Study on fuel properties, spray atomization, combustion and emissions reduction characteristics of bioethanol blended diesel fuel in a diesel engine

Abstract

This paper focuses on investigating the mixing stability, the fuel properties, the overall spray behavior, the atomization performance, the combustion- and the exhaust emissions- characteristics of diesel-bioethanol blended fuels in a diesel engine with a common-rail injection system. The verification of the application possibility of bioethanol blended diesel fuel to the compression ignition diesel engine is also one of purposes of this study. In this study, the phase separation between diesel and bioethanol fuels was overcome through the addition of biodiesel fuel and the prevention of the contact with air. The fuel density, kinematic viscosity, and the surface tension of diesel-bioethanol blended fuels decreased with the increase of bioethanol contents because the bioethanol fuel has the low fuel density, kinematic viscosity, and the surface tension. The cloud point and pour point slightly increased with bioethanol fuel. In addition, the heating value and cetane number decreased by the addition of the bioethanol fuel. In order to investigate the overall spray behavior of diesel-bioethanol blended fuels, the spray visualization system and the droplet measuring system (phase Doppler particle analyzer, PDPA) were used. The blending of bioethanol fuel to the pure diesel fuel slightly affected the spray tip penetration and the spray cone angle (decrease of spray tip penetration and increase of spray cone angle), whereas the droplet size of diesel-bioethanol blended fuels decreased with an increase in the bioethanol contents. The mean droplet size decreased as the bioethanol blending ratio increased because of the low kinematic viscosity and low surface tension of bioethanol blended fuels. The effects of bioethanol contents on the combustion characteristics and the exhaust emission reduction were investigated in a single-cylinder diesel engine and four-cylinder diesel engine with a common-rail injection system. The exhaust emissions such as NOx, HC, CO, soot and nano-sized particle distribution were investigated by using the emission analyzer and SMPS with CPC and DMA. The increased bioethanol blending ratio extends the ignition delay. The bioethanol- diesel blending causes a small decreases in soot emissions because of high oxygen content of the bioethanol fuel and reduces ISNOx emissions because of the low combustion temperature caused by the high heat of evaporation of bioethanol fuel and low LHV. The ISHC emission increased by the increase of the bioethanol blending ratio, and the ISCO emission generally increased with the increase of the bioethanol blending ratio. On the other hand, the increase of bioethanol blending ratio generally leads to the decrease of the total particles number and mass. However, in the case of DE30 fuel, the total number and mass of nano-sized particles increased due to the increased ignition delay and the occurrence of the ignition in the expansion stroke. The increase of biodiesel blending ratio caused the advance of the ignition delay due to the high cetane number. By the change of the combustion phasing, the IMEP was linearly decreased with biodiesel fuel. In the exhaust emission characteristics, NOx and soot emissions decreased by the low LHV and high oxygen contents in biodiesel fuel, respectively. In addition, it can be observed that the HC emission decreased by the increase of biodiesel fuel due to the oxygen in biodiesel fuel. On the other hand, based on the experimental results in a single cylinder diesel engine, the combustion and exhaust emission characteristics were investigated in the four-cylinder diesel engine in order to verify the application possibility of diesel-bioethanol blended fuel to the commercial diesel engine. In a multiple injection test, the increase in the bioethanol blending ratio caused the increase of the ignition delay in the pilot combustion. However, the influence on the main combustion is insignificant. The increase in the pilot injection amount causes a significant increase in the combustion pressure of the pilot injection. The NOx emissions show an increasing trend according to the retardation of the pilot injection timing and the increase in the pilot injection amount. The NOx emission can be reduced through bioethanol blending and the application of EGR. The CO emission increases with the advance in pilot injection timing, the increase of the pilot injection amount, and the increase of the bioethanol blending ratio. In addition, an increase in the bioethanol blending ratio and an advance of the pilot injection timing increase the HC emissions. An increase in the pilot injection amount induces a slight increase in the HC emissions. In this study, synthesizing above results and discussions about diesel-bioethanol blended fuel, the atomization performance can be improved, and the mixture with high quality can be obtained, the NOx and soot emissions can be reduced through the application of diesel-bioethanol blended fuel. Therefore, it is certain that the diesel-bioethanol blended fuel is one of most strong alternative fuels, and it is also an eco-friendly fuel replacing the conventional diesel fuel in a near future.