Effects of fuel trapping in piston crevice on unburned hydrocarbon emissions in early-injection compression ignition engines

摘要

Early injection strategy is employed in many advanced combustion concepts of modern compressionignitionengines to promote the fuel-air premixing, but it could result in fuel spray impingement on thecylinder wall and potential fuel trapping in the crevice regions. The impact of the fuel trapping on theformation and distribution of unburned hydrocarbon (UHC) in the advanced compression-ignition conceptsis not well understood. In this study, the planar laser-induced fluorescence (PLIF) technique wasapplied in a single-cylinder optical engine to visualize the fuel distribution before combustion and theformaldehyde formation during combustion. A three-dimensional computational model was establishedto explore the detailed mechanism of UHC formation in the piston crevice. Three cases with injectiontimings of 20 °, 40 ° (SOI-40), and 100 ° (SOI-100) were compared. The PLIF and simulation results indicatethat, under early injection conditions, the squish region and piston crevice trap a considerable amountof fuel, resulting in increased UHC emission and reduced engine work, and the main combustion zoneresides in the squish region. The simulation shows that the amount of UHC formation from the trappedfuel is highly dependent on the local equivalence ratio distribution formed by different injection timings.The local equivalence ratio within the piston crevice region for the SOI-40 case exceeds 2 before combustion;the charge sequentially undergoes both low- and high-temperature heat release (LTHR and HTHR)processes and produces less UHC in the crevice region. However, the SOI-100 case produces an overallleaner mixture with a local equivalence ratio lower than 1 before combustion; the charge undergoes LTHRwithout noticeable HTHR and becomes UHC near the cylinder wall. The injector dribbling results in moreUHC formation in the central part of the cylinder under very early injection timing, and thus an accurateinjector dribbling model is required to better reproduce the UHC emissions.