decrease of EGR temperature in case of cooled EGR, while the stable combustion was maintained. The increase of EGR rate or the decrease of EGR temperature results in the reduction of $NO_x$ emission even in the increase of HC emission. Furthermore, decreasing EGR temperature by 180℃ enabled the reduction of exhaust gas temperature by 15℃ in cooled EGR test at 1600rpm / 370kPa BMEP operation, and consequently the reduction of thermal load at exhaust.