Edible and Non-Edible Biodiesel Blends Show Mixed Results for Cutting NOx

Diesel engines remain the workhorses of global transport, but they are also direct emitters of oxides of nitrogen, the pollutant family known as NOx. A new open-access review published in Discover Mechanical Engineering on 19 October 2026 has systematically compared how biodiesel blends made from edible and non-edible oil sources perform against this pollutant, drawing on dozens of engine emission studies conducted mainly on single-cylinder and four-cylinder diesel engines under steady-state conditions. The verdict is nuanced: biodiesel can cut NOx in some configurations, but in many cases it does the opposite, and the outcome depends heavily on feedstock chemistry, blend ratio, and what else is mixed into the fuel.

The stakes are considerable. According to the review, NOx alone accounts for roughly 15 percent of global anthropogenic pollution. In the atmosphere, NOx reacts with water vapour to form nitric acid, a major component of acid rain, and with hydrocarbons in sunlight to produce smog and ground-level ozone. The health evidence cited is stark: an analysis by Hamra and colleagues found that each 10 micrograms per cubic metre increase in NOx exposure was associated with a 4 percent rise in lung cancer cases, while Alves and colleagues reported that a 3 micrograms per cubic metre reduction in exposure corresponded to a 10 to 18 percent decrease in lung-related deaths. Older people, pregnant women and newborn infants face elevated risks, and prolonged exposure has been linked to respiratory and cardiovascular disease.

The review also documents the broader societal toll. Around 10,000 premature deaths in Germany between 2008 and 2015 were attributed to NOx emissions, during a period when the German Federal Environmental Protection Agency measured NOx concentrations of 0.85 grams per kilometre, four times the European limit of 0.18 grams per kilometre. London recorded 420 hospital admissions for respiratory disease linked to NOx-degraded air quality in 2010, and studies in France and Denmark found modest but measurable associations between nitrogen dioxide concentrations and breast cancer incidence, with odds ratios of 1.11 and 1.03 per 10 micrograms per cubic metre respectively.

Biodiesel itself is a mono-alkyl ester of long-chain fatty acids, produced from vegetable oils through trans-esterification. Its fuel properties are close to those of mineral diesel: it has a higher cetane number, which promotes smoother combustion, contains no sulfur, and lubricates the engine better. But it also carries drawbacks, including a lower calorific value that reduces thermal efficiency and a higher viscosity that impairs atomisation. These competing properties explain why its effect on NOx is so inconsistent across studies.

Among edible-oil biodiesels, the review identifies safflower as the standout performer, with reported NOx reductions ranging from 14.0 to 68.2 percent compared with fossil diesel. The largest single reduction came from a study in which safflower biodiesel was blended with kerosene in a 50:50 volumetric ratio; the higher energy density of kerosene prevented sudden in-cylinder temperature rises, cutting NOx by 68.2, 56.9 and 55.1 percent for 50, 75 and 90 percent safflower blends respectively. Soybean biodiesel showed more modest behaviour, with oxidised B20 blends producing about 11 percent more NOx than diesel at retarded injection timing, although raising the methyl oleate content to 76 percent created a NOx-neutral fuel. Sunflower biodiesel blends with polystyrene achieved up to 19.8 percent NOx reduction at 2100 rpm.

Not every edible feedstock fared well. Canola biodiesel blends increased NOx by 8.9 percent in one study, pure canola biodiesel raised it by 14.3 percent in another, and a combination of canola, safflower and waste cooking oil biodiesels with diesel produced a striking 80.5 percent increase. Rice-bran biodiesel blends generally emitted more NOx than diesel unless n-butanol or exhaust gas recirculation was added. Palm biodiesel results were mixed, ranging from a small reduction with 20 percent blends to a 3.9 percent increase at 3000 rpm with 5 percent blends.

The non-edible category produced some of the most dramatic figures in the review. Pongamia pinnata biodiesel operated in dual-fuel mode with biogas achieved a maximum NOx reduction of approximately 84.6 percent at full load, attributed to the low energy density of the gaseous fuel, which lowered the combustion rate. Algae biodiesel also performed strongly: pure Azolla pinnata biodiesel cut NOx by 39 percent at full load in a four-stroke, single-cylinder, direct-injection, water-cooled diesel engine. Jatropha biodiesel dosed with carbon nanotubes at 50 milligrams per litre of diesel reduced NOx by 52 percent at 75 percent load, since the nanoparticles shorten ignition delay and prevent rapid in-cylinder temperature spikes.

The failures are equally instructive. Neem biodiesel emerged as the worst performer, with maximum NOx emissions recorded at 63.75 percent above diesel in the reviewed data, and one quaternary blend study measuring figures up to 78.8 percent higher than diesel at elevated injection pressure. Castor biodiesel B20 blends produced 8 percent more NOx than diesel in a four-stroke, single-cylinder, water-cooled engine. The review attributes these increases to the high oxygen content of many biodiesels, which supplies extra oxygen molecules for combustion, triggers higher heat release, and pushes cylinder temperatures into the range where nitrogen and oxygen combine, above roughly 1300 degrees Celsius.

The most consistent success stories involve additives and engine strategies that cool the combustion environment. Alcohols such as propanol, heptanol, octanol, decanol and butanol exert a charge-cooling effect through their high latent heat of vaporisation, and blends containing them repeatedly lowered NOx. Antioxidants such as butylated hydroxyanisole obstructed peroxyl-free-radical formation, cutting NOx by 49 percent in one algae biodiesel configuration with advanced injection timing. Exhaust gas recirculation, retarded injection timing, optimised injection pressure and nanoparticles that promote controlled combustion all contributed to reductions in individual studies.

The authors conclude that both edible and non-edible biodiesel categories contain promising candidates, but they flag food security concerns associated with diverting edible oils to fuel production, and call for more research on non-edible feedstocks. They also note that the inherent disadvantages of biodiesel, including high viscosity, low calorific value and high wax content, must be addressed through engine modifications and material selection, and that future work should assess the trade-offs between NOx control, other exhaust emissions and engine performance. For now, the review’s practical message is that feedstock choice and blend engineering, rather than biodiesel alone, determine whether a diesel engine runs cleaner or dirtier on NOx.

Subject of Research: Comparative review of edible and non-edible oil biodiesel blends for reducing NOx emissions from diesel engines

Article Title: Comparative assessment of edible and non edible biodiesel for nox reduction

Article References: Khan, M. M., Manimaran, R., Ahmad, M. S., Anand, S., Hasnain, S. M. M., Paramasivam, P., & Ayanie, A. G. (2026). Comparative assessment of edible and non edible biodiesel for nox reduction. Discover Mechanical Engineering, 5(1), Article 196. https://doi.org/10.1007/s44245-026-00386-6

Image Credits: AI Generated

DOI: 10.1007/s44245-026-00386-6

Keywords: biodiesel, NOx emissions, diesel engines, edible oils, non-edible oils, pongamia pinnata, algae biodiesel, safflower, exhaust emissions, alternative fuels, combustion, air pollution