As mentioned the trend of new emission standards is towards zero tailpipe pollutant in real conditions; this is the aim of new standard cycles. With focus on EU regulations, the real conditions are verified for type
110 approval with higher load and colder conditions, for instance the test cycle starting at ambient temperature-7°C [5].
Focusing on SI engines, higher load can lead the engine to operate in a zone characterized by thermo-mechanical stresses for the engine components, such as the turbine, that can be protected with dedicated cooling systems or by means of the enrichment of the mixture.
The first solution is not suitable for all components and in some cases not sufficient. The new EU7 standard forces to limit HC/CO and PM/PN emissions at high load, as a consequence the operation at =l is needed. To fulfill this technical requirement, avoiding performance reduction or engine specific power reduction, some technologies are mandatory (e.g. EGR, Water Injection) and they has been discussed in a previous section.
The monitoring of PM and PN, in real conditions with lower limits, forces the improvement of the GDI combustion process with higher fuel injection pressure (>300 bar) [305, 320 ,321] and the adoption of dedicated exhaust after-treatment devices, like the Gasoline Particulate Filter (GPF) [321]. The support of an electric motor during load transient can help to reduce the PM/PN production by the engine in hybrid electric vehicles, but it can not avoid the GPF application.
Gasoline Particulate Filter
The GPF is an exhaust after-treatment device with the purpose of trapping particulate within its porous honeycomb ceramic substrate (wall-flow filter). Pores generally have a diameter in the range of 10 μm ÷ 30 μm, trapping bigger particles and being passed through by the gaseous components. Ceramic is used for the substrates due to high temperatures which the GPF is exposed to, via extruding a porous ceramic material in order to obtain a monolithic cylinder. Blind parallel channels are made in the substrate, being alternately open/close at their upstream and downstream extremities, forcing gases to pass through the porous walls and to deposit suspended particles, see Figure 6-29. Particle agglomerates cumulate on the filtering surfaces, enhancing GPF capability to trap increasingly smaller particles, but also rising back-pressure.
Figure 6-29 GPF assembly and filtering scheme inside its channels
The GPF is periodically cleaned through an oxidation of the deposits by means a procedure called
«regeneration». Two different kinds of regeneration can be performed, depending on O2 availability and the temperature conditions:
Passive Regeneration, the GPF is capable of auto-regeneration, in case of sufficiently high temperature, without external energy contribution.
Active Regeneration, external energy contribution is needed to reach temperature for GPF regeneration (e.g. expansion injection of fuel to keep the combustion during the exhaust stroke).
The GPF technology is derived by Diesel Particulate Filter (DPF). The following Table 6-4 illustrates a comparison of the main features and summarizes the data published in [237,238].
111 Table 6-4 Differences between GPF and DPF (Veng: engine displacement)
The higher DPF efficiency can be explained with the soot cake formation [238]. In GPF the soot cake is easily eliminated due to passive regeneration, favored by higher temperature during fuel cut-off events.
There are a number of options for the positioning of GPF in the exhaust system, see Figure 6-30 from [239]: in the underfloor position, in a position close to the engine and behind a TWC or in a CC1 (Closed Coupled 1) position close to engine. All concepts respectively offer different advantages and disadvantages and impose various requirements on the coating which are contrasted below.
Figure 6-30 Overview of the possible positioning of the GPF with the integrated TWC functionality in the exhaust system
GPF in the underfloor structure behind a TWC allows the GPF to be used as an “add-on” solution in many applications. The configuration of the application close to the engine can remain unchanged and the diagnosis of TWC in the CC position must not be changed either. A challenge
112 for using the GPF in the underfloor position is the soot regeneration of the filter. It must not only be guaranteed that the necessary temperature for the combustion of the soot is generated, but the necessary oxygen must be provided as well.
GPF in the CC2 position behind a TWC is characterized by good temperature for easy passive regeneration. The packaging is the main challenge, due to the positioning in engine compartment close to the engine. For this reason compact solution can be adopted, with negative effect on the backpressure.
GPF in the CC1 position is a closed-coupled position as the first catalytic component in the exhaust system offers the best conditions with respect to soot regeneration. The filter must have a low light-off temperature even after severe ageing to ensure the emission in cold start, in the relevant cycles as well as under all RDE conditions. This low light-off temperature is primarily important because the filter substrate has a higher thermal mass than a conventional three-way catalytic converter discharge substrate and therefore has disadvantages during cold start. Another challenge is that the complete on-board diagnosis (OBD) must take place using the GPF since the GPF is installed at the CC1 position. This poses special requirements for the oxygen storage capacity of the GPF. For the diagnosis, it must be ensured that an adequate amount of oxygen storage capacity is available after relevant ageing, while the amount in fresh state must not be too high. These prerequisites for the coated GPF make it necessary to develop a special coating that can fulfil the requirements of positioning in CC1.
Electric Heating Catalyst
To cope with the cold start issue, a faster light-off of catalyst is requested. The main technological solution is the Electric Heating Catalyst (EHC), which offers advantages in conventional powertrain and higher electrification ones. In fact in HEVs, in cold starts condition, the thermal engine is maintained on until the catalyst reaches the operation temperature, delaying the pure electric mode operation. The EHC can accelerate the catalyst warm-up and maintaining it warm, extending the electric mode operation.
The EHC system can be enabled by the introduction of 48V power net on board the vehicle, that can easily support the needed peak of power over 4 kW.
In the EHC an electric coil is installed in a metal catalyst, and supplied and heated with energy from the traction battery. The electric catalyst is a thin catalyst disc directly followed by another, often ceramic, catalyst [147]. The resulting heat energy of the first catalyst disc is transported through the complete catalyst by secondary air or the exhaust gas flow. In this way the whole exhaust system is finally heated.
The electrically heated catalyst is already being used in series in some high performance cars. This technology benefits from the increasing electrification of the powertrain, because higher voltages (> 12 V) allow a sufficient heating level (> 4 kW) and higher battery capacity from traction batteries can be used.
As already proven in some studies [147,148], this technology allows a fuel saving versus the conventional light-off management, as illustrated in Figure 6-31.
A similar temperature level can be achieved using electric heating or engine-based catalyst heating, even if the energy used in the EHC is much lower. This is predominantly the effect of the lower mass flow rate in EHC based catalyst heating than engine-based catalyst heating. Temperatures and mass flows at the catalyst using a range of different heating processes. This means that in engine-based catalyst heating the energy needed consists of two contributions: the first one is the energy needed to heat up the mass flow from the temperature reached during normal operation to the desired temperature.
The second is the energy needed to heat up the additional mass flow to the desired temperature. This second contribution is needed to heat up the catalytic converter as fast as possible. Therefore, in order to achieve a defined catalyst temperature, an energy level that is almost three times as high as with electric heating will be necessary (Figure 6-31). Thus, in the context of modern automotive architectures, the EHC appears particularly interesting from the energy requirement point of view.
113 Figure 6-31 Comparison of primary energy used and resulting energy at the catalyst in the first 100s for identical emissions level using both heating processes [147]