Chemistry of Combustion Products Investigation of Combustion Chamber Deposits Formed in a Gasoline Direct Injection Stratified Charge SI Engine

Sammanfattning: The aim of this work was to characterize deposits formed in the combustion chambers of a gasoline direct injection stratified charge SI engine. The engine was run with several types of gasoline fuels and a special reference engine oil. The deposits were analyzed in several ways: thermal desorption (TD) and liquid extraction followed by gravimetric analysis, gas chromatography/mass spectrometry (GC/MS), cyclic fast neutron activation analysis (cFNAA), solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy and solid state ¹H inverse recovery NMR spectroscopy (measuring the spin-lattice relaxation constant, T₁).

Combustion chamber deposits consisted of a volatile and a solid part. TD followed by GC/MS showed that up to 50 weight % of the deposits consisted of the volatiles. The volatiles were dominated by higher n-alkanes originating from the engine oil and contained small amounts of aromatics from gasoline. The results of solid state ¹³C NMR showed that the solid part of the deposits contained a substantial amount of polyaromatics. T₁, from solid state ¹H inverse recovery NMR, of the aliphatics contained in the deposits was measured to be about 0.2 s. As a comparison, the T₁ of the engine oil was measured to be 0.46 s. The results indicate, in agreement with TD and GC/MS, that deposits contain a liquid phase derived from engine oil components.

Up to 30 weight % of the combustion chamber deposits consisted of oxygen. The oxygen in the deposits was bound to different types of carbon compounds as oxygenated aliphatics, carboxyls and carbonyls. The fuels used in this study did not significantly affect the oxygen content and the organic composition of the deposits.

Two methods were developed to study the oxidation of the engine oils. First, soot produced by the oxidation of oils in a thermal reactor was analyzed. The results showed that the reference engine oil degraded via formation of oxidized aliphatics. Soot produced by the reference oil at 550 °C was similar to deposits with respect to organic chemical composition and oxygen content. Second, in situ measurement of the amount of oxygen in oil in a sample container was carried out using fast neutrons. The sample size can be in the volume range of a few hundred milliliters up to several liters. Monte Carlo calculations showed that the optimal radius and height of the sample container were about 10 cm and 44 cm, respectively. This method could be used for quantification of oxygen in oil during a slow oxidation process.