Method of operating a diesel engine and diesel engine with checking of NH3 concentration

Abstract

Various embodiments include a method of operating a diesel engine having an exhaust tract and an SCR-combined diesel particulate filter in the exhaust tract, wherein an aqueous urea solution is introduced into the exhaust tract, and an exhaust gas recirculation apparatus having an exhaust gas recirculation conduit branching off downstream of the SCR-combined diesel particulate filter for performing a low-pressure exhaust gas recirculation comprising: measuring an NH.sub.3 concentration in exhaust gas downstream of the SCR-combined diesel particulate filter; and upon exceeding a threshold value of the measured NH.sub.3 concentration, reducing the low-pressure exhaust gas recirculation rate based at least in part on the measured NH.sub.3 concentration.

Claims

1. A method of operating a diesel engine having an exhaust tract and an SCR-combined diesel particulate filter in the exhaust tract an exhaust gas recirculation apparatus having an exhaust gas recirculation conduit branching off downstream of the SCR-combined diesel particulate filter for performing a low-pressure exhaust gas recirculation comprising: measuring an NH3 concentration in exhaust gas downstream of the SCR-combined diesel particulate filter and upstream of the exhaust gas recirculation conduit; upon exceeding a threshold value of the measured NH3 concentration, reducing the low-pressure exhaust gas recirculation rate based at least in part on the measured NH3 concentration; introducing a first urea solution dose into the exhaust tract upstream of the SCR-combined diesel particulate filter; introducing a second urea solution dose into the exhaust tract downstream of the SCR-combined diesel particulate filter and downstream of the exhaust gas recirculation conduit; and using the measured NH3 concentration to decide when and to what extent the delivery of the second urea solution dose is activated.

2. The method as claimed in claim 1, further comprising increasing a high-pressure exhaust gas recirculation rate of the diesel engine.

3. The method as claimed in claim 1, further comprising measuring the NH3 concentration in the exhaust gas using an NH3 sensor.

4. The method as claimed in claim 1, further comprising measuring the NH3 concentration in the exhaust gas by performing storage determination of an SCR catalyst arranged in the exhaust tract downstream of the SCR-combined diesel particulate filter in conjunction with a measurement from an NOx Sensor arranged downstream of the SCR-combined diesel particulate filter.

5. The method as claimed in claim 4, further comprising measuring the NH3 concentration in the exhaust gas by performing a measurement with a second NOx sensor downstream of the SCR-combined diesel particulate filter and downstream of the SCR catalyst.

6. The method as claimed in claim 1, further comprising measuring the NH3 concentration in the exhaust gas during supply of the aqueous urea solution to the SCR-combined diesel particulate filter or to the exhaust tract upstream thereof.

7. The method as claimed in claim 1, further comprising reducing the delivery of the first urea solution dose until the NH3 concentration is below a first threshold value.

8. The method as claimed in claim 1, further comprising increasing the urea dosing amount when the NH3 concentration is below a second threshold value.

9. The method as claimed in claim 1, further comprising introducing the second urea solution dose according to an NOx sensor signal downstream of the SCR-combined diesel particulate filter.

10. A diesel engine comprising: an exhaust tract; an SCR-combined diesel particulate filter arranged in the exhaust tract; and an exhaust gas recirculation apparatus including an exhaust gas recirculation conduit branching off downstream of the SCR-combined diesel particulate filter for performing a low-pressure exhaust gas recirculation; a sensor measuring a NH3 concentration in the exhaust gas downstream of the SCR-combined diesel particulate filter and upstream of the exhaust gas recirculation conduit; a first urea solution injector to dose urea into the exhaust tract upstream of the SCR-combined diesel particulate filter; a second urea solution injector to dose urea into the exhaust tract downstream of the SCR-combined diesel particulate filter and downstream of the exhaust gas recirculation conduit; and a controller programmed to reduce the low-pressure exhaust gas recirculation rate according to the measured NH3 concentration by adjusting a valve; wherein the controller is further programmed to assess the NH3 concentration to decide when and to what extent the second urea solution injector is activated.

11. The diesel engine as claimed in claim 10, wherein the controller is further programmed to correspondingly increase the high-pressure exhaust gas recirculation rate by adjusting a valve.

12. The diesel engine as claimed in claim 10, further comprising an SCR catalyst arranged in the exhaust tract downstream of the SCR-combined diesel particulate filter.

13. The diesel engine as claimed in claim 10, further comprising an NOx-Sensor downstream of the SCR-combined diesel particulate filter and downstream of the SCR catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The teachings herein are hereinbelow elucidated in detail with reference to two exemplary embodiments in conjunction with the drawing. In the figures:

(2) FIG. 1 shows a schematic representation of a diesel engine having an exhaust tract incorporating teachings of the present disclosure; and

(3) FIG. 2 shows a representation of another example embodiments incorporating teachings of the present disclosure.

