Spark Ignition and Spark Assisted Controlled-Auto Ignition in a Top Entry Direct-Injection Gasoline Engine

An experimental study of the mixture preparation and combustion characteristics of a direct injection gasoline (G-DI) engine operating in both spark ignition (SI) and controlled-auto ignition with spark assistance (SA-CAl) is presented. The air motion, fuel distribution and combustion performance in the engine are described by the application of advanced Laser-based experimental methods and high-speed data acquisition. Determining the correct operation of the engine for spark ignition and compression ignition formed an important part of the research study. The series of experimental studies were conducted on a Ricardo Hydra singlecylinder research engine modified for optical access through the top 20mm section of the cylinder bore, as well as via a 45° mirror and flat window inserted in the crown of the Bowditch piston arrangement. The cylinder head used was based on the Ricardo Flagship concept engine with reverse tumble geometry. Two different interchangeable camshaft sets permitted both high and low valve lift configurations to be utilised. Spark ignition (SI) and controlled-auto ignition with spark assistance (SA-CAl) were achieved in the test programme. Laser-induced fluorescence (L1F), combined with simultaneous flame imaging and Particle Image Velocity (PIV) enabled qualitative in-cylinder fuel concentration measurement during fired operation and quantitative motored air motion characteristics to be determined. Simultaneous acquisition of pressure-based data during the fired studies provided a record of the in-cylinder, intake and exhaust manifold pressures. The addition of K-type thermocouples provided additional temperature information in the engine. The results attained using the above techniques showed that rates of heat release in the SA-CAl combustion mode were considerably slower than those observed in a parallel SI combustion study. This suggested that a key indicator of controlled-auto ignition (CAl) was not present in this study; it is widely considered that CAl heat release rates are of the order of those encountered in SI, when considering 4stroke operation. This result implied that flame-front propagation was the governing mechanism of fuel oxidisation in the SA-CAl mode. However, the results of the PIV studies in the SA-CAl mode suggested that the turbulence intensity was too low to support flame-front propagation in spark-initiated combustion. High-speed photography (HSP) applied in addition to the previously mentioned techniques showed that a hybrid mode of combustion exists in the SA-CAl reaction where flame front propagation could be seen to develop alongside individual sites of autoignition within the cylinder, albeit at a considerably lower rate. Images of fuel distribution in the combustion chamber acquired from the previous studies were correlated to pressure-based combustion stability data. A statistical analysis was conducted of both the SI and SA-CAl sets of data in order to determine cyclic variability within the in-cylinder fuel concentration. The results indicated higher than expected cycle-to-cycle and in-cycle variations in fuel distribution for an early injection, homogeneous charge, combustion mode when compared to those based upon the coefficient of variation in the measured indicated mean effective pressure. The L1F data. from the SA-CAl studies was found to be of far lower intensity, suggesting that the presence of high levels of exhaust residuals acted to attenuate the fluorescence signal. These findings were of particular importance to the application of the L1F technique in this manner. From the findings of the experimental techniques applied it was possible to determine from the mode of combustion encountered, SA-CAl, that a hybrid mode of fuel oxidation was occurring. It was clear that flame-front propagation was taking place, albeit at a far slower rate than in the corresponding Sl mode, as both high EGR rates and lower turbulence intensity was found to attenuate flame propagation, with further heat release occurring through diffusion burning and numerous autoignition points throughout the cylinder through the thermal energy supplied from the resident internal EGR.EThOS - Electronic Theses Online ServiceGBUnited Kingdo