2. SERVICE LIFE OF COMPRESSOR IMPELLER DESIGNED FOR HIGH PRESSURE RATIO
The trend with the diesel engines toward the higher power output will continue, and it is expected that with the two-cycle type, the engines operating at the mean effective pressure of 19 bars will be the mainstream in the near future. Also, the scavenging pressure required for engine operation is expected to reach 3.7 to 3.9 bars.
Every turbocharger manufacturer has so far employed aluminum alloy forging as material for the turbocharger compressor impeller in the generally accepted practice. Now that the compressor impeller of aluminum alloy forging is used in the high-pressure turbocharging application, however, its creep strength under high stressed and at high temperatures has come to emerge as a matter of serious concern.
The marine diesel engines used in the ship propulsion plants are normally operated at 85 to 90% load levels, so the aluminum alloy compressor impeller may be employed as it is for the time being. The stationary diesel engines, however, are run always at 100% load, warranting caution against the compressor impeller creep rupture. Restraining the impeller centrifugal stress and the rise in its temperature, therefore, looms large as an important design consideration in ensuring that the compressor impeller will remain in service for an adequate period of time.
(1) Restraining Impeller Centrifugal Stress
a) Turbocharger Rotating Speed
MET-SD and -SE turbochargers manufactured by Mitsubishi Heavy Industries, Ltd. the authors are affiliated with employ a backwardcurved compressor impeller blading system, in which the impeller blades are curved opposite from the direction of rotation radially along toward the impeller air exit. The angle of the blade curvature as viewed in the radial direction is called the backward curvature angle and exerts significant effect on the compressor pressure ratio. Generally, the smaller the backward curvature angle, the lower the rotating speed for achieving a given compressor pressure ratio, hence the smaller the impeller centrifugal stress becomes and thus the more advantageous for the impeller blade structural strength.
Fig.2 compares the performance curve of a MET66SD turbocharger compressor featuring the impeller blading of large backward curvature angle and that of a MET66SE compressor featuring the impeller blading of small backward curvature angle. it can be seen that running at the same rotating speeds, the MET66SE turbocharger compressor exceeds the MET66SD turbocharger compressor in pressure ratio. Also, the two turbocharger compressor pressure ratios shown plotted on the engine operating line in the performance curves of Fig.2 are shown each correlated with the turbocharger rotating speed in Fig.3.
It can be seen from Figs.2 and 3 that the employment of the compressor impeller blading of small backward curvature angle is effective in restraining the impeller centrifugal stress to a low level and, hence, ensuring a prolonged compressor impeller service life.
