4.5 CONTAINMENT
Burst tests are used to ensure that even in the worst case scenario, in which the compressor or the turbine wheel burst, all parts remain within the turbocharger casings ensuring no risk of injury to personnel. The containment test with the TPS..D/E was carried out by choosing the largest trim of the compressor and the turbine wheel. In order to achieve the required impeller burst speed of approximately 120% of the maximum turbocharger speed, the wheel had to be strongly weakened since the natural burst speed of the unweakened wheel lies over 40% above the maximal turbocharger speed.
After operating the turbocharger for a few minutes at the maximum speed, the inlet conditions were changed to cause the sudden acceleration up to impeller burst speed. At that speed the impeller breaks into two equal pieces while the shaft continues to accelerate until the turbine wheel bursts at its natural burst speed. The design of the compressor casing ensures that all parts of the impeller remain inside the casing while most of the kinetic energy of the turbine pieces is absorbed either by the inner or outer burst protection ring. Fig.2 shows the turbine side protection in detail.
5. ENGINE PERFORMANCE
During the development of the TPS..D/E turbochargers extensive simulations of representative engines' behavior were carried out with ABB's simulation program entitled "SiSy" [3].
When modern engines with very high levels of mean effective pressures are considered, it becomes evident that it is no longer possible to find one suitable matching for both constant and variable speed applications.
A general study has shown that the minimal acceptable level of part load efficiency for safe operation according to the propeller law is a function ofthe full load efficiency. The matching parts of the TPS..D/E turbochargers were developed to alternatively allow a full load and a part load matching on the compressor and on the turbine side as indicated in section 3 of this paper.
Fig.15 shows the curves for a small medium-speed engine, i.e., a Wärtsilä 4L20B supercharged with a TPS48E as shown in Fig.16.
These curves are coupled to a FPP operating with pulse turbocharging. It can be seen that only the part load matching allows a safe operation above the limit of excess air ratio (λv) for heavy fuel, with a marginal loss of full load performance. These initial engine measurements have confirmed ABB's matching philosophy and calculated turbocharger performance level based on component efficiencies.
A similar study was carried out for the transient behavior of the supercharged diesel engine. The needs are similar to those of a part load optimized turbocharger, but the analysis had to include very low pressure ratios.
With the improvement of the thermodynamic matching it is possible to achieve a better acceleration with the TPS..D/E than with the RR.. 1 turbochargers, despite the increased weight of the rotating parts.
The objective of all of these studies was to achieve the best possible performance in respect to part load and transient behavior.
In extreme cases, when the torque demand goes far beyond the propeller law, or with constant pressure turbocharging, the matching of turbocharger components cannot help anymore. For such cases a regulated turbocharging system such as waste gate and air-by-pass is necessary; special options on the turbocharger side (variable turbine geometry, jet assist) can also improve performance.
Fig.15: Operating curves with different turbocharger matchings (propeller applications)
Fig. 16: Wärtsilä 4L20B engine supercharged with an ABB TPS 48 E turbocharger
