The objective of this research was to evaluate the influence of lubricating oil temperature and pressure on compressor performance at low speed. In spite of the use of a specific test bench, the study has demonstrated the difficulty of assessing compressor performance at low speed.
First of all, it is important to stress the influence of the heat given by the lubricating oil, which in the case of low speeds leads to serious errors when trying to estimate the compressor isentropic efficiency. Given that the experiments were conducted on a “cold” test rig, it is certainly unrealistic to hope for correct results at low speeds on hot test rigs. Thus, starting from the experimental results, only those tests performed in quasi-adiabatic conditions were selected in order to draw the compressor characteristics.
This more reliable approach has allowed a better estimation of the compressor mechanical efficiency. As seen, this efficiency, which is about 0.85 at high speeds, does not exceed 0.3 at 30,000 rpm.
Thanks to our test bench equipped with a torquemeter, the difficulty in measuring the power given to the air flow in real conditions (non adiabatic) can be easily overcome if the mechanical efficiency of the turbocharger is known. Unfortunately, the accuracy of theoretical calculations of mechanical power losses is not satisfactory with basic solving methods [15] and [16] and needs to be improved [9].
The present study forms part of a general research program on automotive turbochargers, financed by the French National Research Agency, involving two French automotive manufacturers, two turbocharger manufacturers and three French Universities.
The on-going research concern focuses on calculations with a 2D and 3D CFD software and the concurrent enhancement of the test procedure to improve our test bench results. Experiments have been done with insulation of the compressor and hot air, in order to create “new adiabatic” conditions. Furthermore to validate the computation on bearing losses, journal bearings and thrust bearing, the test bench has been equipped with a special device to control axial forces.