# A Comprehensive Review of the Application Characteristics of Biodiesel Blends in Diesel Engines
## 5 Emission Characteristics
### Unregulated Emissions  
#### 5.2.1 Volatile Organic Compounds  
Volatile organic compounds (VOCs) usually refer to organic compounds with an atmospheric boiling point ranging from 50 \({}^{\circ}\)C to 260 \({}^{\circ}\)C. The most common chemical structures include aldehydes, ketones, alkanes, aromatics, alkenes and halogenated hydrocarbons [12; 115]. Vehicle emissions are an important source of VOCs. VOCs emitted by vehicles generally come from the natural volatilization and incomplete combustion of fuel, and the components emitted from these are very similar [116]. Volatile organic compounds can easily form photochemical smog and secondary aerosols that damage the environment. They can also affect the normal metabolism of the human body and cause acute symptoms such as headaches and allergies [117].  
Most researchers have observed experimental results that the total emissions of VOCs by biodiesel have decreased. [PERSON] et al. tested diesel and waste soybean oil biodiesel blend B20 in three driving modes to simulate engine VOCs emission characteristics under various operating conditions and loads. It was found that B20 releases an average of 61% less total VOCs than conventional diesel, which has more emissions and more types of substances. In addition, there are 27 kinds of volatile organic compounds with emissions exceeding 1 mg kW h\({}^{-1}\) in diesel, and only 13 kinds in B20 [118]. [PERSON] et al. performed experiments and found that combustion of fuels containing canola oil biodiesel fuel can release fewer VOCs as shown as Figure 14, and benzene is the highest emission component in all fuels, reaching 41% [119]. This trend may be due to the higher cetane number and higher flash point of biodiesel, which can reduce the ignition delay. In addition, the increased oxygen content in biodiesel helps the fuel to burn more completely, thereby reducing hydrocarbon emissions [118; 120; 121]. In addition, diesel is mainly composed of linear alkanes, and the conversion into emissions requires a series of complex reactions involving alcohols, carbonyl compounds, acids, and esters. In contrast, biodiesel is dominated by short-chain compounds, which make it easier to directly oxidize esters to carbon dioxide [121; 122].  
Meanwhile, the composition of the released VOCs gases from different fuel blends is also different. For example, [PERSON] et al. found that benzene (25.9%) is the most abundant in B0, nonane (16.0%) is the most abundant in B7, and butanal (18.3%) is the most abundant in B20 [120]. The increase or decrease in VOCs will also change under different loads and operating speeds. Generally, the reduction of VOCs is more pronounced at higher engine speeds [121]. Some conclusions point out that there is a  
Figure 13: Overall decrease in emissions of carbon monoxide (CO), hydrocarbon (HC), soot and NOx achieved by addition of the CeO\({}_{2}\)-MWCNTs nanocatalyst compared to catalyst-free fuels [113].  
positive correlation between VOCs emissions and load, which may be due to higher injection pressure, good atomization and inlet turbulence intensity [12]. Of course, the oil-gas mixture is likely to be thicker due to the large injection volume under high load, which may show the opposite result [121].