# A Comprehensive Review of the Application Characteristics of Biodiesel Blends in Diesel Engines
## 4 Combustion Characteristics  
### Ignition Delay  
Ignition delay (ID) is the one of important parameters in combustion process of fuels, it is defined as the period of time from the start of fuel injection to the start of combustion [69; 70]. The crank angle is generally used as the measurement unit, and the crank angle corresponding to 10% of the combustion mass is regarded as the end point of the ignition delay. The rise of the injector needle means the start of the fuel injection. However, the standard to identify the start of combustion is not unique. The sudden change in the cylinder pressure, heat release rate gradient, or the photoluminescence signal detected by the photocell can be used as a means to identify the start of combustion [25]. The ID is functionally dependent on physical and chemical delays. After being injected, the fuel is atomized, evaporated, and mixed with air resulting in spontaneous combustion. This period is called physical delay. The chemical delay is the period from the beginning of the slow chemical reaction until combustion occurs [71]. The physical delay and chemical delay occur simultaneously and depend on conditions such as ambient temperature and pressure [72]. The chemical delay is generally longer, but if the ambient temperature increases, the rate of chemical reaction will be accelerated, and the chemical delay will be shorter [71].  
ID directly affects the heat release rate and indirectly affects the formation of engine noise and exhaust emissions [70]. [PERSON] et al. found that the ID of the fuel containing karanja biodiesel or roselle biodiesel is shorter than that of diesel [73]. The results are shown in Figure 6. The ID is usually related to the type of fuel and the operating conditions of the engine. Factors such as fuel properties, fuel injection atomization, fuel injection pressure, equivalence ratio, and engine speed may all affect the rhythm of ignition. Most studies found that biodiesel has a shorter ID than that of diesel due to the higher cetane number of biodiesel. Because the high cetane number can reduce the scale of premixed combustion. In biodiesel, some polymer esters with a higher boiling point than diesel will decompose into low molecular weight gas during high temperature injection. This light compound easily evaporates and spreads at the edge of the spray, which ignites the volatile fuel in advance and shortens the ID [28]. Meanwhile, the high viscosity, high density, low calorific value and low volatility of biodiesel also affect the physical delay, although the effect on the chemical delay is not obvious [74]. When the engine load increases, the ID will be shortened as the oil and gas mix at a faster rate because of the higher cylinder temperature, diluted exhaust gas and the excess heat retained in the previous combustion cycle [8; 30]. [PERSON] et al. also found that the ID decreases with an increase in biodiesel content that may be due to the degraded atomization effect, the improved cetane number, the reduced spray cone angle, and the higher oxygen content. The combined effect leads to shortened ignition delay [4; 30]. In some studies, the ID period increases when the engine speed and load increases and this may be attributed to the higher combustion temperature and exhaust gas dilution at higher load and speed. Moreover, the chemical reaction of injected biodiesel at high temperature leads to the decomposition of high molecular weight esters into small molecular weight, which makes volatile compounds burn earlier and delay period shorter [70].