| Resumo : |
With increasing legislation regulating pollutant emissions and greenhouse gases on spark-ignition engines, there is a need for improvement on engine fuel efficiency and use of alternate fuels with potentially beneficial emission characteristics, such as ethanol. There are several technologies available to increase fuel efficiency in gasoline engines, but further development is still needed for flex-fuel and ethanol-optimized applications. By operating a research spark-ignition engine with optical access, at a partial load low-speed condition, combustion performance was evaluated by means of cylinder pressure and heat release analysis, along with high-speed cycle-resolved direct visualization of flame propagation. Direct fuel injection (DI) technology was primarily utilized, tests were performed with Port Fuel Injection (PFI) to obtain baseline results for comparison. Commercially available fuel mixtures of ethanol and gasoline were tested in all cases, E95w5 (95\% vol ethanol, 5\% vol water) and E27 (27\% vol ethanol, 73\% vol gasoline). On PFI engine tests, lower cylinder pressures were registered for E95w5 due to higher charge cooling effect. Air-guided DI showed higher cyclic variation and slightly retarded combustion, especially for ethanol-content fuels likely related to a combination of ethanol and direct injection charge cooling effects and less time for fuel vaporization on DI. Spray-guided DI improves engine performance over previous air-guided DI system, E95w5 still shows lower peak cylinder pressures due to greater air charge cooling. The more favorable fuel injector position in this case is attributed for the improvements in rate of burn and cyclic performance. Optical investigations revealed a tendency for E0 and E27 gasoline fuels to show more flame center of mass displacement throughout propagation, probably linked to the faster fuel vaporization process and thus more compliance to the in-cylinder air flow. Flame roundness or circularity indicated higher values for hydrous ethanol on DI operation, this is aligned with other published results but requires further investigation as to understand the causes. Higher fuel injection pressures seem more beneficial for E95w5, showing improved combustion stability with 200 bar operation, despite the increase in wall-wetting of the piston observed through the transparent cylinder liner. |