Pressure Control of CNG Engines by Noninteger Order Controllers: a New Trend in Application of Fractional Calculus to Automotive Systems

The massive use of electronic control in automotive vehicles improved performance,comfort, safety and reduced pollutant emissions and consumption. In particular, the accurate control of the fuel injected into cylinders allowed the common rail fuel injection system to increase engine performance while reducing emissions, noise and fuel consumption. In this context, compressed natural gas (CNG) engine systems can further reduce emissions to adhere to environmental policy regulations. However, the injection process is strongly non linear, time variant and highly coupled, so suitable control systems must be designed to guarantee the desired performance. This chapter describes how to synthesize and realize non integer order controllers for pressure control in common rail injection systems of CNG engines. The realization is relatively simple and cheap, as required by the industrial application. Namely, not only low sensitivity to parameter variations and load disturbances mustbe achieved, but also a limited cost with respect to implementation by consolidated PID controllers.The CNG common rail injection system includes: a tank storing high pressure gas;a main chamber and a control chamber; a solenoid valve; an electronic control unit; a common rail and electro injectors. The tank feeds the downstreamcircuit. The main and control chambers are separated by a moving piston. Both chambers receive fuel from the tank and send it to the common rail, which is a constant volume accumulator connected to the electro injectors.The inlet flow to the main chamber is regulated by a shutter that is integral with the piston, whose position depends on the equilibrium of the pressures acting on its surfaces. Adjusting the pressure in the control circuit by the solenoid valve regulates the main chamber inflow. Moreover, as the main chamber and the common railhave almost equal pressures, accurate metering of the injected fuel is allowed by setting the injection timings at the same time.This work reports recent advancements in the design and simulation of switched fractional order PI controllers, in which the integral action is of non integer order.Performance, robustness and disturbance rejection are tested by simulation of virtual prototypes based on non linear models. The basic idea is to perform a loop shaping of the open-loop transfer function to obtain frequency domain performance specifications and achieve an optimal feedback system. To this aim, the fractional integrator is profitably used and robuststability of the closed loop system is guaranteed by a D decomposition method. There are several benefitsof the design approach. Closed formulas determine the controller gains by frequency domain specifications and can be used for an automatic synthesis of the controller. Moreover, the realization of the noninteger operator is by an efficient approximationmethod that prevents numerical problems and leads to a rational transfer function characterized by interlaced minimumphase zeros and stable poles, so that a reduced approximation error is obtained and easy implementation is possible. To conclude, the non integer order controllers allow higher accuracyin metering the injected fuel and promptness in setting the rail pressure to the desired reference values.