GB2134408A

GB2134408A - Method for regenerating a diesel engine exhaust filter

Info

Publication number: GB2134408A
Application number: GB08402712A
Authority: GB
Inventors: Wallace Robert Wade; Vemulapalli Durga Nageswar Rao
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1983-02-03
Filing date: 1984-02-02
Publication date: 1984-08-15
Also published as: DE3403505A1; CA1216200A; GB8402712D0; GB2134408B

Abstract

A diesel engine exhaust filter has an upstream pressure monitor whose output is compared with the pressure which a clean filter would exhibit at similar engine speeds, and when the difference between the two pressures exceeds a given value a filter-cleaning cycle is automatically started. This cycle involves a) bypassing the exhaust gas round the filter 10 by opening valve 18, b) supplying hot combustion- supporting gas to the filter to ignite the accumulated carbon therein, c) monitoring the temperature of gas downstream of the filter and shutting off the hot gas when this temperature exceeds a given level. The hot gas may be air or exhaust gas, heated by an electric heater or by adding fuel through nozzle 33 and igniting, either by a glow plug 36 or catalyst. <IMAGE>

Description

SPECIFICATION Method for operating a regenerative diesel engine particulate trap Background of the invention and prior art statement State of the art engine technology may allow a diesel engine to emit as low as .6 g/ml particulates.

However, with more stringent particulate emission requirements to come into effect in 1985-1988, such as at a level of .20 g/ml, the technology cannot meet such lower level of particulate emissions without some form of particulate trap. The most important materials used to date by the prior art for the trap material have included monolithic and fibrous ceramic filter materials (see U.S. patent 4,276,071) and wire mesh (see U.S. patent 3,499,269), each material having its own characteristic mode of trapping.

The particulates emitted and trapped throughout the life of the vehicle by such a trap cannot be stored since the amount can be typically 20 cubic feet for each 100,000 miles of engine use. As the particulates build up, the exhaust system restriction is increased, commonly referred to as back pressure. Thus a means is required to remove the trapped material periodically, commonly referred to as regeneration of the filter. One of the most promising methods found to date is rejuvination of the filter by thermal oxidation of the carbonaceous particles, which incinerate at about 1100 F (550 C).

Normal diesel engine exhaust temperatures rarely reach 1100 F during normal driving conditions.

Therefore an auxiliary temperature elevating means is necessary to carry out thermal oxidation. The types of thermal oxidation means used by the prior art have generally fallen into the following three categories: use of a fuel fed burner (see U.S. patent 4,167,852 and Japanese 55-19934), use of an electric heater (see U.S. patents 4,270,936; 4,276,066; 4,319,896), and detuning techniques which may be combined with any of the above for raising the temperature of the exhaust gas at selective times (see U.S. patents 4,211,075; 3,499,269). These techniques have been used to burn the collected particulates in the presence of excess oxygen.

Each of the prior art systems have certain disadvantages associated with them which is described in more detail in copending U.S. application S.N.- More importantly, each of the prior art systems have been operated by a control system which is totally driver initiated and driver controlled, failing to: (a) initiate regeneration only when the filter optimally needs regeneration; (b) continue regeneration for a period which produces optimum filter life, optimum filtration, and maximum fuel economy for the vehicle; and (c) be quickly responsive to a change in engine conditions to permit the regeneration cycle to take place under conditions of optimum filtration rather than operate, as in the prior art, only when the engine is functioning in a narrow range of conditions.

Summary of the invention The invention is a method of regenerating a filter (and the associated apparatus therefor) used to extract and collect particulates from the exhaust gas of a diesel engine. The method results in more economical operation of the filtration system, less fuel losses when operating the vehicle, and the system is more quickly responsive to engine operating characteristics over a much wider range than that known to the prior art.

The method comprises: (1) Determining when the pressure of the exhaust gas, measured at a station immediately upstream from the filter, exceeds a predetermined variable maximum allowable pressure and then initiating steps 2-4.

(2) Bypassing said exhaust gas around said filter and conveying a flow of a combustion supporting gas to and through the filter at a low flow rate (i.e., 5-7CFM).

(3) Raising the temperature of said combustion supporting gas to a level effective to ignite at least a leading portion of the particulate collection in the filter.

(4) Measuring the temperature of the heated gases exiting from the filter; when the temperature exceeds a predetermined level, shutting off the heated gas conveyed to the filter and terminating the regeneration cycle.

