Particulate Emissions

The heterogeneous fuel-air mixture in the cylinder during the diesel combustion process contributes to the formation of soot particles, one of the most difficult challenges for diesel engine designers. These particles are formed in high temperature regions of the combustion chamber where the air-fuel ratio is fuel-rich and consists mostly of carbon with small amounts of hydrogen and inorganic compounds. Although the mechanism is still not understood, biodiesel reduces the amount of soot produced and this appears to be associated with the bound oxygen in the fuel (McCormick et al., 1997). The particulate level in the engine exhaust is composed of these soot particles along with high-molecular-weight hydrocarbons that adsorb to the particles as the gas temperature decreases during the expansion process and in the exhaust pipe. This hydrocarbon material, called the soluble organic fraction, usually increases when biodiesel is used, offsetting some of the decrease in soot (Sharp et al., 2000). Biodiesel’s low volatility apparently causes a small portion of the fuel to survive the combustion process at light loads, probably as liquid coating the cylinder walls, where it is then released during the exhaust process.

NOx Emissions

A second difficult challenge for diesel engine designers is the emission of oxides of nitrogen (NOx). NOx emissions are associated with high gas temperatures and fuel-lean conditions; in contrast to most other pollutants, they usually increase when biodiesel is used (Sharp et al., 2000). NOx contributes to photochemical smog formation and is difficult to control in diesel engines because measures taken to reduce NOx tend to cause increases in particulate emissions and fuel consumption. The bound oxygen on the biodiesel molecule may play a role in creating a leaner air-fuel ratio in NOx formation regions thus increasing the availability of oxygen for NOx formation. However, the dominant mechanism is probably more complex. Tat et al., 2000, have suggested that changes in the physical properties of biodiesel, such as the speed of sound and bulk modulus, can affect the fuel injection timing and this can increase NOx. Another possible effect is that biodiesel’s reduction in combustion-generated solid carbon reduces the amount of radiative heat loss and thus increases the in-cylinder temperature (Cheng, et al., 2006). Since most NOx formation follows the temperature sensitive Zeldovich, or thermal pathway (Heywood, 1988), the higher in-cylinder temperatures can increase NOx production.

Autoignition Properties

One of the most important properties of a diesel fuel is its readiness to autoignite at the temperatures and pressures present in the cylinder when the fuel is injected. The laboratory test that is used to measure this tendency is the cetane number (CN) test (ASTM D 613). The test compares the autoignition tendency of the test fuel with a blend of two reference fuels, cetane (hexadecane) and heptamethylnonane. Fuels with a high CN will have short ignition delays and a small amount of premixed combustion since little time is available to prepare the fuel for combustion. Most biodiesel fuels have higher CNs than petroleum-based diesel fuels. Biodiesel fuels from more saturated feedstocks have higher CNs than from less saturated feedstocks (Knothe et al., 1997). Biodiesel from soybean oil is usually reported to have a CN of 48–52, while biodiesel from yellow grease, containing more saturated esters, is normally between 60 and 65 (Van Gerpen, 1996). For more details, see Chapter 4.1, Basics of the Transesterification Reaction, and the tables in Appendix A.

Energy Content (Heat of Combustion)

The energy content of the fuel is not controlled during manufacturing. The lower heating value for diesel fuel can vary depending on the refinery in which it was produced, the time of year, and the source of the petroleum feedstock because all of these variables affect the composition of the fuel. Diesel fuels with high percentages of aromatics tend to have high energy contents per liter even though the aromatics have low heating values per kilogram. Their high density more than compensates for their lower energy content on a weight basis. This is of special importance for diesel engines because fuel is metered to the engine volumetrically. A fuel with lower energy content per liter, such as biodiesel, will cause the engine to produce less peak power. At part load conditions the engine operator will still be able to meet the demand for power but a greater volume of fuel will have to be injected. The fuel injection system may advance the fuel injection timing when the fuel flow rate increases, and this can cause an increase in the NOx emissions. In addition to the compressibility effects mentioned earlier, this effect is another reason for the higher NOx emissions observed with biodiesel (Tat et al., 2007).

Biodiesel fuels do not contain aromatics but they contain methyl...