The reduction in the turbocharger performance and hence reduced engine output after turbocharger overhaul is a clear evidence of improper overhauling procedure and assembly. Possible faults during overhauling are following:
- The compressor axial clearances set during the bearing reassembly are not correct and the rotor could be fouling the casing, hence the rotational speed would fall. If the turbocharger rotates at a slower rate, then there will be less air delivered and hence lower pressure.
- The turbocharger casings have not been correctly reassembled, causing air leakage. This leakage would reduce the pressure generated at the turbocharger outlet
- The turbocharger filters were not cleaned, and the restriction would cause an air flow restriction. The pressure ratio is known across the turbocharger at certain operating speeds, would cause the discharge pressure to fall if the intake or suction pressure is low.
The following engine operational information should be gathered in order to detect the possible cause of the problem with turbocharger overhaul:
- Engine power
- Scavenge and exhaust manifold pressures
- turbocharger speed
- Inlet/outlet gas, air and water temperatures
- Differential pressures of the air filter, air cooler, and exhaust gas boiler (if fitted)
Once these readings were taken, they could be checked against previous data from before the overhaul, or even datum readings when the engine and turbocharger were new. Abnormalities would be noted, and this would hopefully indicate the possible defect. Specific faults such as the air filter fouling would be indicated by the measurement of the pressure differential across the filters.
The defects stated would arise due to an incorrect procedure during turbocharger overhaul, and lack of suitable checks to ensure correct reassembly. A new procedure detailing the areas to be checked would be drawn up. This procedure would list the specific checks and measurements that must be taken during reassembly. If a specific fault has previously occurred, then a method to overcome this defect would be recorded in the new procedure.