A number of studies with comparisons of diesel, natural gas, and diesel/biodiesel blend bus emissions have been published (Janulis 2004; Tzirakis et al. 2007; Krahl et al. 2009; Soltic et al. 2009; Coronado et al. 2009). Biodiesel has a good energy return because of the simplicity of its manufacturing process, has significant benefits in emissions as well, and could also play an important role in the energy economy if higher crop productivities are attained (Granda et al. 2007).

Table 5.6 shows the emissions of biodiesel (B20 and B100) for same model compression-ignition (diesel) vehicles (Demirbas 2009a). Emissions of NOx in­crease with increasing biodiesel amounts in blends. The properties of biodiesel and diesel fuels, in general, show many similarities, and therefore biodiesel is rated as a realistic fuel as an alternative to diesel. There are several ways to control NOx in a biodiesel engine. One could run the engine very lean, which lowers the tem­perature, or very rich, which reduces the oxygen supplies, decrease the burn time. Emissions of NOx increase with the combustion temperature, the length of the high — temperature combustion period, and the availability of biodiesel, up to a point.

Alcohols have been used as a fuel for engines since the 19th century. Among the various alcohols, ethanol is known as the most suited renewable, bio-based, and eco-friendly fuel for spark-ignition (SI) engines. The most attractive properties of ethanol as an SI engine fuel are that it can be produced from renewable energy

sources such as sugar, cane, cassava, many types of waste biomass materials, corn, and barley. In addition, ethanol has a higher evaporation heat, octane number, and flammability temperature; therefore it has a positive influence on engine perfor­mance and reduces exhaust emissions. The results of an engine test showed that ethanol addition to unleaded gasoline increases engine torque, power, and fuel con­sumption and reduces carbon monoxide (CO) and hydrocarbon emissions (Demir — bas 2009a).

The biodiesel impacts on exhaust emissions vary depending on the type of biodiesel and on the type of conventional diesel. Blends of up to 20% biodiesel mixed with petroleum diesel fuels can be used in nearly all diesel equipment and are compatible with most storage and distribution equipment. Using biodiesel in a conventional diesel engine substantially reduces emissions of unburned hydrocar­bons, carbon monoxide, sulfates, polycyclic aromatic hydrocarbons, nitrated poly­cyclic aromatic hydrocarbons, and particulate matter. These reductions increase as the amount of biodiesel blended into diesel fuel increases. In general, biodiesel in­creases NOx emissions when used as fuel in a diesel engine. The fact that NOx emissions increase with increasing biodiesel concentration could be a detriment in areas that where ozone forms. The pollutant emissions of ethanol-gasoline blends of 0, 5, 10, 15, and 20% were experimentally analyzed in a four-stroke (SI) engine. The concentration of CO and HC emissions in the exhaust pipe were measured and found to decrease when ethanol blends were introduced. This was due to the high oxygen percentage in the ethanol. In contrast, the concentration of CO2 and NOx was found to increase when ethanol was introduced (Najafl et al. 2009).

Oxygenated diesel fuel blends have the potential to reduce the emission of par­ticulate matter and to be an alternative to diesel fuel. Results obtained showed that the addition of bioethanol to the diesel fuel may be necessary to decrease diesel par­ticulate matter generation during combustion (Corro and Ayala 2008; Yu and Tao 2009). The total number and total mass of the particulate matter of ethanol-diesel blend fuels were decreased by about 11.7 to 26.9% (Kim and Choi 2008).

An experimental investigation was conducted to evaluate the effects of using blends of ethanol with conventional diesel fuel, with 5 and 10% (by vol.) ethanol, on the performance and exhaust emissions of a fully instrumented, six-cylinder, tur­bocharged and after-cooled, heavy-duty, direct-injection, Mercedes-Benz engine. Fuel consumption, exhaust smokiness, and exhaust-regulated gas emissions such as nitrogen oxides, carbon monoxide, and total unburned hydrocarbons were mea­sured. The differences in the measured performance and exhaust emissions of the two ethanol-diesel fuel blends from the baseline operation of the engine, i. e., when working with neat diesel fuel, were determined and compared (Rakopoulos et al.

