Effect of Pulsating Flow Characteristics on Performance and Surge Limit of Automotive Turbocharger Compressors

Abstract

The flow in turbocharger compressors and turbines for automotive engine application is highly unsteady in nature, as it\ud responds to the intake and exhaust manifolds of the internal combustion engine. The optimization of the turbocharger\ud system is therefore a very difficult task, since only steady flow maps are generally provided by turbocharger manufacturer.\ud For several years a specialized components test facility operates at the University of Genoa, particularly suitable to test\ud turbochargers under steady and unsteady flow conditions. The test bench has been continuously upgraded in order to study\ud components under pulsating flow condition by using different layout configurations. A recent set-up makes it possible to\ud study turbocharger compressor under unsteady flow condition by using a rotating valve pulse generator system.\ud Measurements of pressure signals downstream the compressor, instantaneous mass flow rate and turbocharger rotational\ud speed are performed.\ud In the paper the effect of both pulse characteristics (frequency - i.e., engine rotational speed -, amplitude and mean\ud value) on compressor performance is analyzed. In particular the attention is focused on the surge limit position under\ud unsteady flow operation

Non-Intrusive Methodology for Estimation of Speed Fluctuations in Automotive Turbochargers under Unsteady Flow Conditions

“…Here, the ratio between inlet and outlet compressor pressure is shown as a function of the reduced mass flow rate for different compressor speeds. The first aspect to be addressed is that the real operating range of the compressor can exceed the operating region shown in Figure 3 (the surge line can be exceeded) [1,3,6]. To avoid interpolation errors, the compressor map has been extrapolated according to the methodology presented by El Hadef et al [2].…”

“…The balance between power delivered by the turbine and power required by the compressor is related to the turbocharger assembly acceleration through Equation (3). (3) This differential equation can be written in the crank angle frequency domain applying a Fourier expansion. Average values of the Fourier expansion applied to Equation (3) return the usual balance equation for the turbocharger assembly: (4) In this case, the effect of turbocharger speed fluctuations on η o (mechanical efficiency) is neglected, since the amplitude of speed fluctuations is very low with respect to turbocharger speed mean value.…”

“…The first issue to be addressed is the fact that maps provided by the turbocharger manufacturer usually do not cover the lowest rotational speeds, that become predominant in normalized driving cycles [2]. In addition, the most important limit to be taken into account is the lack of information on turbocharger unsteady flow performance, since only steady flow maps are usually available for both compressor and turbine [3].…”

Analysis of the Turbocharger Compressor Surge Margin Using a Hurst-Exponent-based Criterion

“…These measurements are carried out on a gas stand, where the value of B and the throttle characteristic are different from the values on the engine. Furthermore, pulses at the compressor outlet as generated by the engine intake valves, as well as the pipe geometry at the compressor inlet have been shown to influence the onset of surge in [4,5]. The surge limit given by the supplier is therefore not generally valid on the engine, and a safety margin must be incorporated.…”

“…The B parameter is defined as (3) where L c is the intake duct length and ω H is the eigenfrequency of a Helmholtz resonator with an equivalent duct length and plenum volume, shown in Figure 1. The stability criteria are then (4) (5) The throttle characteristic slope ∂ψ T /∂ϕ T is generally positive. Therefore, the compressor is in the stable operating regime for a negative characteristic slope, while the systems stability depends on the value of B and the throttle characteristic slope for a positive compressor characteristic.…”