Combustion process investigations in a DISI engine fueled with butanol through thermodynamic analysis and optical dignostics

Fuel availability and tighter limits for emissions have made the study of alternative energy sources a key development strategy for internal combustion engines. Alcohols are the most widely used biofuels in sparkignition (SI) engines, given that their production, storage and distribution are well known and understood. While methanol has the lowest production cost and ethanol is far less toxic, compared to the first two, biobutanol features properties that are closer to gasoline, such as increased energy density and good compatibility with components used in the automotive industry. In order to provide improved understanding of fuel effects, experiments were performed on a direct injection SI engine operated on gasoline and butanol, with combined analysis of in-cylinder pressure measurements and optical data. The optically accessible power unit was fitted with a commercial cylinder head with four valves per cylinder, and a high pressure wall guided injection system working at 100 bar. A fixed engine speed setting of 2000 rev/min was chosen as a representative value for mid-road load automotive use. Different spark timings, with partially open throttle, were tested. All trials were performed at close to stoichiometric air-fuel ratio, as measured at the exhaust through an oxygen sensor. Optical diagnostics based on UV-visible digital imaging and natural emission spectroscopy were applied in order to study the development of combustion process from the spark ignition to the exhaust valve opening. A custom post-detection procedure applied to digital imaging data allowed to evaluate the flame area from early combustion stages. Results were correlated with volume fraction burned obtained through thermodynamic analysis. Moreover, UV-visible natural emission spectroscopy was applied to investigate the formation and the evolution of the main compounds characterizing the spark ignition and combustion process. Finally, pollutant measurements (HC, CO and NOx) at undiluted exhaust, for gasoline and butanol, were correlated with pressure related data and optical results.