Forced regeneration of a diesel particulate filter

Details Forced regeneration of a diesel particulate filter Forced regeneration of a diesel particulate filter

2002-08-13

2004-12-14

Denion, Thomas (Department: 3748)

Power plants

Internal combustion engine with treatment or handling of...

By means producing a chemical reaction of a component of the...

C060S274000, C060S278000, C060S280000, C060S311000

Reexamination Certificate

active

06829890

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to diesel engines that have diesel particulate filters for treating exhaust gases passing through their exhaust systems. More particularly, the invention relates to engine systems and methods for forcing regeneration of such filters.
BACKGROUND OF THE INVENTION
A known electronic engine control system comprises a processor-based engine controller that processes data from various sources to develop control data for controlling certain functions of the engine. The amount and the timing of engine fueling are two functions that are controlled by an engine control system. A typical diesel engine that comprises fuel injectors for injecting fuel into the engine cylinders under control of an engine control system controls both the duration and the timing of each fuel injection to set both the amount and the timing of engine fueling. In a turbocharged diesel engine, the electronic engine control system also exercises control over turbocharger boost.
An exhaust system of a diesel engine that comprises a diesel particulate filter (DPF) is capable of physically trapping diesel particulate matter (DPM) in exhaust gas passing through the exhaust system from the engine. This prevents significant amounts of DPM from entering the atmosphere.
DPM includes soot or carbon, the soluble organic fraction (SOF), and ash (i.e. lube oil additives etc.). The trapping of those constituents by a DPF prevents what is sometimes seen as black smoke billowing from a vehicle's exhaust pipe. The organic constituents of trapped DPM, i.e. carbon and SOF, are oxidized within the DPF at appropriate times and under appropriate conditions to form CO2 and H2O, which can then pass through and exit the exhaust pipe to atmosphere. The ash collects within the DPF over time, progressively aging the DPF by gradually reducing its trapping efficiency.
One type of known DPF is marketed by Johnson Matthey Company under the trade name “Continuously Regenerating Trap” or (CRT™). An oxidation catalyst is disposed upstream of the DPF. The oxidation catalyst oxidizes hydrocarbons (HC) to CO2 and H2O and converts NO to NO2. The NO2 oxidizes carbon trapped in the DPF. While O2 could be used to oxidize DPM, the high temperatures for accomplishing oxidation make O2 rather impractical for treating diesel engine exhaust without the aid of still another catalyst such as cerium-oxide (CeO2), and as one might expect, the inclusion of a second catalyst would make such an exhaust treatment system even more expensive.
Another type of known DPF is marketed by Englehard Corporation under the trade name DPX™. It is sometimes referred to as a Catalyzed Soot Filter (or CSF). The Engelhard CSF has an additional CeO2 catalyst that eliminates the need for an upstream oxidation catalyst, which in turn reduces the overall size of a DPF and avoids the greater pressure drops present in a two-substrate DPF like a CRT™ filter. In both types of DPF, the oxidation catalyst oxidizes hydrocarbons (HC) and converts NO to NO2, with the NO2 then being used to oxidize the trapped carbon.
The rate at which trapped carbon is oxidized to CO2 is controlled not only by the concentration of NO2 or O2 but also by temperature. Specifically, there are three important temperature parameters for a DPF.
The first is the oxidation catalyst's “light off” temperature, below which catalyst activity is too low to oxidize HC. That temperature is typically around 180-200° C.
The second controls the conversion of NO to NO2. This NO conversion temperature spans a range of temperatures having both a lower bound and an upper bound, which are defined as the minimum temperature and the maximum temperature at which 40% or greater NO conversion is achieved. The conversion temperature window defined by those two bounds extends from approximately 250° C. to approximately 450° C.
The third temperature parameter is related to the rate at which carbon is oxidized in the filter. Reference sources in relevant literature call that temperature the “Balance Point Temperature” (or BPT). It is the temperature at which the rate of oxidation of particulate, also sometimes referred to as the rate of DPF regeneration, is equal to the rate of accumulation of particulate. The BPT is one of the parameters that is especially important in determining the ability of a DPF to enable a diesel engine to meet expected tailpipe emissions laws and/or regulations.
Typically, a diesel engine runs relatively lean and relatively cool compared to a gasoline engine. That factor makes natural achievement of BPT problematic. Therefore, a manufacturer of a DPF for a diesel engine should strive for a design that minimizes BPT, and a diesel engine manufacturer should strive to develop engine control strategies for raising the exhaust gas temperature to temperatures in excess of BPT whenever the amount of trapped particulates exceeds some threshold that has been predetermined in a suitably appropriate manner, such as by experimentation. Using an engine control to raise exhaust gas temperature in this way is called forced regeneration.
Investigation of several methods for initiating a forced regeneration of a DPF has disclosed that retarding the start of main fuel injections seems to be the most effective way to elevate exhaust gas temperature. That method is able to increase the exhaust gas temperature sufficiently to elevate the catalyst's temperature above catalyst “light off” temperature and provide excess HC that can be oxidized by the catalyst. Such HC oxidation provides the necessary heat to raise the temperature in the DPF above the BPT.
The method has been validated in a motor vehicle powered by a diesel engine whose exhaust system has a DPF. The DPF was loaded with soot, and regeneration was forced at low idle, 30 mph, 60 mph and high idle driving conditions. It was discovered that complete DPF regeneration may not always occur, as evidenced by the incomplete removal of all accumulated soot. Although a full understanding of that phenomenon has not yet been attained, the method does offer promise for eventual commercialization.
SUMMARY OF THE INVENTION
The present invention relates to engine systems and methods for accomplishing forced regeneration of DPF's, preferably by retarding timing of engine fueling to create suitable exhaust gas temperatures.
A presently preferred embodiment of the invention disclosed herein comprises several sub-system models, including an engine emissions model, a DPM oxidation model, a DPM accumulation model, and an ash accumulation model. Data for various parameters relating to engine and DPF operation are processed through the models, and results are used to initiate and terminate forced regenerations via regeneration initiation/termination logic. The timing and the duration of a forced regeneration are thereby controlled. Examples of data processed by an engine control system processor for accomplishing this objective include pressure, temperature and O2 concentration data relevant to the DPF.
The forced regeneration process is itself conducted according to an algorithm that processes certain data to adjust both engine fueling and the timing of engine fueling to elevate exhaust gas temperature to a range sufficient to exceed the BPT of the DPF while at the same time striving for transparency of the process to the manner in which the motor vehicle is being driven so that the driver of the vehicle will not perceive that forced regeneration is occurring. The invention inherently accounts for altitude and ambient temperature effects, and it also accounts for DPF aging through use of the ash accumulation model. If a diesel engine has a turbocharger that provides boost, the invention also makes certain adjustments in control of the turbocharger to minimize effects of the elevated exhaust gas temperature on boost as forced regeneration proceeds.
Accordingly, several generic aspects of the present invention relate to a method of imposing a forced regeneration cycle on a DPF that treats exhaust gas passing through an exhaust s

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