# Pollutant Emissions and Combustion Efficiency Assessment of Engines Using Biodiesel
## 1 Introduction
The higher the cetane number, the faster the fuel evaporates, inducing an increase in the pressure inside the cylinder. This faster transition, from liquid to gas, can result in greater output power, allowing higher engine speeds [9]. The compounds that can be found during the exhaust emissions in an engine and their respective effects are listed as follows:  
* Sulfur dioxide (SO2) is a colorless gas that is mainly produced during the burning of fossil fuels containing sulfur. According to the 2014 emissions inventory [10], a high concentration of this contaminant can damage the lungs and generate infections, especially if it is in amounts greater than 1.5 ppm, and it may present with bronchial constriction and respiratory infections [11]. * In addition, nitrogen oxides (NOx) are generated in combustion processes by the reaction between atmospheric nitrogen and oxygen. The term nitrogen oxides generally refers to the sum of the concentrations of nitric oxide (NO) and either nitrogen dioxide or nitrous oxide (NO2) [12]. Carbon monoxide (CO) is a tasteless, colorless, and odorless gas. In the atmosphere, it plays a minor role as a greenhouse gas because of its ability to absorb small amounts of infrared energy radiation [12]. Finally, hydrocarbons (HC) means the collective concept of almost all chemical compounds of carbon and hydrogen. HC emissions are attributed to incomplete combustion of the air and fuel mixture due to a lack of oxygen. Partially oxidized hydrocarbons (e.g., aldehydes, ketones) smell unpleasant and form secondary products that, with the lasting action of specific concentrations, are also considered carcinogens [13]. Biodiesel has become an attractive fuel due to its efficiency and low cost compared to gasoline [14]. In addition to reducing unburned hydrocarbons (HC) and CO particles, it can also increase the amount of NOx [15]. Biodiesel has similar properties to those of diesel; for example, it is similar in cetane number. It has the property of being biodegradable, improving lubrication, and it is not particularly toxic [14]. Many researchers agree that the biodiesel used in internal combustion engines generally causes a decrease in soot, HC, CO, and PM, but it generates an increase in NOx emissions [15; 16; 17]. Recent research has shown the problems caused by the use of biodiesel in internal combustion engines; researchers have concluded that the use of pre-injection systems reduces soot, as well as HC and CO emissions. However, NOx emissions are higher in comparison to the result obtained for mineral diesel [18]. That is why the injection system is important; it is responsible for polluting emissions such as NOx [15]. Another common problem is the characteristics of the combustion chamber, as shown by other research on combustion performance [19]. The use of biofuels in engines, due to their properties, can cause some disadvantages. A specific example is incomplete combustion [20]. The effect of this incomplete combustion is reflected in the increase of some polluting gases in tests made with biofuels in diesel engine generators, showing a reduction in HC and CO but an increase in NOx [21] and a decrease in the emission of suspended particles (PMs) and CO\({}_{2}\)[22]. There are many studies related to electric generators; alternative fuels have been analyzed to find the ones that offer the most significant benefits, with biodiesel considered the best alternative to meet the world's energy demands [23]. Thermal efficiency, engine performance, fuel consumption, and gas emission are the most important parameters to evaluate them. Governments, automotive manufacturers, and biodiesel producers have carried out research and development programs for biodiesel-diesel validation tests [24]. There are physical phenomena in the use of biofuels; for example, when biodiesel and ethanol contain water, they show a reduction in the initial sulfur portion as well as the content of polycyclic aromatic hydrocarbons.