Closed Loop Emission Control On A Diesel Engine

Technology TU Dresden Closed Loop Emission Control Diesel Engine
Closed Loop Emission Control On A Diesel Engine

RDE legislation has led to a growing number of influencing factors with regard to emission formation. In order to safely stay within the mandatory nitrogen oxide emission limits, the Combustion Engines Faculty at TU Dresden has developed a responsive operating strategy in which, depending on the driving situation, the engine control unit uses diverse sub calibrations that satisfy different optimisation foci.

MODIFIED BOUNDARY CONDITIONS

In the field of transportation, the internal combustion engine, especially the diesel engine, is still the most efficient power unit. The current legislation and the related low exhaust emission limits require intensive efforts in the development of future engine concepts. Particularly the check of real driving exhaust emissions, as a part of the new extended passenger car certification process, plays an important role in this context. The significantly larger range of influencing factors under real conditions – for example route, weather or driving style –has a considerable effect on exhaust emissions.

The development, design and optimisation of the engine process, has so far mainly been based on strict defined test cycles like the NEDC. From now on, a global view of the entire combustion engine’s operating range is necessary. Moreover, the integrated system components, especially the exhaust aftertreatment system and their effectively limits, must be considered even more intensively [1].

Until now, the calibration of the engine control is, under identical conditions during the engine operation, largely constant. Additional operating modes are used, among others, only for the periodic regeneration of the exhaust aftertreatment, emergency operation or during the start or warm-up of the engine. Furthermore, the calibration focus has mainly been on adequately high process efficiency and the observance of various operating and burden limits. The emission adjustment was only based on the certification test cycle. This process has to be done almost individually for each vehicle class or family. In future this procedure is, due to the certification based on RDE tests, only possible for a limited extent. The greater degree of freedom, in respect of driving profile and the number of different influencing parameters, makes for example a consideration of a sufficient emission reserve, the spacing from the limit, more difficult. Due to the CO2 target, a calibration of the engine control for “worst case” scenarios or cycles is not possible and at the same time unacceptable to customers. A change to the “Closed Loop Emission Control” as an operating strategy is thus unavoidable [1].

CONCEPTUAL APPROACH

At the chair of combustion engines of the TU Dresden a new method for an engine control system has been developed that in future will ensure the non-exceedance of the required nitrogen oxide emission limit (NOx limit) even during real driving. This method causes a change-over from a predictive, forward-looking operation strategy to an event-orientated, responsive control of the internal combustion engine. Accordingly, the adjustment of the engine control depends on the required exhaust emission limit and the permanent monitoring of exhaust emissions during real driving [2].

(1) Schematic diagram for the regulated adaptation of the engine control to restrain the NOx limit during real driving (© TU Dresden | LVM)

This means in detail: in the specific vehicle a superordinate, universal operating strategy is used which consists of various sub calibrations with different optimisation foci. Depending on the current driving situation the corresponding calibration or an interpolation between neighbouring calibrations is used. As (1) shows schematically, a continuous optimisation between minimally required NOx raw emission (NOx, raw), low fuel consumption ergo CO2 emission and high engine dynamics occurs. The time integral of the emitted NOx emission, based on the current driving distance and the resulting spacing from the required limit, serves as a basis.

METHODICAL TEST TOOLS

For the functional verification on the real engine, an implementation of the developed operating strategy is very complex. Moreover, the performance of freely programmable control units for the running of necessary real-time models must also be questioned. A systematic use of targeted test tools is therefore essential. The experimental test concept in [3] provides a promising alternative. (2) shows the approach of the concept which is based on the reproduction of the real driving at the chassis dynamometer or at the highly dynamic engine test bench. Consequently, vehicles do no longer need to be equipped with portable emission measurement systems. Complex test drives can also be avoided. On the basis of suitable load and vehicle models, the various influencing parameters, for example slope or steering angle, are considered. The tests can be reconstructed and, as often as required, repeated by the use of conventional exhaust measurement systems. Within the testing and development, the technical, financial and time-related expenses for RDE test drives are considerably reducible.

