Abstract
The stringent emission regulations have motivated the development of cleaner fuels as diesel surrogates. However, their different physical-chemical properties make the study of their behavior in compression ignition engines essential. In this sense, optical techniques are a very effective tool for determining the spray evolution and combustion characteristics occurring in the combustion chamber. In this work, quantitative parameters describing the evolution of diesel-like sprays such as liquid length, spray penetration, ignition delay, lift-off length and flame penetration as well as the soot formation were tested in a constant high pressure and high temperature installation using schlieren, OH∗ chemiluminescence and diffused back-illumination extinction imaging techniques. Boundary conditions such as rail pressure, chamber density and temperature were defined using guidelines from the Engine Combustion Network (ECN). Two paraffinic fuels (dodecane and a renewable hydrotreated vegetable oil (HVO)) and two oxygenated fuels (methylal identified as OME1 and a blend of oxymethylene ethers, identified as OMEx) were tested and compared to a conventional diesel fuel used as reference. Results showed that paraffinic fuels and OMEx sprays have similar behavior in terms of global combustion metrics. In the case of OME1, a shorter liquid length, but longer ignition delay time and flame lift-off length were observed. However, in terms of soot formation, a big difference between paraffinic and oxygenated fuels could be appreciated. While paraffinic fuels did not show any significant decrease of soot formation when compared to diesel fuel, soot formed by OME1 and OMEx was below the detection threshold in all tested conditions.
Highlights
Pollutant emission regulations are becoming more stringent every year and strategies to reduce them in compression ignition (CI) engines are being constantly investigated
Among them, hydrotreated vegetable oil (HVO), gas-to-liquid (GTL) and new oxygenated fuels like the oxymethylene ethers with the general structure CH3 –O–(CH2 –O)n –CH3, are the most promising alternatives to replace conventional diesel [8,9,10]
Before entering the quantification and detailed analysis on the effect that fuel has upon the spray characteristics, the combustion process and soot formation, a comparison between results for the dodecane fuel from current study and from similar studies available in the Engine Combustion Network (ECN) database [20] is presented
Summary
Pollutant emission regulations are becoming more stringent every year and strategies to reduce them in compression ignition (CI) engines are being constantly investigated. The efficiency and life cycle of the after-treatment systems are deeply related to the fuel behavior inside the combustion chamber In this sense, active strategies to avoid pollutant formation, as the redesign of the combustion chamber or the study of new combustion concepts [2], the improvement of the mixing formation and injection systems [3], the study of new exhaust gas recirculation schemes as well as the development of cleaner alternative fuels, have great interest and importance to the automotive manufacturers [4]. Lower heating value and higher viscosity, as well as a negative effect on nitrogen oxides (NOx ) are important drawbacks compared to conventional diesel fuel [6,7] These have promoted the study of more appropriate alternative fuels. Among them, hydrotreated vegetable oil (HVO), gas-to-liquid (GTL) and new oxygenated fuels like the oxymethylene ethers with the general structure CH3 –O–(CH2 –O)n –CH3, are the most promising alternatives to replace conventional diesel [8,9,10]
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