JP2001020724A - NOx PURIFICATION SYSTEM FOR DIESEL ENGINE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an adequate amount of urea so as to prevent incomplete NOx purification due to insufficient urea as well as emission of ammonia due to excessive urea and the like. SOLUTION: This purification system has a NOx catalyst 3 provided in an exhaust system of an engine 1, and a urea supplying device 4 which is disposed upstream of the NOx catalyst and supplies aqueous solution of urea as a reducer. This system is also provided with a urea concentration detection means for detecting the concentration of the urea in the aqueous solution of urea, and determines the amount of the aqueous solution of urea to be supplied based on a detected value. Since the solution supply amount is determined according to the actual urea concentration, the required amount of urea can be supplied adequately. The urea concentration detection means preferably consists of a refractive index sensor 12 for detecting a refractive index of the aqueous solution of urea, a temperature sensor 13 for detecting the temperature of the urea solution, and a calculation means 10 for calculating the urea concentration based on detected values of the two sensors.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a NOx purifying apparatus for a diesel engine which purifies NOx with a NOx catalyst while supplying a reducing agent.

[0002]

2. Description of the Related Art In recent years, in the field of diesel engines, a zeolite-based NOx catalyst has been
Equipment for purifying wastewater is attracting attention (SAE Paper 97
0185). As the reducing agent, there are ammonia (NH ₃ ) and urea ((NH ₂ ) ₂ CO), but ammonia is a dangerous substance and its handling is often restricted. Therefore, non-toxic urea is used, and by using this, application to a vehicle engine or the like becomes easy.

[0003]

By the way, urea is stored in the urea tank as an aqueous solution, but the urea concentration may vary each time the tank is refilled. It is also known that when the urea tank is opened, moisture evaporates and the urea concentration changes.

The amount of urea required for purifying NOx in exhaust gas can be determined from engine speed, load, exhaust gas temperature and the like. Therefore, it is sufficient to supply the urea aqueous solution in an amount corresponding to the required urea amount. However, when the urea concentration changes, the actual urea amount does not reach a desired value even if a predetermined amount of the urea aqueous solution is supplied.
NOx purification processing cannot be performed suitably. If the urea concentration is lower than the standard value, the urea amount becomes insufficient and the NOx purification becomes incomplete. Conversely, if the urea concentration is higher than the standard value, the NOx purification can be completed but the urea amount becomes excessive, and the ammonia generated by the decomposition of the urea becomes excessive. Leaks from the tail pipe to the outside, causing irritating odor and the like.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a NOx purification device for a diesel engine provided with a NOx catalyst in an exhaust system of the engine and a urea supply device for supplying an aqueous urea solution upstream of the NOx catalyst. Urea concentration detecting means for detecting the urea concentration of the aqueous urea solution is provided, and the supply amount of the aqueous urea solution is determined based on the detected value.

According to this, since the supply amount of the urea aqueous solution can be determined according to the actual urea concentration, the required amount of urea can be supplied without excess or shortage, and the above problem can be solved.

Here, the urea concentration detecting means includes a refractive index sensor for detecting the refractive index of the aqueous urea solution, a temperature sensor for detecting the temperature of the aqueous urea solution, and an arithmetic operation for calculating the urea concentration based on these two detected values. Preferably, it comprises means.

[0008]

Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 shows a NOx purifying apparatus for a diesel engine according to the present invention. The exhaust system of the diesel engine 1 is composed of an exhaust pipe 2, and the exhaust pipe 2 is branched into two branches in the middle, and a NOx catalyst 3 is provided in each of the two branches. As these NOx catalysts 3, zeolite-based ones are used.

A urea supply device 4 for supplying an aqueous urea solution is provided upstream of the NOx catalyst 3. The urea supply device 4 includes a urea tank 5 for storing the urea aqueous solution, a feed pump 6 for sucking and discharging the urea aqueous solution in the urea tank 5, and an electromagnetic valve for selectively discharging the urea aqueous solution sent from the feed pump 6. And a urea injection nozzle provided in the collecting section 2a of the exhaust pipe 2 located on the upstream side of the branch section, for supplying the aqueous urea solution sent from the supply controlling apparatus 7 into the collecting section 2a. 8 and a compressed air supply device 9 for supplying compressed air to the urea injection nozzle 8. In the urea injection nozzle 8, an aqueous urea solution is ejected by an ejector effect by compressed air.

An electronic control unit (ECU) 10 is provided as control means for controlling the urea supply amount. This is common to engine control. ECU1
To 0, various signals indicating an engine operation state such as an engine rotation and an engine load are transmitted from a rotation sensor, a load sensor, and the like, not shown. A catalyst temperature sensor 11 for detecting the catalyst temperature of the NOx catalyst 3 is provided.
Sent to U10.