DETAILED DESCRIPTION

(4) Various methods incorporating teachings of the present disclosure perform additional low-pressure exhaust gas recirculation rate control according to the NH.sub.3 concentration in the exhaust gas. The higher the NH.sub.3 concentration in the exhaust gas stream exiting the SCR-combined diesel particulate filter, the lower the low-pressure exhaust gas recirculation rate. This makes it possible to optimize urea consumption. The negative effects of NH.sub.3 on the exhaust gas recirculation or the air cooler can also be reduced.

(5) In some embodiments, a high-pressure exhaust gas recirculation rate of the diesel engine is correspondingly increased with the reduction in the low-pressure exhaust gas recirculation rate. Measurement of the NH.sub.3 concentration in the exhaust gas may be carried out using an NH.sub.3 sensor. This sensor is arranged between the SCR-combined diesel particulate filter and the branching point of the exhaust gas recirculation conduit.

(6) In some embodiments, when an additional SCR catalyst is used downstream of the SCR-combined diesel particulate filter and downstream of the branching point of the exhaust gas recirculation conduit so that the ammonia proportion in the exhaust gas which does not pass into the exhaust gas recirculation conduit may be converted into nitrogen in the subsequent SCR catalyst. When such an SCR catalyst is provided the NH.sub.3 concentration in the exhaust gas may then be measured by performing a storage determination of this SCR catalyst arranged in the exhaust tract downstream of the SCR-combined diesel particulate filter in conjunction with a measurement from an NO.sub.x sensor arranged downstream of the SCR-combined diesel particulate filter. The storage determination of the SCR catalyst may be performed using an antenna sensor.

(7) In some embodiments, the NH.sub.3 concentration in the exhaust gas may be measured by performing a measurement with NO.sub.x sensors downstream of the SCR-combined diesel particulate filter and downstream of the SCR catalyst. It will be appreciated that it is always measurement of the NH.sub.3 concentration downstream of the SCR-combined diesel particulate filter and upstream of the branching point of the exhaust gas recirculation conduit that is concerned.

(8) The corresponding NH.sub.3 concentration in the exhaust gas may be measured during supply of the aqueous urea solution to the SCR-combined diesel particulate filter or into the exhaust tract upstream thereof. When an additional SCR catalyst is present measurement is preferably also carried out during supply of the urea solution to this SCR catalyst or the exhaust tract upstream thereof.

(9) In some embodiments, delivery of two urea solution dosings upstream and downstream of the SCR-combined diesel particulate filter is concerned here. For an exhaust tract where a first urea solution dose is introduced into the exhaust tract upstream and a second urea solution dose is introduced into the exhaust tract downstream of the SCR-combined diesel particulate filter in this method variant the NH.sub.3 concentration is measured in the above-described fashion and used to decide when and to what extent the delivery of the second urea solution dose is activated. The higher the NH.sub.3 concentration downstream of the SCR-combined diesel particulate filter, the more the urea solution dosing upstream of the SCR-combined diesel particulate filter is reduced until the NH.sub.3 concentration falls below a relatively high threshold value which is dependent on the SDPF temperature and may be 40 ppm for example. When the NH.sub.3 concentration falls below a second lower threshold value which is dependent on the SDPF temperature and may be 10 ppm for example the urea dosing amount is increased with the first urea solution dose.

(10) The second urea solution dose may simultaneously be introduced therewith according to the NO.sub.x sensor signal downstream of the SCR-combined particulate filter. The higher the NO.sub.x signal, the higher the urea dose. The NO.sub.x sensor signal downstream of the SCR catalyst may be used for controlling the additional urea solution dosing for the correct storage of NH.sub.3 in the SCR catalyst.

(11) In some embodiments, a diesel engine has arranged in the exhaust tract of the diesel engine an SCR-combined diesel particulate filter in which an aqueous urea solution is introduced into the exhaust gas stream and an exhaust gas recirculation apparatus having an exhaust gas recirculation conduit branching off downstream of the diesel particulate filter for performing a low-pressure exhaust gas recirculation. The diesel engine is characterized in that it comprises means for measuring the NH.sub.3 concentration in the exhaust gas downstream of the SCR-combined diesel particulate filter and means for reducing the low-pressure exhaust gas recirculation rate according to the measured NH.sub.3 concentration.

(12) In some embodiments, the diesel engine comprises means for correspondingly increasing the high-pressure exhaust gas recirculation rate. The means for measuring the NH.sub.3 concentration in the exhaust gas downstream of the SCR-combined diesel particulate filter may comprise an NH.sub.3 sensor for example.

(13) In some embodiments, an additional SCR catalyst is arranged in the exhaust tract downstream of the SCR-combined diesel particulate filter. The exhaust gas recirculation conduit branches off from the exhaust tract between the SCR-combined diesel particulate filter and the additional SCR catalyst.

