An Experimental Investigation on Low Speed Pre-Ignition in a Highly Boosted Gasoline Direct Injection Engine

The biggest challenge in developing Turbocharged Gasoline Direct Injection (TGDI) engines may be the abnormal combustion phenomenon occurring at low speeds and high loads, known as low-speed pre-ignition (LSPI). LSPI can trigger severe engine knocks with intensities much greater than those of spark knocks and thus characterized as super knocks. In this study, behavior and patterns of LSPI were investigated experimentally with a highly-boosted 1.5L TGDI engine. It was found that LSPI could occur as an isolated event, a couple of events in sequence, or a trail of events. Although occurring randomly among the engine cylinders, LSPI took place frequently when the engine was operated at low speeds and high loads in the zone where scavenging was employed for boosting engine torques at low speeds, typically <2500 rpm. Frequencies and patterns for LSPI were found to be influenced considerably by engine operation parameters, such as excess air coefficient, engine coolant temperature, valve timing, etc. Intensities of super knocks triggered by LSPI varied with the timing for the pre-ignition. Based on findings in this study, it was proposed that the oil particles as oil loading in the blow by recirculation could be the root cause for LSPI as they cause dirtiness of the combustion chamber roof where the oil particles could be attached and then initiate LSPI in the late compression stroke. It was demonstrated that although LSPI may not be eliminated due to the nature of the triggers, TGDI engines can be operated super-knock free through controlling conditions for combustion and engine cooling. As long as the values of the maximum peak firing pressure in LSPI events and their frequencies are under the design criteria, the engine can tolerate the LSPI events with acceptable knock intensities.