DE3721572A1 - Process for open-loop and closed-loop control of a catalyst - Google Patents

Abstract

Process for metering the NH3 feed to a selective catalyst (1) for reduction of the nitrogen oxides (NOx) contained in the untreated emission of a two-stroke gas engine (M). In the process according to the invention at least 75 % of the NH3 requirement calculated from the power (N) and speed of rotation (n) of the engine (M) is fed to the catalyst (1), as a result of which rapid open-loop control of the NH3 feed is possible. The NH3 feed still lacking to make up the correct NH3 preset value is carried out by closed-loop control, the NOx concentration being measured in the exhaust gases and the NH3 feed being controlled in dependence on the NOx concentration determined. <IMAGE>

Description

The invention relates to a method for metering the ammonia (NH ₃ ) supply into the exhaust line of an internal combustion engine (engine), in particular a two-stroke gas engine, before or in a catalyst for the selective catalytic reduction of nitrogen oxides (NO _x ) in the exhaust gases of the engine, the NO _x concentration in the exhaust gases being measured and being fed to the catalyst NH _{3 as a} function of the NO _x concentration found.

To reduce the nitrogen oxides contained in the raw emissions of internal combustion engines, in particular of stationary two-stroke gas engines, such as those used in compressor systems, so-called SCR catalysts (Selectiv Catalytic Reaction) are used. In such catalysts (e.g. TiO ₂ catalyst, zeolite catalysts or catalysts based on cordierite), ammonia (NH ₃ ) is injected into the exhaust line before or in the catalyst and reacts in the catalyst with the nitrogen oxides on catalytically active Areas largely to harmless connections.

During operation, the amount of NH ₃ injected into the catalytic converter must always be in a certain ratio to the nitrogen oxides contained in the exhaust gases, in order not to exceed the officially prescribed NO _x values after the SCR catalytic converter and on the other hand to prevent NH ₃ slip . NH ₃ slip means the NH ₃ concentration that is still present in the exhaust gases after the SCR catalytic converter. This NH ₃ slip, which is particularly undesirable because of the toxicity of NH ₃ and its pungent smell, always occurs to an unpleasant extent when too much NH _{3 is} injected in relation to the NO _x .

A previously known method consists in regulating the NH ₃ supply into the catalytic converter, the NO _x concentration being measured after the catalytic converter and being fed to the catalytic converter NH _{3 as a} function of the NO _x concentration found. In the case of non-steady-state operating mode, in which the engine's output and speed, and thus the NO _x concentration in the raw emissions of the engine, change rapidly, due to the control time, which is typically on the order of one minute (from sampling to photometric analysis to Correction of the injected NH ₃ value) NO _x and NH ₃ emission peaks, the increased NH ₃ concentration being particularly troublesome due to the unpleasant pungent odor mentioned. This increased NH ₃ concentration or NH ₃ slip occurs in the case of a rapid power reduction, as occurs in practice, for example, when the load changes suddenly, because there it is in relation for about a minute (until the control system stops reducing the NH ₃ supply) too much NH _{3 is} added to the NO _x concentration which has decreased due to the power reduction.

In addition, in the known control in the event of loss of activity of the catalyst, the NH ₃ supply is simply increased in order to keep the NO ₃ concentrations still below a predetermined limit value. However, this results in an inadmissibly high NH ₃ slip. The possibility of building up the control based on the measurement of the NH ₃ slip is ruled out in practice, since sulfur usually contained in the fuel for the engine reacts with NH ₃ immediately with ammonium sulfate and thus falsifies the NH ₃ value, at least with fuels that are not absolutely sulfur-free .

The object of the invention is to provide a method for metering the NH ₃ supply into a catalytic converter, by means of which a specific NO _x concentration and in particular a specific NH ₃ slip in the case of rapid changes in power and speed of the engine Exhaust gases emerging from the catalyst must not be exceeded. In addition, the risk of an inadmissibly increased NH ₃ slip in the event of loss of activity of the catalyst is to be avoided.

This is achieved according to the invention in that an at least 75%, preferably at least 85% of the NH ₃ supply determined and preferably calculated from the power and speed of the engine, preferably calculated, of the NH ₃ supply is independent of the measured via the NO _x concentration in the Exhaust gas control is fed to the catalytic converter and the remaining part of the NH ₃ supply is controlled as a function of the NO _x concentration measured in the exhaust gases.

