The present invention relates generally to diesel particulate filters and, more particularly, to methods and apparatus for estimating soot loading in a diesel particulate filter.
Diesel particulate filter (DPF) systems are becoming more common in many markets, particularly heavy duty truck markets, and will likely remain as an important emission solution for particulate matter for some time to come. There are various methods to estimate the accumulated soot mass in the DPF but, currently, there is only one direct feedback mechanism, namely, a pressure drop (dP) sensor that measures pressure drop across the DPF.
The dP signal can be used to estimate DPF soot mass through a correlation or model, usually based on observation of numerous filters over a period of time, thereby establishing data for what shall be referred to as a “nominal” DPF. The model inputs have several uncertainties. Aside from typical sensor uncertainties such as offset, hysteresis, temperature drift, time drift, etc., there are additional factors that affect model accuracy. These factors include DPF-to-DPF variations, ash loading of the filters, non-uniform soot loading, catalyst sintering, and the like, all of which make it difficult to confidently compare pressure drop data of a subject filter with pressure drops across a nominal DPF to estimate soot loading of the subject DPF based on soot loading of the nominal DPF.
If soot loading of the subject DPF cannot be estimated with a reasonable degree of accuracy, then damage to the DPF may result, or the DPF may cease to perform acceptably to filter particles. If the estimated soot loading of the subject DPF is too low, then the subject DPF may be overloaded with soot before an active regeneration operation is performed, and the regeneration operation may not burn off acceptable levels of the accumulated soot. If estimated soot loading of the subject DPF is too high, then the subject DPF may be subjected to excessive regeneration operations, which can shorten its useful life.
It is desirable to provide a technique for improving the accuracy of soot loading estimates for DPFs.
In accordance with an aspect of the present invention, a method for estimating soot loading of a diesel particulate filter comprises establishing a soot loading model for estimating soot loading of a nominal diesel particulate filter, adjusting the soot loading model for a subject filter to account for differences between the nominal filter and the subject filter, and estimating soot loading of the subject filter using the adjusted soot loading model.
In accordance with another aspect of the present invention, a diesel engine with an exhaust aftertreatment system comprises a subject diesel particulate filter, and a controller, the controller comprising a soot loading model for estimating soot loading of a nominal diesel particulate filter, the controller being adapted to adjust the soot loading model for the subject filter to account for differences between the nominal filter and the subject filter, and being adapted to estimate soot loading of the subject filter using the adjusted soot loading model.
The features and advantages of the present invention are well understood by reading the following detailed description in conjunction with the drawing in which like numerals indicate similar elements and in which:
FIG. 1 is a schematic view of an engine with an exhaust aftertreatment system according to an aspect of the present invention;
FIG. 2 shows differences between pressure drop and volume flow measurements for two different diesel particulate filters; and
FIG. 3 schematically compares pressure drop versus volume flow ranges for a nominal filter, a high resistance filter, and a low resistance filter.
A diesel engine 21 with an exhaust aftertreatment system 23 according to an aspect of the present invention is shown in FIG. 1. The aftertreatment system 23 comprises a diesel particulate filter 25 (DPF) downstream from the engine 21. The DPF 25 will hereinafter be referred to as the “subject” DPF to distinguish it from a nominal DPF used for purposes of comparison. The subject DPF 25 may be a catalyzed filter.
A heater 27 is provided upstream of the subject DPF 25 and is adapted to raise the temperature of the exhaust from the engine 21 entering the DPF to perform active regeneration of the subject DPF. A sensor 29 is provided for sensing at least one condition of the subject DPF 25. The sensor 29 is arranged to send a signal to a controller 31 to cause the heater 27 to initiate an active regeneration operation in response to the signal. The at least one condition can be at least one of pressure drop across the subject DPF 25, fuel consumption of the engine 21, distance traveled by a vehicle including the engine, and operating time of the engine or the filter. The at least one condition corresponds to an estimated level of soot loading of the subject filter 25. The aftertreatment system 23 will ordinarily include other aftertreatment devices, such as a diesel oxidation catalyst (DOC) (not shown). A DOC, however, may also be considered to be a type of heater 27 or may be provided in connection with a separate heater, such as a burner or an electric heater.
The controller 31 is any suitable type of controller, such as a computer. The controller 31 comprises a soot loading model for estimating soot loading of a nominal DPF. The model is typically programmed into the controller based upon observations of actual filters under various operating conditions, e.g., different atmospheric pressures, exhaust temperatures, length of operation, etc. The controller 31 is adapted to adjust the soot loading model for the subject DPF 25 to account for differences between the nominal DPF and the subject DPF. The controller 31 is also adapted to estimate soot loading of the subject DPF 25 using the adjusted soot loading model.
According to an aspect of the present invention, the soot loading model for the subject DPF 25 is adjusted to account for differences between the nominal DPF and the subject DPF. For example, FIG. 2 shows two sets of dP (pressure drop across the DPF) versus volume flow (across the DPF) measurements for two DPFs from two trucks. Each set comprises a plurality of separate measurements taken at different times between start-up and active regeneration of each filter.
