CT Logo

Diesel engines have high efficiency, durability, and reliability together with their low-operating cost. These important features make them the most preferred power source of commercial transport, being employed in trucks, buses, trains, and ships as well as off-road vehicles such as construction machinery and mining equipment. Although they have many advantages, they have a significant impact upon environmental pollution problems worldwide.

Diesel engines are considered as one of the largest contributors to environmental pollution caused by exhaust emissions. Diesel emissions also include pollutants that can have adverse health and/or environmental effects. Most of these pollutants originate from various non-ideal processes during combustion, such as incomplete combustion of fuel, reactions between mixture components under high temperature and pressure, combustion of engine lubricating oil and oil additives as well as combustion of non-hydrocarbon components of diesel fuel, such as sulphur compounds and fuel additives. Common pollutants include unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) or particulate matter (PM). 

The most  astounding proportion of  diesel outflows is  because of NOx.  NOx has  emissions rate above than 50%. In diesel emissions, PM has the second highest percentage. CO and HC are produced due to incomplete combustion and are found in lowest concentration. Moreover, diesel emissions include a little amount of SO2 depending upon the details and quality of fuel.

To  lower the antagonistic impacts of diesel outflows on human health and environment, exhaust after-treatment technologies are considered as best where cleaning process of exhaust gases takes place before they can be released into the environment.

Diesel Oxidation Catalysts, also known as DOCs, are exhaust aftertreatment devices that reduce emissions from diesel fuelled vehicles and equipment. Diesel oxidation catalyst (DOC) converts diesel exhaust emissions into harmless gases by means of catalytic  oxidation. Engine manufacturers have used DOCs in different in-use applications for many years, and DOCs are widely used as a retrofit technology because of their simplicity and limited maintenance requirements. DOCs generally consist of a precious metal coated flow-through honeycomb structure contained in a stainless steel housing. As hot diesel exhaust flows through the honeycomb structure, the precious metal coating causes a catalytic reaction that breaks down pollutants into less harmful components.

The diesel oxidation catalyst (DOC) owes its name to its ability to promote oxidation of exhaust gas components by oxygen, which is present in ample quantities in diesel exhaust. When passed over an oxidation catalyst, carbon monoxide (CO), gas phase hydrocarbons  (HC), the organic fraction of diesel particulates (OF), as well as non-regulated emissions such as aldehydes or PAHs can be oxidized to harmless products, and thus can be controlled using the DOC. The  DOC oxidizes CO, HCs into CO2 and H2O. In modern diesel aftertreatment systems, an important function of the DOC is to oxidize nitric oxide (NO) to nitrogen dioxide (NO2)—a gas needed to support the performance of diesel particulate filters and SCR catalysts used for NOx reduction.

DOC replace mufflers on the engine and no extra modifications are required. DOC's are inexpensive, maintenance-free, and suitable for diesel engines. When properly installed and maintained, DOCs should remain effective for the life of the vehicle, generally five to ten years or 10,000 or more hours of operation. Engine problems with fuel control or oil consumption may quickly deteriorate the performance of a DOC. Consequently, regular engine maintenance is essential to DOC performance.

Diesel particulate filters (DPF) are devices that physically capture diesel particulates to prevent their release to the atmosphere. Diesel particulate filter materials have been developed that show impressive filtration efficiencies, in excess of 90%, as well as good mechanical and thermal durability. Diesel particulate filters have become the most effective technology for the control of diesel particulate emissions—including particle mass and numbers—with high efficiencies.

Due to the particle deposition mechanisms in these devices, filters are most effective in controlling the solid fraction of diesel particulates, including elemental carbon (soot) and the related black smoke emission. Filters may have limited effectiveness, or be totally ineffective, in controlling non-solid fractions of PM emissions—the organic fraction (OF) and sulphate particulates.

DPFs typically use a porous ceramic or cordierite substrate or metallic filter, to physically trap particulate matter (PM) and remove it from the exhaust stream. After it is trapped by the DPF, collected PM is reduced to ash during filter regeneration. Regeneration occurs when the filter element reaches the temperature required for combustion of the PM. “Passive” regeneration occurs when the exhaust gas temperatures are high enough to initiate combustion of the accumulated PM in the DPF, without added fuel, heat or driver action. “Active” regeneration may require driver action and/or other sources of fuel or heat to raise the DPF temperature sufficiently to combust accumulated PM. The frequency of regeneration is determined by the engine's duty cycle, PM emission rate, filter technology and other factors. In addition to regeneration, the filter must be periodically cleaned to remove non-combustible materials and ash. It is important to avoid excessive PM and ash accumulation in a DPF

Diesel particulate filters (DPFs) come in a variety of types depending on the level of filtration required. The simplest form of particulate removal can be achieved using a diesel oxidation catalyst (DOC). DPFs can be either partial, flow-through devices or wall-flow designs which achieve the highest filtration efficiency.

Selective Catalytic Reduction (SCR) is an advanced active emissions control technology system that injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually automotive-grade urea, otherwise known as Diesel Exhaust Fluid (DEF). The DEF sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe, which is then expelled through the vehicle tailpipe.

SCR technology is designed to permit nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is called "selective" because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The chemical reaction is known as "reduction" where the DEF is the reducing agent that reacts with NOx to convert the pollutants into nitrogen, water and tiny amounts of CO2. The DEF can be rapidly broken down to produce the oxidizing ammonia in the exhaust stream. SCR technology alone can achieve NOx reductions up to 90 percent.

SCR catalysts are made from various ceramic materials used as a carrier, such as titanium oxide, and active catalytic components are usually either oxides of base metals (such as vanadium, molybdenum and tungsten), zeolites, or various precious metals. Another catalyst based on activated carbon was also developed which is applicable for the removal of NOx at low temperatures. Each catalyst component has advantages and disadvantages.

SCR technology is one of the most cost-effective and fuel-efficient technologies available to help reduce diesel engine emissions. SCR can reduce NOx emissions up to 90 percent while simultaneously reducing HC and CO emissions by 50-90 percent, and PM emissions by 30-50 percent. SCR systems can also be combined with a diesel particulate filter to achieve even greater emission reductions for PM.