The determination of the NH ₃ requirement from power (fuel supply rate) and speed of the engine is known per se (US Pat. No. 44 03 473). In the known system, however, the entire NH ₃ supply is controlled, ie given exclusively by the NH ₃ requirement calculated from the engine data. A control over the actual NO _x concentration in the exhaust gases is completely absent in the known system, so that due to parameters that are difficult to ascertain, such as the temperature and aging condition of the catalyst or fluctuations in the raw emission, which also determine the true NH ₃ requirement, a desired one exact compliance with the NO ₃ starting concentration cannot be achieved.

The inventive method, the NH ₃ supply is largely (at least 75%) dependent on performance and speed, for example according to a map determined in preliminary tests for a certain engine type, ge controls, while by regulating the remaining part of the NH ₃ supply total NH ₃ supply is regulated from the actually required NH ₃ setpoint. The stated NH ₃ setpoint represents the power and speed-dependent NH ₃ supply at which the raw emission in the catalytic converter always drops to a (predetermined NO _x initial concentration, which will generally be just below the prescribed maximum NO _x concentration, on the one hand to save the relatively expensive NH ₃ and on the other hand to reduce the risk of a larger NH ₃ slip from the outset.

The NH ₃ supply, which is independent of the regulation of the NO _x concentration in the exhaust gas, does not necessarily mean a spatially separate NH ₃ supply (approx. 85% controlled, approx. 15% regulated) in the catalytic converter, although this is also possible appears. Rather, it is contemplated to provide a single spatial NH ₃ feed into the catalytic converter and then to apply a signal to the control input of this metering device, which signal is composed of a controlled partial signal that is at least 75% of the determined NH ₃ -Requires, and additively a regulated partial signal, which corresponds to the NH ₃ supply missing to the NH ₃ setpoint.

It is essential in the process according to the invention that the NH ₃ supply is largely controlled independently of the NO _x concentration, which is an advantageous matter of seconds (in contrast to the approximately one-minute adjustment in the sole regulation of the NO _x concentration in the exhaust gas) It is possible to adapt the NH ₃ supply to a NO _x concentration which has changed due to a change in the engine's output and speed. The fact that the regulated, much smaller proportion of the NH ₃ supply (≲ 25% of the total NH ₃ supply) lags behind for about 1 minute with such a rapid adjustment is hardly a disadvantage. The NH ₃ supply is reduced when the engine power is reduced the temporal lag of the regulated portion of the NH ₃ supply briefly slightly above the NH ₃ setpoint. However, because of the only slight increase compared to the NH ₃ setpoint, this only results in a somewhat lower decrease in the NO _x concentration and no noticeable increase in the NH ₃ slip. When the engine power increases, the NH ₃ supply is just below the NH ₃ setpoint; however, this is also only minor, since only the small, regulated portion of the NH ₃ supply is somewhat too small. The NH ₃ buffer effect, which is usually used for catalyst types, also has a compensating effect here, so that an increase in the NO _x concentration above the permitted value does not occur under any circumstances.

The small proportion of the NH ₃ supply, which is regulated as a function of the measured NO _x concentration, thus does not constitute a disadvantage with regard to the risk of NH ₃ or NO _x emission peaks when there are changes in power, but does offer the essential advantages that the possibility exists , in spite of cross-sensitivities, such as the current temperature and aging status of the catalytic converter or fluctuations in the raw emission, which are not easily ascertainable, to fully exploit the conversion rate of the catalytic converter and, at least in stationary partial operating phases, the specified NO ₃ concentration, which is usually just below the official limit values to reach.

In the case of large stationary engines, in which the method according to the invention is advantageously used, measured instantaneous engine data are usually available, from which the instantaneous power and speed of the engine can easily be determined. Then, according to a preferred feature of the invention, it is advantageous if the NH ₃ requirement is at least temporarily determined essentially from measured instantaneous engine data (power, speed) of the engine. Due to the control, more than 75% of the NH ₃ supply always matches the current NH ₃ setpoint from the start. The remaining portion of the supply of NH ₃ is set to the exact _NOx - concentration corresponding NH ₃ -Sollwert readjusted.

