Turbochargers

With reference to large medium speed diesel engine turbo chargers, describe how the thrust bearing is lubricated

It is lubricated from the engine driven oil pump. Plain bearings in turbos use oil to keep parts from rubbing each other based on the hydrodynamic principle. In hydrodynamic lubrication, the fluid (oil in a turbo) stays between the moving surfaces and prevents metal-to-metal contact.

Describe the indications of surging

Noises from the air compressor side; turbocharger may race intermittently; air discharge pressure will drop and behave erratically.

State the precautions to be taken during water washing

1. During the water washing process the water drains must be open to avoid the water with any contaminants from entering into the engines cylinders. 2. To avoid thermal shock, before washing, engine power should be reduced to allow cooling of hot turbine surfaces. 3. Great care exercised since removal of deposits may affect the balance of the turbine rotor.

Describe how to minimize turbocharger vibration.

1. Idle the engine for two to three minutes to cool down the turbo bearings before shutting off the engine, and thus the (cooling) oil supply.

Describe the 3 causes of surging

1. The engine is operating with one of turbochargers faulty. 2. Fouling on the air side may include restrictions on the impeller and diffuser, intake filter or charge air cooler. 3. The gas side fouling may be in the turbine, nozzle rings, exhaust grids or moving blades.

Sketch a cross section of a radial turbo

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Explain a diffuser

A static diffuser (vane) ring placed around the impeller in Diffuser ring centrifugal pumps. The vanes form a series of divergent nozzles that act to further convert the high velocity of the pumped fluid to pressure energy.

Explain the term surge

Abnormal change in either air or exhaust gas flow to or from the charger.

Explain an inducer

An axial flow impeller with blades that wrap in a helix around a central hub. It serves as a small booster pump for the main impeller and improves efficiency.

Explain the possible causes of turbocharger vibration whist operating at a steady speed

Bearing damage, loss in balance, damaged blade.

Explain the consequences of increase fouling on the turbine blades

Deposits on turbine blades leads to increased resistance to gas flow. Distortion of air passages leads to pressure drop, reducing the capacity of the turbine. Severe conditions lead to excessive rotor thrust. Uneven deposition can unbalance the turbine rotor, causing vibration problems.

Explain how the turbocharger is cooled.

Smaller turbochargers are air cooled; Larger turbochargers circulate cooling water from the main engine through the jacket in the outlet casing surrounding the exhaust gas passage in which the turbine wheel and nozzle ring is situated

Explain how the turbocharger is lubricated

The shaft may rotate in white metal bearings that can be lubricated from the engine driven oil pump. Or, in smaller chargers, the bearings may have their own reservoir which forms part of the turbo charger. These reservoirs usually have round oil level sight glasses with two horizontal lines marked to indicate the high and low levels. Seals are fitted to retain the oil.

With reference to large medium speed diesel engine turbo chargers, explain why a thrust bearing is needed

Thrust bearing is the part of the bearing system that manages thrust loads. Thrust loading pushes the rotating assembly back and forth and is created by pressure differentials in the compressor and turbine housings. Thrust bearing is usually fixed at compressor end. It not only has a direct impact on the working life and reliability of turbocharger but also influences its mechanical efficiency.

Explain a labyrinth gland on a turbocharger

To prevent oil loss past the compressor end, air is bled from the compressor casing via labyrinths on the rear face of the impeller. An air supply also feeds through tubular passages in the centre casing, which in turn pressurises the turbine end preventing exhaust gas and oil leakage, and also serves to cool the turbine disc.

Explain the procedure for cleaning the turbine blades

Water washing. 1. Compressor side: spray a small quantity of fresh water into the air stream at the compressor inlet. 2. Turbine side: spray water into the gas flow, upstream of the protection grids for the turbine.

Describe the function of a main engine turbocharger.

a) The turbo charger supplies a larger mass of air into the cylinder to that of a same cubic capacity normally aspirated engine, which allows for a proportional increase in the amount of fuel that can be injected and burnt in the cylinder thereby providing an increase in the power output of the engine.