For purposes of discussion, the lower set L of dP versus volume flow measurements will be considered to represent data for a nominal DPF, and the upper set U of dP versus volume flow measurements will represent data for a subject DPF. It will be appreciated, however, that differences between a nominal DPF and a subject DPF may be such that a set of dP versus volume flow measurements for the subject DPF may be lower than the set of dP versus volume flow measurements for the nominal filter, such as is shown in FIG. 3, which schematically compares ranges of dP versus volume flow for a nominal DPF with ranges for a high resistance DPF and a low resistance DPF. In FIG. 2, the dashed lines at the bottom of the L and U sets of measurements are provided to assist in showing approximately the dP versus volume flow for a new filter or a filter immediately after active regeneration.
In some aspects of the present invention, adjusting the soot loading model for the subject DPF comprises multiplying the dP versus volume flow curves for the subject DPF by a factor so that they approximate the curves for the nominal DPF. For example, the slope of the dashed line at the bottom of the U measurements could be multiplied by a factor so that it equals the slope of the dashed line at the bottom of the L measurements, if the L measurements reflected dP versus volume flow for a nominal filter. Typically, the factor is determined by comparing dP versus volume flow curves or data points for a nominal filter and a subject filter that have each been de-greened, and that each have substantially no soot loading. This condition is ordinarily the condition after de-greening of a new filter or after complete active regeneration of an older filter. Of course, the factor might be determined in some other ways, such as by comparing dP versus volume flow curves or data points for the nominal filter and the subject filter at some other time, including or instead of just upon initial start-up after de-greening or immediately after active regeneration, or over a period of time.
Soot loading of the subject DPF 25 is estimated by the controller 31 using the adjusted soot loading model. To illustrate according to an aspect of the present invention wherein the dP versus volume flow curves or data points for the subject DPF 25 are multiplied by a factor to more closely conform to the curves or data points for the nominal DPF, soot loading can be estimated for the subject filter 25 by comparing a dP versus volume flow measurement for the subject filter, multiplied by an adjustment factor. Soot loading of the nominal filter at the dP versus volume flow values to which the subject filter measurements were adjusted by multiplying them by the factor can be estimated to correspond to soot loading for the subject DPF 25.
However estimated soot loading of the subject DPF 25 is determined, an active regeneration cycle will ordinarily be initiated when the estimated soot loading of the subject DPF exceeds a predetermined level. For example, if the soot loading model estimates soot loading as a function of pressure drop across the nominal DPF at some volume flow rate, pressure drop and volume flow rate across the subject DPF 25 can be monitored to obtain at least one pressure drop measurement at a particular volume flow rate, and soot loading of the subject DPF can be estimated as a function of the at least one pressure drop measurement at the particular volume flow rate. If the pressure drop measurement, as adjusted by whatever adjustment technique is employed, exceeds a predetermined value, a signal can be sent to the controller 31 which can send a signal to initiate active regeneration, e.g., by activating the heater 27.
According to another aspect of the present invention, soot loading of the subject DPF 25 can be estimated by calculating an error (E(t)) of the soot loading model for the subject DPF at time t according to the equation:
E(t)=sum(X(t)−Xbar)/T EQ. 1
where time t=0, 1, 2, . . . , T
X(t) is a calculated value for soot loading of the subject DPF at a given filter condition,
Xbar is a calibrated value for soot loading of the nominal DPF at the given filter condition. Xbar can be a constant value or a time-varying value based on real measurements or an internal mass model prediction.
At time T, an offset value Z(t) is determined by:
Z(t)=Z(t−1)+G*[E(t−1)−Z(t−1)] EQ. 2
where G is a factor corresponding to reliability of soot loading data between time t=0 and time t=T. The estimated value Xa(t) for soot loading of the subject DPF can be determined according to the equation
Xa(t)=X(t)−Z(t). EQ. 3
Adding an offset value Z(t) is one simple and practical way to adjust the dP soot model to minimize the model error.
There are certainly other methods to adjust the dP soot model than by adding the offset value Z(t). For example, one could adjust the dP soot model parameters used in EQ. 4, below, or even the structure of the model to eliminate the error.
According to another aspect of the invention, the soot loading model determines soot loading via pressure drop across the nominal DPF according to a pressure drop equation:
where dPnom is pressure drop across the nominal DPF,
μ(T) is viscosity of exhaust gas at temperature T,
uwall is wall flow velocity,
wnom. wall is wall thickness of the nominal DPF,
knom. wall is permeability of the nominal DPF,
wsoot is thickness of a soot layer, and
ksoot is permeability of the soot layer.
The soot loading model can be adjusted for the subject DPF 25 by multiplying one or both of wwall or kwall by a factor so that the equation accurately calculates zero soot loading in the subject DPF. For example, if, at start-up or after complete active regeneration, when wsoot should be equal to zero or substantially zero, if dP for the subject DPF 25 at a particular μ(T) and uwall differs from dP at that μ(T) and uwall for a nominal filter, then one or both of wwall or kwall can be multiplied by an adjustment factor so that the adjusted dP equals the dP for the nominal filter at zero soot loading.
The foregoing equation for pressure drop is one possible simplified version of a more complex equation. The simplified pressure drop equation is ordinarily an approximation of a more complex equation considering a host of variables. To illustrate, a more complex pressure drop equation (and assuming incompressible flow and no ash deposits) applicable to at least certain models of filters can be represented by:
ε=porosity of DPF
w=thickness of soot cake
ws=thickness of substrate wall
μ=viscosity of exhaust
Q=volume flow of exhaust
k=permeability of DPF
ρ=density of exhaust
In the present application, the use of terms such as “including” is open-ended and is intended to have the same meaning as terms such as “comprising” and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as “can” or “may” is intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such.