Setup of an Engine Rapid Control Prototyping System for Catalyst Research and Evaluation Testing

To fulfill ever increasingly stringent emission regulations, a great many studies on engine control and catalytic converter performance have been made. Topics of great interest in this area, to name a few, include: the relationship between catalyst light-off time and air-fuel (A/F) ratio; the relationship between forced A/F ratio modulation and catalyst efficiency; the effects of phaseshifted A/F ratio modulation between banks of a dual bank engine, or among cylinders of a single manifold engine on catalyst efficiency; and methods of modeling and measuring the oxygen storage capacity of a catalytic converter by rich-lean transition, A/F ratio sweeping, or other on-line estimation methods. To undertake this type of research, an engine control system with necessary functions, especially with very flexible A/F ratio control capabilities, is needed. Mass production ECU does not provide the flexibility desired and it is also hard to develop and integrate the control algorithms needed for catalyst testing into the existing ECU software. An engine rapid control prototyping system is set up in an engine dynamometer test cell environment to overcome the limitations of mass production ECU and fulfill the requirements for catalyst research and testing. Model-based development methodology is adopted for the design and implementation of necessary software. Control algorithms, including individual bank control of a dual bank engine, A/F ratio modulation of different frequencies and amplitudes, with and without phase shift between banks, A/F ratio rich-lean transition and sweeping etc, are designed using graphical language, automatically converted into executables to run on the real-time target. UDP communication for real-time command and variable exchange between the engine controller and the test cell controller is developed to facilitate testing. The system provides the flexibility and good control performance desired for catalyst research and evaluation testing. Application and results of the system on a 4.6L V8 gasoline engine is given. INTRODUCTION With environmental protection in the forefront of world issues, increasingly stringent emission regulations are being enforced worldwide. Facing these challenging requirements on emissions, OEMs and suppliers have been working in many areas of engine and vehicle development to achieve their goals. Among these areas, the three-way catalytic converter application is of extraordinary importance and is an area where much research and testing has been conducted. Major activities in this area can be categorized into two groups. The first group mainly focuses on the improvement of converter performance through catalyst technology, such as the development of oxygen storage material; improvement of composition and arrangement of precious metals for better conversion efficiency; optimization of the structure to create a catalyst selfregeneration function and mitigate deterioration, etc. The second group includes various activities in engine control, especially in A/F ratio control to increase the performance of the catalyst [1-13]. It is well known that A/F ratio has great effect on catalyst conversion efficiency. Research topics in this area include among others: • The influence of A/F ratio on catalyst light-off time [1,13] • The relationship between forced A/F ratio modulation (frequency and amplitude) and catalyst conversion efficiency [2,3,4,7,8] • The effect of phase-shifted A/F ratio modulation between banks or among cylinders on conversion efficiency [2,3,9] • methods of modeling and measuring the oxygen storage capacity of a catalytic converter by rich-lean transition, A/F ratio sweeping, or other on-line estimation mechanisms [4-7,10-12] We will briefly review the above cases. It is reported that operating an engine with retarded ignition timing combined with lean A/F ratio will decrease catalyst lightoff time and consequently reduce engine emission [1]. It is also reported that perturbing the A/F ratio to make use of the oxygen storage capacity of the catalyst yields better conversion efficiency than just keeping the constant stoichiometric value upstream of the converter [4]. These researches involve controlling the A/F ratio at desired constant value or desired average value with perturbation. For dual bank engines, some research indicates that independently controlling the A/F ratio of each bank to make their lean and rich status just opposite to each other (called phase-shifted A/F ratio modulation) will benefit catalyst efficiency since both rich and lean exhaust species are present simultaneously at the converter and provide a highly reactive mixture to the catalyst [2,3]. It is also claimed that this mechanism will reduce the demand on the oxygen storage capacity of the catalyst since the exhaust from the two banks will combine and achieve a near-stoichiometric mixture prior to entering the converter. This research involves individually control the fuel injection of the banks so that their A/F ratio waveforms have the desired average value, modulation frequency, perturbation amplitude and phase shift. When carrying out measurement of the oxygen storage capacity of the catalyst, commonly used methods involve controlling the engine to run under certain A/F ratio patterns and monitoring the engine and catalyst behavior, such as studying the response delay of a downstream A/F ratio sensor when the engine is going through rich and lean transitions, or measuring the break-through perturbing oxygen quantity when doing engine A/F ratio sweeping [4,5]. These cases require the A/F ratio to go through square wave, or low frequency triangle wave superimposed with higher frequency modulation. We see all these studies and testing require flexible control of A/F ratio — desired constant value, desired average value, various modulation frequencies, desired perturbation amplitude, and with or without phaseshifting between banks. Mass production ECU does not provide the flexibility needed for these purposes. It is hard to develop catalyst testing algorithms and integrate them into the existing ECU software. We set up an engine rapid control prototyping system in an engine dynamometer test cell environment to overcome these limitations. By the use of model-based development methods, algorithms are specified in a high-level graphical language and directly compiled into executables to run on the real-time hardware. The engine control features stated above are easily designed and implemented to fulfill the catalyst study and testing requirements. To facilitate testing, a mechanism for real-time exchange of commands and variables between the engine controller and the test cell controller is desired. UDP communication is developed to fulfill this need. The test cell controller sends commands to the engine controller through the UDP interface so that the engine is put into specific operating modes for testing and measurement. Various engine operating modes are designed and implemented to realize various A/F ratio patterns. Engine control parameters and variables are also transferred to the test cell controller through UDP communication for analysis purpose. The system is applied to a 4.6L V8 gasoline engine. Results of A/F ratio control for various testing needs are presented. The application and results reflect the flexibility and good control performance of the system. This paper is organized into the following four sections: I. System Overview describing the overall design and configuration of the integrated system. II. Engine Rapid Prototyping Controller Setup describing the features of the controller, its hardware and software configurations, and the function modules used for engine control. III. Model-Based Software Development and Application Results describing the development of needed engine control features, especially the algorithms for steady state A/F ratio modulation and phase-shifting, rich-lean transition, and A/F ratio sweeping etc. The results of application on a V8 engine are also given. IV. Conclusion summarizing the features and the performance of the system.