The effects of stratified lean combustion and exhaust gas recirculation on combustion and emission characteristics of an LPG direct injection engine

The combustion and emission characteristics of a stratified lean-burn liquefied petroleum gas (LPG) direct-injection engine are investigated at five representative engine operating points, where passenger vehicles are most frequently operated. A redesigned engine head that makes room for an injection and ignition assembly, comprised of a center-mounted fuel injector and a spark plug positioned in the vicinity of the fuel injector, made the spray-guided LPG lean-burn possible. The experimental results show that the stratified LPG lean-burning achieves a 10.2–27.5% improvement in the brake specific fuel consumption (BSFC), while reducing NO_x emissions by 72.0–83.4% compared to that obtained through homogeneous stoichiometric combustion. Decreased pumping loss and enhanced combustion speed led by the adoption of the stratified lean combustion strategy play a key role in improving the combustion performance. A cooled exhaust gas recirculation (EGR) system was also introduced for further reduction of NO_x emissions for low engine load conditions It was determined that the most adequate EGR rate of 15% magnifies the benefits of the NO_x reduction in the stratified lean-burning, providing additional reductions of 34.4–46.5%. The application of cooled EGR, however, led to slight fuel economy penalties yielding a 1.6–2.5% increase in the BSFC.