TurbochargerTwo stroke crosshead engines must be supplied with air above atmospheric pressure for it to work. Although turbochargers were developed in 1925, it was not until 1950s that large two stroke engines were turbocharged. Pressurized air is needed to scavenge the cylinders of the exhaust gases and supply the charge of air for next combustion cycle was provided by mechanically driven air compressors (roots blower) or by using the space under the piston as a reciprocating compressor (under piston scavenging).

This of course meant that the engine was supplying the energy to compress the air, which meant that the useful work obtained from the engine was reduced by this amount. The amount of energy that can be produced by a diesel engine depends upon amount of fuel burning per cycle and rpm of the engine. Increase in rpm of the engine leads to reduction of efficiency of the propeller for an engine driving a propeller.

Thus increase in power by the route of increase in rpm is not desirable for this type of marine diesel engine. So the first route is preferable. In order to burn more fuel per cycle, the mass of oxygen (thereby the air) supplied to the engine per cycle should be proportionately increased. To ensure that exhaust gases are scavenged properly and sufficient air free of exhaust gases are left after scavenging for compression and ultimate burning the fuel, 200% extra air (depending upon method of scavenging is supplied than the mass required for stoicmetric combustion (this air helps in cooling the liner and exhaust valve).

About 35% of the total fuel energy goes out in the exhaust gas. The turbocharger uses 7% of the total energy (20% of the exhaust gas energy) to drive a single row turbine. The turbine shaft drives a rotary compressor. Air is drawn and compressed. Due to compression, the air temperature rises. Hence it is cooled in a cooler to increase its density and then sent to the air inlet manifold or scavenge air receiver. At full power of diesel engine, the turbocharger may be rotating at > 10000rpm.

Starting the engine An electric motor driven auxiliary blower is provided to supply air during starting of a two stroke main diesel engine because in this condition the turbocharger is not supplying air to the engine to burn the fuel. This automatically cuts out when the charge supplied by the turbocharger is sufficient for combustion.

Advantages of turbocharger By turbocharging a diesel engine following advantages are there:Increased power from the engine of same size or reduction in size of engine with same output.Reduced fuel oil consumptionThermal loading is reduced due shorter more efficient burning period for the fuel, leading to less exacting cylinder conditions.

Turbocharger construction The turbocharger consists of a single stage impulse turbine connected to a radial centrifugal compressor via a shaft. The turbine is driven by the diesel engine exhaust gas, which enters via the gas inlet casing. The gas expands through the nozzle ring where pressure and thermal energy is partially converted to kinetic energy.

This high velocity gas is directed on to the turbine blades, which drives the turbine wheel at high speed. The exhaust gas then passes to the gas outlet casing to the exhaust uptake. On the air side, air is drawn through suction filter and enters the compressor wheel and moves radially and gets accelerated to high velocity.

The high kinetic energy of the air gets converted to pressure energy after passing through diffuser and volute casing of the compressor. The nozzle ring of the turbine is made from creep resistant chromium nickel alloy steel, heat resisting molybdenum chrome alloy steel or nimonic alloy, which will withstand the high temperature and be resistant to corrosion.

Turbine blades are normally a nickel chrome alloy or a nimonic material (an alloy of nickel containing chrome, titanium, aluminium, molybdenum and tungsten), which has good resistance to creep, fatigue and corrosion. Manufactured using investment casting procedure. Blade roots are of a fir tree shape, which gives positive fixing and minimum stress concentration at the conjunction of root and blade.

The root is usually slack fit to allow for differential expansion of the rotor and blade and to assist in damping vibration. On small turbochargers and the latest modern turbochargers, the blades are tight fit in the wheel. Lacing wire is used to dampen the vibrations, which can be a problem. The wire passes through holes in the blades and damps the vibrations due to friction between the wire and 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 subjected to corrosion and can accelerate fouling by products of combustion when burning heavy fuel oil. Failures of cracks emanating from lacing wire holes can also be a problem. All the above can cause 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 ball and roller bearings with their own integral lube oil supply to keep the lube oil 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 exhaust gas economizer.

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 limited life due to creep, which is dictated by final air temperature. Often the temperature of air leaving the impeller can be as high as 2000 C. The life of the impeller under these circumstances may be limited to about 70000 hours.

To extend the life, air temperatures must be reduced. One way of achieving this is to draw the air from outside the engine room, where the ambient air temperature is below that of the engine room. Efficient filtration and separation to remove water droplets is essential and the impeller will have to be coated to prevent corrosion accelerated by the possible presence of salt water. The air casing is also of aluminium alloy and is in two parts.

Bearings are either ball or roller type or plain white metal journals. The ball and roller bearings are 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 (about 8000 hours). Plain journal bearings are lubricated from the main 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 (during power supply failure). 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 the space within the gland. A vent to 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.