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Thermal regeneration of diesel particulate filters requires an energy input to increase the temperature within the reaction zone between soot—solid carbon particles—accumulated in the filter and oxygen (and/or other oxidants) present in the exhaust gas. In general, the energy can be supplied to the exhaust gas, to the filter substrate, directly to the accumulated soot, or through a combination of the above. These different methods may show different energy efficiency, due to a different degree of energy utilization in the reaction zone, as opposed to heat loss. Electricity offers perhaps most flexibility among other forms of energy, and can be applied in the following ways:
The use of an electric heater to increase the temperature of either the exhaust gas or a stream of air used for regeneration is a relatively simple method of triggering the regeneration of a soot laden diesel filter. It can be used with a wide range of filter substrates. In systems utilizing wall-flow monolith filters, the heater is usually placed upstream of the filter substrate. Depending on the filter media, electric heaters may be also incorporated as an integral part of the filter; in those cases the deposited energy may be split in varying proportions between the flowing gas and the filter media.
Based on system configuration and the source of power, electrically regenerated filters can be divided into three groups:
In on-board regeneration systems, the power needed for regeneration is drawn from the vehicle’s electrical system. This is the only practical filter system concept applicable to highway vehicles such as trucks and buses. The prime concern in on-board regeneration systems is energy consumption. The energy required by the heater puts an extra load on the vehicle electrical system, as well as presents an additional source of fuel penalty. The amount of heat needed to increase the exhaust gas temperature by 1°C per 1 hp of the engine rated power can be estimated as follows:
In the most simple concept of the electric particulate filter, so called “full flow” system, the entire stream of exhaust gas is heated, so the filter inlet temperature reaches about 550-700°C as may be required to oxidize the soot. Electrical energy required to heat up the entire exhaust gas stream from a 100 hp engine by 100°C would be 100×100×1.43 = 14.3 kW or 19.2 hp. Assuming that the efficiency of converting mechanical energy at the engine shaft into electricity is 50%, 38.2 hp of the engine power output—or nearly 40%—would be consumed by the filter system. In most real life situations, a 100°C temperature increase would be insufficient to regenerate the filter. It is easy to calculate that increasing the exhaust gas temperature by 250°C would require that approximately 100% of the engine mechanical power output is converted to electricity and consumed by the heater. In other words, the particulate filter system during regeneration may consume no less but the full rated power output of the engine. Clearly, full flow electric regeneration systems are not feasible. Even if the regeneration was performed at a lower engine speed, the energy requirement in the full flow system could not be lowered to realistic levels.
Most of energy consumed by the full flow filter is lost when hot gas is discharged at a high flow rate from the tailpipe. In order to minimize energy consumption, most on-board regeneration systems utilize either a partial flow layout or a hot air regeneration. In the partial flow system, only a portion of the exhaust gas stream is used for regeneration, while most of the gases bypass the filter. In the hot air system, a dedicated blower supplies the regeneration air. The exhaust gas has to bypass the filter for the regeneration period. If exhaust gas filtration is required over 100% of the vehicle operating time, partial flow and hot air systems require dual filter units, so the flow can be switched between the filters for the filtration and regeneration modes.
Shore power systems require that the filter system is connected to an external power source, such as a wall power outlet in a garage or maintenance shop, for regeneration. Thus, the vehicle is immobilized for the duration of the regeneration cycle. Shore power regeneration systems are applicable to vehicle fleets with easy access to the source of external power, e.g., a fleet of construction vehicles operated within a construction site. This type of system would be impossible to use, or at least very inconvenient, on most highway vehicles.
Several variants of the shore power system are possible. In most cases, the filter is regenerated by hot air, but using the vehicle’s exhaust gas while idling is also possible. If hot air is used, the blower and heater in the “pure” shore power configuration are part of the on-board filter system. This configuration (shown in Figure 4) provides the most portability, as the only external connection that is needed is the electrical power. In some systems certain components are moved to an off-board regeneration unit. For instance, the heater may be still a part of the on-board filter, but the blower may be located off-board. In such case, both power cord and air hose connections need to be made for the duration of regeneration. Moving components to external regeneration units allows to save space on the vehicle. Cost saving may be also possible if more vehicles can share the same regeneration unit. However, the use of such systems is even less flexible and in most cases limited to short-range vehicle fleets within one facility, e.g., a fleet of diesel forklifts operated in a single warehouse.
In off-board systems the particulate filter is removed from the vehicle and placed in an off-board unit for regeneration. If spare filters are available, a soot laden filter can be quickly replaced with a regenerated one allowing for less equipment downtime than it is the case in the shore power system, at a price of higher maintenance intensity.
Shore power and off-board systems are maintenance-intensive and their proper operation depends on the timely intervention of the vehicle operator, who must initiate regeneration. These are fundamental shortcomings of these types of filters, which attracted rightful criticism from their users, as well as engine and vehicle manufacturers . To gain general acceptance, particulate filter systems must be fully automated, integrated with the vehicle powertrain, and “invisible” to the vehicle operator.