Abstract
During the last several years a number of turbocharged engines for automotive applications has considerably increased. A turbocharger becomes an integral part of the Diesel engine and an application of direct injection systems operating at high pressure injection makes this tendency still more intensive.
Despite the long period of the turbocharger application for automotive purposes (over 30 years now), good matching of an engine and a turbocharger still requires very precise information concerning the turbocharger behaviour, especially under conditions of the pulsating flow of hot exhaust gases.
In most cases this information can be obtained only experimentally. Precise determination of turbocharger performance under unsteady flow conditions requires proper measurements of thermodynamic and kinetic parameters of a flow across the compressor and turbine stages.
Due to cyclic work of the combustion engine, a flow of gases at the turbine admission is always unsteady and, for the constant point of the engine work, it can be assumed to be a pulsating flow with periodical variation of its parameters.
In the case of the most typical arrangement of a four stroke, four cylinder Diesel engine with the maximal rotational speed of 4500 rpm. (75 Hz), the corresponding pulse frequency will be 150 Hz.
In order to record only the first harmonic of pressure, temperature and fluid velocity for this range of frequencies, we must dispose of transducers having their frequency band at the level of a few hundred Hz.
Tests of turbochargers under pulse flow conditions were carried out in many laboratories and research centres. We can cite e.g.: Bath University of Technology in England (Wallace et al.), Kelo
University in Japan (Kosuge et al.); Laboratoire de Thermodynamique Appliquée aux Machines in
France (Chelot et al.), University of Genoa in Italy (Capobianco et al.), and many others.
From the metrological point of view, the conclusions are very similar. The application of modern pressure and velocity transducers, characterised by high cut-off frequencies (reaching a few hundred kHz) permits proper measurements of these signals. On the contrary, in temperature measurements a principal problem concerns dynamic properties of the sensor. This aspect of temperature measurements will be described further in detail.