For investigations on the developed operation strategy, this test concept is extended by the 1-D engine process simulation. However, the vehicle model is not affected. By using the 1-D simulation, real driving on the road is increasingly virtualised. With regard to resources and hardware, the additional effort remains relatively low.

(2) Method for the prediction of real driving tests on the test bench or by means of engine process simulation (© TU Dresden | LVM)

MODELING

A 2.0-l diesel engine in the average performance segment with a single-stage exhaust gas turbocharger serves as a test vehicle. This unit corresponds to the latest state of the art. Consequently, a 1-D engine process model is created with the help of the GT-Power software. All essential thermodynamic and fluidic properties are part of the model. The internal combustion and the exhaust gas emissions are predictively calculated by using phenomenological models. In order to ensure a satisfactory model quality, a wide validation of static and dynamic operating conditions, based on the measurement data from the highly dynamic engine test bench, is performed. The control of the 1-D model, that is comparable to the control unit of a real engine, is achieved by an additional control model. This control model is built in Matlab/Simulink and integrated into GT-Power via suitable interfaces. Among other things, various maps are part of this model, which for example provide information of target boost pressure or injection timings. The SCR exhaust gas aftertreatment system is also mapped within the 1-D model, thus enabling the calculation of the NOx tailpipe emission (NOx,EoP).

In order to implement the “Closed Loop Emission Control” strategy, the functionality of the control model has to be extended accordingly. Various variants of the engine calibration which differ in design and optimisation, serve as a basis. That relates predominately to the specific variables of NOx raw emission and fuel consumption. The different engine calibration variants are based on investigations on the one-cylinder and the full-engine test bench. These investigations are focused on the influence of the internal NOx formation by combustion parameters. The different variants are specified with V 0.0 to V 1.0 and stored in an additional map dimension. Variant V 0.0 corresponds to low fuel consumption. V 1.0, on the other hand, has a minimal NOx raw emission (< 1 g/(kWh)). An interpolation between neighbouring variants is possible, because of a continuous transition. (3) (bottom left) schematically illustrates the overall functionality.

Among the various possibilities for realising the actual control (selection of the calibration variant), the following results are based on an approach which refer to a stored driving resistance curve and the actual vehicle speed. The set point value – for example the required NOx,EoP limit – is converted into a new set point value for the NOx raw emission in g/(kWh). Therefore, the behaviour of the exhaust gas aftertreatment system is also taken into account for the calculation. For each calibration variant, the expected NOx raw emission, depending on the operating point, is stored as a calculated variable in the control model. The selected calibration variant must then be at least equal to or below the set point. Furthermore, a PI controller modifies the original set point by means of an additional offset. Thus, driving style and different other influencing parameters can be compensated, in order to ensure the non-exceedance of the defined limit, (3) (top right).

(3) Schematic diagram of the control model (© TU Dresden | LVM)

FUNCTIONALITY & PARAMETER VARIATION

On the basis of a RDE cycle, used as an example, the functionality of the developed operating strategy is illustrated. The selected cycle corresponds to the legal requirements and is in the upper third of the permitted range with regard to driving dynamics and slope. (4) shows the calculated results as a comparison between basic control strategy (reference) and adaptive engine operating strategy (AEC). The course of the selected calibration variant for the adaptive strategy is depicted in the lowest chart. The two different colours indicate whether the engine is currently in the fuel consumption-optimised or emission-optimised operation. The SCR system is also considered and gives small benefits for the adaptive strategy during the course of the NOx conversion. Compared to the basis, the defined NOx limit (80 mg/km) is undercut within the urban part (see NOx,EoP). This is caused by the initial reduction of NOx raw emission. At higher vehicle speed, the high NOx conversion ratio can be used again, to allow higher NOx raw emission in favour of lower fuel consumption. High accelerations which lead to a NOx slippage on the SCR system are intercepted by the reduction of the NOx raw emission. Overall, in comparison to the basis, there is no CO2 advantage. However, a calibration of the basis adapted to the RDE cycle would lead to a significantly higher CO2 result.