The variable maximum allowable pressure can be a value mathematically derived by measuring the instantaneous speed of the engine and the instantaneous fuel flow of the engine, then comparing the instantaneous back pressure value with previously measured back pressures for a clean filter operating under the instantaneous speed and fuel flow conditions. If the difference between the two valves is greater than an acceptable AP, then the maximum allowable pressure has been exceeded.

Preferably the temperature of the combustion supporting gas is raised adding fuel to the combustion supporting gas and either igniting the combustible mixture by use of a glow plug, sparking device, or catalyst to facilitate lower ignition temperatures, or by using an electrical heating element to raise the temperature of the gas to ignition temperature. The combustion supporting gas can be either air or exhaust gas.

Advantageously, the variable maximum allowable pressure in the filter is mathematically and experimentally determined to be proportional to particulate collection in the filter in the range of 150-270 mgfin which in a desirable application will permit regeneration to be initiated with a frequency of about 145 miles of vehicle operation. Preferably the pressure may be measured by use of a thin diaphragm transducer that reads the capacitance between a fixed element and the diaphragm. The temperature may be sensed and measured by use of a thermocouple embedded in the trailing portion of the filter.

Alternatively, the temperature of the filter at its inlet portion may be measured. If the temperature of such inlet filter portion exceeds a predetermined level, the fuel supplied, to raise the temperature of the combustion supporting gas, is shut off in advance of the termination of regeneration. Regeneration will proceed by way of the exothermic reaction resulting from combustion of the particulates.

Advantageously, the combustion supporting gas may comprise a supply of fresh air to which is added hydrocarbon fuel by way of an aspirated nozzle to form a combustible mixture. The exhaust gas is bypassed around the filter during such regeneration initiation.

The method may be further refined by shutting off the fuel supply for a delayed period in the event there is a failure to light the combustion supporting gas. This delay period and shut-off of fuel is initiated by sensing the temperature of the gas flow at the inlet to the filter. If it does not exceed a predetermined value, such delay period is initiated.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Summary ofthe drawings Figure 1 is a schematic layout of the essential appararus components of this invention shown associated with a control system, also in schematic form, to illustrate the sequence of steps that must be taken during a regeneration cycle; Figure 2 is a graphical illustration of the cold flow pressure drop as a function of soot loading which illustrates the minimum and maximum parameters for initiating filter regeneration in response to a measured pressure; Figure 3 is a graphical illustration of temperature as a function of time illustrating a temperature map forthefilter during a regeneration cycle for the embodiment of Figure 1;; Figure 4 is a view similar to that of Figure 1, but illustrating an alternative embodiment using an electrical heaterforthe means to raise the temperature of the combustion supporting gas; Figure 5 is a temperature map similar to Figure 3 for the apparatus of Figure 2; and Figure 6 is a graphical illustration of fuel economy as a function of vehicle miles using the trap system of Figure 1.

Detailed description A preferred mode for carrying out the invention is illustrated in Figure 1. The apparatus components necessary to carry out the method aspects will be described first. The apparatus comprises a filter 10, housing for the filter 10a, and ducts 11-12-13 for conveying exhaust gas to a plenum 14 leading to the frontal face 15 of the filter 10. During periods of regeneration, the exhaust gas is bypassed around the filter, through duct 16, and the flow of gas to the filter is replaced by a flow of combustion supporting gas from duct 17.

A flow control means 18 is employed to carry out the selective bypassing of the exhaust gas around the filter and the introduction of the combustion supporting gas to the filter during regeneration. A temperature elevating means 19 is employed to heat the combustion supporting gas during the regeneration period to a temperature level effective to ignite at least the leading portion of the particulate collection within the filter 10. A sequential control means 20 is employed to sense the back pressure of the exhaust gas (at 21) being admitted to the filter during normal engine operation, continuously sense the temperature of the filter at a location 22 near the exit portion of the filter, to continuously sense the temperature of the gases entering the filter at 23, and sense the instantaneous fuel flow to the engine as well as the instantaneous speed of the vehicle engine.These sensed parameters are fed to an onboard computor in control means 20 which mathematically compares such sensed parameters for determining a variable maximum allowable pressure which, if exceeded by the instantaneously measured back pressure, will initiate the regeneration cycle.

In Figure 1 the means for conveying the combustion supporting gas through at least a portion of the filter comprises the plenum chamber 14 into which is fed a supply of compressed airfrom an air pump 25 which is operated electrically off the battery of the vehicle.The supply of air is admitted or not admitted as controlled by an air valve 26.

