Study of the on-engine turbocharger compressor phenomena: surge and heat transfer, by a modelling approach

Abstract

Turbocharging is one of the most widely used techniques in the automotive industry to improve performance and reduce emissions. To optimise the matching between turbochargers and engines, turbocharger manufacturers provide their characteristics by means of maps done with measurements assuming adiabatic conditions. These maps are used in the development phase of engines, which is focused on the performance of peak torque, i.e. improving the performance at high load and steady conditions. However, automotive engines operation is usually at low load and in transient operations due to the urban traffic conditions, which make turbochargers work off-design and under conditions that are not shown in the maps. In this context, two topics were considered: the development of a technique to detect the onset of surge and the impact of heat transfer in turbocharger measurements. To develop these works, modelling was considered. Therefore, a non-dimensional mean-line model of a centrifugal compressor which could generate compressor maps by itself was developed, but after a parameter adjustment analysis, it was concluded that, at least, a one-dimensional model taking into account the Euler equation was needed. Concerning surge, several studies have been especially focused on its avoidance, its detection and its control, whether their approach is experimental or numerical. Nevertheless, after a state of the art analysis, it was determined that this technique was rather a long term project. Finally, a validated model of heat transfer in turbochargers, which was based around conventional experimental maps, but augmented to predict the effects of heat transfer, was analysed to study the influence of the compressor housing conduction and insulation in turbocharger tests. The model was first validated by simulating external insulation and was found to be consistent with findings of other authors. The insulation was found to affect overall heat flows by 20W but did not significantly change the behaviour of the compressor housing. A subsequent sensitivity analysis of the conductivity of the compressor housing showed that a 50% variation in conductivity could cause a 60% change in heat flows to the compressor gas. This highlights the fact that compressor housing is a crucial aspect and more detailed investigations using more intense instrumentation or 3D simulations are required to refine the model in this area.