Simulation of a turbocharged compression ignition engine at low loads and high rates of EGR

In view of the incoming laws concerning the emission limits during test cycles, it is increasingly important reducing emissions at low and medium loads by using exhaust gas recirculation (EGR) techniques. The development of control strategies can be greatly improved by simulation models able to predict accurately the behaviour of the engine and its emissions in the entire field of operation, and specifically at low loads when the turbocharger is operated at low rotational speed and in the vicinity of the surge line. Often the maps of compressor and turbine made available by the manufacturer are not extended to low rotational speeds and low pressure ratios. Methodologies have been therefore developed in order to extend the available characteristic curves to other working points, taking into account the geometry and the rotational speed of the turbocharger. For the compressor, a technique based on the fluid dynamic similitude is proposed. For the extrapolation of the turbine maps, two models based on analysis of the fluid flow at the mean-line are proposed. These methods have been applied to extend the maps of a turbocharger with variable geometry turbine (VGT) mounted on a six cylinder Diesel engine. A test series at low load and variable EGR valve opening has been simulated. Comparisons of simulation results and experiments show that the proposed methods are able to provide a good simulation of the engine points at low load and high rates of EGR where the working points of the turbocharger were found to be located in region of the maps far from the experimental points provided by the manufacturer.