The Effects of Ethanol/Gasoline Blends on Advanced Combustion Strategies in Internal Combustion Engines.

Abstract

This dissertation presents the effects of blending ethanol with gasoline on advanced combustion strategies in internal combustion engines. The unique chemical, physical and thermal properties of ethanol/ gasoline blends can be used to improve the performance and emissions of advanced engine technologies like gasoline direct injection (GDI) also called direct injection spark ignition (DISI), homogenous charge compression ignition (HCCI) and spark assisted homogenous charge compression ignition (SA-HCCI). This work used experimental studies to understand the impact of ethanol and ethanol/gasoline blends on advanced engine strategies and on understanding which of the fundamental properties of ethanol and ethanol blends control engine performance. The technical approach leveraged high speed imaging to study the fuel spray, combustion, ignition, and sooting (where appropriate) properties of the fuels using different optically accessible engine hardware, including HCCI and GDI configurations. The results of the HCCI work indicated stable operating conditions could be extended to leaner mixtures using the ethanol blends, if the effect of charge cooling due to fuel vaporization was anticipated. Ethanol also improved the stability of flame initiation and growth in SA-HCCI, which affected the global autoignition and performance of the engine. The effect of ethanol on these chemically-controlled engine modes was dominated by the impact of the fuel on thermal stratification. Ethanol combustion chemistry appeared to have little impact. Significant reduction in soot formation was observed in the DISI engine studies using ethanol blends compared to a baseline of reference grade gasoline. This was due to combined effects of ethanol on combustion chemistry, where oxygenated fuels suppress the formation of soot precursors, and of ethanol on increasing evaporation and reducing liquid fuel on the piston, where ethanol changed the fuel spray cone angle and spray collapse. In particular, fuel impingement and wetting of the piston surface dominated in-cylinder soot formation, thus the ethanol fuel spray characteristics that reduced interaction of the fuel spray with the piston and enhanced fuel mixing led to less soot formation.