The Effects of Advanced Fuels and Additives on Homogeneous Charge Compression Ignition Combustion and Deposit Formation.

Abstract

HCCI combustion is highly dependent on in-cylinder thermal conditions and the chemical kinetics of the fuel. In addition to the impact on auto-ignition, the fuel properties will also affect the accumulation of deposits on the combustion chamber walls, and this in turn alters the in-cylinder thermal environment. Because the fuels available at the pump differ considerably in composition, strategies intended to bring the HCCI to market must account for the interplay between the fuel chemical components, the HCCI combustion event, and the deposit accumulation in the engine. In an effort to quantify the impacts of fuel composition on HCCI combustion, a large test matrix of fuel blends was investigated in a single-cylinder HCCI engine with re-induction of residual. These fuels were blended from pure refinery streams in an effort to reproduce expected variations of pump gasoline, and were oxygenated with 10% ethanol. The matrix has three dimensions, i.e. the fraction of Olefins (O), the fraction of Aromatics (A), and the Sensitivity (S), defined as the difference between the RON and MON (RON-MON). The impact of fuel composition on performance and thermal efficiency was investigated in a systematic way, such that the obvious impact of the lower heating value is compensated for. The effect of fuel composition and additive packages on HCCI combustion chamber deposit formation was investigated with a second set of refinery stream fuels. The equilibrium CCD thickness was shown to be on the order of 250 micrometers for a highly Aromatic fuel, which is three to five times more than in the case of a low-Aromatic fuel. The subsequent part of the deposit formation study considered two additive packages, namely polybutene amine (PBA) and polyether amine (PEA). The PEA package helped to reduce in-cylinder thickness while the PBA package promoted growth. Finally, microscopic, spectroscopic and diffractometric techniques were employed to quantify physical structure, morphology, chemical composition, and porosity of deposits formed with different fuels and/or additive packages. It is shown that the morphological and chemical dissimilarities among CCD formed with different fuels and additives are not as important to the HCCI combustion event as is the overall thickness.