Highly Dynamic Intake and Exhaust Back Pressure Control

Measuring emissions of internal combustion engines-not only at steady-state conditions, but also with highly dynamic test cycles-is an important issue in modern engine development. Due to the fact that ambient conditions have an essential influence on power and emissions of internal combustion engines, test beds used for such measurements typically incorporate intake air and exhaust back pressure control for reasons of repeatability, accuracy and comparability. As test cycle dynamics get faster and legal pressure tolerances get narrower, pressure control becomes more demanding and simple PI control schemes are pushed to their limits; therefore, more sophisticated control schemes are necessary. In this paper, a linearised model is first derived and then used to both simplify and optimise PI controller tuning. This is done by means of frequency domain methods. Limitations to such controllers and possible approaches to overcome them are discussed. The main limitation is shown to be caused by resonance effects in the pipe. These effects are modelled thoroughly using partial differential equations to obtain a resonance compensation method. This method is both simple and robust with respect to temperature changes, which renders it perfectly applicable to exhaust back pressure control. The proposed controller with resonance compensation is demonstrated in simulation and validated in the course of experiments; the latter show a reduction of pressure fluctuations from ±30 mbar using a standard PI controller to ±3 mbar using the enhanced controller with resonance compensation.