Characteristics of Abnormal Combustion in the Scavenging Zone for a Highly-Boosted Gasoline Direct Injection Engine 2017-01-1721

In order to improve low speed torques, turbocharged gasoline direct injection (TGDI) engines often employ scavenging with a help of variable valve timing (VVT) controlled by the cam phasers. Scavenging improves the compressor performance at low flows and boosts low-speed-end torques of the engines. Characteristics of the engine combustion in the scavenging zone were studied with a highly-boosted 1.5L TGDI engine experimentally. It was found that the scavenging zone was associated with the highest blowby rates on the engine map. The blowby recirculation was with heavy oil loading, causing considerable hydrocarbon fouling on the intake ports as well as on the stem and the back of the intake valves after the engine was operated in this zone for a certain period of time. The low-speed pre-ignition (LSPI) events observed in the engine tests fell mainly in the scavenging zone. The most representative LSPI pattern observed was a sequence of the events, with the event number varying from one to 10 in the sequence. This LSPI pattern could be caused by two types of triggers: the primary trigger was the oil particles entering the engine cylinder, triggering the initial event in the sequence; the following events in the sequence might be induced by a different or secondary trigger, which might be the falloffs of the carbon deposits on the combustion chamber roof and/or on the intake valves under high frequencies of the pressure waves from superknock combustions. It was demonstrated that with proper mixture enrichment, intensities and frequencies of superknock induced by LSPI could be reduced; when the mixture was enriched sufficiently, LSPI events occurred without leading to superknock.