Microstructure of engine deposits and effects of fuel additives

Abstract

The structural characteristics of combustion engine deposits produced from fuel with 22¿4% aromatics by volume have been studied using Raman spectroscopy and gas sorption techniques. The lateral sizes of graphitic crystallites were found to increase slight with the aromatic content of the fuel used. The surface areas of the deposits were evaluated using BET and DR theories. Density functional theory (DFT) was used to evaluate pore size distributions. The deposits have large internal surface areas (in the approximate range 100¿300 m 2 /g) and their structures are highly porous. Walls of pores with widths of about 0.5 nm are primarily responsible for the majority of the internal surface area found in the deposits. Heat treatment at temperatures above 573 K results in increased surface areas because the release of small hydrocarbon fragments from the deposits opens pores, which were not accessible following heat treatment at lower temperatures. This research also investigated the microstructure of engine deposits produced from fuels with commonly used gasoline additives, polyether amine and polybutene amine. The lateral sizes of graphitic crystallites were found to increase upon addition of additives. The surface areas and pore size distribution of the deposits were evaluated by DR theory and density function theory respectively. The surface area of the deposits decrease with increased concentration of the additives, and this process is more efficient for polybutene amines than polyether amines. The structure of the deposits removed from the combustion chamber's cylinder heads is more porous than that of piston top deposits. Likewise, intake valve deposits were seen to be less porous than combustion chamber deposits.