Vegetable oils blended to Diesel fuel are becoming popular. Economic, ecological and even political reasons are cited to decrease dependence on mineral oil and improve CO2 balance. The chemical composition of these bio fuels is different from mineral fuel, having less carbon and much more oxygen. Hence, internal combustion of Diesel + RME (Rapeseed Methyl Ester) blends was tested with particular focus on nanoparticle emissions, particle filtration characteristics and PAH-emissions.
Fuel economy and emissions of bus engines were investigated in traffic, on a test-rig during standardized cycles, and on the chassis dynamometer. Fuel compositions were varied from standard EN 590 Diesel with <50 ppm sulfur to RME blends of 15, 30, and 50%. Also 100 % RME was tested on the test-rig. Emissions were compared with and without CRT traps. The PAH profiles of PM were determined. Particles were counted and analyzed for size, surface, and composition, using SMPS, PAS, DC and Coulometry.
Results show little difference for legislated emissions HC, NOx and PM. Size distribution and chemical composition of the particles however vary with increase of RME in the blend. Particles from RME-combustion tend to be smaller and PM contains much less EC but more HC. Little effect of RME-blends on PAH, 100% RME however shows increased PAH-content of PM at high engine loads. Lower calorific value reflects in fuel consumption.
DPF efficiency remains high, 99.8 % independent of RME content. CRT regeneration is sustained, possibly due to the combination of a highly Pt-coated DOC and DPF, i.e. the DOC “dries” the particles and the DPF only partially receives the high HC content of engine-out particles. NO↠NO2 conversion is high with CRT, as usual, independent of RME-blend ratio.
Overall assessment: the impact of RME on emissions is minor. And despite lower EC particle emission, the particle trap remains indispensable.