Kivevele and Huan [101] evaluated the effect of metal contents and antioxidant additives on the storage stability of biodiesel produced from C. megalocarpus and Moringa oleifera. The produced C. megalocarpus and M. oleifera biodiesels displayed oxidation stability of 2.5 and 5.3 h, respectively. The oxidation stability of C. megalocarpus biodiesel did not meet the minimum requirement prescribed in ASTM D6751 and EN 14214 standards of 3 and 6 h, respectively. However, M. oleifera biodiesel met the minimum requirement of ASTM D6751 but not the EN 14214. In order to improve the storage stability of C. megalocarpus and M. oleifera biodiesels, two synthetic antioxidants—1,2,3 tri-hydroxy benzene (pyrogallol, PY) and 3,4,5-tri-hydroxy-benzoic acid (propyl gallate, PG)—were doped to the biodiesel samples to examine its effectiveness. Moreover, the samples with and without antioxidant were mixed with different transition metals (Fe, Ni, Mn, Co, and Cu) to evaluate the impact of these metals on storage stability of C. megalocarpus and M. oleifera biodiesels. The samples were stored indoor for 6 months in completely closed and open translucent plastic bottles, the oxidation stability were measured every month.

It was found that 200 ppm of PY and PG was enough for C. megalocarpus and M. oleifera biodiesels to meet EN 14112 biodiesel oxidation stability. Moreover, the antioxidant PY was more effective than PG. Fe displayed least detrimental effect on oxidation stability of C. megalocarpus and M. oleifera biodiesels while Cu had great detrimental impact. The samples in the completely closed (airtight) bottles recorded higher oxidation stability compared to the ones kept in the open bottles, because the latter's exposure to air enhanced the oxidation of the samples.

Kivevele et al. [102] evaluate oxidation stability of biodiesel produced from C. megalocarpus oil. It was found that oxidation stability of C. megalocarpus biodiesel did not meet the specifications of EN 14214 (6 h). The effectiveness of three antioxidants: 1,2,3 tri-hydroxy benzene (PY), 3,4,5-tri-hydroxy benzoic acid (PG), and 2-tertbutyl-4-methoxy phenol (butylated hydroxyanisole, BHA) on oxidation stability of C. megalocarpus biodiesel was investigated. The article of Kivevele et al. concluded that the effectiveness of these antioxidants was in the order of PY > PG > BHA.

Agarwal et al. [103] investigated the effectiveness of five cheap and commercially available antioxidants namely 2,6-di-tertbutyl-4-methyl phenol (BHT), 2-tertbutyl-4-methoxy phenol (BHA), 2-tertbutyl hydroquinone (TBHQ), 1,2,3 tri-hydroxy benzene (PY), and 3,4,5-tri-hydroxy benzoic acid (PG) on biodiesel produced from nonedible vegetable oils of karanja, neem, and jatropha. The antioxidants were dosed in concentrations ranging from 100 to 1000 ppm. JB showed higher oxidation stability than karanja and neem-based biodiesel; however, it was still unable to meet the prevailing ASTM biodiesel oxidation stability specifications.

Optimum concentration required to meet the specifications for karanja biodiesel was: PG 500 ppm, PY 300 ppm, and BHA 500 ppm. The optimum concentration to stabilize neem biodiesel was PG 200 ppm and PY 200 ppm and for JB, it was: BHT700 ppm, PG 100 ppm, PY 100 ppm, and BHA 300 ppm. The authors concluded that the effectiveness of antioxidants investigated was: PY > PG > BHA > BHT > TBHQ, for karanja biodiesel, neem biodiesel, and JB.

Ryu [104] investigated the effects of antioxidants on the oxidation stability of biodiesel derived from soybean oil, the engine performance, and the exhaust emissions of a diesel engine. The results show that the efficiency of antioxidants is in the order TBHQ > PG > BHA > BHT > α-tocopherol. The oxidative stability of biodiesel fuel attained the 6 h quality standard with 100 ppm TBHQ and with 300 ppm PG in biodiesel fuel. Exhaust emissions were not influenced by the addition of antioxidants in biodiesel. However, the BSFC of biodiesel with antioxidants decreased more than that of biodiesel without antioxidants.

Imtenan et al. [105] investigated an improvement in emission and performance characteristics of PB–diesel blend with the help of ethanol, n-butanol, and diethyl ether as additives. The blends consisted of 80% diesel, 15% PB, and 5% additive. These additives improved the fuel blend regarding density and viscosity which in turn improved atomization and showed better combustion characteristics through higher engine brake power, lower BSFC, and higher BTE than PB–diesel blend (B20). Diethyl ether showed highest 6.25% increment of brake power, 3.28% decrement of BSFC, and about 4% increment of BTE than 20% PB–diesel blend when used as additive through its low density and viscosity profile with quite a high calorific value. Other two additives also showed interesting improvement regarding performance. All the blends with additives showed decreased NO and CO emission but HC emission showed a slight increment. The authors concluded from this experiment that, ethanol, n-butanol, and diethyl ether are quite effective regarding emission and performance even when they are used only about 5% as additive. The same authors have done similar work in Refs. [106,107]. Palash et al. [108] present an experimental study on a four-cylinder diesel engine to evaluate the performance and emission characteristics of JB blends (JB5, JB10, JB15, and JB20) with and without the addition of N,N-o-diphenyl-1,4-phenylenediamine (DPPD) antioxidant at 0.15% (m). Engine tests were conducted at engine speeds 1000–4000 rpm at an interval of 500 rpm under the full-throttle condition. The results showed that the additives reduce NO_x emissions significantly with a slight penalty in terms of engine power and brake-specific fuel consumption as well as CO and HC emissions. However, when compared to diesel combustion, the emissions of HC and CO with the addition of the DPPD additive were found to be nearly the same or lower. By the addition of 0.15% (m) DPPD additive in JB5, JB10, JB15, and JB20, the reduction in NO_x emissions were 8.03%, 3.503%, 13.65%, and 16.54%, respectively, compared to biodiesel blends without the additive. Moreover, the addition of DPPD additive to all biodiesel blend samples reduced the exhaust gas temperature (EGT). Shahabuddin et al. [109] investigated engine and emission performance of B20 with IRGANOR NPA as a fuel corrosion inhibitor. The final result implied that the biodiesel with some additives (B20 + 1%) produces 1.73% and 9% higher brake power as compared to fuel B20 and OD, respectively; consumes 26% and 6% lower SFC as compared to fuel B20 and OD, respectively; and reduces CO, NO_x, and CO₂ emissions as compared to other fuels. Swaminathan and Sarangan [110] tested fish oil biodiesel in single-cylinder diesel engine blended with diethyl ether (DEE) at 1%, 2%, and 3% to improve the performance and reduce the emission of the engine.

The percentage reduction toward 91% CO; 62% CO₂; 92% NO_x; and 90% C_xH_y were attained when the engine was run at maximum load using fish oil biodiesel with 2% additive with EGR. The authors concluded that the optimum values were obtained with 2% of additives.