Parametric Investigation of NOx Emissions from Biofuels for Compression-Ignition Engines

Abstract

The consumption of oxygenated biofuels such as biodiesel and ethanol is growing as aresult of increased pressure from federal and state agencies to improve air quality. These fuelsprovide substantial reductions in particulate emissions. Unfortunately, biodiesel has been shown toincrease NOX emissions upwards of 10% compared to petroleum diesel. The objective of thisinvestigation was to identify with the aid of heat release analysis and suitable statistical methods thecombustion characteristics that influence NOx emissions from a turbocharged and intercooled dieselengine run on selected biofuels. Test fuels included traditional No. 2 diesel and four biofuelscomprising 100% soy methyl ester biodiesel, 2% biodiesel, 10% ethanol-diesel fuel, and 5% ethanolin biodiesel. The engine was instrumented with an in-cylinder pressure transducer, a fuel injector control current probe, and an optical shaft encoder to trigger data recordings. Exhaust NOXemissions were monitored with a Horiba NOX analyzer. Correlation results indicated that start ofcombustion greatly influenced NOX emissions and varied due to cetane number and start of injection.Start of combustion was highly correlated with other crank angle location-based combustion eventssuch as peak temperature location and peak rate of heat release location. Peak temperature wasnot highly correlated with NOX emissions suggesting thermal NO formation does not adequatelyexplain NOX emissions from biofuels. NOX formation did not demonstrate dependence on fueloxygen content. The addition of small percentages of ethanol to biodiesel or retarding injectiontiming appear to be the most effective methods of reducing NOX emissions when combustingbiodiesel.