Challenging and Future of Homogeneous Charge Compression Ignition Engines; an Advanced and Novel Concepts Review

Abstract

A thorough examination of past and present combustion chemistry results, emission behaviors, the difficulties 
this technique faces, and all controlling parameters, including transient states, are presented, along with the 
potential of HCCI combustion to lower internal combustion engine exhaust emissions, especially NOX and soot 
emissions, and to define the application range of this technique. According to HCCI combustion chemistry and 
emissions analysis, the heavy fuels exhibit two-stage heat release or two-stage combustion process involving 
low temperature oxidation (LTO) stage followed by high temperature oxidation (HTO) stage separated by a 
time delay between them. This is attributed to the negative temperature coefficient (NTC), the HCCI engine's 
advantage of reducing NOX emissions diminishes at high load conditions, HC production decreases with 
increasing engine load, and soot ejection is negligible under all operating conditions. Over a broad range of 
speed and load, the HCCI combustion characteristics are advancedly controlled by valve timing, compression 
ratio, intake air temperature, and EGR. The use of EGR in HCCI operation is restricted to EGR-rates of around 
70%. At this point, response speeds and ignition timing are significantly decreased and delayed, respectively, 
which causes misfiring and the generation of HC emissions. To fully realize the benefits of HCCI engines in the 
future, challenges include homogenizing fuel, air, and recycled burned gases before ignition, controlling 
ignition and combustion timing, and controlling heat release rates.