Even without measuring the motor data (power and speed), the method according to the invention can be used. For this purpose, a further preferred embodiment provides that the NH ₃ requirement is determined at least temporarily from engine control data (power, speed) delivered to the engine via an engine control device. Of course, it is also possible to measure the speed, for example, which is easy, and to derive the power from the engine control data. On the basis of the engine control data instead of the measured actual engine data, it must of course be ensured that the engine control data are at least approximately a measure of the engines which are then set.

Sets to the determination of the NH ₃ -Bedarfes the current engine power and speed as a basis, occurs during rapid lowering of engine performance - as mentioned above - for a short time at a slightly higher than the NH ₃ -Sollwert lying NH ₃ feed. At the currently very high limit value for the NO _x concentration, below which the vehicle is barely driven, the catalyst is not loaded to the end of its conversion rate and the excess of NH ₃ is used essentially only for the NO _x concentration lower further. If the concentration rate of the catalytic converter has to be fully exhausted by future strict exhaust gas standards even in stationary operating phases, or if you absolutely want to rule out any increase in NH ₃ slip, then it can advantageously be provided according to the invention that the NH ₃ requirement already has a preferably adjustable first time period before the submission of engine control data for reducing the engine power is determined essentially in accordance with the intended reduced engine power, and that during a second time period which extends from the beginning of the first time period at least until the setting of the new engine power, 100% of the determined NH ₃ - If necessary, the catalytic converter is supplied and the control that is running via the measured NO _x concentration is invalid. In this variant of the method, the NH ₃ supply is reduced prematurely, which absolutely rules out an increase in NH ₃ slip. Due to the NH ₃ buffering effect of the catalytic converter, which lasts only a short time, an increase in the NO _x concentration above the permissible maximum value is avoided during the switched-off control. It is necessary to switch off the control, since this would otherwise attempt to correct the NH ₃ supply value that was deliberately set too early to the new engine output if the NO _x concentration is still high in accordance with the old engine output.

Conveniently, in connection with such precontrol of the NH ₃ value in the case of a power reduction, it can be provided that the NH ₃ requirement during the second time period in which the regulation is still ineffective again when or after setting the new engine power measured actual engine data (power, speed) is determined. According to this variant of the method according to the invention, the instantaneous power value is practically always used to determine the NH ₃ supply. The feedforward control described above only comes into effect in the case of power reductions, the instantaneous power value for determining the NH ₃ supply being used again immediately after reaching the new low power and the regulation of the remaining portion of the NH ₃ supply coming into effect again.

In order to prevent an increased NH ₃ slip from occurring due to loss of activity of the catalyst, a preferred feature of the invention provides that for a total NH ₃ supply which is well above the characteristic control times of the measured NO _x concentration current period of time is over 100%, preferably over 105% of the NH ₃ requirement determined from the power and speed of the engine, an alarm signal is triggered and / or the engine is switched off.

The method according to the invention is described below by a Embodiment based on a device guidance of the process through the drawings explained. It shows

Fig. 1 is a schematic block diagram of the device for carrying out the invention Ver driving,

Figs. 2 and 3 are each a diagram showing the time course of the engine output of the controlled NH _{x 3} -Anteiles the supply of NH _3, the regulated portion of the supply of NH ₃ or of the entire supply of NH _3, the NO concentration and the NH ₃ slip after the catalytic converter when the engine power is reduced, and

FIGS. 4 and 5, the same graphs as in Figs. 2 and 3, however, when increasing the engine power.

In Fig. 1, a device for performing the method according to the invention is shown schematically. The entire system essentially consists of a supercharged two-stroke gas engine M , the harmful substances contained in the raw emission react largely to harmless compounds in a SCR catalytic converter 1 that acts selectively on NO _x and an oxidation catalytic converter 2 , an NH ₃ metering device 3 , which doses the NH ₃ supply (NH ₃ injection) into the exhaust line, a control device 4 which determines the NO _x concentration behind the SCR catalytic converter 1 and, depending on this NO _x concentration via the control line 5, the NH ₃ - Supply controls, and a control device 6 , which calculates the NH ₃ requirement from power N and speed n of the motor M according to a map and independently of the control device 4 via the control line 7 causes a supply of about 85% of the NH ₃ requirement .

