Abstract
Research and/or Engineering Questions/Objective
An inline 4-cylinder turbocharged 2.0 L PFI engine was modified to operate in D-EGR mode with nominally 25% cooled EGR. To fully explore the peak efficiency potential of this engine architecture, the combustion system was stressed to its limits by increasing the maximum dilution (air and also EGR dilution) until engine instability was reached. The motivation for the added dilution testing arose from the prior test results when bottled H2 / CO was added to a LPL EGR version of this engine [1]. The results indicated that at least 5 percent more cooled EGR could be tolerated by the engine across the operating range. Based on these results, it was also concluded that at least 5-10% lean dilution on top of 25% cooled EGR should be feasible.
Methodology
The following study is separated into two chapters. First, engine performance with added air dilution was investigated. Second, the benefits and challenges of additional EGR dilution in the main cylinders (cylinders that exhaust to the atmosphere) were evaluated.
Results
The results documented that with ~10% lean dilution, further efficiency improvements could be gained at part loads. Engine-out NOx emissions further decreased, however, not to a level where no lean NOx aftertreatment will be required. A secondary issue was large cylinder IMEP imbalances between the lean main cylinders and the rich dedicated cylinder. When additional EGR dilution was utilized, efficiency did only increase at knock limited conditions.
Limitations of this study
At part load, the additional EGR caused unstable dedicated cylinder operation and thus required a reduction in over-fueling. In consequence, H2 and CO production were reduced, leading to unstable main cylinder engine operation. Moreover, both added dilution scenarios further taxed the boosting system.
What does the paper offer that is new in the field including in comparison to other work by the authors?
In prior work, D-EGR engines have not been operated with additional lean or EGR dilution.
Key Words : EGR; D-EGR; dilution; efficiency