Emission control technologies
Significant improvements in engine technology over the last decades have resulted in better fuel economy and lower engine-out pollutant emissions. Despite these improvements, strict emission limits in Europe cannot be met with engine measures only. Therefore, several emission control devices, known as exhaust aftertreatment technologies, have been developed and are used in modern vehicles. Emission control technologies can be broadly grouped into three main categories (EEA, 2016), namely:
Catalytic converters
A catalytic converter is a device that uses a catalyst to convert the main harmful air pollutants in vehicle exhaust emissions into harmless compounds. The catalyst activates certain oxidation and/or reduction reactions, which transform CO, HCs, and NOx into CO2, water and nitrogen.
Traps and adsorbers
Traps and adsorbers are used to control emissions of specific pollutants - usually NOx or HCs - when engine operating conditions may not be ideal for conventional catalysts to achieve their full potential. They store the pollutant for a period of time, but then release it when conditions are suitable for it to react over the catalytic materials.
Filters
Filters are used in both diesel and petrol engines to reduce PM emissions. Based on the engine technology and application specifications, various filter technologies can be used to decrease particle emissions.
Main technologies used in spark ignition engines
Three-way catalyst (TWC)
Has been the primary emission control technology for petrol vehicles since the early 1980s. Although the primary components and function of a TWC have remained relatively constant, each of these components (catalytic coating, substrate, mounting materials) has gone through a continuous evolution and redesign process in order to improve the overall performance. It operates in a closed-loop system including an oxygen sensor to regulate the air to fuel ratio and it oxidizes CO and HC to CO2 and water, while reducing NOx to nitrogen (picture in the right). TWC is still the main technology used to control emissions from petrol engines.
Three-way catalyst (TWC). Source: (Laurenzi and Spigler, 2018)
Gasoline Particulate Filter (GPF)
Is used to filter PM from the exhaust of petrol vehicles. It is an effective technology with high filtration performance under all engine operation points and ambient temperature variation. The filter technology is drawn from the large experience base with diesel particulate filters (DPFs, see below) and comprises of a honeycomb-like filter structure made from synthetic ceramic material.
Gasoline particulate filter (GPF) Source: (AECC, 2017)
Activated carbon canister
Is a trap device used to control evaporative HC emissions from petrol fuel tanks. The canister consists of a plastic case containing the activated carbon, which traps (or adsorbs) the petrol vapor as it is forced out of the fuel tank during heating or refuelling. The adsorbed fuel vapours are then released (or desorbed) into the engine when the vehicle is driven, regenerating the canister. This adsorption/desorption cycle continues for the life of the vehicle.
Activated carbon canister. Source: (EEA, 2016)
Main technologies used in compression ignition engines
Diesel Oxidation Catalyst (DOC)
Looks much the same as a TWC and its construction and composition is similar, although slightly less complex. It converts CO and HCs to CO2 and water but has little effect on NOx. It also decreases the mass of diesel particulate emissions (but not their number) by oxidizing some of the hydrocarbons that are adsorbed onto the carbon particles. It remains a key technology for diesel engines.
Diesel oxidation catalyst (DOC). Source: https://www.attacproject.eu/
Exhaust Gas Recirculation (EGR)
As the name implies, redirects (re-circulates) a portion of engine exhaust back into the engine to cool and reduce peak combustion temperatures and pressures. In most systems, an intercooler lowers the temperature of the re-circulated gases. Hence, combustion temperature in the engine is lowered, thus, inhibiting NOx formation.
Selective catalytic reduction (SCR)
Is an advanced emissions control technology system that reduces NOX by injecting a liquid reducing agent through a special catalyst into the exhaust stream of a diesel engine. The reducing agent is usually urea, commonly known with its commercial name, AdBlue. AdBlue is stored in a separate tank and enables a chemical reaction that converts NOx into nitrogen, water and CO2. A typical diesel car will consume approximately 2.5 to 3 litres of AdBlue for every 100 litres of diesel to meet the Euro 6 standards. SCR is a proven catalyst technology capable of reducing diesel NOX emissions to levels required by current emission standards.
Lean NOx Trap (LNT)
Is used to capture NOX emissions from diesel engines. It is a technically simpler solution compared to SCR as it comprises less components, including a catalyst and an adsorbent, and was used in early Euro 6 vehicles. In modern applications, it can be used in combination with an SCR system for a more efficient control of NOX emissions.
HC absorbers
Are used to 'trim' HC emissions during cold starts, until the catalyst has been warmed-up and can effectively control engine-out HC emissions.
Diesel particulate filters (DPFs)
Are devices used with diesel engines to reduce particle emissions. Depending on engine technology and application specificities, different filter technologies may be used to reduce particle emissions. In the most common type (wall-flow filters), PM is removed from the exhaust by physical filtration using a honeycomb structure similar to a catalyst, but with the channels blocked at alternate ends. The exhaust gas is thus forced to flow through the walls between the channels and PM is deposited on the walls. DPF materials have been developed that show impressive filtration efficiencies, in excess of 90%, as well as good mechanical and thermal durability. DPFs must be cleaned on a regular basis to maintain their filtration efficiency. This process is called DPF regeneration. During a regeneration, usually happening when the vehicle exhaust is hot enough such as on motorways, all PM collected is burnt off. DPFs have become the most effective technology for the control of diesel particulate emissions—including particle mass and numbers—with high efficiencies.
Diesel particulate filter (DPF). Source: (Lázaro et al., 2011)
Another type of hybridized DPF with the addition of SCR is the recently popular SDPF which is one of the most efficient ways to reduce NOx and PM emissions from diesel engines (Czerwinski et al., 2015).
Ammonia Slip Catalyst (ASC)
Is an additional catalyst that improves the overall performance of SCR catalyst system. ASC is very selective to N2 which means that it can convert almost all of the NH3 to N2 rather than in NOx. The ASC also converts HCs and CO to CO2.
Despite the many options available to consumers in the market, the purchase cost of a new low-emitting and fuel-efficient vehicle provides a barrier to some people who wish to reduce their environmental impact. A potential solution is the use of retrofit emission control systems (REC). In this case, REC systems can be fitted to older vehicles (where it is technically feasible) that lack emission control technologies, in order to conform to more recent environmental standards. Retrofitting might still be expensive in cases of very old vehicles without any emissions control as the cost might be disproportionally high compared to the depreciated value of the car.
Retrofit might not always deliver the expected results, especially when there are no efficient mechanisms to verify the quality of the retrofit as well as the emissions released in real word. For this reason, it is suggested a verification of a proper replacement by the specialized staff and a proper maintenance.