Fuel Property Effects on Injection Timing, Ignition Timing and Oxides of Nitrogen Emissions from Biodiesel-Fueled Engines

Biodiesel is an environmentally friendly alternative diesel fuel consisting of the alkylmonoesters of fatty acids. It is obtained from triglycerides through the transesterficationprocess. Biodiesel has been observed to reduce most engine exhaust pollutants with theexception of oxides of nitrogen (NOx), which generally increase by 5 to 15%. The objective ofthis research was to determine the reason for the higher levels of NOx emissions. A conceptmap was developed to show the interrelationships between the fuel and engine variables thataffect NOx production. It was determined that a change in combustion timing caused bychanges in the fuel properties between diesel fuel and biodiesel is an important source of theNOx increase. The properties investigated in this research included the lower heating value,volatility, density, speed of sound, bulk modulus, and cetane number of biodiesel. It was found that half of the start of combustion advance associated with biodiesel originatedfrom a start of injection advance that was split approximately evenly between the automatictiming advance of the pump as it injects more fuel to compensate for the lower heating value ofbiodiesel and the effect of the bulk modulus, viscosity, and density of the fuel. At the sametemperature, the fuel delivery of biodiesel was higher than for petroleum-based diesel fuelbecause of the higher viscosity of biodiesel. At the same viscosity level, it was found that thefuel delivery of petroleum-based diesel fuel was higher than for biodiesel. This was attributed tothe metering orifices in the fuel injection pumps restricting the amount of fuel flow for moredense fuels. The other half of the start of combustion timing advance was due to the highercetane number of the biodiesel.