Nowadays, much greater emphasis is being placed on improving the fuel consumption of automobiles due to the global move to reduce CO₂ emissions. Ideally, an engine should be able to simultaneously offer a high power density and low fuel consumption. High-pressure turbocharging is indispensable to improving the fuel consumption of an engine by enabling downsizing and lean-boost. To this end, there is a demand for a turbocharger with a wide flow range. The characteristics of different turbocharging systems have been evaluated by one-dimensional engine performance simulation. The variable-geometry turbochargers with motor assist are effective at improving the low-speed torque and the transient response of an engine. On the other hand, the surge limit of the compressor restricts the charging pressure at low engine speeds. A key technology for improving the surge limit involves the development of a casing treatment and a variable-geometry compressor. A two-stage turbocharging system offers the double advantage of eliminating the surge limit and improving the transient response. Unsurprisingly, however, the engine system and its control become more complicated. Therefore, it is vital that we develop an optimum turbocharging system to suit the engine specification.

To improve low-end torque and the transient response of a turbocharged engine, there is a need to develop a wide flow range compressor for use in turbochargers. Especially, we must improve the surge limit of the compressor because this restricts the boost pressure rise at low engine speeds.
We studied the surge characteristics of a turbocharger compressor by experiment. Firstly, we developed a casing treatment that featured a curved wall cavity. We then went on to investigate the effects of the dimensions of the casing treatment on the surge limit. As a result, we found that both the surge limit and the compressor efficiency were improved by use of the casing treatment. The installation of the casing treatment reduced the surge flow rate by 30 % relative to a conventional compressor at a pressure ratio of 2.5. Secondly, we investigated the synergy effects of the Variable Inlet Guide Vane (VIGV) with the casing treatment. The surge flow rate was found to have been reduced significantly compared with their being used separately, due to the synergy effect. Also, the surge flow rate was reduced by 59 % relative to a conventional compressor at a pressure ratio of 2.5, again due to the synergy effect. Furthermore, the surge limit with a VIGV setting angle of 80 degrees was not changed by reducing the backward angle of the impeller relative to the radial direction, despite an increase in the choke flow rate. As a result, we were able to develop a compressor with a significantly wide flow range and an impeller with a low level of centrifugal stress.

Yuji Iwakiri, Hiroshi Uchida

A numerical fluid analysis has been used to clarify the cause of the synergistic improvement in the surge limits of a centrifugal compressor when a self-recirculation casing treatment is used together with variable inlet guide vanes.
The following results were obtained by this study.

1) The reverse flow at the tip clearance is closely related to the surge limits, and controlling the reverse flow at the tip clearance is an effective means of improving the surge limits.

2) The surge limit is improved by increasing the recirculation flow rate through the casing treatment.

3) The reverse flow at the tip clearance can be controlled up to the low flow rate region by combining the variable inlet guide vanes with the casing treatment, and as a result, the surge limit is significantly improved.