The means for controlling the flow of exhaust gas from the engine comprises ducts 12-13-16 with a Y-connection effective to normally permit exhaust gas to flow through the branch 13 of the Y into the plenum chamber leading to the filter during normal engine operation, and to pass through the other leg 16 of the Y when regeneration is desired. This means for controlling the flow of either exhaust gas or the combustion supporting gas to the filter is also comprised of the bypass valve 18, which is movable between a first and second position by a vacuum motor 27. The vacuum motor is actuated by a vacuum reservoir 28, which is, of course, created as a result of a vacuum pump 29 energized from the automobile battery. The vacuum is admitted to the vacuum motor by way of a valve 30.

The means for heating the combustion supporting gas to a temperature effective to ignite the leading portion of the particulate collection comprises a fuel line 31 supplied from a pressurized fuel supply controlled by a valve 32, the fuel supply being aspirated from a nozzle 33 as a result of the interjection of air into the nozzle along with the fuel for mixing therein and by the separate supply of air through side vents 34 in the burner housing 35. A glow plug 36 heated with sufficient energy from a control 37 is effective to ignite the combustible fuel/air mixture.

The method utilizing the above apparatus essentially comprises: (1) Determining when the pressure of the exhaust gas (measured at a station immediately upstream from the filter) exceeds a predetermined variable maximum allowable pressure, a series of event for regeneration is initiated. In this step a pressure sensor 21 is placed in the leg of the Y-connection leading to the filter so that the back pressure of the exhaust is continuously monitored.

Back pressure is defined herein to mean exhaust gas pressure in front of the filter. Variable maximum allowable pressure is defined herein to mean a pressure which has been mathematically derived by measuring the instantaneous back pressure, instantaneous rpm of the engine, and the instantaneous fuel flow of the engine and comparing it to the measured back pressure for a clean filter at each of the rpm and fuel flow combinations. When the predicted allowable pressure exceeds a specific pressure drop for any specific combination of rpm and fuel flow, the regeneration cycle can be initiated because it indicates that an optimal collection of particulates is present. Considerable test data has been collected to indicate that a certain minimum amount of particulates must be collected to permit regeneration to effectively take place and propogate and fully clean the trap.This minmum amount is usually in the range of 150-270 mg/in3 (see Figure 3) and will vary depending upon the nature of the filter substrate and its corresponding porosity. It has further been determined that a pressure drop of 40-65 inches of water (approximately 3-5 3-5 inches of mercury) for 110 in3 alter volume is typically associated with said minimum collection of particulates in case of a 1979 Opel vehicle with 2.3 liter engine at steady 40 MPH (or 90 SCFM flow).Therefore, based upon these precalculations, the onboard microcomputer of the vehicle mathematically makes these comparisons between the input data and derives a net electrical signal in the event the measured pressure drop exceeds the predetermined variable maximum allowable pressure.

(2) Next, a flow of combustion supporting gas is conveyed to and through the filter 10. This is brought about by a series of events which include turning on the vacuum pump 29, which when sufficient vacuum is present in the reservoir 28, permits the vacuum solenoid 27 to be turned on, which in turn actuates the bypass valve 18 to a position where exhaust gas is bypassed around the filter and a flow of compressed air would be permitted into the plenum 14. Next, the glow plug 36 is energized. Next, the air pump 25 is energized, causing compressed air to be pumped through the air supply system, and followed by actuation of the air solenoid valve 26, permitting the air to be fed not only through the aspiration vents of the burner unit, but through the nozzle 33 channel.

Next, the fuel solenoid valve 32 is actuated to permit pressurized fuel to be transmitted through the fuel nozzle 33 along with the air creating a combustible atomized fueliair mixture in the plenum 14. Since the glow plug is already activated, the combustible mixture is immediately ignited, elevating the flow of gases to a temperature of at least 1 200"F prior to entering the filter. In the event such ignition does not take place, a thermocouple 23 immediately adjacent the front face ofthefilter senses a temperature that is below the 1100"F normally resulting from ignition and thus sends a signal which shuts off the fuel valve solenoid 32 for a predetermined delayed period of time.After the delayed period of time, the fuel flow is again instigated with the intent that the glow plug would be effective to ignite the combustible mixture and proceed with the ignition of the particulate collection.

Once the heated gas has been generated to the ignition temperature of the particulates, the propogation of the burning particulates will proceed over an expected period of time, which is normally about two minutes (see Figure 3), the peak temperature within the filter progressing along the axial extent thereof.

The method results in more economical operation of the filtration system and less fuel losses when operating the vehicle (see Figure 6). The system is more responsive to engine operating characteristics over a much wider range than that known to the prior art because in the regeneration mode the particulate filter is isolated from the engine flow and is independent of the engine operation.

