# A Comprehensive Review of the Application Characteristics of Biodiesel Blends in Diesel Engines
## 4 Combustion Characteristics
### Heat Release Rate  
The heat release rate (HRR) indicates the rate at which the fuel releases chemical energy during the combustion process in the diesel engine [31]. It can be used to determine the start of combustion, the difference in fuel fraction and combustion rate during the premix period [34]. The engine performance parameters can be well understood and controlled through HRR. According to the first law of thermodynamics, it can be calculated by [79]:  
\[\frac{dQ_{ combustion}}{d\theta}=\frac{dQ_{net}}{d\theta}+\frac{dQ_{wall}}{d \theta}=\frac{1}{k-1}V\frac{dP}{d\theta}+\frac{k}{k-1}P\frac{dV}{d\theta}+ \frac{dQ_{wall}}{d\theta}, \tag{5}\]  
where, \(\frac{dQ_{ combustion}}{d\theta}\) is the heat release rate, \(\theta\) is the crank angle, \(k\) is the ratio of specific heat, and \(Q_{wall}\) means the heat exchange of the wall.  
[PERSON] et al. studied the combustion characteristics of moringa oleifera biodiesel-diesel blends at six engine speeds (1500, 2000, 2500, 3000, 3500, 4000 rpm) in a four-cylinder turbocharged diesel engine with total cylinder volume of 1.461 L. They reported that the peak HRR of MOB50 (50% moringa oleifera biodiesel +50% petroleum diesel by volume) was 2.06% lower than baseline diesel at 3000 rpm, which was the largest difference across all engine speeds [26]. Studies [32; 54] also showed that biodiesel decreased the HRR peak value, which is shown in Figure 8. However, a few studies [34; 59] observed almost the opposite phenomenon. The HRR of fuel varies with engine test conditions, and the relative trends of diesel and biodiesel under different operating conditions are not unique. Most studies showed that fuels containing biodiesel components tend to start burning earlier than pure diesel and have a lower peak HRR [4; 7; 59]. This is because the shorter ID of biodiesel makes it burn earlier than diesel. In addition, biodiesel has a higher bulk modulus and viscosity. A higher bulk modulus results in lower compressibility, so the pressure response in the fuel injection system is more sensitive. A larger viscosity helps reduce losses during the fuel injection process, making it beneficial to spray early [80]. Although the HRR of diesel is slower than that of biodiesel, it is often observed that diesel has a higher peak HRR in the premixed combustion stage. Many results show that the peak HRR decreases with increasing biodiesel content [33; 38; 52]. This may be due to the greater density and viscosity of biodiesel, which results in slower vaporization of the fuel, which affects the release of premixed combustion heat. The lower heating rate peak of biodiesel may be attributed to its lower calorific value and density [26]. Some researchers also found that sometimes the peak HRR of biodiesel is greater, probably because the higher oxygen content promotes good air-fuel mixing [32; 34; 59]. Biodiesel and diesel have similar trends in the diffusion combustion stages, and the excess oxygen content of biodiesel allows the burn to continue in the later period [31].  
Figure 7: CP changes for biodiesel with loads at 21, 24 and 27\({}^{\circ}\) bTDC. Adapted with permission from [67]. Copyright © 2016, Fuel, published by Elsevier Ltd., Amsterdam, The Netherlands.