Particle Reduction in LPG Lean Stratified Combustion by Intake Strategies

Abstract

Lean stratified combustion shows high potential to reduce fuel consumption because it operates without the intervention of a throttle valve. Despite its high fuel economy potential, it emits large amounts of particulate matter (PM) because the locally rich mixture is formed at the periphery of a spark plug. Furthermore, the combustion phasing angle is not realized at MBT ignition timing, which can bring high work conversion efficiency. Since PM emission and work conversion efficiency are in a trade-off relation, this research focused on reducing PM emission through achieving high work conversion efficiency. Two intake air control strategies were examined in this research; throttle operation and late intake valve closing (LIVC). The experiment was conducted in a single cylinder spray-guided direct injection spark ignition (SG-DISI) engine with liquefied petroleum gas (LPG). The injected fuel amount was fixed so as to investigate the effect of each strategy. The throttle valve strategy decreased the filter smoke number (FSN) as the restriction of inhaled air was increased. By contrast, the number of particles was increased. When the LIVC strategy was applied, both FSN and the number of particles were decreased; additionally, stabilized combustion was realized. Combustion visualization was performed in an optical engine. When the throttle valve was applied, the locally rich mixture was reduced due to the low ambient pressure. As a result of inhaled air reduction, the global fuel-air ratio was increased. Therefore, randomly distributed high luminous flame was detected. In the case of LIVC, the high luminous flame was not detected near the spark plug due to its lower ambient pressure and increased in-cylinder flow. In conclusion, the LIVC strategy could achieve low PM emission with reduced sacrifice of IMEP.