The aim of the present work is the study of the combustion process in Gasoline Direct Injection (GDI) engine fuelled with ethanol mixed with gasoline at percentages of 10 and 85. The characterization has been made in terms of performance and emission for different injection pressure conditions and the results correlated to the unperturbed non-evaporating evolution of the fuel injected in a pressurized quiescent vessel.
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 camera in order to allow the visualization of the fuel injection and the combustion process. The engine is equipped with solenoid-actuated six-hole GDI injectors, 0.14 mm hole diameter, 9.0 g/s @ 10 MPa static flow.
A single injector has been characterized in terms of mass flow rates and spatial-temporal evolution of the fuel in the vessel, controlled in pressure and temperature, at pressures ranging from 5.0 to 15.0 MPa. The jets have been illuminated by flashes and frozen by a synchronized high-speed CMOS camera. The images, processed off-line, have given the main parameters of the spray evolution to be compared with those in the fired engine.
Optical imaging in the engine showed that E10 spray has been more affected from the air motion and in-cylinder pressure than E85, especially at higher injection pressure. At increasing of the injection pressure, flame front speeds show different behaviors for each fuel. In particular, while E10 flame speed increases, E85 one slows down. However E85 shows best results in term of combustion speed and performance, both higher IMEP with lower cycle to cycle variability and UHC emissions. E85 combustion shows also less diffusive flames than E10, and consequently a lower amount of soot measured at the exhaust.
Thus this work, points out the necessity of a careful optimization of injection pressure at the changing of the fuel and of engine operating condition in order to better exploit the capability of ethanol blend in reducing emission and in performance improving.