In particular, the present apparatus is provided with a urea concentration detecting means for detecting the urea concentration of the aqueous urea solution. The urea concentration detecting means includes a refractive index sensor 1 for detecting a refractive index of the aqueous urea solution.
2 and a temperature sensor 13 for detecting the temperature of the aqueous urea solution. The ECU 10 calculates the urea concentration based on these detected values, and determines the supply amount of the aqueous urea solution required for NOx purification. As described above, the ECU 10 forms the calculating means of the present invention.

Hereinafter, a method of purifying NOx by the present apparatus will be described. First, the required amount of urea is determined based on the following reaction formula.

(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂ 4NH ₃ + 4NO + O ₂ → 4N ₂ + 6H ₂ O 4NH ₃ + 2NO ₂ + O ₂ → 3N ₂ + 6H ₂ O Thus, urea (NH ₂ ) ₂ CO is decomposed to become ammonia NH _3, this ammonia NH ₃ results in substantial reduction action.

The amount of ammonia corresponding to the current amount of exhausted NOx is determined from the graph of FIG. That is, the required urea amount is determined from this graph and the above reaction formula. However, since the discharged NOx amount is indirectly obtained from the engine speed, the load, and the like, the ECU 10 actually determines the necessary urea amount based on the rotation, the load, and the like.

Next, the ECU 10 calculates the urea concentration of the aqueous urea solution according to the map shown in FIG. This map is E
The urea concentration is stored in the CU 10 and can be uniquely calculated from the refractive index and the temperature of the aqueous urea solution. According to this, the urea concentration tends to increase as the temperature of the aqueous solution increases, and to increase as the refractive index increases.

After the required urea amount and the current urea concentration are obtained in this way, the ECU 10 calculates the required urea aqueous solution amount from these values. Then, a control signal is sent to the supply control device 7, the control signal is appropriately switched, and an amount of the urea aqueous solution equal to the required amount of the aqueous solution is injected from the urea injection nozzle 8.

As described above, the present apparatus detects the urea concentration of the aqueous urea solution in advance, and determines the supply amount of the aqueous urea solution based on the detected urea concentration. The amount of the urea aqueous solution can be calculated without excess or deficiency. As a result, urea can be supplied without excess or shortage, and NO
x It is possible to eliminate problems such as incomplete purification and ammonia discharge due to excessive urea. Further, the target NOx purification rate can always be secured.

As described above, the embodiments of the present invention are not limited to those described above. In the present embodiment, the urea concentration is calculated from the relationship between the refractive index of the aqueous urea solution and the temperature according to the map in FIG. 3, but the urea concentration is calculated from the relationship between the temperature and the specific gravity of the aqueous urea solution according to the map in FIG. You may make it. According to this, the urea concentration tends to increase as the temperature of the aqueous solution increases, and to increase as the specific gravity increases.

Alternatively, although not shown, the concentration may be calculated from the relationship between the temperature and the viscosity of the aqueous urea solution.
The concentration may be calculated by measuring the water content of the aqueous urea solution.

[0021]

According to the present invention, an excellent effect is achieved in that the supply of urea can be carried out without excess or deficiency, and incomplete purification of NOx due to insufficient urea and ammonia discharge due to excessive urea can be prevented.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a NOx purification device according to an embodiment.

FIG. 2 is a graph showing a relationship between an exhausted NOx amount and a necessary ammonia amount.

FIG. 3 is a urea concentration calculation map of an aqueous urea solution.

FIG. 4 is another urea concentration calculation map of the urea aqueous solution.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diesel engine 3 NOx catalyst 4 Urea supply device 10 Electronic control unit 12 Refractive index sensor 13 Temperature sensor

Claims (2)

[Claims] An NOx catalyst and an Nx catalyst are provided in an exhaust system of an engine.
In a NOx purification device for a diesel engine provided with a urea supply device for supplying a urea aqueous solution upstream of an Ox catalyst, a urea concentration detecting means for detecting a urea concentration of the urea aqueous solution is provided, and the urea aqueous solution is provided based on the detected value. A NOx purification device for a diesel engine, characterized in that a supply amount of NOx is determined. 2. A urea concentration detecting means for detecting a refractive index of a urea aqueous solution, a temperature sensor for detecting a temperature of the urea aqueous solution, and a calculating means for calculating a urea concentration based on both detected values. The NOx purification device for a diesel engine according to claim 1, comprising: