Investigation of effect of flame propagation and thermal conditions on the ultra-lean mixture ignition through a rapid compression machine 2019-01-0963
Compression ratio and specific heat ratio are two dominant factors influencing engine thermal efficiency. Therefore, ultra-lean burn may be one method to deal with increasingly stringent fuel consumption and emission regulations in the approaching future. To pursue high efficiency and clean combustion, innovative combustion modes have been applied on research engines including homogeneous charge compression ignition (HCCI), spark-assisted compression ignition (SACI), and gasoline direct-injection compression ignition (GDCI), etc. Compared to HCCI, SACI can extend the load range and more easily control combustion phase while it is constrained by the limit of flame propagation. For SACI with ultra-lean burn in engines, equivalence ratio no more than 0.6, rich-fuel pockets around spark plug and supercharging are essential for flame stability and dynamic performance requirement. In order to investigate the effect of flame propagation and thermal conditions on ultra-lean combustion, experiments using lean iso-octane mixture were carried out in a rapid compression machine (RCM) together with high-speed photography to record the propagation of flame and reaction front, which can indicate combustion modes under various thermal conditions. Knocking intensity (KI) has also been analyzed to evaluate intensity of heat release and regions related to knock-free and super-knock were found. Results show that flame propagation can enhance knock intensity in intermedium temperatures from 800 K to 1000 K while volumetric ignition will be dominant over 1000 K. Knock-free areas will occur under 30 bar covering most negative temperature coefficient (NTC) regimes. Detonation and supersonic spontaneous deflagration can exist under ultra-lean conditions at effective temperatures from 850 K to 900 K, and effective pressures more than 20 bar, which is worth paying attention to for lean-burn engine application.