(14) In some embodiments, the diesel engine has a first urea solution metering means upstream and a second urea solution metering means downstream of the SCR-combined diesel particulate filter.

(15) In some embodiments, there is an NO.sub.x sensor arranged downstream of the SCR-combined diesel particulate filter and an NO.sub.x sensor arranged downstream of the SCR catalyst.

(16) FIG. 1 shows a schematic representation of a diesel engine 1 having an exhaust tract 2. Those parts of the diesel engine not of interest for the invention described here have not been provided with reference numerals. Arranged furthest upstream in the exhaust tract 2 is a diesel oxidation catalyst 3 which may have a conventional configuration. Arranged downstream of the diesel oxidation catalyst 3 in the exhaust tract is an SCR-combined diesel particulate filter (SDPF) 4, which may be a customary diesel particulate filter having an integrated SCR catalyst. SCR here stands for selective catalytic reduction. Such an SCR-combined diesel particulate filter 4 operates as described at the outset, wherein a supplied aqueous urea solution in the catalyst brings about an extensive conversion of the exhaust gas into CO.sub.2 and NH.sub.3 and the produced ammonia reduces the nitrogen oxides (NO.sub.x) in the exhaust gas to afford nitrogen (N.sub.2) and water.

(17) An SCR catalyst 5 which operates in the same way as described hereinabove is arranged in the exhaust tract downstream of the SCR-combined diesel particulate filter 4. A tank for an aqueous urea solution has the reference numeral 8. Two feeding means 7 run from the tank 8 to the exhaust tract 2, wherein the feeding means 7 comprise dosing means which inject the aqueous urea solution into the exhaust tract upstream of the SCR-combined diesel particulate filter 4 and of the SCR catalyst 5 respectively.

(18) For emissions reduction, the diesel engine 1 further comprises an exhaust gas recirculation apparatus which has an exhaust gas recirculation conduit 11 which branches off from the exhaust tract 2 between the SCR-combined diesel particulate filter 4 and the SCR catalyst 5. The exhaust gas recirculation conduit 11 supplies a portion of the exhaust gas to the intake air of the diesel engine, thus bringing about a corresponding emissions reduction. Further sensors (lambda sensors, No.sub.x sensors, etc.) may be provided in the exhaust tract and shown in FIG. 1.

(19) As mentioned hereinabove NH.sub.3 is converted into N.sub.2 and H.sub.2O in the SCR-combined diesel particulate filter 4. However, this is not necessarily the case in all operating modes and therefore undesired NH.sub.3 can also exit the SCR-combined diesel particulate filter 4. There is therefore a danger that a portion of the NH.sub.3 passing into the exhaust tract downstream of the diesel particulate filter 4 can enter the exhaust gas recirculation conduit 11 and thus be mixed with the intake air of the diesel engine. This is the case in low-pressure exhaust gas recirculation mode.

(20) In order to largely prevent this there is in the exhaust tract 2 downstream of the SCR-combined diesel particulate filter 4 and upstream of the branching point of the exhaust gas recirculation conduit 11 an NH.sub.3 sensor 6 which measures the NH.sub.3 concentration in the exhaust gas downstream of the SCR-combined diesel particulate filter 4. When the measured NH.sub.3 concentration exceeds a threshold value a control means (not shown) provided with the corresponding sensor signal reduces the low-pressure exhaust gas recirculation rate according to the measured NH.sub.3 concentration, for example by gradually closing a valve means (not shown). This prevents NH.sub.3 passing through the SCR-combined diesel particulate filter 4 from being admixed with the recirculated exhaust gas.

(21) Furthermore, the NH.sub.3 concentration measured by the NH.sub.3 sensor 6 is used to decide when and to what extent the delivery of the second urea solution dose is activated. This second urea solution dose is injected into the exhaust gas stream via the second feeding means 7 shown on the right-hand side in FIG. 1 upstream of the SCR catalyst 5. The higher the NH.sub.3 concentration downstream of the SCR-combined diesel particulate filter, the more the urea solution dosing upstream of the SCR-combined diesel particulate filter is reduced until the NH.sub.3 concentration falls below a relatively high threshold value of for example 40 ppm. When NH.sub.3 is then below a second low threshold value of for example 10 ppm the first urea solution feeding means can in turn increase the urea dosing amount.

(22) FIG. 2 shows an embodiment of the teaching herein which the NH.sub.3 concentration is likewise measured downstream of the SCR-combined diesel particulate filter 4. However, concentration measurement is achieved here not by means of an NH.sub.3 sensor but by means of an NO.sub.x sensor 10 downstream of the SCR-combined diesel particulate filter 4 and an NO.sub.x sensor 10 downstream of the SCR catalyst 5. It is further noted that the SCR-combined diesel particulate filter 4 and the SCR catalyst 5 are each provided with an antenna sensor 9.