Formation mechanisms of combustion chamber deposits

Abstract

Combustion chamber deposits are found in virtually all internal combustion engines after a few hundred hours of operation. Deposits form on cylinder, piston, and head surfaces that are in contact with fuel-air mixture during the engine cycle. The effects of deposits include increased engine-out NOx emissions, octane requirement increase, and changes in flame speed and thermal efficiency. A framework is developed for examining the physical and chemical processes that contribute to the formation of combustion chamber deposits. First, a hypothesis for the general mechanism of deposit formation is developed from a review of previous work on this issue. The key features of this mechanism are formation of deposit precursor species from fuel and air as the flame quenches at the engine wall, diffusive and convective transport of these species to the wall, and condensation or adsorption at the wall surface. The experimental system and methodology developed in this work are meant to provide insight into the interactions between these processes, and in particular to study the chemical mechanisms that contribute to the formation of deposit precursor species. A cooled low pressure flat flame burner is used to produce steady-state propane-air flames doped with toluene, a known deposit forming species.

(cont.) Profiles of concentrations and temperature are measured using infrared spectroscopy and gas chromatography techniques. In conjunction with the experiments, a one-dimensional numerical model is developed, capable of simulating flame quenching with deposition over a range of conditions extending from the low pressure, steady state burner experiments to high pressure, rapid transient engine conditions, using chemical mechanisms of precursor formation that may be determined experimentally. Modeling of deposition with simplified chemical mechanisms reveals that deposition by condensation can reproduce trends observed in experiments by other researchers; however, adsorption could still be a contributing factor. Experimental observations of toluene-doped flames show the formation of oxygenated compounds such as benzaldehyde and benzofuran, which are likely deposit precursor candidates. The methodology developed in this thesis shows promise for determining deposit precursor identities and formation mechanisms for important fuel components, and for clarifying the role of gas-phase processes in the formation of combustion chamber deposits.