3.3 TURBOCHARGER

Exhaustive performance measurements were carried out on ABB's performance test rigs with different turbine and compressor specifications. A typical result of these tests is shown in Fig.6 comparing the TPS57D/E with the VTR251 turbocharger which is known and still used in the Japanese market. A remarkable improvement in efficiency and pressure ratio is recognized. The measured efficiency levels combined with the high pressure ratio capabilty of the IPS..D/E turbochargers provide a very competitive performance level for this type of turbocharger design.

　

Fig.6: Turbocharger efficiency as a function of compressor pressure ratio for the TPS 57 D/E and the VTR251 along< a typical engine operating line

　

4. MECHANICAL TESTING

　

To ensure turbocharger reliability, ABB Turbo Systems Ltd. has developed a stringent mechanical qualification program which includes:

　

・extended dynamic blade strain measurements on the compressor and the turbine

・high cycle fatigue tests on the turbine blades

・evaluation of shaft motion for all possible bearing clearance combinations

・load carrying capacity and limits of the thrust bearing tests

・compressor and turbine end seal oil leaking tests

・turbine casing thermal cycling tests

・hot shutdown tests

・containment tests for compressor and turbine casing

　

All new turbochargers including the TPS..D/E series had to pass these tests in order to be released for sale. Sections 4.1 to 4.5 describe some of these tests and summarize their results. Some of the results also include field results from turbochargers which have reached 10,000 running hours.

　

4.1 BLADE STRAIN MEASUREMENTS

The purpose of blade strain measurements is to confirm that no dangerous resonance amplitudes occur which could lead to severe damage of the turbocharger. The dynamic blade strain measurements were carried out on both compressor wheels and on the turbine wheel. The locations of the strain gauges were chosen based on finite element calculations of the natural frequencies and modes to determine areas of maximum strains.

The strain gauge signals were transmitted from the rotating shaft to the automatic signal acquisition and processing system by means of high-speed mercury slip rings which had been specially developed for this purpose. For both compressors, the measured strains were clearly below the safety limit; the one percent failure limit at 108 cycles of the aluminum alloy used for the compressors was chosen as the safety limit. In order to prove the reliability of the turbine for pulse supercharging the above mentioned tests had to be carried out with a partial admission of the nozzle ring to simulate the pulse effect on the test rig. The reliability of this method was confirmed by additional measurements on an engine which was supercharged with a pulse system. In addition to the blade strain measurements, high cycle fatigue tests were also carried out to verify the operating safety of the turbine. Two shafts were run in the critical resonances until at least 1.5 × 10⁷ load cycles were accumulated for each resonance. All these test results were built into a computer code which can determine turbine safety and reliability based on the pressure amplitudes measured in exhaust pipes. A corresponding analysis of modem engines (see Fig.7) shows that the pressure amplitudes occuring in exhaust pipes can be safely handled by the TPS..D/E turbine impeller.

　

Fig.7: Dynamic turbine stresses of TPS 57 D/E

　

4.2 BEARING RELIABILITY

　

According to ABB turbocharger design guidelines the radial turbocharger shaft motions in operation must remain below 40% of the radial clearance to ensure the reliable operation of the turbocharger.

For the qualification of the TPS57E turbocharger shaft motion measurements were performed with all extreme combinations of clearances and two different rotor unbalance levels, one of which represents the maximum level expected in operation with a turbine contaminated by combustion residues from heavy fuel operation. The lubrication oil pressure was varied within the expected limits of the engine operation conditions during these tests. The resultant maximum shaft motions for the most critical clearance combination are shown in Fig.8. During all tests only synchronous vibrations were present; no self-excited vibrations due to oil film instabilities occurred as explained in [4].

It is known that under severe running conditions - particularly on engines running on HFO or waste gas - this extremely smooth running behavior could be disturbed by the following parameters:

1. oil coking in the bearings resulting in restricted movement of the bearing bushes and blocked wedges in the thrust bearing

2. erosive wear from particles in the lubrication oil increasing the bearing clearances

3. corrosion of the bearing material

Consequently the impact of these phenomena also has to be considered, in addition to the qualification of the turbocharger in the new condition.

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