Detailed Analysis of Particulate Matter Emitted from Biofueled Diesel Combustion with High EGR

Difference of engine combustion characteristics, species and amount of exhaust gas and PM (particulate matter consisted of SOF and Soot and Ash), and especially PM oxidation characteristics were studied when diesel fuel or bio-fuel, here PME (palm oil methyl ester) as an example, was used as a fuel. The fueling rate was adjusted to obtain the same torque for both fuels and engine was operated under several range of EGR (Exhaust Gas Recirculation) ratio. Under such conditions, PME showed shorter ignition delay time and lower R.H.R (rate of heat release) under 0-40% EGR ratio. With respect to engine exhaust gas species, CO, NO, THC and HCHO, CH₃CHO concentration was almost the same when the EGR ratio is higher than 35% (Intake-Air/Fuel: A/F=20). However, PME also showed lower exhaust gas emission when the EGR ratio is higher than 30%. PME showed lower PM emission compared to diesel fuel especially at 30-40% EGR ratio, maybe because of higher combustion performance due to oxygen in the HC molecules.

The PM exhausted from diesel and PME fueled combustion was analyzed by XRD (X-Ray Diffraction) and DTA/TG (Differential Thermal Analysis/Thermal Gravimetric Analysis) in air atmosphere. Lower crystallization degree was suggested by XRD measurement and sharper exothermic peak was observed by DTA in the soot (one extracted from PM by Soxhlet extraction method) originating from PME. Also DTA/TG measurement (in 1% oxygen) for each fuels suggested that PME can be oxidized easily than diesel fuel. These results suggested that oxygen containing PME fuel is easy to be oxidized and the soot from such PME fuel can be easily oxidized because of its lower crystallization degree.