Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
1999-04-23
2001-02-13
Walberg, Teresa (Department: 3742)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S286000, C060S288000, C060S298000
Reexamination Certificate
active
06185935
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for controlling the temperature of an exhaust gas treatment system in the exhaust system of an internal combustion engine in the region of a predetermined intended temperature, comprising a device for varying the temperature of the exhaust gases flowing through the exhaust gas treatment system, by means of which the exhaust gas treatment system is supplied with colder or hotter exhaust gases in dependence on a control signal of an engine control unit, and also comprising a device for determining the actual temperature of the exhaust gas treatment system and a device for determination of a quantity which approximately represents a value for the input of energy into the exhaust gas treatment system, and to an apparatus for putting the method into practice.
BACKGROUND AND PRIOR ART
This kind of temperature control is necessary in particular in the case of nitrogen oxide traps (NO
x
traps) such as are employed for exhaust gas purification in engines operating with a weak mixture (lean bum engines). While conventional three-way catalytic converters produce satisfactory conversion results over a relatively wide range of temperatures, and in the last few years it has been possible to reduce their sensitivity to overheating, known nitrogen oxide traps only operate in a satisfactory manner in a relatively restricted range of temperatures. At lower temperatures the nitrogen oxide trap does not absorb the pollutants efficiently enough. At too high temperatures the absorbed pollutants are partly set free again, thus impairing the purification efficiency. At still higher temperatures the nitrogen oxide trap can suffer accelerated ageing or even be permanently damaged. In view of the fact that under different engine conditions exhaust gas temperatures can vary in a range from 200 to 1000° C., control of the exhaust gases flowing through the exhaust gas treatment system is necessary.
In German patent application 197 03 295.8, which was not published at the date of the present application, a method for controlling the temperature of a catalytic converter having a nitrogen trap is proposed in which the temperature of the nitrogen oxide trap is measured and the nitrogen oxide trap is operated in the weak mixture mode in a temperature range corresponding to the optimum efficiency of the nitrogen oxide trap by a device for varying the temperature of the exhaust gas. The device for varying the exhaust gas temperature can be an exhaust gas line having two pathways for the gas which have different heat losses from the exhaust gas and can be selected by means of a valve controlled by an actuating element. To perform the control it is proposed to employ a PI control element.
It has been found that the temperature sensor in the exhaust gas path should be located not before but preferably after at least one element of the nitrogen oxide trap. This arrangement has the particular advantage that if something goes wrong a threat of overheating can be investigated and prevented. Thus if ambient air penetrates into the exhaust system through a leak in the exhaust gas path before the nitrogen oxide trap, then in the case of engine operation with an enriched mixture (e.g. at full load) exothermal reactions occur with unburned components of the fuel, which can lead to overheating of the substrate in the nitrogen oxide trap. If the temperature sensor is located before the nitrogen oxide trap, the risk of such overheating cannot be detected.
However, locating the sensor after at least one element of the nitrogen oxide trap (e.g. between two elements [so-called “bricks”]), has the disadvantage that owing to the thermal inertia of these elements the control exhibits very long time lags. Thus a change in the exhaust gas temperature only leads to a corresponding change in the actual temperature measured at the temperature sensor after a delay of some 10 seconds. Such systems can only be controlled with difficulty by conventional PI controls, since unacceptable control oscillations occur. To handle such systems with long time lags so-called time lag controls have been proposed. However, in the case of time lag controls it is necessary to have a mathematical model of the system to be controlled and an advance estimate of the lag. Under unusual operating conditions, or if a fault develops, these models can fail, so that uncontrollable control response can occur.
OBJECT OF THE INVENTION
The object of the invention is to provide a process of the kind referred to above in which, by simple means, stable temperature control can be obtained under all operating conditions irrespective of the long time lags.
SUMMARY OF THE INVENTION
To achieve this object it is proposed that the actual temperature is compared with the intended temperature in a two-step control element which produces an output signal indicating cooling or heating of the exhaust gas, and that from this output signal a modified output signal is produced and supplied to the varying device, such that in case of an output signal which indicates heating of the exhaust gas, if the energy input value is increasing the signal is modified in a direction effecting cooling of the exhaust gas and/or that in the case of an output signal which indicates cooling of the exhaust gas, if the energy input value is decreasing this signal is modified in a direction effecting heating of the exhaust gas. As a result of the output signal from the two-step control element member being modified on the basis of the energy input value, there is a limitation of the control output, thus avoiding overshooting of the temperature of the exhaust gas treatment system. Preferably a control output limitation is provided both in the case of an output signal indicating exhaust gas heating and of one indicating exhaust gas cooling. However, a control output limitation may also be provided in only one of these two cases.
In an advantageous embodiment of the invention, it can be provided that the modification takes place by modulation of the output signal by means of a squarewave signal the duty factor of which, in the case of an output signal indicating heating of the exhaust gas, is varied (starting from a duty factor indicating a steady heating) in the direction of an increasing cooling of the exhaust gas when a predetermined engine operating condition is exceeded and the energy input value is increasing, and/or the duty factor of which, in the case of an output signal indicating cooling of the exhaust gas, is varied (starting from a pulse control factor indicating a steady cooling) in the direction of increased heating of the exhaust gas if a predetermined engine operating condition is fallen below and the energy input value is decreasing. The square-wave signal preferably has a period (e.g. 10 seconds) such that the control actuator can simply follow it mechanically. The oscillations of the exhaust gas temperature caused by the modulation are eliminated by the thermal inertia of the exhaust gas treatment system.
It can further be provided that the determination of the energy input value takes place at least using the mass air flow in the intake system of the internal combustion engine, which approximately represents the current mass flow of exhaust gas. The mass flow of exhaust gas is an approximate measure of the heat energy supplied to the converter per unit of time. The greater this energy input, the more quickly does the material of the nitrogen oxide trap adapt to the exhaust gas temperature and the more severely must the control output limitation be curtailed. However, further engine operating parameters can also be taken into account in the limitation of the control output: for example, the vehicle speed, which has a considerable effect on the heat losses through the airflow at the nitrogen oxide trap.
It may further advantageously be provided that the duty factor is determined using a functional relationship, in dependence on the energy input value and the output signal of the two-step control element, and using the intended
Dickers Guido
Grieser Klemens
Ford Motor Company
Fuqua Shawntina
Lippa Allan J.
Walberg Teresa
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