Investigation of combustion and particle number (PN) emissions in a spark induced compression ignition (SICI) engine for ethanol-gasoline blends

Abstract

Spark induced compression ignition (SICI) is a promising method to achieve high thermal efficiency as well as a robust control of combustion. With the increase of compression ratio, the popularization of bioethanol and the development of NO_x emission control technologies for lean combustion, it is easier to realize high-efficiency and clean SICI combustion under medium-to-high load. Combining split injection with various excess air ratio (λ), the effects of ethanol in fuels with the same RON value (93# and 80#E15) on combustion characteristics, indicated thermal efficiency and PN emissions were investigated in a high compression ratio (15.5), single-cylinder, four-stroke engine. For comparison, the performance of pure anhydrous ethanol (E100) was also studied in this engine. The results show that ethanol-contained fuels can accelerate the flame propagation but suppress the heat release rate (HRR) of autoignition, and autoignition does not even exist in E100 combustion. Consequently, CA50 is advanced but combustion duration is prolonged for ethanol-contained fuels. It is highlighted that the predominant contribution of ethanol in improving the indicated thermal efficiency is the optimization of CA50 rather than the combustion iso-volume. In addition, the knock intensity is effectively suppressed, and the combustion process can be optimized by split injection with appropriate EOI2 (the end of the second injection) and split ratio for gasoline and ethanol-blended fuel, which further improve the indicated thermal efficiency. PN emissions exhibit a generally positive correlation with knock intensity in SICI mode and SICI mode can reduce the particles generated by the spray-wall impingement and ultra-rich mixture near the wall. Lean combustion can simultaneously suppress flame propagation and autoignition and is more effective in PN emissions reduction for fuels of 93# and 80#E15 within the cycle-to-cycle variation (COV) limitation rather than E100. However, E100 demonstrates two orders of PN emissions smaller than 93# and 80#E15 due to its high oxygen fraction content.

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