The goal of this thesis is to develop a control strategy for the TWC in order to maintain the required conversion rates under all operating conditions through- out the TWC’s lifetime. This includes permanent monitoring of the parameters which are relevant for the TWC dynamics as well as an adaptation algorithm which accounts for the changing of these parameters during the TWC’s life- time. The diagnostics of TWCs, i. e., the link between the monitored parame- ters and the actual conversion efficiency of the TWC is only discussed briefly.
Control-oriented TWC modelling often addresses mainly the oxygen storage dynamics. In this thesis, also the deactivation of the TWC by reducing species is discussed. This plays an important role for the dynamics of the ratio between HC, CO, and H2 downstream of the TWC. This ratio significantly influences the λsensor signals. This impact, especially on the switch-type sensors, is thoroughly investigated. Additionally, a method is presented which allows the
adaptation of the controller to the changing dynamics of the TWC because of ageing.
The setup of the addressed system corresponds mainly to the one shown in Figure1.4. Thus, only port-injected spark-ignited engines are discussed. This thesis focuses on the use of a wide-rangeλsensor upstream and a switch-type λsensor downstream of the TWC. In addition to the sketched system, it is as- sumed that a temperature sensor for the raw exhaust gas or at least an accurate estimate is available. Auxiliary systems such as secondary air injection, can- ister purge, exhaust gas recirculation, turbochargers or other devices are not discussed. Only the air-to-fuel ratio or rather the amount of injected fuel is considered as an actuator. The spark advance and dwell angles are not taken into account as additional degrees of freedom.
Modern exhaust gas aftertreatment systems consist of two TWCs in the ex- haust path. Mounted close to the engine, the close-coupled device is designed for fast light-off in order to minimise the hydrocarbon emissions which arise during warmup. It is usually reasonably small and robust against the thermal stress occurring close to the engine. The underfloor catalyst is mounted further downstream. It is larger in volume and often designed to efficiently remove the NOxemissions. This thesis only covers aspects of the close-coupled TWC.
The extension of the concept to systems with two TWCs is straightforward.
The thesis is divided into a process modelling part (Chapters 3 and 4), a control-oriented modelling part (Chapter5) and a control part (Chapter6). The Chapters are structured as follows:
In Chapter 3, a detailed process model of the TWC is developed in order to gain insight into its dynamic behaviour. Here, the focus is on the oxygen storage and the deactivation dynamics. It is shown how these dynamics drive the transients of the most important exhaust gas components, especially the ones which contribute to the sensor output downstream of the TWC.
Chapter4focuses on theλsensors. A detailed process model of the switch- type sensor is developed in order to obtain a thorough understanding of its working principle and also of the advantages and difficulties of its use. The wide-range sensor is addressed shortly in the second part of this chapter. It is demonstrated that the signal can be significantly distorted. It is further dis- cussed why the switch-type sensor is preferred downstream of the TWC.
Based on the process models, a control-oriented model of the TWC and the λsensors is presented in Chapter5. The TWC model accounts for both the most important dynamics and the exhaust gas components which mainly drive or distort the sensor signals. The sensor model considers the influence of the exhaust gas components provided by the TWC model. Based on the simpli- fied model, an extended Kalman filter is developed which allows the online estimation of the relative oxygen level in the TWC, the deactivation, and the
oxygen storage capacity. These quantities are crucial for both the control and the diagnosis of the TWC.
The state observer allows the use of a model-based controller of the TWC.
This is presented in Chapter6. Different strategies are discussed which address various issues of combinedλand TWC control. An LQ controller with an integrator extension is developed which not only accounts for the TWC control but also estimates the offset of the wide-rangeλsensor located upstream of the TWC. Apart from the controller, the question of the setpoint management in terms of emission-optimal operation is discussed, as well.
Finally, some conclusions are drawn and an outlook with recommendations for future research is presented.
All measurements presented throughout this thesis have been performed on the engine test bench of the Measurement and Control Laboratory at ETH Zürich. A description of this test bench with all the measurement devices and sensors is given in Chapter2.
All measurements presented in this thesis have been performed on the engine test bench of the Measurement and Control Laboratory at ETH Zürich. This test bench is equipped with an AUDI V6 30V engine with a displacement of 2.8 litres. The power output is142 kWat6000 rpm. The engine is connected to a highly dynamic direct current generator which can be run in speed or torque controlled mode. A detailed description of the setup can be found in [73] and [88]. The generator can be run in all operating conditions, i. e., it can be used both to brake and to drag the engine, for example to emulate the use of the engine as a brake. Thus, driving cycles such as the FTP cycle (cf. Section1.2) can be run on the test bench. The FTP cycle measurements presented in this thesis are based on chassis dynamometer measurements at the Robert Bosch GmbH, where the engine speed and the throttle angle were recorded during a test on a vehicle equipped with the same engine. The vehicle used for the recording had a manual transmission, which leads to a frequent occurrence of fuel cut-offs during deceleration periods. Figure2.1shows a comparison of the recorded (setpoint) and the actually obtained engine speed and throttle an- gle during a significantly transient period of the FTP cycle. The agreement of the signals is excellent, which shows that the conditions occurring in the cycle can be emulated well. A major advantage of this procedure is the very accu- rate reproducibility of the driving cycle. This facilitates the comparison of the different control strategies for the three-way catalytic converter. In order to en- hance this reproducibility, also the trigger for the fuel cut-off has been recorded.
Thus, fuel cut-offs always occur at the same time in all cycle measurements.
In its present configuration, the engine is not equipped with any turbocharger or exhaust gas recirculation. The camshaft timing and the intake manifold vol- ume may be varied, but this has not been used in any experiment presented here.
The test cell is not air conditioned, which leads to slightly varying ambi- ent conditions, depending on the time of the year. Usually, the ambient tem- peratures during the measurements are between 25◦Cand35◦C. Ambient pressure and humidity only vary little. Since no cold start experiments were conducted, no special attention was paid to the ambient conditions.
The engine has two exhaust gas lines which are completely separated. Each line is equipped withλsensors and three-way catalytic converters and is con-
0 5 10 15
α th [%]
170 172 174 176 178 180
0 1000 2000 3000
time [s]
n eng [rpm]
setpoint measured value
Figure 2.1: Comparison of the setpoints with the actual values of the throttle angle and the engine speed between170 sand180 s. Notice that the time scale differs from the original FTP cycle because data recording starts prior to the actual cycle.
trolled independently. The experiments presented here have been performed on one exhaust gas line, only.