## 1 Introduction  
Using internal combustion engines provide convenient transportation. However, air pollution and depletion of the resources caused by internal combustion engines are serious problems. The pollutant emissions from internal combustion engines, such as nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbon (HC), and particulate matter (PM), are subject to strict regulation [1]. Further, global efforts are underway to reduce CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O, the GHGs (greenhouse gasses) that affect global warming and climate change problems [2; 3]. The exhaust gas emitted from diesel engine contains higher amount of NOx and particulate matter that causes of severe environmental problems affecting human health [4]. Regulated emissions are not the only pollutants from engines. There are volatile organic compounds (VOCs) that are emitted in small quantities but make photo-chemical smog from a reaction with nitrogen oxide and have an important role in the formation of ozone [5; 6; 7; 8]. VOCs are emitted from various pollution sources, among them, vehicles using internal combustion engines are known to be the major source of VOCs in metropolitan areas with high densities of people [7; 9]. Thus, VOCs are known as the precursors of photochemical smog and ozone [6; 7]. The important characteristic of VOCs is toxicity, and some VOCs are toxic for human and animals [10]. Representative toxic VOCs are benzene, toluene, ethylbenzene, and xylene in the aromatic family [11; 12; 13]. Benzene is classified as a class 1 carcinogen by the World Health Organization (WHO) and the InternationalAgency for Research (IARC) [14]. Long exposure to benzene may cause bone marrow damage, aplastic anemia, and leukemia. Ethylbenzene is known as a potential carcinogenic material. Toluene and xylene are harmful and classified as group d, which means non-carcinogenic. Toluene may cause eye, nose, and throat irritation as well as headaches and dizziness by impacting the central nervous system. Long exposure to xylene irritates eyes and may cause blindness. [PERSON] et al. [15] reported that long exposure to toluene, xylene, and ethylbenzene may cause chronic nerve damage. For these reasons, the risk rate for emissions from diesel engines was raised to group 1 from group 2a by IARC and WHO in 2012 June [16]. Many researchers [5; 7; 9; 17; 18] analyzed harmful VOCs by collecting air from major cities around the world. [PERSON] et al. [17] studied VOCs in Seoul, Korea from 1997 to 1999 and showed that 58% of aromatic VOCs were emitted from vehicles. Also, [PERSON] et al. [9] reported that the major source of VOCs was vehicles, and toluene was emitted the most among aromatic VOCs from vehicles. [PERSON] et al. [19] studied the VOCs from light-duty diesel vehicles with a chassis dynamometer and showed that aromatic VOCs represented the high portion of pollutants. [PERSON] et al. [7] checked the levels of VOCs using light-duty diesel vehicles under different regulation levels operated on real roads in a big city in China. They reported that benzene is the most common material among the aromatic VOCs. The development of catalysts for reducing aromatic VOCs is ongoing using ceria, CeOY and MnOx, and Manganese [20; 21; 22]. One of the effective ways to slow the depletion of resources and reduce pollutant emissions is to use biofuel. The representative biofuels are biodiesel and bioethanol [3]. The physical properties of biodiesel are similar to petroleum diesel so that it can be used without mechanical modification of diesel engines [3; 23]. Biodiesels contain 10-12% oxygen by weight, which can improve combustion efficiency [24; 25]. In contrast, the calorific value is lower than petroleum diesel, which results in more fuel consumption than using a petroleum diesel to produce the same output. The high viscosity, density, and surface tension can deteriorate the combustion because of poor atomization of injected fuel, which means droplet sizes are much larger [26; 27; 28].