Effect of Thermodynamic Conditions on Spark Ignition to Compression Ignition in Ultra-Lean Mixture Using Rapid Compression Machine

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 achieve 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 (φ), rich-fuel mixture around spark plug, and supercharging are three essentials for combustion stability. In order to investigate the effect of flame propagation and thermodynamic conditions on ultra-lean combustion, SACI experiments using lean iso-octane mixture were carried out in a rapid compression machine (RCM) together with high-speed photography to capture flame propagation and reaction front, which indicate combustion modes under various thermodynamic conditions. Knocking intensity (KI) was used to evaluate heat release degree and distinguish non-knock/conventional knock/super-knock regions. Test results show that flame propagation could enhance knock intensity in intermediate temperatures from 800 K to 1000 K due to flame compression. While volumetric ignition becomes dominant over 1000 K due to short ignition delay. Non-knock region occurs under 30 bar mainly due to influence of negative temperature coefficient (NTC). Detonation and supersonic reaction front occur under ultra-lean conditions at effective temperatures from 850 K to 900 K and effective pressures more than 20 bar, which should be suppressed for lean-burn engine application.