A technical review on low temperature combustion alternatives for ultra-low emission vehicles

Oxides of nitrogen (NOx) emissions have always been considered as the primary concern for the development of diesel engines. Several post-treatment technologies have also been developed to fulfil the stringent standards for NOx emissions that cannot be controlled by fuel composition and combustion reaction. At present, the aftertreatment technologies employed include Selective Catalytic Reduction (SCR), Lean NOx Traps and SCR filtering. The current review article summarizes the investigations conducted earlier about the reduction of NOx emissions from lean-burn engines and discusses the benefits and limitations of different types of SCRs. Among the available technologies, the urea SCR process is ultimately the most efficient process using Ad blue technology. Urea SCR process uses NH3 reduction agents to achieve 98% conversion efficiency. The article discussed some of the operating parameters such as the nature of catalytic material, the range temperature of the catalyst and the mixing of NH3 and NOx. Hybrid SCR such as the metals-impregnated SCR (M-SCR, HC- SCR, SCR + LNT) moderately increases the catalytic activity and converts the exhaust gas into less harmful gases even at low temperature. The novelty of the work, debates about the outrivaling importance of oxidation catalyst of the modern diesel exhaust purification systems, techniques for enhancement and the optimization of oxidation catalysts that are aimed at NOx and hydrocarbon conversion and the methods for reducing the Platinum Group Metal (PGM) content in these catalytic systems via combination devices for thermal energy and further optimization and material cost attenuation. There exists a requisite for continuous investigation of PM control technology that comprises of regenerating systems to reduce the complexity of these systems and concomitant fuel economic penalties. Future research works should be conducted extensively with different additives, substrate materials, regenerating systems, and controls to develop optimal devices that are capable of reducing the diesel particulate emissions levels.