An Analysis of Intake Valve Deposits from Gasolines Containing Polycyclic Aromatics 912378
Polycyclic aromatic hydrocarbons such as anthracene and pyrene in gasolines are believed to be one of the causes of deposits in internal combustion engines. One source of these compounds is heavy reformate, a high-octane gasoline component. This blending stream can be redistilled at added expense to remove these heavy compounds, commonly referred to as reformer bottoms or reformer polymer. Removing this material also improves the color and gum content of the gasoline.
In this study, ten fuels with various concentrations of reformate and reformer bottoms were run on a standard intake valve deposit test cycle using a 1987- vintage, 2.5-liter, four-cylinder, throttle-body-injected engine. It was found that characterizing the amount of Ultraformer bottoms by the anthracenes + pyrenes (A+P) concentration in the finished gasoline provided an excellent correlation (cc = 0.95) to the deposits formed. Naphthalenes concentration did not correlate with deposit formation. This may indicate that only the three- and four-ring aromatics are involved in deposit formation while the two-ring aromatics are not. None of the gasolines contained detergent deposit control additives since it was found that they would mask the test results.
Results indicate that as the reformate concentrations in the gasoline increase, as measured by A+P concentration, intake valve deposits also increase. The deposits appear to be an aromatic-aliphatic polymer, becoming more aromatic at higher deposit weights. The aromatic cluster size, however, remains constant at three to four rings for all the samples. This suggests that the polycyclics are not condensed into a graphite-like structure. Metallic additives from the lube oil contributed a nearly constant weight to the deposits and therefore a decreasing fraction of the total deposits as deposit weight increased. The deposits also contained 10-15% oxygen and 1-2% nitrogen.