Vehicle Emissions Reduction

Biodiesel

Biodiesel is produced by reacting vegetable or animal fats with methanol or ethanol to produce a lower viscosity fuel that is similar in physical characteristics to diesel, and which can be used neat or blended with petroleum diesel in a diesel engine. Engines running on biodiesel instead of or blended with petroleum diesel tend to have lower black smoke and CO emissions, but higher NOx and possibly higher emissions of particulate matter. These differences are not very large, however. Other advantages of biodiesel include high cetane number, very low sulfur content, and the fact that it is a renewable resource. Disadvantages include high cost, reduced energy density (resulting in lower engine power output), and low flash point, which may make it hazardous to handle. The effects of biodiesel on engine performance and emissions over a long time in actual service are not well documented.

Although there are no published field test data on engine emissions, performance and durability for vehicles using blended or neat biodiesel, there are some reports in the literature on short-term effects measured in the laboratory.

The general consensus of these studies is that blended or neat biodiesel has the potential to reduce diesel CO emissions (although these are already low), smoke opacity, and measured HC emissions. However, the studies show an increase in NOX emissions for biodiesel fuel when compared to diesel fuel at normal engine conditions. The higher NOX emissions from biodiesel-powered engines are partly due to the higher cetane number of biodiesel, which causes a shorter ignition delay and higher peak cylinder pressure. Some may also be due to the nitrogen content in the fuel. The reduction in smoke emissions is believed to be due to better combustion of the short chain hydrocarbons found in biodiesel, as well as the effects of the oxygen content. Other data have also shown that mixing oxygenates with diesel fuel helps to reduce smoke.

As for the HC emissions, research shows a reduction in HC emissions when biodiesel is used. However, the effect of the organic acids and/or oxygenated compounds found in biodiesel may affect the response of the flame ionization detector, thus understating the actual HC emissions. The behavior of these compounds with respect to adsorption and desorption on the surfaces of the gas sampling system is not known. Thus more studies are needed to understand the organic constituents in the exhaust gases from biodiesel-powered engines before firm conclusions can be drawn regarding the effects on HC emissions. There is controversy concerning the effect of biodiesel on particulate matter emissions.

The cost of biodiesel fuel is one of the principal barriers making it less attractive to substitute for diesel fuel. Since the heating value for biodiesel is less than that for diesel, more fuel must be burned to provide the same work output as diesel fuel. This adds further to the cost disadvantage of biodiesel.