Structural characterization of carbonaceous engine deposits
View/ Open
Date
2010Author
Pinto da Costa, José Mário Cerqueira
Metadata
Abstract
Carbonaceous engine deposits tend to accumulate on most of the inner
surfaces of the car engine. The presence of these deposits leads to a deteriorated
efficiency of the engine and a number of adverse effects, such as higher
propensity of the engine to knock. It has been proposed that selective adsorption
of some of the fuel components in the porous deposits (and changing
composition of the pre-combustion fuel) could be a contributing mechanism of
the diminished efficiency of the engine. This, as well as other mechanisms of the
deposits action, crucially depend on the porous structure of the material.
Therefore, the aim of this investigation is to develop a method, which is able to
accurately characterize the internal porous structure of the engine deposits and
predict their adsorption properties at different conditions. This should allow us
to assess whether the selective adsorption of fuel components is indeed a
plausible contributing mechanism to the diminished performance of the engine.
Accurate characterization of the engine deposits faces several difficulties
due to their complex porous structure and chemical composition. A widely
adopted approach in the characterization of activated carbons, which combines
molecular simulation, specifically grand canonical Monte Carlo (GCMC) in slit
pores, and experimental adsorption isotherms, is the starting point for the
method suggested in this work. In this thesis, we will demonstrate that, by
systematic modification of the solid-fluid interaction in the molecular
simulation, we are able to correctly account for the chemical structural
heterogeneity of the samples used. The new parameters of solid-fluid interaction
allow us to extract representative pore size distributions and investigate the
adsorption properties under different conditions of temperature and pressure,
based on the obtained pore size distribution. Specifically, using the experimental
data from a single ethane isotherm at 278K we accurately predict ethane
adsorption at other temperatures and in different samples. Additionally, the
proposed method is able to predict the adsorption of more complex
hydrocarbons, i.e. n-butane and isobutane. The performance of the method is
assessed by comparing the simulations results with the experimental adsorption
measurements data on the engine deposits samples.
Another important capability of the method is that it enables us to
generate adsorption predictions of two key components commonly used to
represent the combustion properties of the fuel, n-heptane and isooctane. We
explore the equilibrium adsorption properties of these components based on the
determined pore size distributions of the deposit samples. The results presented
in the thesis highlight the importance of the adsorption in the internal porous
structure of the engine deposits.
The present study reinforces the value of molecular simulation combined
with a limited number of experimental measurements, to accurately characterize
heterogeneous carbonaceous materials and to make predictions at different
conditions with sufficient precision.