CFD-Guided Combustion Strategy Development for a Higher Reactivity Gasoline in a Light-Duty Gasoline Compression Ignition Engine

Paper #:
  • 2017-01-0740

Published:
  • 2017-03-28
Abstract:
Partially-premixed combustion (PPC) enabled through gasoline Compression Ignition (GCI) shows a promising potential to achieve high fuel efficiency with low engine-out oxides of nitrogen (NOx) and particulate matter (PM) emissions. However, it faces technical barriers to meet the need for simultaneously mitigating combustion efficiency loss at low load as well as containing maximum pressure rise rate (MPRR) and soot at high load. In addition, GCI typically requires high EGR rate at medium-to-high load and therefore poses challenges on the air system development and transient engine operation. The current study aims to utilize 3-D computational fluid dynamics (CFD) combustion analysis to guide the development of a viable full-load range combustion strategy using a higher reactivity gasoline that has a research octane number (RON) of 70. RON70 was selected as it has the potential to offer a good balance between low load and high load GCI operation. The analysis was conducted through closed-cycle, sector engine combustion simulations. Piston bowl geometry was designed to accommodate different combustion strategies and the pent-roof cylinder head design of the base engine. The engine has a geometric compression ratio of 14.5. Detailed combustion optimization was focused on 6 and 18 bar IMEPg at 1500 rpm through a Design of Experiment (DoE) approach. Key design factors include fuel injection pressure, injector nozzle configuration (i.e., # of nozzle holes, nozzle total flow area, and included spray angle), injection strategy, and swirl ratio. Overall, at 6 bar gross indicated mean effective pressure (IMEPg), high efficiency clean PPC operation can be achieved using two different strategies: (a) early triggering injection with a narrow spray angle; and (b) late triggering injection with a wide spray angle and higher injection pressure. Moreover, strategy (a) preferred low swirl ratio, while higher swirl ratio was more beneficial for strategy (b). Compared to strategy (b), strategy (a) generally showed better fuel-air mixing and slightly improved fuel efficiency. When increasing engine load to 18 bar IMEPg, strategy (a) was limited by excessive pressure rise rate with the early triggering injection. Retarding the triggering injection was helpful, but it resulted in higher soot and deteriorated fuel efficiency when using a narrow spray angle. In contrast, by carefully balancing between premixed combustion and mixing-controlled combustion, strategy (b) led to robust mixed-mode combustion operation (i.e., premixed combustion combined with mixing-controlled combustion). NOx emissions targets were identified with the assumption to use appropriate aftertreatment systems, such as selective catalytic reduction (SCR). High fuel efficiency was obtained while maintaining reasonably low levels of soot and pressure rise rate. Furthermore, with the relaxed NOx targets, the combustion system became more robust and less demanding on EGR delivery, thereby offering benefits towards practical application of the GCI concept. In addition, following these combustion strategies, viable GCI operation was also demonstrated at 1500 rpm-10 bar IMEP, 1500rpm-14 bar IMEP, and 2000 rpm-24 bar IMEPg.
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