The influence of engine lubricating oil on Diesel nanoparticle emissions and kinetics of oxidation

Earlier work [1] shows that kinetics of Diesel soot oxidation is different from that of ethylene diffusion flame soot oxidation [2], possibly due to metals from lube oil. This study investigates the influence of metals on soot oxidation and the exhaust particle emissions using lube oil dosed fuel (2 % by volume). This method does not simulate normal lube oil consumption, but is used as a means of adding metals to particles for oxidation studies. This study also provides insight into the effect of systems that mix lube oil with fuel to minimize oil change service. The HTO-TDMA (High Temperature Oxidation-Tandem Differential Mobility Analyzer) technique [1] was used to measure the surface specific oxidation rate of Diesel particles over the temperature range 500-750°C. Diesel particles sampled from the exhaust stream of a Diesel engine were size segregated by differential mobility and oxidized in situ in air in a heated flow tube of known residence time and temperature profile. The change in particle diameter after oxidation took place was measured and converted into the surface specific oxidation rate. The pre-exponential factors increased by about a factor of two, whereas the activation energy determined for the lubrication oil dosed Diesel particles was very close to that of undosed Diesel particles (108 kJ mol^-1, [1]). This suggests that the increase of active sites by the presence of Ca (and possibly by other metals included in the additive package of the lube oil) results in the faster oxidation rate. Particle size measurements show that particle volume emissions, which are roughly proportional to particle mass, decreased by about a factor of two, but that particle number emissions increased by an order of magnitude with lube oil dosed fuel.