The essence of the method according to the invention is that at least 75%, preferably 85%, of the NH ₃ requirement determined from the power N and speed n of the engine M are fed to the SCR catalytic converter 1 independently of the regulation via the NO _x concentration. (= faster control), while only the remaining NH ₃ supply 1 <25%) to the NH ₃ setpoint at which the NO _x concentration contained in the raw emission in the SCR catalytic converter is caused by reaction with the supplied NH ₃ a required NO _x output concentration drops, is regulated (= slower regulation). Thus in turn the NH ₃ feed can be even with rapid changes in power of the motor M always exactly on the corresponding NH ₃ -Sollwert meter, which both _NOx - be avoided and NH ₃ -Emissionspeaks. On the other hand, despite difficult to detect cross-sensitivities such as temperature, aging of the SCR catalytic converter 1 or fluctuations in the raw emission, the fine control always enables such an NH ₃ supply which always leads the NO _x concentration exactly to a predetermined NO _x concentration.

The system shown works as follows:

In steady-state operation, the engine control device 8 defines a constant power N and speed n of the engine M charged via the exhaust gas turbocharger 10 via the engine control line 9 . The measured, current engine data (power N , speed n) are supplied to the demand computer 12 of the control device 6 via a measuring line 11 . This calculates an output signal from the measured engine data, which is fed via the switch S and the control line 7 to the NH ₃ metering device and via which an NH ₃ supply of 85% of the NH ₃ requirement calculated by the demand computer acts.

At the same time, the NO _x measuring unit 13 of the control device 4 continuously measures the NO _x concentration. In the actual control unit 14 , the actual value of the NO _x concentration is compared with the preset nominal value of the NO _x concentration and, depending on the comparison result, the NH ₃ supply via the control line 5 and the NH ₃ metering device 3 is corrected, which ultimately corrected the NO _x concentration. The addition of the control signals on the control line 7 and the control signals on the control line 5 takes place in point 15 .

The structure of the NH ₃ metering device 3 is shown only in a highly schematic manner and is briefly described. All that is essential is the task of the NH ₃ metering device, namely to effect a specific NH ₃ supply via the injection line 18 as a function of the signal present on the input line 16 . For this purpose, an electrically actuable valve 19 is provided, which ultimately metered the NH ₃ supply from the NH ₃ storage and delivery unit 20 . A control device 21 receives, apart from the input signal via a flow meter 22 to the actual value of the supply of NH ₃ and controls these via the valve 19 to the input signal corresponding value.

In the event of an intended reduction in the power of the motor M , the pilot control unit 23 takes over the control of the NH ₃ supply instead of the demand computer 12 by the switch S shown schematically being flipped into the position not shown in FIG. 1. In practice, the switch S will be implemented electronically.

As is apparent from Fig. 2, the Vorsteuerein unit 23 reduces the controlled portion of the NH ₃ supply (or the signal on the control line 7 ) for a period of time t ₁ (t ₁≈20 sec) before the real reduction in power N. to a value that corresponds to the intended new performance. To prevent the control from trying to readjust, the control line 5 is switched off via the on / off switch 24 for a period of time t ₂ (≈60-80 sec), with which the pilot control device determines 100% of the NH ₃ supply. This pilot control prevents an increased NH ₃ slip in any case (see FIG. 3 below), since the NH ₃ supply is actually too low at the moment. This causes a slight increase in the NO _x concentration during the time period t ₁ , which, however, as can be seen from FIG. 3, never reaches the NO _x limit value. Thereupon the engine power N and with it the NO _x concentration really decreases within t ₃ (t ₃ ≈12 sec). After lowering the performance of the required computer 12 again takes over the control of the supply of NH _3, the NH ₃ feed causes 85% of the corresponding to the new power N NH ₃ -Bedarfs. Since the control is still switched off, the NO _x concentration increases slightly until the control is switched on. The control device is then switched on again, which corrects the NH ₃ supply in such a way that the NO _x concentration drops to approximately 10% below the NO _x limit value. During the power reduction, the total NH ₃ slip never increased and the NO _x concentration remained below the limit. After reducing the power, the NO _x concentration behind the SCR catalytic converter is the same as before, and the NH ₃ supply is in any case lower due to the lower NO _x raw emissions.

