# A Comprehensive Review of the Application Characteristics of Biodiesel Blends in Diesel Engines
## 5 Emission Characteristics
### Regulated Emissions
#### 5.1.4 Nitrogen Oxides  
Compared with gasoline engines, diesel engines have significant nitrogen oxides (NO\({}_{\rm X}\)) emissions. Most studies suggest that there are three main sources of NO\({}_{\rm x}\) whose main ingredients are NO (95%) [108]. They are described in the following: (1) thermal NO: N\({}_{2}\) and O\({}_{2}\) in air can react through a series of chemical steps called the Zeldovich mechanism at high cylinder temperatures. The increase in the residence time and the temperature of the mixture in the cylinder may lead to an increase in NO\({}_{\rm x}\) emission; (2) prompt NO: under oil rich conditions, the intermediate products containing hydrocarbons are produced first in fuel combustion. CH and CH\({}_{2}\) can react with N\({}_{2}\), and the substances containing CN compounds continue to oxidize to NO\({}_{\rm x}\); (3) N\({}_{2}\)O pathway: in this mechanism, N\({}_{2}\) directly reacts with oxygen to produce N\({}_{2}\)O [108]. For diesel engines, the formation of NO\({}_{\rm x}\) is mainly influenced by the mechanism affected by the local temperature in the combustion mixture, the residence time at high temperatures, and the local air/fuel ratio in the cylinder [59; 88; 109]. NO\({}_{\rm x}\) is both harmful to the human respiratory system, and contributes to the formation of photochemical smog [84]. Thermal NO: \[\rm N_{2}+O\leftrightarrow NO+N\ \ \ \ N+O_{2}\leftrightarrow NO+O\ \ \ \ N+OH \leftrightarrow NO+H\] (6) Prompt NO: \[\rm CH+N_{2}\leftrightarrow HCN+N\ \ \ \ C_{2}+N_{2}\leftrightarrow 2 CN\ \ \ \ CN+O_{2}\leftrightarrow NO+CO\] (7) N\({}_{2}\)O pathway: \[\rm O+N_{2}+M\leftrightarrow N_{2}O+M\ \ \ \ \ N_{2}O+O\leftrightarrow NO+NO\] (8)  
Many researchers have found that NO\({}_{\rm x}\) emissions from biodiesel are greater than that from traditional diesel under different loads [4; 56; 59; 89]. [PERSON] et al. tested the combustion and emission characteristics of canola biodiesel blends on a four-stroke naturally aspirated single-cylinder stationary DI diesel engine. It was found that the average NO\({}_{\rm x}\) emissions of all loads of each fuel B20, B15, B10 and B5 increased by 8.9%, 7.8%, 6.3% and 3.3%, respectively [91]. When testing rapeseed oil biodiesel, [PERSON] et al. found that the NO\({}_{\rm x}\) emissions of B25, B50, B75, and B100 were 14.4%, 21.6%, 28.5%, and 32.9% higher than that of diesel at maximum braking power, as shown as Figure 12[38]. The effect on NO\({}_{\rm x}\) formation seems to be mainly due to the effect of molecular structure on the spontaneous ignition delay that occurs after the fuel is injected into the combustion chamber, and the flame temperature during fuel combustion is relatively low [104]. In addition to the higher cetane number, the higher oxygen content in biodiesel is an important factor in NO\({}_{\rm x}\) formation. Excessive hydrocarbon oxidation and high oxygen content of biodiesel lead to excessive local temperature and corresponding excess air in the cylinder, which increases NO\({}_{\rm x}\) emissions [56; 59; 88; 91; 95]. Moreover, as more fuel is injected, an oil-rich core is generated, which may combine with oxygen in the oil to affect NO\({}_{\rm x}\) emissions, especially under high load conditions [59]. [PERSON] et al. also pointed out that the higher percentage of unsaturated fatty acids in biodiesel fuels, which have higher adiabatic flame temperatures, lead to higher NO\({}_{\rm x}\) emissions [88]. Longer hydrocarbon chain lengths promote higher adiabatic flame  
Figure 11: Particulate matter (PM) concentrations for all tested fuels under different speeds and loads [102]. temperatures [104]. The double bonds of fatty acids promote the formation of hydrocarbon free radicals [91,94]. However, a few scholars have reported some cases of NO\({}_{\rm x}\) reduction. NO\({}_{\rm x}\) emission characteristics were evaluated in a four-stroke, single-cylinder air-cooled diesel engine fueled with microalgae chlorella protothecoides biodiesel-diesel blends by [PERSON] et al. [96].