Turbocharger
• 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 engine
• Thus 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.
• 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.
• If the compressor is driven from the
crankshaft of the engine, the system is
called ‘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.
• 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.
• 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%.
• 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.
Constant pressure turbocharging
of two-stroke engines
The two-stroke engine is neither self-aspirating
nor self-exhausting. It relies on a positive
pressure drop between the inlet and exhaust
manifold in order to run at all. The scavenging
process, in which fresh charge is forced in and
residual gas goes out, is the key to a successful
two-stroke engine. It follows that the two-stroke
engine is far more dependent on a reasonable
pressure drop being developed across the
cylinder than is the case with the four-stroke
engine. The four-stroke engine will work with an
adverse pressure gradient, the two-stroke will
not.
• To obtain reasonable scavenging, it is
essential to pass some air right through
the system and to waste, since
considerable mixing of air and combustion
products takes place. The amount of
‘excess air’ required would depend on
scavenging efficiency of the system being
used and many other factors. The amount
will vary from 10% to 40% (from uniflow to
cross scavenging).
• The consequence to the turbocharger is twofold. Firstly,
the gas in the exhaust manifold will be diluted with cool
scavenge air lowering the turbine inlet temperature.
Secondly, a penalty must be paid for compressing the
excess air since, although it will be expanded through
the turbine in due course, only part of the energy will be
regained, due to compressor and turbine inefficiencies.
Thus not only has the turbocharger have a more difficult
job than on a four-stroke engine, since it must provide a
positive pressure drop across the cylinder, but it is
required to work under the adverse conditions mentioned
above. It was for these reasons that turbocharged twostroke diesel engines first appeared with some form of
compressor assistance, and many now operating on the
constant pressure system, still retain it.
• The exhaust temperature will be lower for
two-stroke engines. Typical values of
mean turbine inlet temperature on highly
rated two-stroke and four-stroke engines
at full load might be 4000C and 4500C
respectively. For a pressure ratio of 2.5:1,
a four-stroke engine will perform
adequately if the turbocharger efficiency is
50% or more, whilst the two-stroke engine
will require a turbocharger efficiency of
over 55%.
• A large, well designed turbocharger can
develop 60-65% overall efficiency, but
some allowance of about 5% must be
made for a reduction in efficiency in
service (due to fouling- build up of dirt and
carbon). In practice this means that a well
designed engine having a good
scavenging system, with little pressure
loss, will run satisfactorily at full load
provided that it has a well matched,
efficient turbocharger.
• At part load, the turbine inlet temperature
will fall due to the lower air-fuel ratio.
Turbocharger efficiency will probably also
fall since the turbocharger would usually
be matched for optimum working
conditions near full load. If the turbine inlet
temperature drops below 3000C and the
turbine efficiency below 55%, then the
engine will stop. Thus all two-stroke
engines using the constant pressure
turbocharging system require some
additional aid for scavenging for starting
and part load operation.
• In the past, the underside of the piston
was used in cross-head type engines, as
compressor placed in series with the
turbocharger compressor. As engine and
turbocharger design have improved, the
contribution of the scavenge pump to
overall compression has reduced, so that
simple electrically driven fans are used
today. These are switched off once the
engine load and speed are such that the
turbocharger can provide the necessary
positive pressure differential across the
cylinders.
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.
• 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.
• 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.
• 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.
OPERATIONAL CHECKS
• Turbocharger Maximum RPM – within limits
• High air temperature reduces efficiency
• More than 100 mm water drop across air suction
filter indicates fouling
• Low cooling water temperature – drop in thermal
efficiency and cold corrosion on gas side.
• High cooling water temperature – hot corrosion on
gas side and reduced life of components
• Reduced exhaust gas temperature drop across
turbine – fouling of nozzle and blades – hence drop
in turbocharger efficiency
LUB OIL SYSTEM – Ball & roller bearings
1.
2.
3.
4.
5.
Level checked and maintained
Bearings supplied with luboil, specially after luboil
renewal
Maximum allowable temperature not exceeded.
(Luboil temperature depends on turbocharger
rpm, suction air inlet temperature and cooling
water temperature at turbine end).
Colour of luboil closely monitored – dark colour
indicates exhaust gas leakage. Gas leakage to be
rectified and luboil renewed at the earliest.
Foaming of luboil to be checked. (Foaming of
luboil is caused by Contamination). Luboil to be
renewed.