(4) Model comparison of basic engine control and adaptive engine control (AEC) for an RDE trip (© TU Dresden | LVM)

The major benefit of the developed operating strategy is the adaptive adaptation of the engine control to the current boundary conditions. Regarding the selected RDE cycle, (5) shows an example of the results for different loads. Contrary to the basis, the new operating strategy completely compensates the expected increase in NOx emission, regardless of the vehicle mass.

(5) Influencing parameter variation (vehicle mass) – comparison between basic and adaptive engine control (AEC) (© TU Dresden | LVM)

SUMMARY & OUTLOOK

The “Closed Loop Emission Control” method used on diesel engines in passenger cars is a fundamental new operating strategy. This patent-pending method is primarily intended to ensure the non-exceedance of the permissible NOx emission within the legal driving cycles as well as in real driving, independently from external influencing parameters. A further premise is the possibility to keep the following as minimal as possible: phlegmatisation of the internal combustion engine, increase in CO2 emission and overall system costs. For this purpose, an engine calibration which adaptively adapts to the prevailing boundary conditions, driving style and various other disturbing influencing factors, is used. This avoids a “worst case” engine calibration as well as an inflationary increase in complexity of the exhaust aftertreatment system. Ideally, regardless of the vehicle and transmission, only one fundamental calibration of the control system for each basic engine is required. The unique effort for each developed diesel engine increases remarkably. However, with regard to the use within the entire vehicle fleet, the development period can be reduced and considerable savings in costs are noticeable, (6). Compared to the existing procedure for the engine calibration, the new operating strategy can achieve, in addition to general economic matters, further advantages. For example local (urban/rural) or temporally (day/night) varying emission limits as well as a general tightening of the exhaust emission legislation can directly be taken into account. The mechanism and the aging of the exhaust aftertreatment are also included and balanced accordingly [4].

(6) Classification and comparison of the developed method with regard to the effort on development of the engine control and compliance with the emission requirements (© TU Dresden | LVM)

Although the functionality of this method has been demonstrated by means of numerical simulation, a proof under real conditions and in a real vehicle still needs to be done. Moreover, in terms of the certification process it is also necessary to experience to which extend this operating strategy finds acceptance by the authorities. Operation strategies which have a negative effect on the exhaust emission behaviour in favour of fuel consumption during real driving can currently be registered only in a limited way. Furthermore, an applicability of the method for gasoline or gas engine and the expansion to other exhaust gas components is possible.


REFERENCES

[1] Zellbeck, H.: Closed Loop Emission Control. In: MTZ worldwide 78 (2017), No. 6, p. 90

[2] Walter, R.; Roß, T.; Werner, R., Zellbeck, H.: Eine Methode zur Stickoxidoptimalen Betriebsstrategie für Pkw-Dieselmotoren. 8th Emission Control, Dresden, 2016

[3] Zellbeck, H.; Walter, R.; Stiegler, M.; Roß, T.: RDE – Real Driving at the High Dynamic Engine Test Bench. In: MTZ worldwide 76 (2015), No. 2, pp. 42-47

[4] Walter, R.: Dieselmotorische Betriebsstrategie zur prozessoptimalen Ausschöpfung eines fahrstreckenvariablen Stickoxidkontingents unter realen Fahrbedingungen. Dresden, Technical University, dissertation, 2017


AUTHORS

DR.-ING. ROBERT WALTER was Research Assistant at the Combustion Engines Faculty of TU Dresden (Germany).

Dr.-Ing. Tilo Roß is Chief Engineer at the Combustion Engines Faculty of TU Dresden (Germany).

Prof. Dr.-Ing. Hans Zellbeck holds the Chair of the Combustion Engines Faculty of TU Dresden (Germany).