Investigation of an optimal exhaust gas recirculation rate on a four-stroke spark-ignited LPG engine

Abstract

To determine the optimal exhaust gas recirculation (EGR) rate that can optimize a spark-ignited (SI) liquefied petroleum gas (LPG) engine's performance without compromising emissions, the effects of EGR rates on combustion performance and emission characteristics of a four-stroke SI LPG engine were investigated. Experimental investigations with 0 % and fixed 13.9 % EGR rate were conducted on the SI LPG engine. Afterward, three-dimensional (3D) simulation cases of EGR rates ranging between 0 % and 30 % were modeled while maintaining the SI LPG engine's constant initial operating conditions. Simulation results were validated by the experimental engine results and showed a good agreement. For in-cylinder temperature and pressure, the changes are slightly insensitive between 0 % and 15 % EGR rate resulting to the reduction of engine output power by less than 10.25 %. Between 0 % and 15 %, NOx was significantly reduced, however, an EGR rate of 10 % and 15 % resulted in 1.85g/kWh and 0.83g/kWh NO emissions which are in compliance with NOx Tier III control requirements of the experimental engine. CO emissions were insignificant between 10 % and 15 % EGR rate with an increase of 2.23 %. Soot emissions were significantly lower between 0 % and 15 % EGR rates, but increased sharply at 20 %, 25 %, and 30 % EGR rates by 0.012, 0.022, and 0.028g/kWh respectively which challenges the assumption that SI LPG engines are clean fuels with minimal soot. The results offer valuable insights that promote the use of the EGR strategy at enhancing the SI LPG engine's combustion performance and controlling exhaust emissions.