METAL NANOPARTICLE FUEL ADDITIVES
Metal nanoparticle additives or nanofuel such as aluminium have been used in solid form for rockets for many years as a way of increasing the performance, since they have the ability to increase the volumetric heat release of the propellants. Fine particles and microparticles of aluminium, as well as boron and zinc, have also been numerously investigated as a potential fuel additive. Cerium oxide is known to behave as an oxygen-storing agent in the diesel exhaust catalyser. Advances in fabrication and characterisation of nanoparticles have allowed more detailed research into the relationship of particle size and structure with performance. Recently, the introduction of nanotechnology has led to significant developments in the field of energetic materials. Nanoscale energetic materials, due to their surface area and unique thermal properties, are known to exhibit many advantages, where the oil and automobile industries can adopt to solve practical problems.
Some study of nanofuel additive with diesel combustion engines reports CO and NOx have been reduced up to 20.5 and 13 %, respectively, and HC reduced by 28 %. The same study also indicates a 3 % fuel consumption reduction accompanied with 6 % improvement in the engine power. Another study reports 70% reduction in HC emission and 41 % reduction in CO emission for biodiesel with nanoenergetic fuel additive. It is evident from all the studies that the diesel engine performance improves appreciably with nanofuel additives for all the cases of diesel, biodiesel and emulsified fuels.
There has been considerable debate over the mechanism of nanoparticle ignition, and the conditions at which nanoparticles burn. For example, considering aluminium, the most commonly researched metallic additive, current theoretical models cannot fully explain nano-aluminium ignition in certain environmental conditions and particle size ranges.
As with many nano-additives, the main concern is their environmental impact. The dosage amount of nanoparticles for improvement fuel efficiency and the quality of exhaust emissions should not cause environmental issues if the nano-additives are carried into the exhaust gases themselves. The estimation of the optimum dosage is essential for every nano-additive.
One debated example of metal fuel additive is Methylcyclopentadienyl manganese tricarbonyl (MMT) due to the automobile system failing issues. As a result, U.S. environmental protection agency has put a limit of at 8.3 mg Mn/l in using MMT into fuel.
It is more generally believed that NOx emission increases with the addition of nanoparticles. The reason is due to the addition of nanoparticles which improved the combustion process and thereby releasing higher heat release rate. At higher temperature, the formation of NOx increases without involving a quenching mechanism.
Consequently, a minimum dosage of metal nanoparticles is preferable, hence the overall toxicity of the emissions will be negligible in the presence of the exhaustion particulate filter system.