State of the art wall-flow filters are now included in many vehicles. From passenger cars to trucks, diesel or gasoline and in future also gas applications, and these are also installed in off road vehicles. Filters have positively contributed to the reduction of particulates from vehicles.
Diesel Particulate Filters (DPFs)
Particulate filters were initially used with diesel engines to remove diesel particulate matter (PM). Based on engine technology and application specificities, different filter technologies may be used to reduce particulate emissions.
In wall-flow filters, particulate matter is removed from the exhaust by physical filtration using a honeycomb structure similar to an emissions catalyst substrate but with the channels blocked at alternate ends.
The exhaust gas is thus forced to flow through the walls between the channels and the particulate matter is deposited as a soot cake on the walls. Such filters are made of ceramic (cordierite, silicon carbide or aluminium titane) materials.
Ceramic wall flow filters almost completely remove the carbon particulates, including fine particulates of less than 100 nanometres (nm) diameter with an efficiency >95% in mass and >99% in number over a wide range of engine operating conditions. The latest European emissions limit values (i.e. Euro 5 and 6) are set on both mass and number to account for the number and size of particulates, which are thought to be more critical indicators of health impact. AECC test programmes show that both Diesel Particulate Filters (DPF) and Gasoline Particulate Filters (GPF) give low real-world Particle number (PN) emissions under all driving conditions.
A catalytic coating can also be integrated onto the filter substrate to increase the catalyst volume within the entire system. This is the case for SCR on DPF.
Diesel Particulate Filter Regeneration
Since the continuous flow of soot into the Diesel Particulate Filter would eventually block it, it is necessary to ‘regenerate’ the filtration properties of the filter by burning off the collected particulate on a regular basis.
Trapped particulate burns off at lower exhaust temperatures using the powerful oxidative properties of NO2. This might be possible without the need of active regeneration control. Alternatively particulates can burn with oxygen when the temperature of the exhaust gas is periodically increased to higher temperatures with active measures, e.g. through late post-injection in the combustion engine.
PN emissions might increase towards the end of the regeneration as the filtration efficiency drops due to the temporary decrease of the thickness of the soot layer.
Regulation EU 2017/1154 establishes the conditions for tests with vehicles equipped with periodically regenerating systems that require a periodical regeneration process in less than 4 000 km of normal vehicle operation. The diesel particulate filter is an example of such system. This regulation prescribes the application of a so called Ki factor or Ki offset to represent the results of the emission testing of the vehicle under real driving conditions including the regeneration event. A Ki factor of 2-3 is reported in the Handbook of Emissions Factors (HBEFA), but emissions still stay well below the limit of 6×1011 #/km when taking this factor into account. HBEFA furthermore shows a positive trend towards Euro 6d vehicles.
Regulated tailpipe PN measurements do not take into account the particulates smaller than 23nm. However, the Diesel Particulate Filter captures these smallest particulates well because of the diffusion mechanism, confirmed by low sub-23nm measurements as AECC has demonstrated in its testing programmes.
The most successful methods to achieve regeneration include:
- Incorporating an oxidation catalyst upstream of the filter that, as well as operating as a conventional oxidation catalyst, also increases the ratio of NO2 to NO in the exhaust.
- Incorporating a catalytic coating on the filter to facilitate the burning of particulates
- Using very small quantities of fuel-borne catalyst, such as ceria or iron additive compounds added to the fuel, injected by an on-board dosing system. The catalyst, when collected on the filter as an intimate mixture with the particulate, allows the particulate to burn at lower exhaust temperatures (around 350°C instead of 650°C) and increases the combustion reactivity (typically 2-3 minutes) while the solid residues of the catalyst are retained on the filter as ashes.
- Place a fuel injector in the exhaust line upstream of the DPF.
- Electrical heating of the adsorber either on or off the vehicle.
Gasoline Particulate Filter (GPF)
Gasoline Direct Injection (GDI) is a key technology of gasoline engine development to reduce CO2 emissions while improving torque and power output. However, the drawback of GDI engines is an increase in particle number (PN) emissions compared to conventional Port Fuel Injection (PFI) engines.
Most of the GDI particulates are formed during the cold-start phase, catalyst heating mode and dynamic engine modes. Therefore, the injection system, including the injection operating programme (e.g. number of injections, timing, and amount of injection) has been further developed in order to improve the air-fuel mixture in the cold-start phase. Furthermore, internal engine measures such as improving mixture homogenisation and minimising the amount of injected fuel striking the engine walls helps to avoid the formation of particulates. Thus, the latest GDI vehicles can achieve the PN limit of 6×1011/km on the regulatory test cycle (NEDC or WLTC). However, the RDE procedure also includes particle counting in a wide range of engine map operation. Gasoline Particulate Filter (GPF) technology has been derived from successful experience with DPF and ensures control of ultrafine particulates from GDI engines under real-world driving conditions.
A catalytic coating can also be integrated onto the filter substrate to increase the catalyst volume within the entire system. This is the case for TWC on GPF.