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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.
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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.
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Power of a two stroke diesel engine = pm x L x A x N x no. of
cylinders Where pm = mean effective pressure;L = stroke of the
engine;A = cross sectional area of the cylinder; N = revolution per
second of the engineThus to increase the power of the engine of
given swept volume i.e. the power to weight and volume ratios of
the engine we have to increase either mean effective pressure or
the revolution per second of the engine. The approach of increasing
power output by increasing speed is unattractive, due to rapid rise
of mechanical and aerodynamic losses, and the corresponding fall in
brake thermal efficiency.
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For increasing the mean effective pressure, more fuel has to be
burnt during the cycle of the engine, which requires higher
quantity of air per cycle of the engine. The purpose of
supercharging is to increase the mass of air trapped in the
cylinders of the engine, by raising its density. A compressor is
used to achieve the increase in air density.
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Two methods of supercharging can be distinguished by the method
to drive the compressor. If the compressor is driven from the
crankshaft of the engine, the system is called mechanically driven
supercharging or often just supercharging. If a turbine drives the
compressor, which itself is driven by the exhaust gas from the
cylinders, the system is called turbocharging. The shaft of the
turbocharger links the compressor and the turbine, but is not
connected to the crankshaft of the engine.
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The advantage of the turbocharger, over a mechanically driven
supercharger, is that the power required to drive the compressor is
extracted from the exhaust gas energy rather than the crankshaft.
Thus turbocharging is more efficient than mechanical supercharging.
However the turbine imposes a flow restriction in the exhaust
system, and therefore the exhaust manifold pressure will be greater
than atmospheric pressure.
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If sufficient energy can be extracted from the exhaust gas, and
converted into compressor work, than the system can be designed
such that the compressor delivery pressure exceeds that at turbine
inlet, and the inlet and exhaust processes are not adversely
affected. For a compressor pressure ratio of 5, allows to increase
the specific power output of the engine by 400%.
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Compressor characteristic and the surge limit
Centrifugal compressor characteristics are similar to those of
centrifugal pumps. At a constant RPM, the characteristic would
appear similar to the figure. At constant speed the discharge
pressure first rises as volumetric flow increases and then drops
off rather sharply. The compressor efficiency curve also rises to a
peak, although at any constant this peak is to the right of the
pressure peak. The power consumed by the compressor is related to
the product of discharge pressure and flow rate.
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In the region to the right of the peak in pressure curve,
operation will be stable: in this region a momentary drop in
volumetric flow rate, for example, perhaps brought on by a
momentary reduction in engine speed, will be countered by a rise in
pressure, with little or no effect on the turbine. In the region to
the left of the pressure peak, a momentary drop in volumetric flow
rate will be accompanied by a drop in discharge pressure and a
reduction in compressor power consumption.
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Operation in the unstable area to the left of the pressure peak
may result in compressor surge. As the pressure at the compressor
discharge falls below that downstream, the flow can reverse. The
result can simply be a pulsation if the situation is not severe or
of long duration, or the reversed flow can continue to the air
intake and become audible, ranging in volume from a soft sneezing
to a very loud backfiring sound.
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Obviously, operation in the surge region should be avoided;
consequently, turbocharger designers establish a line, called a
surge limit, through the pressure characteristics slightly to the
right of the peak. Similar data as previous figure are obtained at
several constant speeds covering the range of operation, and
plotted together on the same axes. The resulting compressor
performance map is shown.
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Cleaning Turbochargers in operationPeriodic cleaning reduces or
even prevents contamination, allowing significantly longer
intervals between overhauls. The proposed cleaning method, carried
out periodically, will prevent a thick layer of dirt from forming.
A thick layer of dirt can cause a drop in efficiency and increased
unbalance on the compressor side of the turbocharger, which could
influence the lifetime of the bearings.
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The compressor wheel of the turbocharger can be cleaned during
operation by spraying water into the air inlet casing. The dirt
layer is removed by the impact of the injected water. Since the
liquid does not act as a solvent there is no need to add chemicals.
The use of saltwater is not allowed, as this would cause corrosion
of the aluminium compressor wheel and the engine. Water is injected
from a water vessel that holds the required quantity of water.
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ProcedureThe best results are obtained by injecting water during
full-load operation of the engine, i.e. when the turbocharger is
running at full speed. The complete contents of the water vessel
should be injected within 4 to 10 seconds. Successful cleaning is
indicated by a change in the charge air or scavenging pressure, and
in most cases by a drop in the exhaust gas temperature.
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If cleaning has not produced the desired results, it can be
repeated after 1 0 minutes. The interval between compressor
cleanings will depend on the condition of the turbocharger suction
air. It can vary from 1 to 3 days of operation. If a very thick
layer has built up and it cannot be removed using the method
described, it will be necessary to dismantle the turbocharger in
order to clean the compressor side. Since the dirt layer is removed
by the kinetic energy of the water droplets, the engine has to be
run at full load.
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Cleaning the TurbineThe combustion of heavy fuel in diesel
engines causes fouling of the turbine blades and nozzle ring. The
result of this fouling is reduced turbine efficiency and engine
performance as well as an increase in the exhaust gas temperature,
Experience has shown that the contamination on the turbine side can
be reduced by regular cleaning in operation, and that such cleaning
allows longer intervals between turbocharger overhauls.
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Some of the deposits have their origin in soot, molten ash,
scale and unburned oil, partially burnt fuel and sodium
vanadyl-vanadat. Investigations have shown that most of the
residues are caused by the calcium in the lube oil reacting with
the sulfur from the fuel to form calcium sulfate during the
combustion process.
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The quantity of the deposits depends on the quality of the
combustion, the fuel used, and the lube oil consumption. The
frequency with which cleaning has to be carried out depends on the
extent of the contamination on the turbine side.Two cleaning
methods existWet cleaning (water injection)dry cleaning (solid
particle injection)
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Procedure for wet cleaningThe exhaust gas temperature before the
turbine should be in the range of 200 to 4300 CThe boost pressure
should be above 0.5 bar to prevent water entering the oil chamber
on the turbine side.The quantity of injected water will depend on
the exhaust gas temperature, water pressure, size of the
turbo-charger and number of gas inlets.
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Water should be injected for 5 to 10 minutes. Check if the water
has entered the turbine parts by opening the drain of the gas
outlet casing. Water flowing out provides assurance that enough
water has passed the nozzle ring and the turbine blades. The
interval between turbine cleanings will depend on the combustion,
the fuel used and the fuel oil consumption. It can vary from 1 to
20 days of operation. Principle:The dirt layer on the turbine
components is removed by thermal shock rather than the kinetic
energy exerted by the water droplets.
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Procedure for dry cleaningThe exhaust gas temperature before the
turbine should not exceed 5800 C.Dry cleaning has to be carried out
more often than water cleaning as it is only possible to remove
thin layers of deposits. A cleaning interval of 1 to 2 days is
recommended.To ensure effective mechanical cleaning, granulated dry
cleaning media are best injected into the turbine at a high
turbocharger speed.The quantity needed will vary from 0.2 l to 3 l,
depending on the size of the turbocharger.
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Experience has shown that the best results are achieved with
crushed nut-shell or granulate.Principle:The layer of deposits on
the turbine components is removed by the kinetic energy of the
granulate causing it to act as an abrasive. Experience has shown a
combination of the two to be very effective, especially in the case
of 2-stroke engines.