**2. Parameters and metrics of study**

Biodiesel fuel has been investigated by researchers with varying outcomes. **Table 4** shows some problems and possible solutions related to biodiesel fuel. Some important properties of biodiesel are cloud and pour point, storage stability, viscosity, acid value, cetane number. These are the properties of biofuel that must meet the set ASTM standard of biofuel. These properties are deficient when considering using biodiesel in CI engines, particularly in a cold, temperate, regions. The cloud points of ethyl ester produced from use cooking oil, linseed oil, canola, sunflower, and rapeseed oil are −1°C, −2°C, −1°C, −1°C, and −2°C, respectively. Lang et al. (2001) [49] reported that the cloud point of ethyl esters of linseed oil, canola, sunflower, and rapeseed oil were −2°C, −1°C, −1°C, and −2°C, respectively,whereas the corresponding methyl esters had cloud points of 0°C, 1°C, 1°C, and 0°C. Currently, internal combustion engine are controlled by compression ignition(CI) engine and the spark ignition (SI) engine. The operations of Spark engine (SI) is done by premixed charge near stoichiometric air-fuel ratio (Ø ~ 1), the flame is spread with the aid of a spark plug, the throttling effect of the charges into the cylinder leads to low thermal efficiency at partial load. Moreover, with the high temperature at peak load, cause generation of massive NOX, this have advert effect on the environment, is form a major challenge of the SI engine applications. The compression ratio of CI engine is between 12 and 24, in addition CI engine, is characterized by turbulent flame, diffused flame, auto-ignition via elevated pressure and temperature around the top dead center. The CI engine has a higher thermal efficiency compared to a SI engine. The major part of its operation is ignition delay (this is the time difference between the start of injection and the self-ignition). The ignition delay mechanism is controlled by physical and s chemical kinetic processes. The physical process proceeds sequentially through droplet formation, collision, break-ups, evaporation, and vapor diffusion. The chemical kinetic process involves species and radical formation proceeding through low-temperature reaction (LTR), negative temperature coefficient (NTC), and high-temperature reaction (HTR). The challenge is that the time scales for the physical processes are often larger than those of the chemical processes, hence ignition often commences before the physical processes are completed. This inevitably leads to a complex system of charge, flame, and thermal stratification that produces both high NOX in some regions and high soot precursors and unburnt hydrocarbon (UHC) in others. Research focus, therefore, has been to hydroprocess biodiesel fuel into HDRD to mitigate NOX, UHC, and soot

*A Comparative Evaluation of Biodiesel and Used Cooking Oil as Feedstock for HDRD… DOI: http://dx.doi.org/10.5772/intechopen.104393*


**Table 4.**

*Problems, causes, and possible solutions regarding biodiesel products.*

formation (as is the case with the SI engine). From the foregoing, it is clear that there are many challenges facing the efforts to utilize biodiesel as a fuel in transportation.