Alternatively, the fuel system controls (31-32-3334-36) can be eliminated. The glow plug 36 is replaced by an electrical heating element 50 disposed adjacent the inlet face 15 of the filter 10 (see Figure 4). A control system 51 employing a thermocouple control 52 is added to limit the surface temperature of the heating element. The temperature map of the heater element and forthe inlet and outlet ofthefilter,forthistype of embodiment, is shown in Figure 5.

Claims

1. A method of regenerating a filter of exhaust gas for a diesel engine of a vehicle which has become laden with particulates from said exhaust, comprising determining when the pressure of the exhaust gas, measured at a station immediately upstream from said filter, exceeds a predetermined variable maximum allowable pressure, thereby initiating the following series of events:: (a) bypassing said exhaust gas around said filter and conveying a flow of combustion supporting gas to and through said filter at a low flow rate; (b) raising the temperature of said combustion supporting gas to a level effective to ignite at least a leading portion of the particulate collection in the filter; (c) measuring the temperature of the heated combustion supporting gases exiting from the filter, and when the temperature of said heated combustion supporting gases exceeds a predetermined level, shutting off said flow of heated combustion suporting gases conveyed to said filter to bring about termination of the regeneration cycle.

2. The method as described in claim 1, in which said flow of combustion supporting gases is at a rate of 5-7 CFM (cubic feet per minute).

3. The method as described in claim 1, in which said predetermined variable maximum allowable pressure corresponds to the pressure of collected particulates in said filter in an amount of 150-270 mg/in3.

4. The method as in claim 1, in which said predetermined variable maximum allowable pressure is a function of accumulated fuel, mileage of the vehicle, and instantaneous sensed pressure drop across the filter.

5. The method as in claim 1, in which said predetermined maximum allowable pressure is selected to have said filter regenerated at intervals of about 145 miles of travel of the vehicle.

6. The method as in claim 1, in which said pressure of the exhaust gas is sensed and measured by use of a thin diaphragm transducer that reads the variation in capacitance between a fixed element and the diaphragm.

7. The method as in claim 1, in which the temperature of said combustion supporting gas is raised by addsing fuel thereto and igniting the resulting mixture.

8. The method as in claim 7, in which said mixture is ignited by a heated catalyst supporting member effective to suppress the temperature required for ignition of said mixture.

9. The method as in claim 1, in which said combustion supporting gas is compressed air.

10. The method as in claim 1, in which said combustion supporting gas is exhaust gas from said engine.

11. The method as in claim 1, in which said combustion supporting gas is air inducted into said engine to which fuel is added after such gas becomes an exhaust gas for raising the temperature thereof.

12. An apparatus for controlling the regeneration cycle of a filter of exhaust gas for a diesel engine of a vehicle which has become laden with particulates from said exhaust, comprising: (a) means for conveying combustion supporting gas through at least a portion of said filter; (b) means for permitting the flow of either said exhaust gas or said combustion supportng gas to and through said filter, said flow permitting means regulating the flow of said combustion supporting gas through said filter at a low rate; (c) means for heating said combustion supporting gas when flowing to and through said filter to a temperature of at least 1000"F;;and (d) means for sequentially (i) actuating said flow permitting means to initiate bypass of exhaust gas around the filter and flow of combustion supporting gas through the filter in response to a sensed pressure of said exhaust exceeding a predetermined variable maximum allowable pressure, (ii) actuating said heating means to raise the temperature of the combustion supporting gas to ignite said particulates in said filter, and (iii) to shut off said heating means in response to a sensed temperature in the trailing portion of said filter which exceeds a predetermined value indicating regeneration of the filter is complete.

13. The apparatus as in claim 12, in which said flow permitting means comprises solenoid actuated flow control valves effective to switch between the supply of exhaust gas and compressed air to said filter.

14. The apparatus as in claim 12, in which said heating means comprises a burner which adds hydrocarbon fuel to said combustion supporting gas and ignites the mixture.

15. The apparatus as in claim 12, in which said heating means comprises an electrically heated catalyst supporting member.

16. A method of regenerating a filter of exhaust gas for a diesel engine of a vehicle which has become laden with particulates from said exhaust, comprising determining when the pressure of the exhaust gas substantially herein described with reference to and as illustrated by the accompanying drawings.

17. An apparatus for controlling the regeneration cycle of a filter of exhaust gas for a diesel engine of the vehicle which has become laden with particulates from said exhaust substantially herein described with reference to and as illustrated by the accompanying drawings.