Conceptual Design of a Variable Geometry, Axial Flow Turbocharger Turbine

Abstract

The modern automotive industry is under strict regulations to reduce emissions to comply with the Kyoto Protocol, a universally acknowledged treaty aiming at reducing exhaust gas emissions. In order to achieve the required future emission reduction targets, further developments on gasoline engines are required. One of the main methods to achieve this goal is the application of engine downsizing. Turbocharging is a cost-effective method of downsizing an engine whilst reducing exhaust gas emissions, reducing fuel consumption and maintaining prior performance outputs. For these reasons, the turbocharging is becoming the most widely adopted technology in the automotive markets. In 2012, 32% of passenger and commercial vehicles sold had a turbocharger installed, and is predicted to be 40% of 2017 [1]. Even if the engine turbocharging is a widespread technology, there are still drawbacks present in current turbocharging systems. The main problem is overcoming the issue of turbo-lag, which is the poor initial response of the turbocharger to the driver commands due to its inertia. Indeed, the system turbine plus compressor is characterized by an own rotational inertia, therefore, the turbocharger will take a certain time to accelerate and produce the desired boost when a higher amount of exhaust gas is sent to the system. In this work, an innovative solution to the turbo-lag phenomenon will be analyzed: a vaneless stator-axial flow turbine. The proposed turbine configuration would improve the transient response of the system since the axial turbine has intrinsically a lower inertia than the radial turbine as stated by the research works of Ford first [2] and Honeywell after [3]. The whole design process is presented in this paper and particular relevance has been given to the thermo-fluid-dynamic aspect of the machine. Several CFD investigations have been carried out in order to deeply understand the new turbine behavior and, thanks to a 1D model of the target engine, it has been possible to validate the new design simulating the performance of the system engine + turbocharger.