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Analysis of Cyclic Variability and the Effect of Dilute Combustion in a Gasoline Direct Injection Engine
Abstract:
The pressing need to improve U.S. energy independence and reduce climate forcing fossil fuel emissions continues to motivate the development of high-efficiency internal combustion engines. A recent trend has been to downsize and turbocharge automotive spark-ignited engines coupled with direct fuel injection to improve engine efficiency while maintaining vehicle performance. In-line with recent trends in state-of-the-art engine technology, the focus of this study is lean and EGR dilute combustion in a gasoline direct injection (GDI) engine.
The lean and dilute operating limits are defined by combustion stability typically in terms of COVIMEP so experiments were carried out on an automotive size single-cylinder research engine to characterize combustion stability. From a 20,000 cycle sequence analysis, lean operating conditions exhibit binary high- to low-IMEP cycle sequences. This may be because the cycle-to-cycle feedback mechanisms are physically limited to one or two cycles. Longer sequences are observed with EGR likely because the EGR loop introduces a much longer transient feedback mechanism.
Results from a sensitivity study with induced variability show that COVIMEP is far less sensitive to ignition perturbation than to injection perturbation while the sensitivity is similar in terms of efficiency. This is true for both lean and EGR dilute operation; however, one difference that stands out with EGR is the sensitivity of COVIMEP to ignition perturbation. Increased sensitivity to ignition perturbation is likely the result of longer ignition delay which is critical for early flame kernel development.