SAE International The Effect of EGR on Low-Speed Pre-Ignition in Boosted SI Engines 2011-01-0339

The spark ignition (SI) engine has been known to exhibit several different abnormal combustion phenomena, such as knock or pre-ignition, which have been addressed with improved engine design or control schemes. However, in highly boosted SI engines - where the engine displacement is reduced and turbocharging is employed to increase specific power - a new combustion phenomenon, described as Low-Speed Pre-Ignition (LSPI), has been exhibited. LSPI is characterized as a pre-ignition event typically followed by heavy knock, which has the potential to cause degradation of the engine. However, because LSPI events occur only sporadically and in an uncontrolled fashion, it is difficult to identify the causes for this phenomenon and to develop solutions to suppress it. Some countermeasures exist that OEMs can use to avoid LSPI, such as load limiting, but these have drawbacks. As an alternative measure to suppress LSPI, the benefits of operating the engine with various levels of Exhaust Gas Recirculation (EGR) have been explored. Besides a reduction in general engine knock tendencies, recent engine tests indicate that light to moderate levels of EGR (5 - 10%) reduce the incidence of LSPI dramatically. Additionally it was recognized that the addition of about 6% EGR allows an increase in fueling amount of 5-10% and an increase in engine torque of 10-13% (when operating with knock limited spark advance) without increasing the frequency and severity of LSPI. By operating the engine with 10% EGR an increase in fueling rates of 15% and engine torque of 17% was possible while reducing LSPI frequency by roughly 80% and LSPI intensity by approximately 30% over non-EGR conditions.