2. LITERATURE REVIEW
2.7. Diesel engine operation using biodiesel-biodiesel mixtures
Biodiesel-diesel blends are very common in the literature. However, relatively few studies have been done on biodiesel-biodiesel blends. Previous studies on biodiesel-biodiesel blending are reviewed from different perspectives in this section.
Benjumea et al (2011) conducted an experimental study on a 4-cylinder turbocharged direct-injection diesel engine to observe the effects of degree of unsaturation (DU) of palm oil and linseed oil biodiesels. According to the study, no significant effect of degree of unsaturation was reported on engine performance. However, the combustion start time of linseed oil biodiesel with an iodine value of 185.4 was 2°CA later than the palm oil biodiesel which had an iodine value of 52. Moreover, peak heat release rate (HRR) of the linseed oil biodiesel was 40% higher than the palm oil biodiesel. In terms of the exhaust emissions, higher iodine value biodiesel released approximately 40% higher NOx and slightly higher smoke opacity compared to low iodine value biodiesel. The palm and linseed biodiesel blends reported to have a linear relationship between the base fuels for all test parameters.
Rajkumar and Thangaraja (2019) tested the karanja and coconut biodiesels in the fractions of K100, K80C20, K60C40, K40C60, K20C80 and C100. The effect of retarding Start of Injection (SOI) as a result of different fuel properties such as bulk modulus of compressibility and degree of unsaturation was investigated in a four-cylinder turbocharged diesel engine equipped with a mechanically operated injection system. The results of the study showed an increasing trend in the peak pressure with the increasing fraction of the karanja biodiesel, which had a higher degree of unsaturation and bulk modulus compared to coconut biodiesel. The trend was similar in terms of the NO emission i.e. K100, K80C20, K60C40, K40C60, K20C80 and C100 fuels had NO emissions of 765, 732, 695, 672, 628, and 590 ppm at 2500 rpm, 80% load, respectively. However, more studies are required to support these results as the karanja biodiesel used in the study was not neat but hydrogenated. The 200 ml of karanja biodiesel was diluted in 800 ml of methanol in the presence of 1 gram palladium catalyst.
Sanjid et al. (2016) blended kapok and moringa biodiesels with diesel and tested them in a 4-cylinder diesel engine. Not only the 10% and 20% biodiesels blended with diesel (i.e. KB10, KB20, MB10 and
MB20), but also 5% and 10% of neat biodiesels blended with each other along with 90% and 80% diesel (i.e. KB5MB5 and KB10MB10) and analysed. According to the authors, biodiesel-biodiesel- diesel blends had comparable engine performance with neat biodiesels and diesel. KB5MB5 and KB10MB10 biodiesel-biodiesel-diesel blends had between 14% and 17% increased NO emissions compared to diesel. Moreover, the same blends gave 1% and 2% higher CO2 emission compared to diesel respectively. However, significant reductions on HC and CO emissions were reported as 38% and 31% for the KB10MB10 blend, respectively.
Dos Santos et al (2018) studied the traceability of biodiesel-biodiesel mixtures collected from different regions of Brazil. Fourier Transform Infrared Spectroscopy (FTIR) was used to analyse the various ratios of biodiesels obtained from soybean, tallow, palm and cotton. This preliminary study showed that biodiesels produced from different regions of Brazil can have different properties. The authors stated that biodiesel-biodiesel blending can improve fuel properties and they also recommend future works in multiple feedstock biodiesel blending.
Usta et al. (2005) highlighted the importance of soapstock feedstock, which is a side product of edible oil extraction process, as they are cheap, highly available and sustainable. However, these products may have high free acid values which make biodiesel production from soapstock more difficult compared to vegetable oils. Hence, in their study, hazelnut soapstock was blended with waste sunflower oil in 50/50 ratio to reduce the overall fatty acid content of the feedstock. The viscosity of the biodiesel obtained was very high at 25 mm2/s; thus the biodiesel was blended with the diesel. According to results, 17.5% biodiesel blend gave the best engine performance. Similarly, in another study, Can (2014) reduced the high free fatty acid content (10%) of the WCO collected from fast food restaurants by blending them with the WCO obtained from cooking factories. The results of the study showed that 10% blend of the obtained biodiesel with diesel provided; 4% increase in BSFC, 2.8% reduction in BTE, 8.7% increase in NOx emission, 29 % decrease in HC and 51% reduction in CO emission compared to diesel.
Sharma and Ganesh (2019) analysed two different biodiesel blends produced from linseed, karanja, palm and sunflower biodiesels in a one-cylinder diesel engine. The first blend was composed of 25% of all the four biodiesels; whereas the second blend contained 37.5% of palm and karanja biodiesels and 12.5% of linseed and sunflower biodiesels. They reported that the first blend decreased the NOx emission by 20% but increased the smoke intensity by 30%. In addition, CO2 emission was reported 35% lower compared to diesel. The study pointed to the importance of biodiesel-biodiesel blending technique which can solve NOx emission disadvantage of biodiesels. Similary, Mehta and Jeyaseelan (2014) reported 14% reduction in NO emission with the 80% palm biodiesel - 20% karanja biodiesel blend in a 4-cylinder turbocharged direct-injection CI engine.
Fadhil et al (2017) blended castor seed oil and waste fish oil feedstock at the ratios of 90/10, 80/20, 70/30, 60/40 and 50/50 to improve the transesterification process. They reported that 50/50 oils blend was found optimal blend for transesterification and best conditions were provided as 0.5 weight% KOH, 1:8 molar ratio of methanol, 32⁰C temperature and 30 minutes of stirring at 600 rpm. Although the biodiesels were not tested in any engine, this study proved that feedstock blending is a promising technique to improve the transesterification process as maximum yield was obtained at lower reaction temperature.
Summarising the biodiesel-biodiesel blending studies, it is clear that this technique is really promising for the future research. According to literature, one of the main advantageous of biodiesel-biodiesel blending could be enhancing the utilisation of cheap, highly available but low fuel quality feedstock such as soapstocks and high free acid content waste cooking oils. No study was found about the effects of using a waste cooking oil and animal fat biodiesel-biodiesel blend in a diesel engine.