Exhaust Gas Turbocharging System

Construction:-

The sketch shows a typical turbocharger fitted in large slow speed engines.

1. The main parts are the turbine rotor with blades and the impeller mounted on a common shafted enclosed in a volute casing.
2. The exhaust gas enters the water-cooled turbine casing through a grid, which prevents any broken pieces of piston ring from entering the turbine.
3. The exhaust gas passes through nozzles where the pressure energy is converted to kinetic energy.
4. The gas impinges upon the turbine blades, rotates the turbine wheel, and comes out of the exhaust gas outlet.
5. At the blower end, the impeller draws air from the engine room atmosphere through the silencer air filter.
6. The air inlet passage called Inducer. It guides the air smoothly to the impeller.
7. The impeller discharges the air radially through the diffuser and volute casing to the air cooler.
8. The diffuser and a volute casing have divergent shape, which reduces the speed of the air passing through them. This reduction in speed converts kinetic energy into pressure energy.
9. The turbine casing is water-cooled, but in some designs, the casing is air-cooled.
10. The rotating parts are supported on spring-mounted ball bearings at the ends.
11. Turbine driven pumps lubricate the turbochargers. There are two pumps which are on the blower side and other on the turbine side. The pumps direct the oil to the bearings.
12. Labyrinth glands prevent leakage of air and exhaust gas.
13. On the exhaust gas side the labyrinth gland is supplied with sealing air under slight pressure from the blower. This prevents exhaust gas from escaping to the oil space.

Operation:-

The sketch shows SULZER RTA series engine scavenging system.

1. The piston is at the top dead centre. During the downward stroke of the piston, when the crank is about 95° past the top dead centre, the exhaust valve begins to open.
2. When the crank is about 50° before the bottom dead centre the scavenge ports begin to open.
3. In a 2-Stroke engine, the scavenge and exhaust valve timing is symmetric about the dead centres. They are closed when the crank is about 50° past the bottom dead centre. The exhaust valve closes about 70° past the bottom dead centre.
4. The exhaust gas passes through the turbine side of the turbocharger via the cylinder exhaust valve and exhaust gas manifold.
5. The exhaust gas drives the turbine. It also passes through the economiser, where it is used to convert the feed water into steam.
6. The turbine is connected to a centrifugal blower. The rotation of the turbine rotates the blower. The blower rotation causes air from the engine room to pass through an air filter, cooler, water separator and delivers to the scavenge air receiver.
7. The scavenge air receiver has an inner and an outer compartment with a set of non-return valves between them.
8. From the outer compartment, air enters the inner compartment. From the inner compartment, the scavenge air enters the cylinders when the piston uncovers the scavenge ports towards the end of its downward stroke.
9. An electric motor drives the auxiliary blower.
10. During manoeuvring, auxiliary blower draws air from the outer compartment and delivers to the inner compartment via another set of non-return valves.
11. The operation of the blower is automatic. The blower starts whenever the scavenge air pressure drops below a preset limit. It stops automatically when the pressure is sufficient.
12. If the turbocharger breaks down, the auxiliary blower delivers sufficient air to run the engine at reduced power to reach the next port.

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Authored By:- Cdt. Swapnil Pawar, TMI