Diesel Particulate Filters (DPFs) and Gasoline Particulate Filters (GPFs)
Particulate filters are generally used with diesel engines to remove diesel particulate matter (PM), but in principle can be used with other types of engine/fuel combinations, such as Direct Injection petrol engines also characterized by a high number of ultrafine particles. Based on engine technology and application specificities different filter technologies may be used to reduce particles emissions.
Particulate wall-flow filters
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 titanate) honeycomb materials.
Wall-flow particulate filters
Wall-flow filters trap most ultra-fine exhaust particulate.
Ceramic wall flow filters remove almost completely the carbon particulates, including fine particles 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) gave real-world PN emissions below a Conformity Factor of 1.0 under the conditions tested.
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 particles are formed during the cold-start phase, catalyst heating mode and dynamic engine modes. Therefore the injection system including injection operating programme (e.g. number of injections, timing, and amount of injection) has been further developed in order to improve air-fuel mixture in the cold-start phase. Furthermore, internal engine measures such as improved mixture homogenization and minimized amount of injected fuel striking the walls helps to avoid the formation of particles. Thus latest GDI vehicles can achieve the PN limit of 6×1011/km on the regulatory test cycle (NEDC or WLTC). The RDE procedure will however also include particle counting in a wide range of engine map operation. The Gasoline Particulate Filter (GPF) technology has been derived from successful experience with DPF and is available. It ensures control of ultrafine particles from Gasoline Direct Injection engines under real-world driving conditions.
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 normal exhaust temperatures using the powerful oxidative properties of NO2 and can burn in oxygen when the temperature of the exhaust gas is periodically increased through post-combustion. 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 lower the temperature at which particulate burns
- Using very small quantities of fuel-borne catalyst, such as ceria or iron additive compounds added to the fuel thanks to 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 kinetic (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 trap either on or off the vehicle.