Effect of Aromatics on Combustion Stratification and Particulate Emissions from Low Octane Gasoline fuels in PPC and HCCI mode

Paper #:
  • 2017-24-0086

  • 2017-09-04
Compared to conventional Compression Ignition (CI), both of Homogeneous Charge Compression Ignition (HCCI) and Partially Premixed Combustion (PPC) concepts have shown high efficiency with low soot emissions. However, soot measurements are rarely investigated and correlated with in-cylinder combustion in an optical diesel engine. The objective of this study is to investigate the effect of addition of toluene (aromatic) to primary reference fuel, PRF60, on combustion stratification and particulate emissions. Experiments are performed in an optical CI engine at a speed of 1200 rpm for TPRF0 (60% iso-octane + 40% n-heptane), TPRF20 (33.5% iso-octane + 46.5% n-heptane + 20% toluene) and TPRF40 (6% iso-octane + 54% n-heptane + 40% toluene). TPRF mixtures are prepared in such a way that the RON of all test blends are same (RON = 60) to account for the influence of aromatics in TPRF mixtures. The motored pressure at TDC is maintained at 35 bar and fuelMEP is kept constant at 5.1 bar. Single injection strategy with a fuel injection pressure of 800 bar is adopted for all test fuels. Start of injection (SOI) is swept from late (-5 CAD aTDC) to early (-180 CAD aTDC) fuel injection timings, representing various modes of combustion via CI, PPC and HCCI. High-speed video of the in-cylinder combustion process is captured and one-dimensional stratification analysis is performed from the intensity of images. In addition, particle size, distribution and concentration are measured and linked with the in-cylinder combustion images. Results show that combustion phasing decreases from CI to PPC and then attains a constant value in HCCI mode. When compared to HCCI, combustion is more stratified for PPC to improve the combustion and reduce the emissions. The soot mass concentration is higher for late injection timings (-5 CAD aTDC to -10 CAD aTDC) due to reduced premixing. For injection timings in PPC and HCCI region, the soot mass concentration was significantly reduced as premixing is improved due to longer ignition delay. The particle number is lower for the late injection and becomes higher as the injection timing advanced to PPC and HCCI mode. While the soot particles are almost nuclear model with the size range of 5nm~15nm and as combustion transit from CI via PPC to HCCI, the particle size becomes larger.
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