# Pollutant Emissions and Combustion Efficiency Assessment of Engines Using Biodiesel
## 1 Introduction  
The industrial revolution established new forms of production, such as serial and mass production. Since then, the use of fossil fuels has been an indispensable energy resource. However, some of the consequences of its use are damage to the environment and the health of living beings. An example of this is Mexico City, which is one of the most polluted places in the world, where the implementation of the Kyoto protocol was carried out in 1997. This protocol establishes the percentagesof reduction in greenhouse gas emissions, stating that a reduction of at least 5.25% in CO2 emissions is required, a request that the federal government undertakes to comply. The government has previously tried to implement programs aimed at reducing pollution levels. The best known was the program \"Hoy no circula\", introduced in 1989, in which cars that did not reach the emission criteria could not be used on a given day of the week, depending on the last number of the vehicle license plate. Analyzing the impact of this program with a regression discontinuity design, [PERSON] et al. mentioned that this policy was not effective [1]. There was cause for alert when Mexico, in 2011, was in 13 th place among the top 15 countries generating the most greenhouse gases (GHGs), being responsible for 2% of total GHGs released into the atmosphere, one of the main factors responsible for global climate change [2]. According to the Global Database on Air Pollution in Urban Environments, reported by the World Health Organization in 2016, Mexico City ranked as the seventh most contaminated city by fine suspended particles [3]. In the same year, the Mexico City government began a reforestation program, where the sanitation and reforestation of avenues and ridges were carried out with suitable species for climatic and urban conditions. According to the World Health Organization (WHO) in 2008, around 1.3 million people die annually because of air pollution; 3.7 million died in 2012. Additionally, more than two million premature deaths were related to air pollution [4]. It has been estimated that 24% of the global disease burden and 23% of all deaths can be attributed to environmental factors [5]. Experts who have worked for the Mexico City government have said that the pollution rate is derived from the increase in the number of vehicles, the majority of which circulate daily. In comparison to the engine using gasoline, diesel engines are widely used in medium and heavy applications due to their lower fuel consumption, lower emissions of carbon monoxide (CO), and unburned hydrocarbons (HC) [6]. For these reasons, particles causing the greenhouse effect should be minimized; hence, the use of biofuels, especially biodiesel, may represent a different option. The raw material used in the biodiesel production process is varied (e.g., different types of vegetable oils and animal fats, recycled oils), making the corresponding chemical reaction result in a multiplicity of esters of different fatty acids in variable proportions, all of them called biodiesel [7]. Transesterification is a chemical reaction that has shown the best results in obtaining biodiesel. A triglyceride reaction (composed of a molecule of glycerol esterified by three molecules of fatty acids) is contained in vegetable oil, animal fat, and light alcohol (methanol or ethanol). Glycerin products and esters derived from the three starting fatty acids are obtained as biodiesel. In general, methanol is used as an alcohol replacement, in which case biodiesel will be composed of methyl esters [7]. Transesterification depends mainly on the amount of alcohol, catalyst, pressure, time, FFA (free fatty acids), and the amount of water. Oils with a large amount of free fatty acid are difficult to process because they will form a soap solution in the presence of the catalyst [8]. In terms of fuels for diesel engines, there are several factors affecting suitability. A critical characteristic is the number of cetanes (CN) that is used as a measure of combustion quality.