Principles And Construction Of Turbocharging consist of a single-stage impulse turbine connected to a centrifugal impeller via a shaft. The turbine is driven by the engine exhaust gas, which enters via the gas inlet casing. The gas expands through a nozzle ring where the pressure energy of the gas is converted to kinetic energy. This high-velocity gas is directed onto the turbine blades where it drives the turbine wheel, and thus the compressor at high speeds (10 -15000 rpm). The exhaust gas then passes through the outlet casing to the exhaust uptakes.
On the airside air is drawn in through filters, and enters the compressor wheel axially where it is accelerated to high velocity. The air exits the impeller radially and passes through a diffuser, where some of the kinetic energy gets converted to pressure energy. The air passes to the volute casing where a further energy conversion takes place. The air is cooled before passing to the engine inlet manifold or scavenge air receiver.
The nozzle ring is where the energy in the exhaust gas is converted into kinetic energy. It is fabricated from a creep resistant chromium-nickel alloy, heat-resisting moly-chrome nickel steel or a nimonic alloy which will withstand the high temperatures and be resistant to corrosion.
Turbine blades are usually a nickel-chrome alloy or a nimonic material (a nickel alloy containing chrome, titanium, aluminium, molybdenum and tungsten) which has good resistance to creep, fatigue and corrosion. Manufactured using the investment casting process. Blade roots are of fir tree shape which gives positive fixing and minimum stress concentration at the conjunction of root and blade. The root is usually a slack fit to allow for differential expansion of the rotor and blade and to assist damping vibration. On small turbochargers and the latest designs of modern turbochargers, the blades are a tight fit in the wheel.
Lacing wire is used to dampen vibration, which can be a problem. The wire passes through holes in the blades and damps the vibration due to friction between the wire and blade. It is not fixed to each individual blade. The wire can pass through all the blades, crimped between individual blades to keep it located, or it can be fitted in shorter sections, fixed at one end, joining groups of about six blades. A problem with lacing wire is that it can be damaged by foreign matter, it can be subject to corrosion, and can accelerate fouling by-products of combustion when burning residual fuels. Failure of blading due to cracks emanating from lacing wire holes can also be a problem. All the above can cause an imbalance of the rotor.
The turbine casing is of cast iron. Some casings are water-cooled which complicates the casting. Water-cooled casings are necessary for turbochargers with the ball and roller bearings with their own integral LO supply (to keep the LO cool). Modern turbochargers with externally lubricated journal bearings have uncooled casings. This leads to greater overall efficiency as less heat energy is rejected to cooling water and is available for the exhaust gas boiler.
The compressor impeller is of aluminium alloy or the more expensive titanium. Manufactured from a single casting it is located on the rotor shaft by splines. Aluminium impellers have a limited life, due to creep, which is dictated by the final air temperature. Often the temperature of the air leaving the impeller can be as high as 200°C. The life of the impeller under these circumstances may be limited to about 70000 hours. To extend life, air temperatures must be reduced. One way of achieving this is to draw the air from outside where the ambient air temperature is below that of the engine room. Efficient filtration and separation to remove water droplets are essential and the impeller will have to be coated to prevent corrosion accelerated by the possible presence of saltwater.
The air casing is also of aluminium alloy and is in two parts.
Bearings are either of the ball or roller type or plain white metal journals. The ball and roller bearings are mounted in resilient mountings incorporating spring damping to prevent damage due to vibration. These bearings have their own integral oil pumps and oil supply and have a limited life (8000 hrs). Plain journal bearings are lubricated from the main engine oil supply or from a separate system incorporating drain tank, cooler and pumps. Oil is supplied in sufficient quantity to cool as well as lubricate. The system may incorporate a header tank arrangement to supply oil to the bearings whilst the turbocharger comes to rest should the oil supply fail. A thrust arrangement is required to locate and hold the rotor axially in the casing. In normal operation, the thrust is towards the compressor end.
Labyrinth seals or glands are fitted to the shaft and casing to prevent the leakage of exhaust gas into the turbine end bearing, or to prevent oil being drawn into the compressor. To assist in the sealing effect, air from the compressor volute casing is led into space within the gland. A vent to the atmosphere at the end of the labyrinth gives a guide to the efficiency of the turbine end gland. Discolouring of the oil on a rotor fitted with a roller bearing will also indicate a failure in the turbine end gland.
A labyrinth arrangement is also fitted to the back of the compressor impeller to restrict the leakage of air to the gas side.
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