Combustion Improvement of Diesel Engine by Alcohol Addition - Investigation of Port Injection Method and Blended Fuel Method

摘要

Alcohol fuels that can be produced from cellulose continue to become more widely used in gasoline engines. This research investigated the application of alcohol to diesel engines with the aims of improving the combustion of diesel engines and of utilizing alternative fuels. Two methods were compared, a method in which alcohol is injected into the air intake system and a method in which alcohol is blended in advance into the diesel fuel. Alcohol is an oxygenated fuel and so the amount of soot that is emitted is small. Furthermore, blended fuels have characteristics that help promote mixture formation, which can be expected to reduce the amount of soot even more, such as a low cetane number, low viscosity, low surface tension, and a low boiling point. Ethanol has a strong moisture-absorption attribute and separates easily when mixed with diesel fuel. Therefore, 1-butanol was used since it possesses a strong hydrophobic attribute and does not separate easily. In this research, the low soot characteristics of alcohol were utilized and a method of improving combustion was tested that raised the premixed compression ignition combustion ratio while suppressing combustion noise. In addition, the possibility of improving thermal efficiency and reducing NO_x was also investigated. The results showed that at middle and high loads the tradeoff between NO_x and thermal efficiency was improved and that NO_x could be significantly reduced at a constant thermal efficiency. A 73% reduction in NO_x was achieved with the port injection method, while a 61% reduction was achieved with the blended fuel method at middle load when using a butanol volume ratio of 50%. On the other hand, thermal efficiency became worse at low load, even at constant NO_x, due to increases in the unburned fuel loss and cooling loss. However, in the blended fuel method, it was possible to reduce NO_x by 48% at low load by adding a cetane number improver and thereby improving the ignition attribute. The results of simulated testing using the New European Driving Cycle (NEDC) showed that NO_x could be reduced by 38% without diminishing thermal efficiency using the port injection method, while NO_x was only reduced by 24% and thermal efficiency diminished by 0.1 point using the blended fuel method. If a cetane number improver was added to the blended fuel to improve the ignition attribute and thermal efficiency, the results showed 37% reduction in NO_x with constant thermal efficiency, which approximately equal to port injection method. However, even further improvement is necessary in order to reach the levels specified in the Euro 6 emissions regulations. Furthermore, this research also analyzed the differences in soot generation by visualizing the combustion. The results showed that even under conditions that simulated the same ignition delay, both methods generated less soot from initial combustion in comparison to diesel fuel. Post injection of blended fuel was also effective in reducing soot.