Patent Application
20080314032
The present invention relates to engine exhaust stream particulate filters and, more particularly, to diesel engine exhaust stream particulate filters. A diesel particulate filter (sometimes abbreviated herein as a “DPF”) is a device designed to remove diesel particulate matter or soot from the exhaust gas stream of a diesel engine.
Diesel engines, during combustion of the fuel/air mixture, produce a variety of particles, generically classified as diesel particulate matter, due to incomplete combustion. The composition of the particles varies widely depending upon engine type, age and the emissions specification that the engine was designed to meet.
Historically, diesel engine emissions were not regulated until 1987 when the first California Heavy Truck rule was introduced capping particulate emissions at 0.60 g/BHP hour. Since then, progressively stricter standards have been introduced for diesel engine particulate emissions. While particulate emissions from diesel engines were first regulated in the United States, similar regulations have also been adopted by the European Union, most Asian countries, and the rest of North and South America.
A DPF cleans an exhaust gas stream by forcing the gas stream to flow through the filter. There are a variety of diesel particulate filter technologies on the market. Each is designed around similar requirements:
Filters generally require more maintenance than catalytic converters. Particulates trapped by the filter will eventually clog the pores. This increases the pressure drop across the filter which, when it reaches or exceeds a critical value, is capable of reducing the efficiency of the engine. Regular filter maintenance or regeneration therefore becomes necessary.
Regeneration is the process of removing accumulated particulates from a filter. This is done either passively or actively by intentionally increasing the temperature of the trapped particulates. On-board active filter management can employ a variety of strategies, for example:
On-board active regeneration systems consume extra fuel, whether through burning the fuel to heat the DPF or providing extra power to the associated electrical system. Typically a computer monitors one or more sensors that measure back-pressure and/or temperature and, based on pre-programmed set points, makes decisions on when to activate and end the regeneration cycle. Running the regeneration cycle too often, although keeping the back-pressure in the exhaust system low will use extra fuel. Not regenerating the DPF sufficiently frequently can increase the risk of engine damage, can reduce engine efficiency due to high back pressure, and can result in excessive regeneration temperatures and possible DPF failure.
Typically without the use of catalysts, diesel particulate matter combusts when temperatures around 600° C. and above are attained. The start of combustion causes a further increase in temperature. In some cases the combustion of particulate matter can raise the temperature of the DPF above a threshold temperature that can cause damage to the DPF. Unlike spark-ignited engines, which typically have less than 0.5% oxygen in the exhaust gas stream upstream from the emission control device(s), many diesel engines typically have 8% to 18% oxygen in the exhaust stream pre-filter. While the amount of available oxygen makes fast regeneration of a filter possible, it can also contribute to runaway regeneration problems.
The particulate filter can be divided into segments which can be regenerated at different times by the selective introduction of a fuel into the particular segment(s) being regenerated, while the engine exhaust stream continues to flow though all segments of the filter including those that are being regenerated. Regenerating a segment or portion of the filter at a given time, compared to the entire filter, reduces the required mass flow rate of the fuel used for regeneration. If, for example, syngas is used as the fuel, this approach can offer the advantage of reducing the size and cost of a syngas generator required by the system. Furthermore, there are advantages to regenerating segments of the filter sequentially in a continuous cycle, so that syngas is being directed into at least one filter segment at a given time during operation of said particulate filter. This enables an essentially continuous requirement for the syngas stream and can offer the advantage of reducing the fluctuations in demand for the syngas stream.
If a DPF is segmented into equal segments of the substantially same dimensions and structure, when all segments are equally loaded with soot the flow rate of exhaust gas passing through each of the segments will be about the same. For example, if the DPF is segmented into four quadrants then about 25% of the exhaust gas flow will pass through each segment when all segments are equally loaded with soot.
If only one segment has been regenerated, then more than 25% of the exhaust gas stream will pass through the “clean” segment, thereby leaving less exhaust gas to pass through each of the other segments when they are being regenerated. If a fixed mass flow rate of fuel is supplied during regeneration, the result is that the fuel-to-exhaust gas ratio will be higher in the next segment to be regenerated, and the temperature associated with combusting a fixed amount of fuel, for example syngas (a mixture of hydrogen and carbon monoxide), will be higher and thus potentially damaging to the DPF's catalyst, washcoat and/or substrate.
