Thermal characterization and heat transfer study of a gasoline homogeneous charge compression ignition engine via measurements of instantaneous wall temperature and heat flux in the combustion chamber.

Abstract

An experimental study was performed to provide qualitative and quantitative insight into combustion characteristics and gas to wall heat transfer in a gasoline-fueled Homogeneous-Charge-Compression-Ignition (HCCI) engine. The single-cylinder engine utilized exhaust gas re-breathing to obtain large amounts of hot residual gas needed to promote ignition. In-cylinder pressure and exhaust emission measurement were employed for combustion diagnostics by heat release analysis. Fast response thermocouples were embedded in the piston top and cylinder head surface to measure instantaneous wall temperature and heat flux, which provided critical information for a thorough understanding of the heat transfer process. A tested HCCI engine had great sensitivity to thermo-chemical conditions, for instance, wall temperature, intake charge temperature, A/F ratio, and internal residual fraction. The engine had less sensitivity to changes in flow motion via different Swirl-Control-Valve (SCV) settings. However, the SCV fully closed setting was an exception demonstrating significantly retarded burn rate due to the residual gas heat loss in the intake port and its subsequent re-circulation. Local heat flux measurements indicated very small spatial variations in the case of operation with fully premixed fuel-air charge. Variations of the heat flux at some locations in the case of direct injection were attributed to fuel impingement and fuel film dynamics. Very low spatial variations of heat flux enabled the use of the spatially averaged heat flux for evaluating global heat transfer correlations. One of the most popular models, the Woschni expression, was shown to be inadequate for the HCCI engine. The problem was traced back to the flame propagation term in the correlation that was not appropriate for HCCI combustion. Consequently, a modified model was proposed which significantly improved the features of the predicted crank-angle resolved heat flux profile, and showed very good agreement in terms of magnitude and phasing over the range of conditions.