Model-Based Analysis and Optimization of Turbocharged Diesel Engines with a Variable Geometry Compressor and Turbine System

In the last few years, the application of downsizing and turbocharging to internal combustion engines has considerably increased due to the proven potential of this technology to increase engine efficiency. Variable geometry turbines have been largely adopted to optimize the exhaust energy recovery over a large operating range. Two-stage turbocharger systems have also been studied as a solution to improve engine low-end torque and efficiency, with the first units currently available on the market. However, the compressor technology is today still based on fixed geometry machines, which are sized to efficiently operate at the maximum air flow and therefore lead to poor efficiency values at low air flow conditions. Furthermore, the surge limits prevents the full capabilities of VGT systems to increase the boosting at low engine speed. In order to increase the compressor efficiency at low engine speed without compromising the operation at medium-high engine speed, variable geometry compressor (VGC) systems have been recently considered as a future design option for automotive turbochargers, considering either a variable inlet guide vanes device or with a variable geometry diffuser. In order to effectively understand the potentials of such technology, this paper presents analysis, optimization and control results for an automotive Diesel engine equipped with variable geometry compressor and turbine. Starting from a validated model of the engine air path dynamics, a detailed design optimization study was conducted, showing how the VGC can be used to increase the stability range of the compressor by means of significantly shifting the surge limit of the machine while retaining potentials for improving the engine performance. Based on the outcome of the above study, a model-based control strategy for the VGC actuation was formalized. The developed control algorithms were tested in simulation, showing how this actuator can be used to optimize boost pressure and air mass flow rate of the engine while preventing the compressor surge.