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

The objective of this study was to investigate the effect of aromatic on combustion stratification and particulate emissions for PRF60. Experiments were performed in an optical CI engine at a speed of 1200 rpm for TPRF0 (100% v/v PRF60), TPRF20 (20% v/v toluene + 80% PRF60) and TPRF40 (40% v/v toluene + 60% PRF60). TPRF mixtures were prepared in such a way that the RON of all test blends was same (RON = 60). Single injection strategy with a fuel injection pressure of 800 bar was adopted for all test fuels. Start of injection (SOI) was changed from early to late fuel injection timings, representing various modes of combustion viz HCCI, PPC and CDC. High-speed video of the in-cylinder combustion process was captured and one-dimensional stratification analysis was performed from the intensity of images. Particle size, distribution and concentration were measured and linked with the in-cylinder combustion images. Results showed that combustion advanced from CDC to PPC and then attained a constant value in HCCI mode. In PPC and HCCI region, the soot mass concentration was significantly reduced as premixing was improved due to longer ignition delay. The particle number was lower for the late injection and becomes higher as the injection timing advanced to PPC and HCCI mode. While the soot particles were almost nuclear model with the size range of 5nm~17nm and as combustion transited from CDC via PPC to HCCI, the particle size became larger. For TPRF blends, the increased intake air temperature was required to maintain same combustion phasing as that of PRF60. With the addition of toluene to PRF60, the soot concentration increased, which was in-line with the increased intensity (yellow) of combustion images. The degree of stratification was higher for TPRF20 and TPRF40 when compared to PRF60.