Examination of Causes of Wetstacking in Diesel Engines

The focus of this work is to understand how diesel engines “wetstack.” Mainly through the work which will be discussed here, the root cause appears to be the presence of polycyclic aromatic hydrocarbons (PAHs) in the exhaust and accumulation on the small clearance between the exhaust valve stem and the valve guide. Experimental data which support this premise was obtained by operating an engine at no load and collecting exhaust soot particulate on a filter paper onto which PAHs were adsorbed. Supercritical carbon dioxide extraction was then used extract PAHs for analysis. The extract was processed by GC/MS to quantify PAHs. Because of the high unburned fuel carry-over at these operating conditions, it was necessary to use tandem MS to isolate each PAH from fuel carry-over in the spectra. In order to model processes for a better understanding of potential formation and condensation of PAHs on cool surfaces in the engine exhaust, calculations using computer software were performed at conditions corresponding to the experimental operating conditions. These included chemical reaction rate modeling to estimate exhaust composition, computing the condensation temperature of the calculated exhaust mixture and finally examining the concentration of PAHs in the exhaust using a bubble point-dew point calculation. The findings are that wetstacking appears to occur at low engine loads because of two related factors: 1) high PAH production and high unburned fuel carry-over at very lean combustion conditions corresponding to low loads, and 2) corresponding low exhaust temperatures leads to condensation of unburned fuel and PAHs on cool exhaust components, namely the exhaust valve stem.