2008) . Diesel emissions were measured from an automotive engine using anhy­drous bioethanol blended with conventional diesel, with 10% ethanol in volume and no additives. The resulting emissions were compared with those from pure diesel (Lapuerta et al. 2008)

The results of the statistical analysis suggest that the use of E10 results in sta­tistically significant decreases in CO emissions (—16%); statistically significant increases in emissions of acetaldehyde (108%), 1,3-butadiene (16%), and ben­zene (15%); and no statistically significant changes in NOx, CO2, CH4, N2O, or formaldehyde emissions. The statistical analysis suggests that the use of E85 results in statistically significant decreases in emissions of NOx (—45%), 1,3-butadiene (—77%), and benzene (—76%); statistically significant increases in emissions of formaldehyde (73%) and acetaldehyde (2,540%), and no statistically significant change in CO and CO2 emissions (Graham et al. 2008).

Biofuels are important because they replace petroleum fuels. There are many benefits for the environment, economy, and consumers in using biofuels. The ad­vantages of biofuels such as biodiesel, vegetable oil, bioethanol, biomethanol, biomass pyrolysis oil as engine fuel are liquid nature-portability, ready availabil­ity, renewability, higher combustion efficiency, lower sulfur and aromatic content and biodegradability. The biggest difference between biofuels and petroleum feed­stocks is oxygen content. Biofuels have oxygen levels of 10 to 45%, while petroleum has essentially none, making the chemical properties of biofuels very different from those of petroleum. Oxygenates are just preused hydrocarbons having a structure that provides a reasonable antiknock value. Also, as they contain oxygen, fuel com­bustion is more efficient, reducing hydrocarbons in exhaust gases. The only disad­vantage is that oxygenated fuel has less energy content.

Combustion is the chemical reaction of a particular substance with oxygen. It is a chemical reaction during which from certain matters other simple matters are produced. This is a combination of inflammable matter with oxygen from the air ac­companied by heat release. The quantity of heat involved when one mole of a hydro­carbon is burned to produce carbon dioxide and water is called the heat of combus­tion. Combustion to produce carbon dioxide and water is characteristic of organic compounds; under special conditions it is used to determine their carbon and hy­drogen content. During combustion the combustible part of fuel is subdivided into volatile parts and solid residue. During heating it evaporates together with some carbon in the form of hydrocarbons, combustible gases, and carbon monoxide re­lease by thermal degradation of the fuel. Carbon monoxide is mainly formed by the following reactions: first from a reduction in CO2 with unreacted C,

CO2 C C! 2CO (5.2)

and, second, from the degradation of carbonyl fragments (-CO) in fuel molecules at temperatures of 600 to 750 K.

The combustion process is started by heating the fuel above its ignition temper­ature in the presence of oxygen or air. Under the influence of heat, the chemical bonds of the fuel are cleaved. If complete combustion occurs, the combustible ele­ments (C, H, and S) react with the oxygen content of the air to form CO2, H2O, and, mainly, SO2.

If insufficient oxygen is present or the fuel and air mixture is insufficient, then the burning gases are partially cooled below the ignition temperature and the com­bustion process stays incomplete. The flue gases then still contain combustible com­ponents, mainly carbon monoxide (CO), unburned carbon (C), and various hydro­carbons (CxHy).

The standard measure of the energy content of a fuel is its heating value (HV), sometimes called the calorific value or heat of combustion. In fact, there are multiple values for the HV, depending on whether it measures the enthalpy of combustion (AH) or the internal energy of combustion (AU), and whether for a fuel containing hydrogen product water is accounted for in the vapor phase or the condensed (liquid) phase. With water in the vapor phase, the lower heating value (LHV) at constant pressure measures the enthalpy change due to combustion (Jenkins et al. 1998). The HV is obtained by the complete combustion of a unit quantity of solid fuel in an oxygen-bomb colorimeter under carefully defined conditions. The gross heat of combustion or higher heating value (GHC or HHV) is obtained by the oxygen — bomb colorimeter method as the latent heat of moisture in the combustion products is recovered.