When there is an increase in performance, the needs calculator always has12  control. Here would be a previous increase in NH -Usage of course undesirable. The regulation lags about a minute after they respond, which results in that the NH_3rd-Delivery when performance increases (cf.Fig. 4) during the periodt _4th (t _4th≈24 sec) lags a bit, which is a slight increase in NO_xConcentration like this inFig. 5 is shown, can cause. As a result of that but 85% of the NH_3rd-Delivery controlled with performance and only about 15% regulated is the extent of the lag the NH_3rd-Delivery low due to the inertia of the regulation and thus the increase in NO_xConcentration also low, the NH_3rd-Buffering effect of the catalyst here also has a balancing effect.

To the curves in FIGS. 2 to 5 would be noted that these are highly schematic courses that can vary considerably in practice. It is important that both with the increase ( FIGS. 4 and 5) and with the reduction ( FIGS. 2 and 3) of the power N the NO _x concentration never exceeds the NO _x limit value and the disturbing NH ₃ slip tends to decrease than rising.

The SCR catalytic converter 1 does not have to be arranged behind the turbocharger 10 and the oxidation catalytic converter 2 , which serves to oxidize non-methane hydrocarbons and CO. Since the oxidation catalyst 2 has a higher efficiency at high temperatures, it is, however, favorable to arrange this catalyst close to the engine M. If the SCR catalytic converter were arranged in front of the turbocharger 10 , an oxidation catalytic converter would have to be installed between the SCR catalytic converter 1 and the turbocharger 10 due to the highly corrosive effect of NO ₃ .

The NH ₃ requirement is determined in the demand calculator by ongoing calculation from the measured engine data. The calculation can be carried out analog or digital. The pilot control unit can also calculate the NH ₃ requirement. But it is also possible that the pilot unit, which is only in power reduction in force, has a simpler structure and can only deliver a few discrete signal values to the NH ₃ metering device, these signal values then only approximately corresponding to the NH ₃ supply is part of the actual performance. However, the essence of the pilot control unit is not that the preset NH ₃ value corresponds exactly to the reduced power, but rather the fact that the NH ₃ supply is reduced before the real power reduction.

Claims (6)

1. Method for metering the ammonia (NH ₃ ) feed into the exhaust line of an internal combustion engine (engine), in particular a two-stroke gas engine, before or in the case of a catalyst for the selective catalytic reduction of nitrogen oxides (NO _x ) in the exhaust gases of the engine , the NO _x concentration in the exhaust gases being measured and being fed to the catalyst NH _{3 as a} function of the NO _x concentration found, characterized in that at least 75%, preferably at least 85%, of the power (N) and the speed (n) of the engine (M) determined, preferably calculated part of the NH ₃ supply, which is calculated based on the NH ₃ requirement, is supplied to the catalytic converter ( 1 ) and the rest, regardless of the control running via the measured NO _x concentration in the exhaust gases Part of the NH ₃ supply is regulated depending on the NO _x concentration measured in the exhaust gases. 2. The method according to claim 1, characterized in that the NH ₃ requirement is at least temporarily determined essentially from measured instantaneous motor data (power N , speed n) of the motor (M) . 3. The method according to claim 1 or 2, characterized in that the NH ₃ requirement is determined at least at times from a motor control device ( 8 ) to the motor (M) given engine control data (power N , speed n) . 4. The method according to claim 3, characterized in that the NH ₃ requirement already a preferably adjustable first time period (t ₁ ) before the delivery of engine control data to reduce the engine power (N) essentially according to the intended reduced engine power (N ) is determined, and that during a second time period (t ₂ ), which extends from the beginning of the first time period (t ₁ at least until the setting of the new engine power (N) , 100% of the determined NH ₃ requirement is fed to the catalyst ( 1 ) are and the current regulation over the measured NO _x concentration is invalid. 5. The method according to claim 2 and 4, characterized in that the NH ₃ requirement during the second period (t ₂ ), in which the control is still ineffective, when or after setting the new engine power (N) again the measured actual motor data (power N , speed n) is determined. 6. The method according to any one of claims 1 to 5, characterized in that for a total NH ₃ supply, the time during a period well above the characteristic rule of the current over the measured NO _x concentration control over 100%, preferably about 105% of the NH ₃ requirement determined from the power (N) and speed (n) of the engine (M) , an alarm signal is triggered and / or the engine is switched off.