The interaction between unburnt hydrocarbons and soot in diesel exhausts
The potential health risk of diesel particulate (DP) has stimulated research into its physical and chemical composition. Its interaction with unburnt hydrocarbons (UHC) at exhaust temperatures was studied (i.e. composition and microstructure), at varying engine conditions. A hot whole exhaust filtration system was developed to collect DP on Pallflex TX-40 PTFE coated filters (for minimal artefact formation) down the exhaust of a Ricardo E6/T IDI diesel engine. Electron microscopy (SEM and TEM) and a gravimetric BET method determined particle size, specific surface area (SSA) and pore character. An in vacuo gravimetric thermal degassing (TD) apparatus was constructed to extract adsorbed volatiles (filter extractable sample - FES). The volatile FES was trapped and analysed by gas chromatography and identified as fuel and oil derived UHC's. Ultrasonic and soxhlet extraction techniques were employed for comparison studies. DP are graphitic carbonaceous aggregates of 30-40nm mean particle diameter. Structural analysis indicated that slit-shaped pores (Type II isotherm) were formed between crystallite layers. Highly adsorbed pore-bound FES fractions were identified (fuel i n ultramicropores, 0.355-lnm; fuel/oil in supermicropores, 1-2nm), trapped by overlapping crystallite van der Waal's fields. Engine load influenced micropore adsorption and DP SSA. High loads with high combustion temperatures, efficiently pyrolysed fuel, producing DP with little adsorbed FES and SSA's of 100m² /g. Low loads with lower in-cylinder temperatures, formed less DP and more fuel survived, producing soots of low SSA(<20m² /g). Between aggregated particles, 'ink-bottle' mesopores (2-50nm) were evident (Type IV isotherm) where fuel FES was weakly adsorbed by temperature dependent chemical scavenging as exhaust temperature declined , reducing SSA and increasing particle size. Thermal degassing was more efficient than soxhlet or ultrasonic extraction methods, because the solvent methods failed to penetrate the smallest pores. TD increased soot SSA, greatest for low load samples (by 200m²/g) compared to high load samples (by 50m² /g). TD was highly advantageous for DP extraction and allowed progressive removal of volatiles. A modern DI engine showed structurally similar soots, but the lower DP emissions produced high relative %FES for all engine conditions giving low SSA's. The research findings are related to cylinder and environmental processes for engineers and environmental scientists to improve control strategies.