Molybdenum-Doped Titanium Dioxide Nanoparticles Boost Ethanol-Diesel Engine Efficiency

Diesel engines remain the workhorses of global transport and industry, but they face mounting pressure to burn cleaner fuels without sacrificing the power and efficiency that make them indispensable. A research team led by Mohamed A. Ismail, Kamran Shah, Khaled Alnamasi and M.S. Gad, publishing in the open-access journal Results in Engineering, has now reported a detailed experimental study of a deceptively simple idea: blending small amounts of ethanol into diesel fuel and then supercharging that blend with a specially engineered nanoparticle catalyst, molybdenum-doped titanium dioxide. The results reveal a delicate trade-off between efficiency and emissions, and they point to a precise formulation that appears to strike the best balance.

Ethanol has long attracted attention as a renewable, oxygen-rich additive for diesel. Because the alcohol molecule carries roughly 33 percent oxygen by weight, it can supply extra oxygen inside the combustion chamber, promoting more complete oxidation of fuel and cutting soot-forming reactions. It is cheap, biodegradable and can be produced from biomass. Yet ethanol also brings stubborn problems: it has a lower heating value than diesel, a low cetane number that delays auto-ignition, and a high latent heat of vaporization that cools the charge. Blends can also suffer from phase separation, because diesel dissolves only limited amounts of ethanol. The new study confronted these issues directly by testing blends containing 5, 10 and 15 percent anhydrous ethanol in a water-cooled, four-cylinder direct-injection diesel engine running at a constant 1500 rpm under varying loads.

The baseline results confirmed the known penalties of ethanol blending. Mean effective pressure fell by an average of 3, 7 and 13 percent for the E5, E10 and E15 blends respectively, while brake specific fuel consumption rose by 4, 7 and 15 percent as the engine burned more fuel to compensate for the lower energy content. Brake thermal efficiency declined in step, dropping 4, 7 and 15 percent. Peak cylinder pressures slipped from 71 bar for pure diesel to 66 bar for E15, and peak heat release rates fell from 49 to 44 joules per degree of crank angle. Ignition delay lengthened by up to 21 percent, and combustion duration stretched by as much as 15 percent, reflecting ethanol’s charge-cooling effect and sluggish ignition chemistry.

The emissions story, however, was more encouraging. Carbon monoxide emissions dropped by an average of 6, 12 and 20 percent for the three blends, thanks to the extra oxygen that ethanol delivers to fuel-rich zones where incomplete combustion would otherwise occur. Nitrogen oxides also fell, by 5, 10 and 15 percent, because ethanol’s evaporative cooling suppresses the high flame temperatures that drive thermal NOx formation through the Zeldovich mechanism. The E5 blend, in particular, showed good miscibility and remained stable for several days, whereas the E10 blend was only metastable and the E15 blend separated within minutes to hours, underscoring why low-level blending remains the practical choice.

That is where the nanoparticles enter. The team synthesized molybdenum-doped titanium dioxide using flame spray pyrolysis, a technique in which titanium isopropoxide and molybdenum hexacarbonyl precursors are atomized through a nozzle and combusted in a methane-oxygen pilot flame, producing roughly spherical particles about 10 nanometers across. Three doping levels, 1, 3 and 5 percent molybdenum relative to titanium, were prepared and dispersed at a fixed concentration of 50 milligrams per liter in the E5 blend using ultrasonic homogenization. Transmission electron microscopy and X-ray diffraction confirmed uniform, phase-pure anatase particles, with the 3 percent doped material showing peak broadening that signals smaller crystallites, more lattice distortion and a higher density of oxygen vacancies.

