Thermodynamic and optical characterizations of a high performance GDI engine operating in homogeneous and stratified charge mixture conditions fueled with gasoline and bio-ethanol

UltraViolet-visible imaging measurements were carried out in a gasoline direct injection (GDI) engine in order to investigate the spray and combustion evolution of gasoline and pure bio-ethanol fuel. Two different starts of injection, early injection (homogeneous charge) and late injection (stratified charge), were tested in two different engine conditions, 1000 rpm idle and 1500 rpm medium load as representative point of urban new European driving cycle (NEDC). Measurements were performed in the optically accessible combustion chamber made by modifying a real 4-stroke, 4-cylinder, high performance GDI engine. The cylinder head was instrumented by using an endoscopic system coupled to high spatial and temporal resolution cameras in order to allow the visualization of the fuel injection and the combustion process. All the optical data were correlated to the in-cylinder pressure-based indicated analysis and to the gaseous and solid emissions. Wide statistics were performed for all measurements in order to take into account the cycle-to-cycle variability that characterized, in particular, the idle engine condition. Optical imaging showed that gasoline spray was more sensible to air motion and in-cylinder pressure than ethanol’s, for all the investigated conditions. The stratified flame front for both fuels was about 40% faster compared to homogeneous in the first phase, due to the A/F ratio local distribution. It leads to better performance in terms of stability and maximum pressure, even if the late injections produce more soot and UHC emissions due to fuel impingement. Ethanol combustion shows less diffusive flames than gasoline. A lower amount of soot was evaluated by two color pyrometry method in the combustion chamber and measured at the exhaust.