Exhaust and silencer after-treatment reduce emissions by a significant margin. Optimising engine design and fuel systems help a great deal and after-treatment strategies can be seen as an interim step whilst advancement in engine technology catches up. This level of emission control is what the stage IV regulations and other local regulations aims to achieve. The following methods have already gained practical levels of commercialization in various applications:
Selective catalytic reduction (SCR): This is a very effective method for curbing NOx emissions and is basically aqueous urea injection into the exhaust stream passing over a suitable catalyst.
SCR can reduce NOx up to 98 percent. Systems consist of an SCR catalyst, urea injection system, urea tank, pump and a control system.
Diesel oxidation catalyst (DOC): DOC is a flow through device that consists of a canister containing a honeycomb-like structure or substrate. The substrate has a large surface area that is coated with an active catalyst layer. DOC is capable of achieving over 95 percent reduction in CO and HC. More specifically, DOCs utilize palladium and platinum catalysts to reduce the particulate matter (PM), hydrocarbon based soluble organic fraction (SOF), and carbon monoxide content of diesel exhaust by simple oxidation.
Diesel particulate filter - Particulate matter is the most visible form of pollutant coming out of an exhaust. It is the main constituent of black smoke or soot which has led to the massive reduction of diesel engines on a non-commercial basis. DPF’s or particulate matter (PM) traps are designed to physically capture PM from the exhaust stream. They can either be simple mechanical filters requiring frequent replacement, or they can be catalytic filters that provide periodic or continuous oxidation (regeneration) of the trapped particulates into CO2. PM traps with continuous regeneration have already reached a high level of product placement in the field and are employed on stationary diesel engines in areas with strict PM emissions regulations. Ultra-low sulphur diesel fuel is needed to prevent contamination of the conversion catalysts. However, filtration efficiencies up to 90 percent have been demonstrated.