As progressively more and more segments are regenerated, the ratio of syngas to exhaust gas will increase further, thereby resulting in higher temperatures and more of a chance of damage to the DPF. The present approach utilizes the benefits of a segmented particulate filter while addressing this issue.
In a preferred method for regenerating a segmented engine exhaust stream particulate filter, the heat value ratio of the engine exhaust stream to the fuel stream introduced into each individual segment during regeneration is maintained substantially consistent among segments. This offers the advantages of reducing the risk of thermal damage to the filter during regeneration (by decreasing the variation in the maximum regeneration temperatures reached in each segment), increased regeneration consistency between the segments, and reduced fuel consumption.
In one aspect, the filter comprises a plurality of filter segments that have differing physical properties or structural characteristics such that the engine exhaust stream to fuel stream ratio is maintained substantially consistent among segments during their regeneration. The segments can be designed and constructed so that if they were each supplied with a gas stream under the same conditions, the mass flow of that gas stream through each of them would be different. Such filters are described herein as “unequally segmented filters”.
In another aspect, the mass flow rate of the fuel stream or syngas stream introduced to each individual segment can be varied to enable a substantially consistent heat value ratio of the streams introduced into each segment during regeneration.
Thus, in preferred embodiments an engine exhaust stream particulate filter comprises at least two filter segments wherein the segments are structured so that if they were each supplied with a gas stream under identical conditions, the gas stream mass flow through each of the segments would be different. For example, the segments can differ from each other in at least one structural characteristic selected from the group consisting of cross-sectional area, longitudinal filter length, filter porosity and filter cell density. Preferably, the segments differ from each other in at least one structural characteristic such that the engine exhaust stream mass flow rate through each segment during their regeneration is approximately equal. In compact designs, the filter segments can be housed in a common enclosure.
In embodiments of a method for operating a particulate filter comprising a plurality of filter segments, the method comprises:
In some embodiments of the above method, the engine exhaust stream mass flow rate through each segment is different so that the heat value ratio is maintained substantially consistent among segments during their regeneration. The segments can be structured as described above, so that if they were supplied with a gas stream under identical conditions, the gas stream mass flow through each one of the plurality segments would be different.
In other embodiments of the above method, the heat value ratio is maintained substantially consistent among segments during their regeneration by varying the mass flow rate of the hydrogen-containing gas stream that is introduced to regenerate each segment. For example, the mass flow rate of the hydrogen-containing gas stream that is introduced to regenerate each segment can depend upon its position in a regeneration sequence
In the above-described methods, the hydrogen-containing gas stream can be introduced into the plurality of segments sequentially in a regeneration sequence.
In the above-described methods, the duration of regeneration of each segment can be substantially the same.
The above-described methods can be employed in a non-continuous regeneration cycle or in a continuous regeneration cycle in which the hydrogen-containing gas stream is being introduced into at least one filter segment at a given time, so that at least one filter segment is undergoing regeneration during operation of the particulate filter.
In embodiments of an engine and exhaust after-treatment system, the system comprises:
Preferably the filter segments are structured so that if they were each supplied with a gas stream under identical conditions, the gas stream mass flow through each of the segments would be different.
a is a simplified end view of a diesel particulate filter divided into four equal segments.
A diesel particulate filter (DPF) can be segmented into at least two segments, for example, by providing a baffle on the upstream side of the filter monolith, or by having separate filter segments housed in a common enclosure or not. This enables the introduction of a fuel, for example, a syngas stream, selectively into different portions or segments of the filter and the regeneration of each segment separately or independently from another. With this type of filter regeneration of the segments can be conducted sequentially and in a continuous cycle. In another variation the segments can be regenerated in sequence, one after the other, and then the DPF can be operated for a period without any segments being regenerated, with the regeneration sequence repeated only when regeneration is required. The engine exhaust stream flows through all segments of the DPF, whether the segment is being regenerated or not. As each segment is regenerated, the mass flow of the engine exhaust stream through the segment being regenerated tends to change as a result of the removal of particulate matter. This in turn can alter the heat value ratio of the engine exhaust stream to the fuel stream during the regeneration of subsequent segments.
The following discussion describes a shortcoming of an “equally” segmented DPF, in which the segments are regenerated sequentially in a continuous or non-continuous cycle. For example, if a DPF is loaded with soot and there are four equally divided segments the total engine exhaust stream flow will be equally split with about 25% of the total engine exhaust stream flowing through each segment. After regeneration of the first segment, the first segment which now has less soot will have a lower resistance to the flow of the engine exhaust stream.
