Abstract
Abstract: According to the protocol of Kyoto, one of the major challenges for the car manufacturers is to decrease the emissions CO2 in order to struggle greenhouse effect. One solution consists in "downsizing" by reduction of engines displacement. The use of high supercharging engines makes possible to use smaller engines with high specific power output which allows lower consumption in case of urban and extra-urban traffic. At least, turbochargers with high compression ratios are required with a wide functioning area which leads to use the compressor close to its surge limit. When the surge limit is crossed, gas flow becomes unsteady and high pressure fluctuation occurs, which leads to vibrations unacceptable for an automotive driver. The surge limit of a centrifugal compressor is highly dependant to the geometrical configuration of its associated system (network). During these instabilities a "surge cycle" is described with frequency and amplitude depending of upstream and downstream configuration. Tests have been done on our turbocharger test bench. The resistivity of the downstream duct is controlled by a valve. The upstream compressor configuration corresponds to the geometry usually met on the intake circuit air of the automotive engine. Three volumes of plenum (air filter box) have been experimented. In this paper is presented the influence of the resistivity on downstream circuit and upstream configuration on the amplitude and frequency of the instationnary fluctuations at low rotational speed, in the surge area. The results of these tests series highlight the mutual influence of the upstream and downstream configuration in term of frequency and amplitudes fluctuations of surge cycles. Indeed, as well as the valve is closed, the surge frequency decreases more especially when the volume of plenum increases. Moreover, the amplitude fluctuations of surge cycle are less important when the volume of plenum increases.
Keywords: surge, centrifugal compressors, turbocharger, downsizing, intake circuit.