Emissions-Controlled Diesel Engine
Legislation concerning the pollutant emissions of diesel passenger cars is becoming increasingly restrictive, especially for NOx and particulate matter (PM). A novel approach to control the pollutant emissions in diesel engines is thus proposed that extends the standard feedforward control in the air and fuel path with a true emissions-feedback structure. In order to prove the feasibility of such an approach, a multivariable emissionsfeedback controller is designed for the NOx emissions and for the air/fuel ratio, which is used as an indicator for the PM emissions, due to the fact that compact and low-cost PM sensors are not yet commercially available. The controlled inputs are the command signal of the exhaust gas recirculation (EGR) valve and the injection timing. Besides the boost pressure that is regulated independently by the standard controller, those are the inputs that most significantly affect the formation of NOx and PM emissions. Moreover, with this choice, the resulting 2×2 system is reduced to a convenient triangular structure. Since the new sensors are sensitive to pressure variations, they have to be placed in the low-pressure tract of the exhaust manifold, i.e., downstream of the turbine. Obviously, time delays due to the gas transport now become relevant for the control system. The multivariable controller proposed consists thus of two separate internal model SISO control (IMC) loops developed with a simplified model of the plant and connected by a decoupling term. The IMC structure has the advantage that can compensate for the time delays in the control loop. Since the controller has to work well in the entire range of operating points, a scheduling of its parameters is necessary. The parameters of the controller are thus generated automatically starting from the engine model in order to reduce the efforts of calibrating the control system in the entire operating range of the engine. The validity and feasibility of the proposed control structure is demonstrated experimentally. A test-bench comparison between the emissionscontrolled and the standard engine shows that, with this new control structure, the tracking performance and the driveability of the engine during a driving cycle are at least as good as those of the standard engine. The benefits of the novel approach are several; the two most important are the following: 1) The engine can operate in a small range of uncertainty for the NOx and PM emissions even in the case of relaxed manufacturing tolerances and ageing of the injectors and of the EGR valve, and 2) the fuel consumption of the engine can be reduced substantially, if the setpoints for the emissions controller are determined by following an adequate control strategy based on the maximization (within the legislated limits) of the NOx emissions.