LUB OIL SYSTEM- Plain sleeve bearing
1.
Luboil pressure within specified range.
2.
Luboil return line flow checked from the sight glass.
3.
Level of the luboil in reservoir tank to be maintained.
4.
Check for the non return valve in the lube oil supply
line to the turbocharger. Fit the non return valve in the
oil supply line after consulting the management office,
if not provided.
5.
The lube oil system with plain sleeve type bearings are
normally fitted with an auto clean (motorized drive )
filter . Please check regularly for operation of the motor
and pressure differential ,clean filter regularly.
VIBRATIONS AND NOISE
1.
Closely monitor Turbocharger vibrations. If the
vibration monitor is fitted, check the indicator for
abnormal readings. Any indication of abnormality
should be immediately attended and rectified by
stopping the engine. Check the foundation bolts,
all casing bolts and pipe connections for tightness.
2.
Listen to any unusual noise and surging of
turbocharger, erratic operation can hence be
determined at an early stage, engine to be
stopped and problem rectified.
CLEANING TURBOCHARGER
DURING OPERATION
1. Clean Blower side daily with fresh water to
keep blower impeller clean.
2. Clean Turbine side with dry cleaning method if
provided, at least once in three days at full
load or once in a week with wet water washing
method at reduced output.
3. Drain accumulated moisture from the gas
outlet casing periodically.
4. The exhaust gas outlet casing drain valve to
be kept in open position during the period
when engine is stopped.
RUN DOWN TIME
• After the engine has stopped the rotor of the
turbocharger keeps turning for sometime due to
its moment of inertia. The run down time is
indicative of the mechanical condition of the
turbocharger. When rotor comes to a sudden
stop, it is usually due to mechanical damage of
bearings, or rubbing of compressor wheel or
turbine wheel against stationary parts, or foreign
matter having become wedged between moving
parts.
ROUTINE CHECKS
1.
Check periodically the anti corrosion plugs in the
cooling water spaces of the turbine casings and
replace if necessary. Check cooling water spaces of
the turbine casing for contamination / deposits.
2.
Inspect Turbocharger grating fitted in the exhaust
manifold on the turbocharger gas inlet side for fouling.
3.
With integral feed oil, renew luboil every 500 hrs and
not longer than 1000 hrs. if mineral oil is used, (if
synthetic oil is used then renew after 5000 hrs).
Ensure oil spaces are cleaned.
4.
In case of Mineral oil, oil sample to be landed for
analysis every 3 months along with other luboils.
PREVENTIVE MAINTENANCE
1.
Exhaust gas leakages and lube oil losses in the engine
room should be rectified to keep atmosphere clean.
Suction of contaminated air causes deposits on filter
meshes and blower thus reducing turbocharger
efficiency.
2.
Poor combustion in engine can lead to erosion of
nozzle ring and turbine blades and formation of
deposits on the nozzle ring and blades. This will lead
to a drop in turbocharger efficiency and increased
vibrations due to rotor imbalance.
3.
Avoid running engine on part loads for extended period
of time. Run the engine on full load for four hours for
every 24 hours of part load operation. During the four
hours of full load operation clean Blower side with
fresh water and dry wash turbine side, if provided.
4.
Use clean recommended luboil, keep oil spaces clean.
5.
Carry out major overhaul as per the intervals specified
in the manual. At every major overhaul turbocharger to
be dismantled, all parts to be cleaned and inspected,
bearings to be replaced. In case of turbocharger with
ball and roller bearings with integral oil feed,
pump assembly to be renewed.
lube oil
6.
Check axial play of the rotor as per instructions given in the
manual.
7.
Check Nozzle ring and turbine blades, shroud ring for
erosion, mechanical damages and fuel / carbon deposits.
8.
Check Blower wheel for deposits and mechanical damages.
9.
Sealing air passages, balance passages and sealing
bushes must be kept clean and Sealing bushes must be
firm in their casing. Check labyrinth strips for damage.
Replace / repair if necessary.
10. Rotor to be dynamically balanced and balancing witnessed
by Chief Engineer during every major overhaul.
IMPORTANT NOTE
• NEVER TAKE UP MAIN ENGINE TURBO
CHARGER OVER HAUL BY SHIP’S
ENGINEERS. INSTEAD MAKER
SERVICE ENGINEER MUST BE
PRESENT DURING COMPLETE
OVERHAULS.