01 Advanced combustion modes for IC engines

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Advanced combustion modes for IC engines Possible strategies to increase more the efficiency of IC engine acting on the combustion process. Advantage and drawbacks of the following new combustion modes: SI engines: CAI (Controlled Auto-Ignition Combustion) CI engines: HCCI (Homogeneous charge compression ignition), PCCI (premixed charge compression ignition). Increasing engine efficiency acting on the combustion process requires: - Lean mixtures (and/or) high EGR - Quasi constant-volume combustion In this way it will be possible to achieve: - Very reduced pollutant emissions (reduced combustion temperatures, reduced CO and HC because of lean mixtures). - High efficiencies (> 45%), since combustion takes place in a small crank angle degrees interval, around TDC, at quasi-constant volume conditions. How this can be achieved: - Kinetically-controlled combustion of a homogeneous (or nearly homogeneous) mixture. How to do this? - SI engines (homogeneous charge): Controlled Auto-Ignition Combustion (CAI). - CI engines (direct-injection): Homogeneous Charge Compression Ignition (HCCI) or Premixed Charge Compression Ignition Combustion (PCCI). SI engines: - Compression ratio: 8 –13 - Fuels with high RON (difficult to be auto-ignited) Diesel engines: - Compression ratio: 14 –22 - Fuels with high CN (easy auto-ignition) HCCI HCCI engines, under development, try to reach the main advantages of SI engines: To be supplied by homogeneous and premixed charge, in order to avoid particulate and NOx emissions. In a Diesel engine, a fuel element in the central core of the spray is heated by the combustion process that occurs around it, with a high oxygen deficiency (φ= 2-3). At constant φ, the element follows the path marked by the red arrows, up to adiabatic flame temperature Tflame,…