Impact of Particle Characteristics and Engine Conditions on Deposit-Induced Pre-Ignition and Superknock in Turbocharged Gasoline Engines

Low Speed Pre-Ignition (LSPI), also referred to as superknock or mega-knock is an undesirable turbocharged engine combustion phenomenon limiting fuel economy, drivability, emissions and durability performance. Numerous researchers have previously reported that the frequency of Superknock is sensitive to engine oil and fuel composition as well as engine conditions in controlled laboratory and engine-based studies. Recent studies by Toyota and Tsinghua University have demonstrated that controlled induction of particles into the combustion chamber can induce pre-ignition and superknock. Afton and Tsinghua recently developed a multi-physics approach which was able to realistically model all of the elementary processes known to be involved in deposit induced pre-ignition. The approach was able to successfully simulate deposit induced pre-ignition at conditions where the phenomenon has been experimentally observed. This approach allowed characterization of the impact of particle characteristics, bulk charge properties and engine conditions and provided valuable insight into behavior observed in engine and bench tests.

The objective of this study is to develop a conceptual model for deposit-induced pre-ignition based on first principles, and to provide the industry a framework to help model this complex phenomenon and to help guide optimization of engine design, control and fuel and lubricant compositions for real world LSPI suppression. In this work, multi-physics simulations were conducted that incorporated accurate representation of heat and mass transfer, particle oxidation and gas-phase autoignition and specific characteristics of the particle that dominate pre-ignition were identified. The study was extended to simulate the effect of bulk charge properties and engine conditions on pre-ignition. The results show that pre-ignition timing, and therefore superknock, is particularly sensitive to key deposit characteristics and engine operating conditions, consistent with results of experimental studies that have been reported in literature. The results of this study help elucidate the phenomenon of deposit induced pre-ignition that has been reported but is not well understood. Furthermore, the results of this study will aid the industry in development of improved fluid formulations and engine design that are needed to suppress superknock in real-world turbocharged engines.