Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
1998-11-09
2001-04-03
Denion, Thomas (Department: 3745)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S286000, C060S303000
Reexamination Certificate
active
06209313
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention lies in the field of exhaust gas purification. In particular, the invention pertains to a method of reducing the NO
x
content in the exhaust gas of a diesel internal combustion engine. The applicable system includes a catalytic converter for the catalytic after-treatment of the exhaust gas according to the SCR principle. A reducing agent is supplied into the exhaust gas upstream of the catalytic converter. A control unit determines the control parameter of the diesel engine, in particular the start of injection of the fuel, the idling speed, and the respective catalytic-converter efficiency and calculates the desired value for the reducing agent feed in dependence on the catalytic-converter efficiency.
The selective-catalytic-reduction method, or SCR method, is suitably used to reduce the NO
x
content in the exhaust gas of a diesel internal combustion engine. In the process, a reducing agent is injected into the exhaust gas at a point upstream of a catalytic converter. Any NO
x
contained in the exhaust gas is thus reduced to N
2
. Ammonia may be used as the reducing agent. For reasons of ease-of-handling, an aqueous solution of urea is normally used. See, for example, U.S. Pat. No. 5,665,318 (German patent application DE 44 36 415 A).
In the urea SCR system, the desired quantity of the reducing agent metering is continuously calculated by the control unit of the diesel internal combustion engine. To this end, the control unit requires the instantaneous catalytic-converter efficiency. The latter is determined from operating parameters and the catalytic-converter temperature. The catalytic-converter temperature may in turn be determined, for example, by a model computation from the exhaust gas temperature and the exhaust gas mass flow.
The effectiveness and efficiency of the exhaust gas treatment depends on the catalytic-converter temperature.
FIG. 3
shows a typical characteristic of the catalytic-converter efficiency as a function of the catalytic-converter temperature. The catalytic-converter efficiency at which the NO
x
reduction is effected reaches a maximum within the temperature range around 250° C. and drops steeply toward lower temperatures. The catalytic-converter efficiency approaches zero at temperatures below 120° C. If aqueous urea solution is used as the reducing agent, the metering may only be effected if the exhaust gas temperature is high enough for the decomposition of urea for forming the ammonia required for the catalytic conversion. The decomposition of urea is catalytically assisted and starts at temperatures above 130° C. Lower temperatures mean, therefore, that, firstly, no reducing agent can be metered and, secondly, the catalytic converter is ineffective.
The catalytic exhaust gas treatment in a diesel internal combustion engine, specifically for the NO
x
reduction, is ineffective directly after a cold start of the engine. The catalytic reaction does not start until the exhaust gas temperature increases. During coasting operation, the exhaust gas cools down relatively quickly, so that, for instance during the acceleration of a vehicle after downhill sections, the catalytic NO
x
reduction is virtually ineffective, which results in an undesirably high NO
x
emission.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method of reducing the NO
x
content in the exhaust gas of a diesel internal combustion engine, which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which increases the catalytic-converter efficiency in the SCR system more quickly if the catalytic-converter temperature is too low.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of reducing the NO
x
content in the exhaust gas of a diesel internal combustion engine, which comprises:
treating exhaust gas of a diesel internal combustion engine in an SCR catalytic converter;
feeding reducing agent into the exhaust gas upstream of the catalytic converter in a flow direction of the exhaust gas;
determining, with a control unit, control parameters of the diesel internal combustion engine and a respective catalytic-converter efficiency;
calculating a setpoint value for a reducing agent feed in dependence on the catalytic-converter efficiency; and
if the catalytic-converter efficiency drops below a predetermined value, changing the control parameters of the diesel internal combustion engine with the control unit (
1
) as a function of the catalytic-converter efficiency such that a NO
x
content in the exhaust gas upstream of the catalytic converter is reduced and the exhaust gas temperature is increased.
In accordance with an added feature of the invention, the control parameters include a start of fuel injection into the diesel internal combustion engine and an idling speed of the engine.
In accordance with an additional feature of the invention, if the catalytic-converter efficiency drops below the predetermined value, an injection instant of the fuel is delayed. In the alternative, or in addition, if the catalytic-converter efficiency drops below the predetermined value, an idling speed of the diesel internal combustion engine is increased.
In accordance with another feature of the invention, which is applicable to a diesel engine that is operated with cutoff of the fuel feed during a coasting operation, the coasting cutoff is deactivated if the catalytic-converter efficiency drops below the predetermined value.
In accordance with a further feature of the invention, which is applicable to a diesel engine equipped with exhaust gas recirculation, an exhaust gas recirculation rate is increased if the catalytic-converter efficiency drops below the predetermined value.
In accordance with again a further feature of the invention, the catalytic-converter efficiency is determined from control parameters of the diesel internal combustion engine, including the exhaust gas mass flow and a measured exhaust gas temperature.
In accordance with a concomitant feature of the invention, the exhaust gas mass flow is calculated by a model computation incorporating an engine speed and an intake pressure or a boost pressure.
In other words, the catalytic-converter efficiency is calculated from engine data, such as air mass, operating temperature or load. In selective catalytic reduction, these calculations are necessary for adjusting the reducing agent feed. With due regard to the catalytic-converter efficiency, by intervention in the control parameters, in particular the start of injection, injection characteristic, exhaust gas recirculation rate, or valve control, the NO
x
emission can be reduced at the expense of fuel consumption minimization. By the intervention in the control parameters, the NO
x
content in the untreated exhaust gas is reduced on the one hand and, on the other hand, due to the reduction in the efficiency of the diesel internal combustion engine, the exhaust gas temperature increases more quickly, which in turn increases the catalytic-converter efficiency. The lower efficiency in a cold internal combustion engine can be accepted, since measures for quick heating of the internal combustion engine are required anyway for reasons of comfort.
In the context of spark-ignition engines, it has become known from an article “Assessment of Stoichiometric GDI Engine Technology” in AVL Conference Engine and Environment, 1997, p. 106-07 to delay the injection instant at the expense of extra consumption of fuel in order to reduce the nitrogen-oxide emissions.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of reducing the NO
x
content in the exhaust gas of a diesel internal combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from t
Hofmann Lothar
Pajonk Gunther
Weigl Manfred
Wissler Gerhard
Denion Thomas
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
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