The present approach reduces the variation in the heat value ratio of the streams flowing through the various segments during regeneration, by factoring into the filter design and/or regeneration technique the change in the engine exhaust stream flow rate through the segments that occurs as they are regenerated or cleaned.
In one embodiment of an improved segmented filter design, a diesel particulate filter is divided into at least two segments having different cross-sectional areas.
Unequal segmentation of the diesel particulate filter can reduce the variation in the heat value ratio, and thus the variation in temperature of the regeneration process, among individual segments as they are regenerated. It can also substantially reduce the overall fuel consumption associated with filter regeneration. For example, if the first segment AA was at approximately 65° compared to an equally segmented filter segment at 90°, the mass flow rate of the engine exhaust stream flowing through the first segment would be 65/90 or approximately 72% of the amount it would be in an equally segmented filter segment. Therefore, the required flow rate of the fuel stream or syngas stream would be about 72% of the amount required for an equally segmented filter, in order to maintain a similar heat value ratio. In the foregoing example the overall fuel saving would be about 28%.
The table in
The data shown in
In other embodiments of improved segmented filter designs, other physical properties or structural characteristics of the filter segments can be different from one another, besides or in addition to their cross-sectional area. For example, the mass flow of the engine exhaust stream can be made more consistent among the different segments during regeneration by having segments with differing filter cell density, differing filter porosity and/or differing longitudinal filter lengths or volumes.
Ceramic wall-flow monoliths used for DPFs can be manufactured by extruding a large unitary section or by cementing or bonding together multiple smaller sections or “bricks” to form one complete larger section. The DPF in
In embodiments of a regeneration technique for a segmented particulate filter, the heat value ratio during the regeneration of an individual segment can be held substantially consistent among segments by controlling and varying the mass flow rate of the fuel stream introduced to each individual segment. The mass flow rate of the fuel stream can be adjusted depending on the mass flow rate of the engine exhaust stream flowing through an individual segment during regeneration to maintain the desired heat value ratio. This technique can reduce the overall fuel consumption associated with filter regeneration, but can cause a fluctuating demand for fuel (for example, syngas) for regeneration purposes, and requires a more complex control system. The technique can be used with an equally segmented particulate filter to reduce the variation in the heat value ratio, and thus the variation in temperature of the regeneration process, among individual segments as they are regenerated. It can also be used with unequally segmented particulate filters of the types described herein, to provide further non-passive control of the heat value ratio.
The present segmented filter designs and regeneration techniques can provide some or all of the following advantages over those used in conventional DPFs:
In the above described embodiments the segments can be regenerated by various methods, for example, conducted sequentially until all segments have been regenerated, conducted in a specific order, conducted in a continuous cycle, conducted in a non-continuous cycle, and/or conducted only when regeneration is of the filter required. The regeneration process can be controlled through an open-loop control method and/or a closed loop control method employing sensors and/or pre-determined regeneration algorithms.
As particulates collect in filter 121, the flow of the engine exhaust stream is impeded, increasing the backpressure of the engine exhaust stream upstream of filter 121. An optional sensor can be employed to monitor the temperature of the engine exhaust stream and can be located near the engine outlet, for example, sensor 133. An optional pressure sensor 131 monitors the pressure of the engine exhaust stream upstream of filter 121 and can be located along conduits 111 or 113. An optional pressure sensor 132 monitors the pressure of the engine exhaust stream downstream of filter 121 and can be located along conduits 114 or other optional conduits located downstream of filter 121 (not shown in
The following paragraphs describe control strategies that can be used to initiate and terminate regeneration of filter 121. These strategies are applicable to unsegmented filters, and equally divided segmented filters or unequally divided segmented filters of the type described herein.
In preferred control strategies, controller 130 initiates a regeneration process for filter 121 based on employing one of equations (1), (2), (3) or (4).
Initiate DPF regeneration when:
n<[(P1−P2)/P3], (1)
n<[(P1−P2)/P3] and t1>x1 (2)
n<[(P1−P2)/P3] and T1>y1 (3)
n<[(P1−P2)/P3] and t1>x1 and T1>y1 (4)
where
Alternatively, in equations (1), (2), (3) or (4), P1 can be employed in place of P3.
In other embodiments of a control strategy, controller 130 initiates a regeneration process for filter 121 based on employing one of equations (5), (6), (7) or (8).
n<P1 (5)
n<P1 and t1>x1 (6)
n<P1 and T1>y1 (7)
n<P1 and t1>x1 and T1>y1 (8)
Alternatively, in equations (5), (6), (7) or (8), P1−P2 can be employed in place of P1.
In yet other embodiments of a control strategy, controller 130 initiates a regeneration process for filter 121 based on employing one of equations (9), (10), (11) and (12).
n<[(P1−P2)/m1], (9)
n<[(P1−P2)/m1] and t1>x1 (10)
n<[(P1−P2)/m1] and T1>y1 (11)
n<[(P1−P2)/m1] and t1>x1 and T1>y1 (12)
where m1=mass flow of engine exhaust stream.
Alternatively, in equations (9), (10), (11) or (12), P1 can be employed in place of P1−P2.
Alternatively, in equations (9), (10), (11) or (12), m2 can be employed in place of m1
where m2=mass flow of engine intake air stream.
In equations (1) through (12) the parameters that are actually monitored and associated signals sent to the controller can be indicative of another parameter, for example, in equation (9) a pressure sensor can be employed to monitor the engine exhaust stream in order to indicate the mass flow rate of the engine exhaust stream.
In further embodiments of a control strategy, controller 130 initiates a regeneration process for filter 121 based on employing at least one of equations (1) through (12) and at least one of equations (13) (14).
T1<y2, (13)
λ>[O2], (14)
where
In preferred embodiments of a control strategy for terminating regeneration, controller 130 terminates a regeneration process for filter 121 based on at least one of equations (15), (16), (17) or (18).
Terminate DPF regeneration when:
T2>y3, (15)
dT2>z (16)
t2>x2 (17)
P4(P1−P2) (18)
P4<P3 (19)
where
Preferably in the above described control strategies, DPF assembly 120 is divided into equally or unequally divided segments which enables the regeneration of at least one segment or portion of filter 121 at a given time. In
In
Various algorithms can be used for controlling regeneration of a segmented DPF. The algorithm can include factors such as:
Table 1 illustrates examples of various regimes that can be used to control the regeneration of an equally or unequally segmented DPF assembly. One or more of these regimes can be used.
As the syngas stream exits conduit 143, 144, 145 or 146, and enters a respective chamber created by baffle 122, the syngas stream mixes with and is carried by a portion of the engine exhaust stream through the respective segment of filter 121. The remaining portion of the engine exhaust stream flows through the remaining segments of filter 121, where particulates are trapped, exits DPF assembly 120 and is then released to the atmosphere via exhaust conduit 114. As the syngas stream and engine exhaust stream mixture flows through a segment of filter 121 it undergoes combustion reactions and heats the segment of filter 121, enhancing the regeneration process. The combustion reactions can be promoted by a catalyst (not shown in
In
(1) Regeneration process of DPF;
(2) De-sulfation process of LNT;
(3) Regeneration process of LNT.
Furthermore, syngas generator assembly 140 can from time to time require a regeneration process which would be a higher priority over the above stated processes.
The present filter segmentation designs and techniques have the following potential commercial applications, end-uses and/or markets (present and future):
The fuel employed to regenerate the DPF can be another fuel (other than syngas) for example diesel, gasoline, natural gas, propane, ethanol, methanol or kerosene can be used.
The engine can be a lean burn combustion engine fueled by suitable fuels, for example, diesel, fuel oil, kerosene, natural gas, propane, liquefied petroleum gas (LPG), methanol, ethanol or gasoline. The engine system can comprise additional devices which utilize a syngas stream for example, a lean NOx trap, selective catalytic reactor (SCR), diesel oxidation catalyst (DOC) and or a fuel cell. A diverter valve can be used to selectively direct the flow of syngas stream to such additional devices.
While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.
This application is related to and claims priority benefits from U.S. Provisional Patent Application Ser. No. 60/938,106, entitled “Control System And Method For Regenerating A Diesel Particulate Filter”, filed on May 15, 2007, and U.S. Provisional Patent Application Ser. No. 60/953,856, entitled “Engine Exhaust Stream Particulate Filter With Unequal Segmentation”, filed on Aug. 7, 2007, each of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60938106 | May 2007 | US | |
| 60953856 | Aug 2007 | US |