The mechanistic rationale is elegant. Molybdenum atoms substituting into the titanium dioxide lattice create defect sites and oxygen vacancies that improve oxygen mobility, allowing the particles to act as oxygen buffers that release lattice oxygen into fuel-rich pockets during combustion. Redox-active molybdenum and titanium states facilitate electron transfer, accelerating oxidation reactions, while the nanoparticles’ high surface-area-to-volume ratio promotes secondary atomization of fuel droplets, micro-explosion behavior and faster evaporation. Unlike largely inert additives such as alumina, which mainly improve thermal conductivity, the doped catalyst actively participates in combustion chemistry, and unlike ceria, which relies on a single redox couple, the doped titania combines oxygen storage with tunable electronic structure.

The engine measurements showed that these laboratory-scale effects translate into measurable gains. Adding the doped nanoparticles to E5 cut brake specific fuel consumption by an average of 4, 9 and 3 percent for the 1, 3 and 5 percent doping levels, and raised brake thermal efficiency by the same margins, with the 3 percent formulation delivering the largest improvement of 9 percent. Ignition delay shortened dramatically, by 14, 21 and 28 percent, and combustion duration contracted by 9, 14 and 19 percent. Peak cylinder pressure and heat release rate climbed by up to 2.5 percent, and exhaust gas temperature fell by as much as 9 percent at higher loads, indicating that more of the fuel’s energy was being converted into useful work rather than escaping as heat.

The trade-offs were equally clear. Carbon monoxide emissions rose by 3, 8 and 15 percent relative to plain E5 as doping increased, a consequence the authors attribute to changes in fuel viscosity and specific gravity that degrade air-fuel mixing and create locally rich, cooler zones where carbon monoxide cannot fully oxidize to carbon dioxide. Nitrogen oxides increased by 6, 12 and 18 percent, because the nanoparticles accelerate radical generation and raise flame temperatures, intensifying the thermal Zeldovich reactions that produce NOx above roughly 1800 kelvin. The 3 percent doping level emerged as the optimum: it recovered most of the efficiency lost to ethanol blending while keeping the emissions penalties comparatively modest, and its particle morphology showed the best dispersion with the least agglomeration.

The authors are careful to frame the scope of their claims. The tests ran at a single engine speed, each condition was repeated three times with coefficients of variation below 3 percent for performance and below 5 percent for emissions, and the overall experimental uncertainty was calculated at about 2.1 percent. Long-term engine durability, injector deposits, lubricant degradation and storage stability were not assessed, and the team notes that a full life-cycle assessment would be needed to confirm genuine environmental benefit, since nanoparticle synthesis and ethanol production carry their own upstream burdens. A simple cost analysis suggests that low doping levels of 1 to 3 percent remain economically plausible, while higher concentrations escalate fuel costs sharply.

Even with those caveats, the study offers a compelling demonstration that catalyst design at the nanoscale can reshape the combustion of renewable fuel blends. By engineering oxygen vacancies and redox sites directly into a stable oxide support, the researchers showed that a well-chosen additive can claw back the efficiency that ethanol costs a diesel engine, while preserving much of its emissions advantage. The next steps, the authors suggest, include mapping the dosage window from 25 to 100 milligrams per liter, integrating the nanofuel with exhaust gas recirculation or selective catalytic reduction to tame NOx, and running the extended endurance tests that will determine whether molybdenum-doped titanium dioxide can move from the laboratory bench to the fuel tank.

Subject of Research: Effects of molybdenum-doped titanium dioxide nanoadditives on the combustion, performance and emissions of ethanol-diesel blends in a diesel engine

Article Title: Improving diesel engine performance, combustion and emissions using ethanol blends with Mo-doped TiO 2 nano additives

Article References: Ismail, M. A., Shah, K., Alnamasi, K., & Gad, M. (2026). Improving diesel engine performance, combustion and emissions using ethanol blends with Mo-doped TiO2 nano additives. Results in Engineering, 32, Article 113242. https://doi.org/10.1016/j.rineng.2026.113242

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113242

Keywords: diesel engine, ethanol blends, nanoadditives, titanium dioxide, molybdenum doping, combustion, brake thermal efficiency, NOx emissions, carbon monoxide, flame spray pyrolysis, oxygen vacancies, alternative fuels